Design, Fabrication, Performance Testing, and Modeling of Diffusion Bonded Compact

Heat Exchangers in a High-Temperature Helium Test Facility

DISSERTATION

Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University

By

Sai K. Mylavarapu, M.S.

Graduate Program in Nuclear Engineering

The Ohio State University

2011

Dissertation Committee:

Prof. Xiaodong Sun, Advisor

Prof. Tunc Aldemir

Prof. Richard N. Christensen

Prof. Richard S. Denning

Copyright by

Sai K. Mylavarapu

2011

ii

Abstract

Very High-Temperature Reactor (VHTR) is a leading candidate for the U.S. Department

of Energys Next Generation Nuclear Power Plant (NGNP) project. It is a helium gas-

cooled reactor with very high reactor core outlet temperatures (800-950oC) and offers

high-efficiency electricity generation and a broad range of process heat applications,

such as coal liquefaction, coal gasification, and oil recovery from shale. To efficiently

transfer the core thermal energy to a secondary fluid, high-temperature and high integrity

intermediate heat exchangers (IHXs) with high effectiveness are required. While there is

no proven IHX concept for NGNP applications yet, a concept called printed circuit heat

exchangers (PCHEs) appears most promising. The current research focuses on the

design, fabrication, thermal-hydraulic performance testing, and modeling of PCHEs

under high operating temperatures and pressures.

PCHEs are plate-type heat exchangers, fabricated by photochemical machining and

diffusion bonding. In the current research work, both these fabrication techniques have

been demonstrated on Alloy 617 plates, a high-temperature candidate material for VHTR

structural components. Two counter-current flow PCHEs have been designed and

fabricated using Alloy 617 plates and are installed in a small-scale high-temperature

helium test facility (HTHF). The HTHF has been designed and constructed at The Ohio

iii

State University as part of this research to facilitate experiments at temperatures and

pressures up to 800oC and 3 MPa, respectively.

Microstructural and mechanical characterizations studies performed on diffusion bonded

Alloy 617 specimens are discussed. This study provided confidence from a safety view

point insofar as the operation of the heat exchangers, under high temperature and pressure

conditions in the HTHF. Performance testing of the two counter-flow PCHEs in the test

facility has been completed at varied operating temperatures, helium pressures, and

helium flow rates. The PCHE inlet temperature and pressure were varied from 85-

390oC/1.0-2.7 MPa for the cold side and 208-790oC/1.0-2.7 MPa for the hot side,

respectively, while the mass flow rate of helium was varied from 15 to 49 kg/h. The

maximum helium temperature that has reached at the exit of the main heater is 823oC.

This range of mass flow rates corresponds to PCHE channel Reynolds number of 950-

4,100 for the cold side and 900-3,900 for the hot side (corresponding to laminar and

laminar-to-turbulent transition flow regimes). The experimental data have been analyzed

for the pressure drop and heat transfer characteristics of the heat transfer surface of the

PCHEs and compared with the available models and correlations in the literature. In

addition, numerical and theoretical treatment of hydrodynamically developing and

hydrodynamically fully developed laminar flow through a semicircular duct is presented.

In summary, the PCHE testing at high temperatures and pressures and the experience

accumulated during the design and construction of the HTHF and the PCHEs will be of

value to the high-temperature reactor research.

iv

Dedication

This document is dedicated to my parents.

v

Acknowledgments

This work would not have been possible without the support of many people.

At the outset, I am greatly indebted to my advisor, Prof. Xiaodong Sun, for his expert

guidance and support throughout the duration of my study. I have learned immensely

from him and I am very thankful for his comments and suggestions during the research

meetings. I truly appreciate his confidence in me and allowing me to work on this

challenging project. He has been an excellent mentor and a guide.

I gratefully thank Prof. Christensen for his candid comments and expert discussion during

all the research meetings. His expertise on heat exchangers and experimental facilities

have been of great help.

I am very thankful to my committee members, Prof. Tunc Aldemir and Prof. Rich

Denning, for their comments and suggestions during my candidacy exam.

The support from the U.S. Department of Energy and Idaho National Laboratory for this

research work is gratefully acknowledged. My sincere appreciation and very many

thanks to the NGNP Project Manager, Mr. Michael Patterson for all his support.

vi

My heartfelt thanks to the NE faculty and NE/ME staff for their wonderful support during

my stay here at Ohio State. It has been a true pleasure and an honor knowing them. I

would like to thank all my friends and colleagues in the Nuclear Engineering Program.

My sincere thanks and appreciation to Noah Needler for helping me during the initial

stages of the project. I would like to personally thank David Arcilesi, Benjamin Doup,

Tae Kyu Ham, and Richard Glosup (order is not important) for making laboratory life

(and graduate life) interesting. All four have helped me a lot in one way or another

during the construction of the facility. Thanks to Richard Glosup and Benjamin Doup for

helping me with the experiments at various stages and willing to stay long nights during

the experiments. Thanks also to Ran Li for helping me with some of the CAD drawings.

I would be remiss if I did not acknowledge the support of Ralph Orr and Grace Hines.

Ralph Orr was instrumental in ensuring that all my purchase orders are expedited

(probably to escape my constant nagging).

Special thanks to my wife, Sai Prasanna Jayanthi for her continued support and help

throughout this period. She had to endure long hours of my absence during the

experiments and the dissertation. Finally, my warmest thanks to Dr. Satya Seetharaman

and Deepa for their wonderful company during our stay at the Buckeye Village.

vii

Vita

2003................................................................B. Tech. Mechanical Engineering,

Jawaharlal Nehru Technological University, Hyderabad, India

2004-2006 ......................................................Research Assistant, Indian Institute of

Technology, Bombay, India

2008................................................................M.S. Nuclear Engineering, The Ohio State

University

2007 to present ..............................................Graduate Research Associate, Nuclear

Engineering Program, The Ohio State University

Journal Publications

1. Mylavarapu, S.K., et al., 2009, "Investigation of High-Temperature Printed Circuit Heat Exchangers for VHTRs," Journal of Engineering for Gas Turbines and Power, Transactions of the ASME, 131(6), pp. 062905-0107. 2. Mylavarapu, S.K., et al., 2011,"Fabrication and Design Aspects of High- Temperature Compact Diffusion Bonded Heat Exchangers," Nuclear Engineering and Design, doi:10.1016/j.nucengdes.2011.08.043

Fields of Study

Major Field: Nuclear Engineering

viii

Table of Contents

Abstract ............................................................................................................................... ii

Dedication .......................................................................................................................... iv

Acknowledgments............................................................................................................... v

Vita .................................................................................................................................... vii

List of Tables ................................................................................................................... xiii

List of Figures .................................................................................................................. xiv

Chapter 1 : Introduction ...................................................................................................... 1

1.1 Motivation ................................................................................................................ 1

1.2 Intermediate Heat Exchanger .................................................................................... 3

1.3 Research Objectives .................................................................................................. 5

1.4 Previous Work ........................................................................................................... 7

1.5 Dissertation Organization ........................................................................................ 11

References for Chapter 1 ............................................................................................... 13

Chapter 2 : High-Temperature Helium Test Facility and Printed Circuit Heat Exchangers

........................................................................................................................................... 15

ix

2.1 Overview ................................................................................................................. 15

2.2 High-Temperature Helium Test Facility ................................................................. 16

2.2.1 Introduction ...................................................................................................... 16

2.2.2 Description of the Test Facility ........................................................................ 16

2.2.3 Design Aspects of OSU HTHF ........................................................................ 21

2.2.4 Simplified Stress Analysis for Loop Piping Design Calculations .................... 28

2.2.5 High-Temperature Helium Test Facility Components ..................................... 29

2.2.6 Instrumentation ................................................................................................. 39

2.2.7 Quality Assurance ............................................................................................. 41

2.3 Leak Testing ............................................................................................................ 43

2.4. Printed Circuit Heat Exchangers: Fabrication and Design Aspects ................... 45

2.4.1 PCHE Fabrication Techniques ........................................................................ 45

2.4.2 Design Aspects of PCHEs ................................................................................ 55

2.5 Microstructural and Mechanical Property Characterization of Diffusion Bonded

Specimens...................................................................................................................... 59

2.5.1 Microstructural Examination ............................................................................ 60

2.5.2 Mechanical Property Testing ............................................................................ 64

References for Chapter 2 ............................................................................................... 67

x

Chapter 3 : Developing and Fully-Developed Laminar Flow in a Semicircular Duct:

Theoretical and Computational Analysis .......................................................................... 71

3.1 Overview ................................................................................................................. 71

3.2 Dimensionless Groups and Basic Definitions ......................................................... 72

3.3 Scale Analysis for Laminar Flow in a Semicircular Duct ....................................... 76

3.3.1 Fully-Developed Laminar Flow ....................................................................... 77

3.3.2 Hydrodynamically Developing Laminar Flow ................................................. 79

3.4 Fully-Developed and Hydrodynamically Developing Laminar Flow in a

Semicircular Duct: Analytical and Numerical Treatment ............................................. 82

3.4.1 Analytical Solution for Laminar Fully Developed flow in a Semi-circular Duct

................................................................................................................................... 83

3.4.2 Computational Model: Results and Discussion ................................................ 92

References for Chapter 3 ............................................................................................. 103

Chapter 4 : Performance Testing of PCHEs in the High-Temperature Helium Test

Facility ............................................................................................................................ 105

4.1 Overview ............................................................................................................... 105

4.2 Heat Exchanger Performance Variables ............................................................... 105

4.3 Thermal-Hydraulic Models/Correlations .............................................................. 114

4.4 Procedure for Determining the Flow Friction Characteristics of the PCHE Heat

Transfer Surface .......................................................................................................... 118

xi

4.4.1 Contributions to Pressure Drop ...................................................................... 121

4.4.2 Discrepancy in Differential Pressure Across Hot Side of PCHE1 ................. 129

4.5 Proof of Linear Temperature Profiles ................................................................... 131

4.6 Magnitude of the Wall Conduction Resistance ..................................................... 133

4.7 Uncertainty Analysis ............................................................................................. 136

4.8 Flow Maldistribution ............................................................................................. 147

4.9 PCHE Performance Experiments in the HTHF: Results and Discussion ............. 149

4.9.1 Experimental Procedure ................................................................................. 150

4.9.2 Experimental Test Matrix ............................................................................... 151

4.9.3 Pressure Drop and Heat Transfer Characteristics ........................................... 155

Chapter 5 : Conclusions, Technical Challenges, and Future Work ................................ 181

5.1 Summary and Conclusions .................................................................................... 181

5.2 Technical Challenges ............................................................................................ 185

5.3 Areas for Future Research ..................................................................................... 188

Bibliography ................................................................................................................... 191

Appendix A : Room and Elevated Temperature Leak Testing Procedure ...................... 197

A.1 Introduction .......................................................................................................... 197

A.2 Leak Testing Methods .......................................................................................... 198

A.3 Procedure .............................................................................................................. 199

xii

Appendix B : Experimental Data .................................................................................... 214

xiii

List of Tables

Table 2.1. Nominal Chemical Composition of High-Temperature Alloys ....................... 23

Table 2.2. Electrical characteristics of the heaters ........................................................... 31

Table 2.3. Heater design specifications ........................................................................... 32

Table 2.4. Cooler design specifications ........................................................................... 34

Table 2.5. Haskel 8AGD-2.8 gas booster specifications ................................................. 36

Table 2.6. Specifications of the pressure reducing regulator ........................................... 37

Table 2.7. Surface roughness of the interior surface of a representative flow channel ... 54

Table 2.8. Basic geometric and characteristic parameters for the PCHEs ....................... 57

Table 2.9. Tensile test results for diffusion bonded Alloy 617 specimen ......................... 65

Table 3.1. Flow parameters for hydrodynamically developing flow in a semicircular duct

........................................................................................................................................... 99

Table 4.1. c1 as a function of l/Do .................................................................................. 126

Table 4.2. Ranges and accuracies of instruments used in the HTHF ............................ 138

Table 4.3. Fluid properties and their uncertainties ........................................................ 139

Table 4.4. PCHE experimental test matrix of helium flow, temperature, and pressure . 152

Table B.1. Experimental Data of PCHE1 (HX1) and PCHE2 (HX2) in HTHF ............ 214

xiv

List of Figures

Figure 1.1 Schematic of the very high temperature reactor ................................................ 3

Figure 2.1 Layout of the high-temperature helium test facility ........................................ 19

Figure 2.2 Photographs of the high-temperature helium test facility: (a) Low-temperature

side and (b) High-temperature side ................................................................................... 20

Figure 2.3. ASME allowable design stresses the HTHF candidate materials .................. 24

Figure 2.4. Pressure design thickness requirement for a nominal 1 inch, seamless Alloy

800H pipe at different pressures ....................................................................................... 26

Figure 2.5. Kanthal RAC Fibrothal tube heater with embedded heating element ........... 31

Figure 2.6. Cooler for cooling helium gas ....................................................................... 33

Figure 2.7. Haskel 8AGD-2.8 gas booster ....................................................................... 36

Figure 2.8. Photochemical machining process flow chart ............................................... 47

Figure 2.9. Photochemically etched Alloy 617 plates (a) straight pattern and z pattern,

(b) channel cross-section ................................................................................................... 48

Figure 2.10. (a) Diffusion bonded Alloy 617 heat exchanger block and (b) heat exchanger

with headers welded .......................................................................................................... 52

Figure 2.11. PCHE flow distribution header of Alloy 800H construction ...................... 52

Figure 2.12. Three-dimensional contour map of the interior surface of a channel .......... 54

Figure 2.13. Illustration of PCHE channel cross-section .................................................. 55

xv

Figure 2.14. Geometry of the straight channel (cold side) and z-channel (hot side) plates

used in the current PCHE design (all dimensions in inches) ............................................ 56

Figure 2.15. Representative SEM micrograph of the diffusion bond interface revealing

the presence of second phase particles that bound the Ni interlayer and separating the

Alloy 617 plates ................................................................................................................ 61

Figure 2.16. EDS spectrum indicating the chemical composition of the particles and

adjacent Alloy 617 matrix. ................................................................................................ 62

Figure 2.17. Inverse pole figure OIM micrograph depicting the grain structure across the

diffusion bond joint between plates of Alloy 617 ............................................................. 63

Figure 2.18. (a) SEM micrograph of a failed specimen crept at 900C/207 MPa and (b)

higher magnification view of the flat and brittle facture surface. ..................................... 66

Figure 2.19. Creep test result of Alloy 617 diffusion bonded specimen .......................... 67

Figure 3.1. Illustration of boundary layer development in a circular pipe ...................... 76

Figure 3.2. Circular sector duct......................................................................................... 83

Figure 3.3. Duct of semicircular cross-section ................................................................. 84

Figure 3.4. Plot of dimensionless axial velocity at various angular locations ................. 87

Figure 3.5. Mesh density on a cross-section ..................................................................... 94

Figure 3.6. Apparent Fanning friction factor as a function of dimensionless axial distance

........................................................................................................................................... 96

Figure 3.7. Incremental pressure drop number as a function of dimensionless axial

distance ............................................................................................................................. 97

xvi

Figure 3.8. Ratio of the maximum axial velocity to mean axial velocity as a function of

dimensionless axial distance ............................................................................................. 98

Figure 3.9. Plot of blown-up view of the axial velocity as a function of duct length ...... 99

Figure 3.10. Contours of fluid axial velocity in the fully-developed region in a

semicircular duct ............................................................................................................. 100

Figure 3.11. Ratio of apparent Fanning friction factor to fully developed Fanning friction

factor in a semicircular duct ............................................................................................ 101

Figure 4.1. Nomenclature for fluid stream temperatures in PCHE ................................. 109

Figure 4.2. Thermal circuit for heat transfer in a heat exchanger ................................... 110

Figure 4.3. Local Nusselt number as a function of dimensionless axial distance for a

semicircular duct ............................................................................................................. 117

Figure 4.4. Differential pressure measurement location across the hot and cold sides of

the PCHE ........................................................................................................................ 120

Figure 4.5. Geometry of the header ............................................................................... 126

Figure 4.6. Plot of pressure drop across the hot and cold sides of PCHE1 and PCHE2

under isothermal test conditions ..................................................................................... 130

Figure 4.7. Energy balance for a counter flow heat exchanger ...................................... 131

Figure 4.8. Plot of numerically estimated wall temperatures on the hot and cold fluid

sides of the PCHE investigated ....................................................................................... 135

Figure 4.9. Experimental test matrix of temperatures, pressures, and flow rates till date

......................................................................................................................................... 153

Figure 4.10. Plot of steady-state mass flow rate recorded by a Venturi flow meter ...... 154

xvii

Figure 4.11. Plot of quasi steady-state temperatures in PCHE2 ..................................... 155

Figure 4.12. Isothermal friction factor for the hot and cold fluid sides of PCHE1 ....... 157

Figure 4.13. Isothermal friction factors for the hot and cold fluid sides of PCHE2 ...... 158

Figure 4.14. Fanning friction factors as a function of Reynolds number for the cold side

......................................................................................................................................... 162

Figure 4.15. Fanning friction factor-Reynolds number product as a function of cold side

Reynolds number ............................................................................................................ 162

Figure 4.16. Plot of Nu as a function of Re for the cold side ......................................... 165

Figure 4.17. Hot (z-pattern) and cold (straight) side of the heat exchanger .................. 166

Figure 4.18. Plot of friction factor as a function of Re for the hot side .......................... 168

Figure 4.19. Plot of Nu as a function of Re for the hot side ........................................... 171

Figure 4.20. Comparison plot of Fanning friction factor for the hot and cold sides of

PCHE1 ............................................................................................................................ 172

Figure 4.21. Comparison plot of Fanning friction factor for the hot and cold sides of

PCHE2 ............................................................................................................................ 173

Figure 4.22. Plot of Fanning friction factor for PCHE1 and PCHE2 as a function of

Reynolds Number ........................................................................................................... 174

Figure 4.23. Comparison plot of Nusselt number as a function of Reynolds number for

hot and cold sides of PCHE1 .......................................................................................... 175

Figure 4.24. Comparison plot of Nusselt number as a function of Reynolds number for

hot and cold sides of PCHE2 .......................................................................................... 176

xviii

Figure 4.25. Effectiveness of the heat exchangers as a function of the number of transfer

units for PCHE1 and PCHE2 .......................................................................................... 177

Figure 4.26. Plot of heat load as a function of Reynolds number ................................... 178

Figure A.1. Schematic of the process chilled water line..................................................204

Figure A.2. Compressed air line schematic ................................................................... 205

Figure A.3. Safety Switches ............................................................................................ 206

Figure A.4. Electrical Enclosure ..................................................................................... 206

Figure A.5. UDC 1200 Controller .................................................................................. 207

Figure A.6. UDC 2500 Controller .................................................................................. 209

Figure A.7. Acuvim II Power meter ............................................................................... 210

1

Chapter 1 : Introduction

1.1 Motivation

The Very-High-Temperature Reactor (VHTR) is a potential candidate reactor concept for

the Next Generation Nuclear Plant (NGNP). The reactor design is a graphite-moderated,

helium-cooled, prismatic or pebble-bed core, thermal neutron spectrum reactor with a

once-through uranium fuel cycle and core outlet temperatures of 900-950oC [1.1]. The

VHTR concept, with a projected plant design service life of 60 years, is being researched

not only due to its near-term deployment potential but also because of its applicability

beyond the electrical grid by providing industry with carbon-free, high-temperature

process heat for a variety of applications, including hydrogen production, petroleum

refining, bio-fuels production, and production of chemical feed stocks for use in the

fertilizer and chemical industries. Fig. 1.1 shows a conceptual layout of the VHTR

illustrating the nuclear heat source and the associated power conversion unit and the

hydrogen generation plant [1.1].

The operating conditions of the VHTR represent a major departure from the existing

light-water cooled reactor technologies. The components of the heat transport system of

2

VHTR will be subjected to elevated temperatures for long times where adequate and

reliable performance of materials is critical. Of all the high-temperature metallic

components, the one most likely to be heavily challenged in the NGNP will be the

intermediate heat exchanger (IHX) [1.2-1.4]. It is a major component of the Heat

Transport System (HTS) of the NGNP and directly affects the system overall efficiency.

It must be robust enough to effectively transfer the heat between the Primary Heat

Transport System (PHTS) and the Secondary Heat Transport System (SHTS). The

current Technology Readiness Level (TRL) status issued by NGNP to all components

associated with the IHX for a reduced ROT of 750-800oC is 3 on a scale of 1 to 10, with

1 being the least matured [1.3].

This warrants a substantial technical development effort before the IHX is ready for full

commercialization. The current research aims to address this by investigating a promising

potential IHX concept called printed circuit heat exchanger (PCHE) for its design,

fabrication, performance testing, and modeling under high operating temperatures and

pressures. To facilitate testing at high temperatures and pressures, a high-temperature

helium test facility (HTHF) has been designed and constructed. Two PCHEs have been

designed and fabricated using Alloy 617 plates and have been tested for their thermal-

hydraulic performance in the HTHF.

3

Figure 1.1 Schematic of the very high temperature reactor [1.1]

1.2 Intermediate Heat Exchanger

The purpose of IHX is to efficiently and reliably transfer the heat generated in the core to

the power conversion system and for high-temperature process heat applications [1.2].

Several candidate materials and candidate configurations exist for the IHX; however,

their applicability for NGNP has not been analyzed or confirmed to date. Compact heat

exchangers such as PCHEs offer a promising alternative to conventional shell and tube

heat exchangers for NGNP applications. PCHEs are plate-type heat exchangers in which

fluid flow channels are photochemically etched on flat metal plates. The etched plates are

4

then stacked together in a particular configuration and diffusion bonded together to form

a heat exchanger block. Flow distribution headers are then welded on to the PCHE block

to form the complete heat exchanger core. In this work, PCHEs fabricated using high-

temperature Alloy 617 plates and straight channels have been investigated.

PCHE as a Potential Design Option for IHX of VHTRs

VHTRs require high-temperature and high efficiency heat exchangers to effectively

transfer the heat from primary helium to the secondary fluid (helium, nitrogen/helium

mixture or a molten salt). Gas coolants typically have low heat transfer capability due to

their low volumetric thermal capacity and thermal conductivity. This necessitates the

requirement of a heat transfer surface with a very high surface area density (650 to 1300

m2/m3), i.e., a compact heat transfer surface. Compared to a non-compact heat

exchanger, compact heat exchangers, such as PCHEs, are characterized by a large surface

area density, resulting in reduced space, weight, support structure and footprint, energy

requirements and cost, as well as an improved process design [1.5]. Furthermore, due to

the nature of the fabrication techniques involved, PCHEs possess high-pressure

containment capability and with the right selection of structural materials can be designed

for high-temperature service applications as well. All these factors have a great influence

on the VHTR plant layout and design. Moreover, PCHEs have a sound technology base

in that they are being extensively used in demanding non-nuclear applications, albeit at

much lower temperatures, such as offshore oil platforms as gas coolers, compressor after

5

coolers, etc. In light of the above, PCHEs have a tremendous potential to be an excellent

choice for IHX.

To the authors knowledge, only two PCHEs have been manufactured in the U.S. from

high-temperature materials, i.e., the two fabricated by the Ohio State University [1.6].

When this research started about 5 years ago, no heat exchangers in the U.S. were tested

under high-temperature helium conditions typical of VHTRs even though there was a

lack of experimental data in the open literature for PCHEs operating at temperatures and

pressures encountered in NGNP. Furthermore, if the IHX is fabricated by diffusion

bonding, the bond strength may become the controlling factor for life and is a key to the

performance capability of a high-temperature heat exchanger. In summary, it is

important to address key issues related to design and fabrication of compact heat

exchangers, such as PCHEs, and experimentally investigate their thermal-hydraulic

performance under high operating temperatures and pressures.

1.3 Research Objectives

Even though there has been some amount of prior experimental research performed with

the PCHEs at low temperature conditions, there was no database on their thermal-

hydraulic performance at operating conditions of VHTRs. The current research aims to

fill the void in the PCHE performance database under high-temperature and moderate

pressure conditions. In what follows, the objectives of the current research are outlined.

Design and construct a high-temperature helium facility that can enable thermal-

hydraulic performance testing of heat exchangers for temperatures and pressures

6

up to 800oC and 3 MPa, respectively. (The test facility has been designed and

constructed and leak tested with helium at close to full design pressure and design

temperatures).

Design and fabricate countercurrent flow PCHEs with straight channels using

Alloy 617 plates. (Two PCHEs with straight flow channels have been designed

and fabricated by diffusion bonding of Alloy 617 plates).

Perform mechanical and material characterization studies on diffusion bonded

Alloy 617 specimens.

Develop a test matrix for the PCHE testing and perform thermal-hydraulic

performance testing of the PCHEs in the high-temperature helium facility at

temperatures and pressures up to 800oC and 3.0 MPa, respectively.

Develop an experimental database for PCHE thermal-hydraulic performance

under intermediate-to-high-temperature environments. Analyze and reduce the

experimental data for friction and heat transfer characteristics of the heat

exchangers. Benchmark the experimental data against available data and models

for straight flow channels of semi-circular cross-section. Straight channels are

employed in the current design as this geometry is relatively better understood.

Investigate hydrodynamically developing and hydrodynamically fully-developed

laminar flow through a semicircular duct and provide relations for determining

the hydrodynamic entrance length in a semicircular duct and the friction factor (or

pressure drop) in the in the hydrodynamic entrance region for laminar flow

through a semicircular duct.

7

1.4 Previous Work

PCHE is a commercial product developed by a foreign vendor for off-shore applications

where compactness is very important [1.7]. PCHEs for these applications are of stainless

steel construction and are typically designed and fabricated to less restrictive ASME

Code requirements. On the contrary, any potential IHX design concept for NGNP

applications should (most likely) conform to more stringent ASME III Code

requirements. Southall et al. [1.8] discusses different compact heat exchanger

configurations, such as printed circuit, formed plate, and hybrid heat exchangers along

with their potential applications for the high-temperature reactors. According to him,

formed plate heat exchangers (FPHEs), fabricated from corrugated sheets, are more

economical for low pressure applications. PCHEs, however, are capable of sustaining

higher design pressure than FPHEs. A hybrid heat exchanger, on the other hand, has the

attributes of both the PCHEs and FPHEs and is more suited for applications where one

heat exchanger must satisfy design requirements for two very different fluids.

In the literature, some research work was performed with the PCHEs. Nikitin et al. [1.9]

and Ishizuka et al. [1.10] investigated, both numerically and experimentally, the heat

transfer and pressure drop characteristics of a 3-kW PCHE in a supercritical CO2 loop

with mass flow rates varying from 40-80 kg/h. The PCHE employed in their study was

manufactured by a commercial vendor and had plates stacked in a double banking

arrangement with a cold channel plate sandwiched between two hot channel plates and

consisted of herringbone type channel flow passages. In their study, the hot and cold side

8

inlet temperatures were varied from 280-300oC and 90-108oC, respectively while the hot

and cold side inlet pressures were varied from 2.2-3.2 MPa and 6.5-10.5 MPa,

respectively. For the PCHE tested, they correlated local heat transfer coefficient and

pressure drop as a function of Reynolds number. A maximum heat exchanger

effectiveness of around 98% was reported in their study. However, their experimental

operating temperatures only reached 300oC and are far from high-temperature reactor

(HTR) requirements. Song et al. [1.11] performed experiments at low Reynolds numbers

with a commercial PCHE using air as the working fluid. For the PCHE tested, they

developed heat transfer and friction factor correlations and examined the adaptability of

Hesselgreaves correlation [1.12] to PCHE type heat exchanger.

Figley [1.13] developed a numerical model of a straight channel PCHE and performed

numerical analysis to investigate its thermal-hydraulic performance for varied operating

conditions and generate predictive data for the PCHEs fabricated at The Ohio State

University. The PCHE model dimensions are representative of the PCHEs fabricated and

installed in the high-temperature helium test facility. The operating conditions are

representative of the maximum design operating temperature and pressure for the high-

temperature helium test facility. The computational results of the convective heat

transfer coefficient and pressure drop from the PCHE model agree well with the available

models in the literature. Kim et al. [1.14] investigated, both numerically and

experimentally, the thermal-hydraulic performance of the PCHE using the KAIST

Helium Test Loop with mass flow rates varying from 40-100 kg/h. The PCHE employed

9

in their experimental study is of Alloy 800H construction and fabricated by a commercial

vendor. In their study, the operating pressure range was 1.5-1.9 MPa and the hot and

cold side inlet temperatures were varied from 25-550oC and 25-100oC, respectively.

Based on the experiments, a global fanning factor and a global Nusselt number were

proposed for the PCHE tested. Furthermore, they validated their 3-D numerical model

against the KAIST helium experimental data.

Pra et al. [1.15] carried out steady and transient tests on a PCHE recuperator mock-up

and investigated their thermal-mechanical behavior in an air test loop for conditions

typical of High Temperature Reactor (510oC). The PCHE was supplied by a commercial

vendor and had convoluted flow passages (wavy or herringbone type). The steady-state

analysis of the PCHE was performed to evaluate its thermal-hydraulic performance and

examine its ability to reach HTR recuperator specifications. An effectiveness of 95%

was reported in their study. Furthermore, transient tests were performed to determine the

number of thermal load cycles required to generate a failure in the heat exchanger. For

the transient tests, the PCHE mock-up was subjected to severe temperature variations or

thermal shocks representative of a HTR recuperator. During the cold shock, the

temperature was varied from 510 to 108oC within 5 s. In a similar manner, a hot shock

was generated by varying the temperature from 180 to 510oC within 3 minutes. This

process was repeated 100 times and no fatigue failure of the mock-up was noticed.

Although these tests are less stringent compared to the IHX requirements for NGNP, they

however show that PCHE has a good potential for high temperature applications.

10

To reduce the high pressure drop associated with the herringbone type channel

configuration and improve the performance advantage, numerical studies have been

carried out on alternative channel geometries as well. Tsuzuki et al. [1.16] developed a

new PCHE with S-Shaped fins and performed a numerical analysis to evaluate the heat

transfer and pressure drop characteristics. In their analysis, they considered supercritical

CO2 as the hot-side fluid and H2O as the cold-side fluid. The inlet temperatures of the

hot and cold fluid are 118 and 7oC and their inlet pressures are 11.5 and 0.25 MPa,

respectively. Their analysis indicated a similar heat transfer performance and a reduced

pressure drop compared to zigzag PCHEs. Kim et al. [1.17] performed a three

dimensional numerical analysis to investigate heat transfer and pressure drop

characteristics of supercritical CO2 flow in a PCHE model that incorporates airfoil shaped

fins instead of zigzag or herringbone type channels. Numerical comparison between

airfoil fin and zigzag PCHE channel configurations indicated an appreciable reduction in

pressure drop in the case of the airfoil fin PCHE. However, the heat transfer rate per unit

volume is comparable in both the configurations. The reduction in pressure drop was

attributed to the streamlined shape of airfoil fins that help suppress generation of

separated flow.

In light of the above discussion, it can be concluded that most of the work on PCHEs is

computational and there are very limited experimental data on their thermal-hydraulic

performance under very high operating temperatures and pressures. Notwithstanding the

aforementioned works on PCHEs, to the authors knowledge, however, a detailed

11

thermal-hydraulic performance of the PCHEs with helium as the working fluid has not

been experimentally investigated for the temperatures and pressures typical of VHTRs.

Besides, there is essentially no experimental data available for the performance of a

PCHE when the operating conditions deviate from the nominal design condition.

Furthermore, the PCHEs employed in the aforementioned experimental studies are

developed by a commercial foreign vendor and the U.S. fabrication capability of PCHEs

has not been fully explored and confirmed. The current research addresses these issues

and provides an extensive database of the thermal-hydraulic characteristics of PCHEs

under high operating temperatures and pressures.

1.5 Dissertation Organization

This dissertation is comprised of five chapters and two appendices.

Chapter 1 introduces the problem statement and provides motivation for the current

research, provides some background information on the heat exchangers, lays down the

objectives of the current research work, and provides a compilation of the previous work

performed on PCHEs.

Chapter 2 provides the design and fabrication aspects of the high-temperature helium test

facility and the printed circuit heat exchangers. The simplified stress analysis performed

on the test facility piping and the heat exchangers are discussed. The processes related to

the design and fabrication the PCHEs, such as the photochemical machining and

12

diffusion bonding techniques are discussed. Microstructural characterization and

mechanical testing of representative diffusion bonded specimens are discussed.

Chapter 3 addresses the hydrodynamically developing and fully-developed laminar flow

in a semi-circular duct. The method of scale analysis [1.18] has been employed for

providing order of magnitude estimates for the friction characteristics in

hydrodynamically fully-developed and hydrodynamically developing laminar flow in a

semicircular duct. Following this, an analytical and numerical treatment of these flows in

a semicircular duct is presented and the results discussed.

Chapter 4 discusses the performance testing of the PCHEs carried out in the HTHF.

Various heat transfer and pressure drop correlations applicable for circular and semi-

circular ducts in the laminar, turbulent, and laminar-to-turbulent transition flow regimes

are discussed. The data reduction procedure for the friction and heat transfer

characteristics are described in detail. The experimental data has been benchmarked

against available models and correlations in the literature and the friction and heat

transfer characteristics of the heat transfer surface of the heat exchangers have been

determined.

Chapter 5 summarizes the important results and findings of this dissertation research, the

technical challenges and the lessons learned in this research, and provides

recommendations for future work.

13

Appendices detailing the leak test procedure at room and elevated temperatures for the

high-temperature helium test facility (Appendix A) and PCHE experimental data

(Appendix B) follow.

References for Chapter 1

[1.1] Idaho National Laboratory Homepage, Available online at http://www.inl.gov/research/very-high-temperature-reactor/, accessed October 12, 2011. [1.2] Natesan, K., Moisseytsev, A., and Majumdar, S., 2009, "Preliminary Issues

Associated with the Next Generation Nuclear Plant Intermediate Heat Exchanger Design," Journal of Nuclear Materials, 392, pp. 307-315.

[1.3] INL, 2009, "Next Generation Nuclear Plant Project Technology Development

Roadmaps: The Technical Path Forward for 750-800oC Reactor Outlet Temperature," INL/EXT-09-16598, Idaho Falls, ID.

[1.4] INL, 2004, "Design Features and Technology Uncertainties for the Next

Generation Nuclear Plant," INEEL/EXT-04-01816, Idaho Falls, ID. [1.5] Shah, R.K. and Sekulic, D.P., 2003,"Fundamentals of Heat Exchanger Design," John Wiley & Sons, Hoboken, New Jersey: NJ. [1.6] Mylavarapu, S., Sun, X., Figley, J., Needler, N.J., and Christensen, R.N., 2009, "Investigation of High-Temperature Printed Circuit Heat Exchangers for VHTRs," Journal of Engineering for Gas Turbines and Power, Transactions of the ASME, 131(6), pp. 062905-0107. [1.7] Southall, D., Le Pierres, R., and Dewson, S.J., 2009, "Design Considerations for Compact Heat Exchangers," Proceedings of ICAPP '09, paper no. 8009. [1.8] Southall, D. C. and Dewson, S. J., 2010, "Innovative Compact Heat Exchangers," Proceedings of ICAPP'10, paper no. 10300. [1.9] Nikitin, K., Kato, Y., and Ngo, L., 2006, "Printed Circuit Heat Exchanger

Thermal-Hydraulic Performance in Supercritical CO2 Experimental Loop," International Journal of Refrigeration, 29(5), pp. 807-814.

14

[1.10] Ishizuka, T., Kato, Y., Muto, Y., Nikitin, K., and Ngo, L., 2005, "Thermal-Hydraulic Characteristics of a Printed Circuit Heat Exchanger in a Supercritical CO2 Loop," NURETH-11, pp. 218-232.

[1.11] Song, S.C., 2005, "Thermal-Hydraulic Performance of a Printed Circuit Heat Exchanger in an Air Test Loop," M.S.Thesis, KAIST, Daejeon, Korea. [1.12] Hesselgreaves, J.E., 2001, "Compact Heat Exchangers: Selection, Design, and Operation," Pergamon Press, New York: NY. [1.13] Figley, J.T., 2009, "Numerical Modeling and Performance Analysis of Printed Circuit Heat Exchanger for Very High Temperature Reactors," M.S. Thesis, The Ohio State University, Columbus, OH. [1.14] Kim, I.H., No, H.C., Lee, J.I., and Jeon, B.G., 2009, "Thermal-Hydraulic Performance Analysis of the Printed Circuit Heat Exchanger using a Helium Test Facility and CFD Simulations," Nuclear Engineering and Design, 239, pp. 2399- 2408. [1.15] Pra, F., Tochon, P., Mauget, C., Fokkens, J., and Willemsen, S., 2008, "Promising Designs of Compact Heat Exchangers for Modular HTRs using the Brayton Cycle," Nuclear Engineering and Design, 238(11), pp. 3160-3173. [1.16] Tsuzuki, N., Kato, Y., and Ishizuka, T., 2007, "High Performance Printed Circuit Heat Exchanger," Applied Thermal Engineering, 27(10), pp. 1702-1707. [1.17] Kim, D.E., Kim, M.H., Cha, J.E., and Kim, S.O., 2008, "Numerical Investigation

of Thermal-Hydraulic Performance of New Printed Circuit Heat Exchanger Model," Nuclear Engineering and Design, 238(12), pp. 3269-3276.

[1.18] Bejan, A., 2004, "Convection Heat Transfer." John Wiley & Sons, Inc., Hoboken, New Jersey: NJ.

15

Chapter 2 : High-Temperature Helium Test Facility and Printed Circuit Heat Exchangers

2.1 Overview

The design and construction of the high-temperature helium test facility is discussed in

this chapter. The HTHF is primarily intended for performance testing of heat exchangers

with different configurations, however, it is designed with sufficient flexibility to allow

testing of other high-temperature components of VHTR such as valves, gaskets, etc. The

test facility components and its working will be explained in detail. A stress analysis of

the test facility piping shows that the test facility is conservative and can be safely

operated at the design temperatures and pressures. In addition, the design details and

fabrication aspects of two high-temperature Alloy 617 printed circuit heat exchangers are

presented in detail. Both the PCHEs have been installed in the HTHF for performance

testing. Furthermore, some microstructural and mechanical characterization studies

performed on Alloy 617 diffusion bonded specimens are discussed.

16

2.2 High-Temperature Helium Test Facility

2.2.1 Introduction

The high-temperature helium test facility has been designed and constructed to facilitate

performance testing of heat exchangers at temperatures and pressures up to 800oC and 3

MPa, respectively [2.1]. The original design goal of the test facility maximum

temperature was 900oC [2.2], which has been reduced due to the reduced reactor outlet

temperatures (750-800oC) in the new DOE VHTR design specifications [2.3]. The

facility has been designed with sufficient flexibility to accommodate testing of heat

exchangers with different configurations and other critical components of VHTR, such as

valves, instruments, gaskets, and piping under high-temperature conditions. In designing

and constructing the facility, the requirements of ASME B31.3 Process Piping Code [2.4]

and ASME VIII and IX of the Boiler and Pressure Vessel Code [2.5] were followed.

Welding and post-weld examination (radiograph and penetrant tests) on the weld joints

was performed as per ASME Section IX. The facility, however, is not designed and

constructed to more restrictive ASME III code. Two counter-flow PCHEs are installed in

series in the facility for thermal-hydraulic performance testing under a wide range of

operation conditions.

2.2.2 Description of the Test Facility

Figure 2.1 shows a schematic of the high-temperature helium test facility and Fig. 2.2

shows photographs of the HTHF, fully insulated and with a protective steel enclosure

17

around it. The test facility is initially vacuumed to the desired vacuum pressure of -14

psig using the vacuum pump. Following this, the facility is pressurized to the desired

pressure with helium gas obtained from the helium gas cylinder. After the facility is

charged with helium, the gas booster is turned on to circulate the helium gas in the test

facility piping and its components. Originally, a gas compressor was chosen in the

design, but was not adopted due to its very high cost. A gas chromatography was

considered in the design stage to provide a means of monitoring the helium environment

during testing. However, due to economic considerations, it is not used in the current

design. A 5-gallon surge volume tank and an inline pressure reducing regulator/valve

(PRV) located downstream of the booster help mitigate the pressure fluctuations due to

the reciprocating action of the booster and ensure a stable helium flow in the test facility.

After exiting the PRV, helium gas is heated by a pre-heater with a maximum capacity of

about 23 kW. The pre-heater is a combination of three radiant type heaters, each having

a maximum capacity of 7.6 kW, and arranged in a 3-phase delta configuration. Helium

gas leaving the pre-heater with a desired temperature is then forwarded to the cold side of

the first PCHE (labeled PCHE1) where it exchanges thermal energy with the respective

hot side. The helium gas exiting the cold side of PCHE1 is then forwarded to the cold

side of the second PCHE (labeled PCHE2). The facility is designed such that the fluid

exiting the cold side of the second PCHE enters the hot side of PCHE2 through a main

heater that heats the helium to around 900oC when operated at its maximum heating

capacity of 23 kW. The main heater is also a combination of three radiant type heaters

and is electrically configured similar to the pre-heater. Following this, the fluid enters the

18

hot side of PCHE1 where it exchanges energy with the respective cold side. After

transferring energy to the cold side, the helium gas enters a cooler, where it transfers heat

to the process chilled water and gets cooled down to the inlet temperature of the gas

boosters, i.e., around 35oC. There are two bypass lines in the facility; one bypasses the

cold side of PCHE1 and the other bypasses the hot side of PCHE1. These bypasses help

realize different flow rates on the hot and the cold sides of PCHE1.

19

Figure 2.1 Layout of the high-temperature helium test facility

20

(a)

(b)

Figure 2.2 Photographs of the high-temperature helium test facility: (a) Low-temperature side and (b) High-temperature side

The facility is well instrumented with various sensors. Pressure transducers are installed

for measuring the gage pressure: a) at the inlet of the hot and cold fluid sides of both the

heat exchangers, b) at the upstream location of the three venturi flow meters, c) at the

inlet and exit of the PRV, and d) on the return line of the process chilled water line. In a

similar manner, differential pressure transducers are installed in the facility to measure

the differential pressure: a) across the hot and cold fluid sides of the PCHEs and b) across

the upstream and throat location of the three venturi flow meters. K-type Alloy 800H-

sheathed thermowells are used for measuring the temperature: a) at the inlet and exit of

the hot and cold fluid sides of the heat exchangers, b) at the upstream location of the

Venturi flow meters, c) at the exit of the heaters, and d) at the exit of the cooler. Three

Venturi flow meters measure the volumetric flow rates of helium gas flowing through the

21

loop. Additionally, two high-temperature flow sensors designed by Delta M Corporation

are installed in the facility for prototype design testing and cross benchmark of the flow

measurements. A turbine flow meter installed on the process chilled water side of the

cooler allows monitoring the flow rate of the process chilled water. The inlet and exit

temperatures of the process chilled water are measured by two RTD temperature sensors.

Along with the information of the flow rate and inlet and exit temperatures of the chilled

water, the rate of the energy being removed by the chilled water can be calculated.

2.2.3 Design Aspects of OSU HTHF

In what follows, the design aspects and features of OSU HTHF are discussed. This

includes material selection for the piping, piping pressure design thickness estimation,

and a simplified piping stress analysis for the design operating conditions.

Candidate Materials for the OSU HTHF

The high-temperature helium test facility must withstand high-temperatures for a

considerable period of time without significant mechanical property degradation and

resist corrosion/oxidation and erosion from the helium coolant. Research grade helium

(99.999% pure) is used in the facility. The leading candidate materials that can withstand

such high temperatures are nickel-based superalloys. Various high-temperature materials

were reviewed for their high-temperature mechanical properties (tensile, creep and creep-

fatigue properties), physical properties (thermal conductivity and thermal expansion),

environmental resistance, fabrication and joining technology, availability and economics

22

[2.2, 2.6]. This assessment is carried out to identify an appropriate candidate material for

the helium test facility piping and the heat exchangers for operating conditions typical of

VHTRs. Finally, four primary candidate alloys, listed below, are identified and assessed

for use at design temperatures of 800-900oC.

Alloy 617

Alloy 230

Hastelloy X

Alloy 800H

Among these alloys, Alloy 617 is a prime candidate for VHTR structural components,

such as piping, reactor internals, and intermediate heat exchanger (IHX). Table 2.1 lists

the nominal chemical composition of these alloys along with their Unified Numbering

Scheme (UNS) numbers and ASME specifications for seamless pipe and plate [2.4-2.5,

2.7-2.8]. All these alloys are Ni-base superalloys with the exception of 800H, which is

an iron-base superalloy [2.7].

ASME Allowable Design Stresses for Candidate Materials

The material selection for the test facility and heat exchangers is primarily based on

allowable stresses from the ASME Section II, Part D approved for ASME Section VIII,

Division I construction (non-nuclear construction) [2.5]. The design and construction of

OSU HTHF does not conform to ASME Section III, Subsection NH and is, therefore, not

intended for nuclear service. Figure 2.3 compares the ASME allowable design stresses at

different temperatures for the materials listed in Table 2.1 and is applicable for both

23

seamless pipe and plate product forms. The ASME allowable stresses are based on the

average stress to cause rupture in 100,000-hour operation (about 11.4 years) in air [2.5,

2.9]. The code uses 0.67 times this average stress at each temperature to define the

allowable stress [2.5, 2.9]. The HTHF would be operated for a time much less than 105

hours and it is safe to infer that the test facility design based on allowable stresses for 105

hours of operation is very conservative.

Table 2.1. Nominal Chemical Composition of High-Temperature Alloys [2.4-2.5, 2.7-2.8]

Alloy UNS Number

Product Form Spec No.

Nominal Chemical Composition (wt.%)

617 N06617 Seamless pipe & tube SB-167 52Ni-22Cr-13Co-9Mo Plate, sheet, strip SB-168

230 N06230 Seamless pipe & tube SB-622 57Ni-22Cr-14W-2Mo-La Hastelloy X N06002 Seamless pipe & tube SB-622 47Ni-22Cr-9M0-18Fe

800H N08810 Seamless pipe & tube SB-407 33Ni-42Fe-21Cr

24

Figure 2.3. ASME allowable design stresses the HTHF candidate materials [2.4, 2.5]

It should be mentioned that among these alloys, only Alloy 800H is ASME Code Section

III (nuclear service) certified for use in applications with temperatures up to 760oC and

that neither Alloy 617 nor Alloy 230 is currently approved for ASME Section III

applications. However, all these three alloys are approved for Section VIII, Division I

construction. As for Alloy HX, only a limited database exists for ASME III applications.

It is however certified for ASME VIII. In light of the above, all the design and

construction pertaining to this facility is based on ASME Code applicable for non-nuclear

service. From Fig. 2.3, it is evident that only alloys 617, 230, and 800H are approved for

temperatures up to 982oC while Alloy HX is approved for temperatures up to 900oC.

Furthermore, on comparing the allowable design stresses (or rupture strengths) for 105

300 400 500 600 700 800 900 10000

20

40

60

80

100

120

140

160

Temperature (oC)

Allo

wab

le S

tress

(MP

a)

Alloy 617Alloy 230Alloy 800HAlloy HX

25

hours of operation at temperatures greater than 800oC, it is clear that Alloys 617 and 230

are the most suitable materials for high-temperature applications. At these temperatures,

the materials are in creep-rupture failure regime, and as such have a finite life. At the

time when this study on materials was carried out, Alloys 617 and 230 were available

only in plate configuration (and not in tube or pipe configuration), which precluded their

usage for the test facility piping. Therefore, from the considerations of availability and

economics, Alloy 800H was selected for the test facility piping. The selection dictated

the maximum allowable working pressure and temperature of the test facility.

Figure 2.4 provides the rationale for designing the test facility for a maximum allowable

working pressure and temperature of 3 MPa and 800oC, respectively. The test facility

piping size is 1 inch NPS and is based on an economic design velocity of 25 m/s. Noting

the fact that the maximum wall thickness for a commercially available 1 inch NPS Alloy

800H pipe is 6.35 mm (corresponds to a pipe schedule of 160), it can be noted from Fig.

2.4 that the required pressure design thickness (minus the sum of mechanical allowances

and erosion and corrosion allowances) corresponding to 3 MPa and 870oC is close to 6.35

mm. Accounting for erosion plus corrosion allowance of 1 mm, it can be inferred from

Fig. 2.4 the test facility can be safely operated for temperatures and pressures up to 850oC

and 3 MPa, respectively.

However, due to the high-temperature nature of the proposed experiments, the test

facility design temperature was scaled down to 800oC from a safety stand point. The

26

pressure design thickness required for operating pressures greater than 3 MPa and at

800oC exceeds the commercially available wall thicknesses. In summary, the test facility

is designed and constructed to facilitate experiments at temperatures and pressures up to

800oC and 3 MPa, respectively. The test facility can enable testing at temperatures lower

than 800oC and pressures greater than 3 MPa and the corresponding temperatures and

pressures can be estimated by following a similar approach.

Figure 2.4. Pressure design thickness requirement for a nominal 1 inch, seamless Alloy 800H pipe at different pressures

550 600 650 700 750 800 850 900 950 10000

2

4

6

8

10

12

14

16

18

Temperature (oC)

Pre

ssur

e D

esig

n Th

ickn

ess

(mm

)

P = 1 MPaP = 2 MPaP = 3 MPaP = 3.5 MPaP = 4 MPaP = 5 MPaSch.160 Pipe

6.35 mm

27

Pressure Design Thickness

The pressure design thickness in Fig. 2.4 was estimated based on the Boardman

expression from ASME B31.3 [2.4]. The minimum required thickness of the pipe mt

including allowances is given by the following expression

mt t c (2.1)

where t is the pressure design thickness in mm; c is the sum of mechanical, corrosion

and erosion allowances and is typically taken as 1 mm. The pressure thickness required

for a pipe is determined by the Boardman expression [2.4] as

2i

i

PDt

SE PY

(2.2)

where iP is the internal design gage pressure in MPa; D is the pipe outer diameter in

mm; S is the pipe material allowable design stress in MPa; E is the quality factor (for a

seamless pipe 1E ), and Y is the stress-temperature compensating factor. The E factor

is an allowable pressure stress penalty based on the method of manufacture of the pipe

[2.4, 2.9]. It reflects the quality of the longitudinal weld in seam-welded pipe and has a

value ranging from 0.6 for furnace butt welded (FBW) to 1.0 for seamless pipe (SMLS).

The Y factor is included to account for the non-linear reduction in allowable stress at

design temperatures above 482oC [2.9]. Other expressions (Lame and Barlow's

Equation) available in ASME B31.3 [2.4] for estimating the pressure design thickness

were used for the purposes of comparison.

28

2.2.4 Simplified Stress Analysis for Loop Piping Design Calculations

A simplified stress analysis is performed to verify the piping pressure design thickness

calculations performed in the previous section. For thick walled cylinders, the hoop or

tangential stress, h , is calculated as [2.10-2.12]:

2 2 2

2

1

1h

o o oi o

i

o

i

r r rr

rr

P Pr rSE

(2.3)

where ir and or denote the inner and outer radii of the pipe, and iP and oP denote the

uniform internal and external gage pressures, respectively. For the current design, since

the external pressure on the piping is atmospheric, it is reasonable to assume zero

external gage pressure, i.e., 0oP . The maximum tangential stress always occurs on

the inner surface. The required radius ratio so that the maximum tangential stress is less

than or equal to the allowable stress, S , can be calculated from Eq. (2.3) by replacing r

with ir . When the internal pressure exceeds the external pressure, the limiting ratio is

given by

.2

i

i

o

oi

S PS P P

r

r

(2.4)

The thickness-to-diameter ratio can be expressed as,

2

o i

i

r rtd r

(2.5)

For the current design with a nominal line size of 1 inch and a pipe schedule of 160 and

using the allowable design stress value of 9.68 MPa [2.4, 2.5] for Alloy 800H at the

29

design temperature of 850oC, we have the following values for the required and available

thickness-to-diameter ratios.

required

1.39 0.22

i

o i

tS PS P P d

(2.6)

and, available

0.31.6oi

r

r

td

(2.7)

From Eq. (2.6) and Eq. (2.7), we have the following:

oravailable required

2i

o i

o

i

r

r

S PS P P

t td d

(2.8)

It can be concluded from the above analysis that it is an acceptable mechanical design

and the test facility can be safely operated at pressures and temperatures of 3 MPa and

850oC, respectively. However, as mentioned earlier, the test facility design temperature

was scaled down to 800oC to make the design more conservative. Furthermore, the

allowable stresses (creep-rupture strength) are based on 105 hours of operation at the

respective temperature, which is far greater than the time this test facility would be

operated. Therefore, it is a very conservative and safe design.

2.2.5 High-Temperature Helium Test Facility Components

The design of the test facility components was primarily dictated by the high operating

temperatures, availability, and economics. A case in point would be the high-temperature

valves. A high-temperature valve rated to at least 800oC and 3 MPa at the exit of the

main heater or before the hot inlet of the second heat exchanger would provide more

flexibility in terms of the test facility operation but was not included in the design due to

30

economics. Operating within the available budget and at the same time ensuring the

integrity of the facility components for use at high temperatures and pressures is a

challenge. The design process became iterative in that it involved repeatedly modifying

the design to fit the component/material availability and budget. In the following

subsections, the principal components of the OSU HTHF are discussed.

Electric Heaters and Power Controllers

Kanthals FIBROTHAL standard RAC tube modules [2.13] were employed as heating

source for heating the helium flowing through the test facility piping and its components.

Figure 2.5 shows a Kanthal FIBROTHAL RAC tube heater with the embedded heating

element and the ceramic fiber insulation. Table 2.2 lists the electrical specifications of

the FIBROTHAL RAC tube heater [2.13]. Each FIBROTHAL heater consists of

vacuum-formed ceramic fiber components with radiating heating element embedded into

it. The embedded heating element is a Kanthal Grade A-1 heating element with a

nominal chemical composition (% by wt.) of 22% Cr, 5.8% Al, and balance Fe. The

heating elements are designed for a maximum element temperature of 1300oC.

31

Figure 2.5. Kanthal RAC Fibrothal tube heater with embedded heating element [2.13]

Table 2.2. Electrical characteristics of the heaters [2.13]

Heater

Voltage (V) Power (W)

at 60 A

Voltage (V) Power (W)

at 72 A

Voltage (V) Power (W)

at 85 A

Resistance R20 (Ohm)

RAC 70/500

63.1 3786

75.8 5454

89.5 7608

1.008

As mentioned earlier, the heating system in the HTHF comprises of a pre-heater and a

main-heater. The pre-heater and the main-heater are each a combination of three heaters

wired in a 3-phase delta configuration with each individual heater having a maximum

heating capacity of 7.6 kW. Therefore, a total of six heaters with a total maximum

heating capacity of about 46 kW act as the heating source to deliver helium to the

PCHEs.

32

Two phase angle type SCR (Silicon Controlled Rectifier) power control units, one for the

pre-heater and the other for the main-heater, control the amount of power input to the

heaters. Two Acuvim II multifunction digital power meters continuously meter and

monitor the current and voltage data fed to the heaters with an accuracy of 0.2% of the

reading. In addition, two Honeywell UDC 2500 temperature controllers with a

thermocouple input and a 4-20 mA output, monitor and control the fluid temperature at

the exit of the heaters by providing continuous feedback to the SCR controllers and help

in realizing the desired heater outlet temperatures. Furthermore, six microprocessor-

based UDC 1200 limit controllers with a thermocouple input and a relay output that is

continuously fed to the power controller provide necessary safety by preventing

overheating of the heating elements. Table 2.3 lists the general design specifications of

the pre-heater and the main heater.

Table 2.3. Heater design specifications

Specification Pre-Heater Main Heater Nominal inlet

Temperature range (oC) 70-100 400-650

Maximum outlet temperature (oC) 350 850 Maximum mass flow rate (kg/h) 45 45

Working Fluid (Gas) He He Power (kW) 23 23

Cooler

The cooler is a tube-in-tube heat exchanger and uses process chilled water (PCW) as a

cooling medium to cool the hot helium gas (working fluid) exiting from the hot side of

PCHE1. The hot helium flows through the inner tube while the water flows in the

33

annular region. The working fluid is cooled to the inlet temperature of the gas booster,

rated for a maximum inlet temperature of 100oC. Figure 2.6 shows the schematic of the

cooler installed in the HTHF. The specifications of the cooler are tabulated in Table 2.4.

A turbine flow meter is installed on the PCW line to measure the volumetric flow rate of

the cooling water. Two RTD sensors, at the inlet and exit of the PCW line, facilitate the

measurement of the inlet and exit temperatures of the process chilled water.

Figure 2.6. Cooler for cooling helium gas

The inner tube and the outer annulus of the cooler is constructed using SS316. The inner

tube through which hot helium flows is a 1/2 in. (12.7 mm) diameter tube with a wall

thickness of 0.065 in. (1.65 mm). The wall thickness of the inner tube through which hot

helium flows should be sufficient to be able to withstand temperatures up to 450oC

without compromising its integrity. For the purposes of the design, the maximum

operating pressure and temperature are taken as 510 psig and 450oC, respectively. The

34

maximum allowable stress for SS316 at this temperature is 12.7 kpsi [2.4]. The pressure

design thickness required for the inner tube of the cooler is estimated from Eq. (2.2) as

0.41 mm. The required pressure design thickness is less than the tube wall thickness of

1.65 mm and hence the cooler can be safely operated without compromising its integrity.

Table 2.4. Cooler design specifications

Parameter Inner Tube Outer Tube Outside Diameter (inch) 1

Fluid Type Helium Water (PCW) Inlet Temperature (oC) 450 20

Outlet Temperature (oC) 100 27 Mass Flow (kg/s) 0.022 1.382

Volumetric Flow (Lpm) 389.9 83.28 Pressure Drop (kPa) 82.26 93.32 Heat Transfer (kW) 40.4

Effectiveness 0.81

Gas Booster

The purpose of the gas booster is to provide the driving head for circulating helium gas in

the test facility piping and its components. Initially, a compressor was used in the design

but was not adopted due to its higher cost. Currently, one gas booster has been installed

in the HTHF. A gas booster differs from the compressor in that it is air-driven and

requires no electrical motive force. Haskels 8AGD-2.8 model gas booster [2.13], shown

in Fig. 2.7, was selected for the HTHF. This is a single stage, double acting, high flow,

air driven, and reciprocating piston type non-lube-oil free gas booster. Table 2.5 lists the

specifications of 8AGD-2.8 gas booster [2.14].

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A gas booster essentially consists of an air drive section (drive cylinder) and a gas barrel

section (boost cylinder) isolated from each other by appropriate seals. The piston in the

drive cylinder is attached to the piston in the boost cylinder. As the drive piston

reciprocates, it compresses the gas in the boost cylinder. The boost cylinder is double-

acting, i.e., it pulls gas on one side while pumping it out on the other side. The maximum

pressure boost is equal to the drive piston area divided by the boost piston area multiplied

by the pressure feeding the drive cylinder. In other words,

do a sb

AP P P

A (2.9)

where Ad, Ab, Po, Pa, and Ps are the drive piston area, boost piston area, gas outlet

pressure, drive pressure, and gas supply pressure, respectively. For the gas booster

8AGD-2.8, the approximate area ratio of the air drive piston area to gas piston area is 2.8.

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Figure 2.7. Haskel 8AGD-2.8 gas booster [2.14]

Table 2.5. Haskel 8AGD-2.8 gas booster specifications [2.14]

Maximum Rated Gas Supply (psig) 800 Maximum Rated Gas Outlet (psig) 800

Static Outlet (Stall) Pressure Formula 2.8a sP P

Piston Displacement (in3/cycle) 125 Minimum Inlet Gas Pressure (psig) 100

Maximum Outlet Gas Pressure (psig) 800 Maximum Air Drive Pressure (psig) 130

Booster Cycling Rate (cycles/min) for continuous operation 60 Maximum Compression Ratio 25:1

The flow rate and discharge pressure of helium exiting the booster can be controlled by

throttling the drive air flow rate and/or regulating the drive air pressure. The leakage rate

from the gas booster is 0.1 SCFH (Standard Cubic Feet per Hour).

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Pressure Reducing Regulator/Valve

A pressure reducing regulator/valve was installed at the exit of the 5-gallon tank located

downstream of the gas booster. It is an air-loaded regulator and its function is to

maintain and control the outlet pressure within limits as other conditions vary and ensure

a stable flow of helium in the test facility. The specifications of the PRV are listed in

Table 2.6.

Table 2.6. Specifications of the pressure reducing regulator

Loading Mechanism Air Actuated, Non-Venting Approx. Air Load to Output

Ratio 6.25:1

Material of Construction SS316 Maximum Inlet Pressure

(psig) 600

Outlet Pressure (psig) 0-500 Temperature Rating (oC) -26-150

During initial experiments prior to the installation of the PRV, helium flow oscillations

with more than 30% variation were noticed during operation and were very

unpredictable. It was noticed that the helium flow oscillations were caused by the gas

booster operation in that it is designed to cycle at variable rates based on the desired

outlet pressure at the exit of the booster. The PRV helped smoothen the pressure spikes

and the oscillations in the helium flow by providing a nearly constant helium pressure

downstream of the regulator and therefore ensured a stable helium flow in the test facility

piping. The flow is now very stable with flow variations less than 1%.

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Vacuum Pump

A DuraVaneHV RVR002H high vacuum pump has been installed on the low temperature

side of the test facility near the helium charging line. Before every experimental run, the

facility is vacuumed to -14 psig to minimize the amount of air in the helium working

fluid. After sufficient vacuum is obtained, the facility is charged with the working fluid

to the desired pressure.

Working Fluid

Helium gas is employed as the working fluid for performance testing of the PCHEs in the

HTHF. High purity research grade helium (99.999% pure) has been used for the

experiments in the HTHF.

High-temperature Valves

Six high-temperature SS316 needle valves provided by Swagelok, Inc., have been

installed in the HTHF. These valves are rated for temperatures up to 650oC and utilize a

high-temperature Grafoil packing as a seal. The valves are installed at locations in the

facility that do not experience temperatures greater than 650oC, the maximum

temperature rating of the valves. The valves have however been a primary source of leak

at high temperatures. The leak is primarily attributed to the drying to nickel anti-seize

used during the valve assembly. This resulted in the failure of the packing material

leading to loss of helium through leakage. As a temporary solution, the valve packing

material was replaced whenever the valve failed.

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2.2.6 Instrumentation

Controls for the Pre-Heater and Main Heater

As mentioned earlier, the heating system employed in the HTHF comprises of a pre-

heater and a main heater. Two phase angle type Silicon Controlled Rectifier (SCR)

power control units, one for the pre-heater and other for the main heater, control the

amount of power input to the heaters. Two Acuvim II multifunction digital power meters

continuously meter and monitor the current and voltage data fed to the heaters with an

accuracy of 0.2% of the reading. In addition, six microprocessor-based UDC 1200 limit

controllers with a thermocouple input ( from the thermocouples located near the heating

elements) and a relay output that is continuously fed to the power controller provide

necessary safety by preventing overheating of the heater elements. Furthermore, two

Honeywell UDC 2500 temperature controllers with a thermocouple input and a 4-20 mA

output monitor and control the fluid temperature at the exit of the heaters by providing

continuous feedback to the SCR controllers and help realize the desired outlet

temperatures.

Temperature Sensors

The temperature sensors provided by Weed Instruments are socket-weld standard-duty

type thermowells of Alloy 800H construction and house ASME special tolerance K-type

thermocouples. Eleven such thermowells are used for the measurement of helium

temperature at various locations in the facility. The thermocouples used in the facility

have special tolerance with an accuracy of 1.1oC or 0.4% (whichever is greater) in the

40

measurement range of 0 to 900oC. The thermocouples were calibrated by comparison

technique with a standard platinum resistance thermometer (SPRT) having traceability to

ITS-90. The calibration was performed with a thermocouple furnace in accordance with

ASTM E220-86 [2.15]. The maximum uncertainty of the SPRT used for calibration is

0.03oC. In addition, two ultra precise 4-wire RTD sensors with 1/10 DIN accuracy

(0.012oC) are installed in the facility to measure the inlet and exit temperatures in the

process chilled water line.

Pressure and Differential Pressure Sensors

Honeywell ST3000 smart pressure transducers with a 4-20 mA DC output are used for

measuring the helium pressures and differential pressures at/across different locations in

the facility. To measure the pressure or pressure differential in the loop at temperatures

above the operating range of transducers, the pressure transducer is isolated from the

pressure source by a long length of coiled tubing so that the helium temperature in the

sensing line (tubing) at the transducer is sufficiently reduced. All the pressure and

differential pressure transducers have been calibrated using standards whose accuracies

are traceable to NIST (National Institute of Standards and Technology). The accuracy of

the pressure transmitters are 0.375 psig. The accuracy of the differential pressure

transducers are 0.075% of calibrated span or upper range value (URV), whichever is

greater.

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Flow Meters

Three Venturi type flow meters measure the volumetric flow rates of helium gas flowing

through the facility. The flow meters have calibration information traceable to NIST

standards. Additionally, two high-temperature flow sensors designed by Delta M

Corporation are installed in the test loop for prototype design testing and cross

benchmark of the flow measurements against the Venturi flow meters. Furthermore, a

turbine flow meter installed on the process chilled water-side of the cooler allows

monitoring the flow rate of the process chilled water with an accuracy of 0.11% and has

calibration information traceable to NIST standard as well.

Data Acquisition System

All data acquisition (DAQ) and process control tasks are managed by a PC executing

LabView 8.5 under Windows XP. The DAQ system consists of NI compact DAQ

chassis and five NI 9211 modules, one NI 9205 module, and one NI 9217 module. The

NI 9211 module is a 24-bit, 4-channel thermocouple input module; NI 9205 is a 10 V,

16-bit, 32-channel single-ended or a 16-channel differential analog input module; and NI

9217 is a 4-channel, 24-bit, 100 RTD analog input module. A DC excitation power

supply is used to power all gage pressure and differential pressure transducers.

2.2.7 Quality Assurance

The Thermal Hydraulics Laboratory (THL) at OSU has a QA procedure in place that is

consistent with the QA guidelines provided by the U.S. Department of Energy. Personnel

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performing research on OSU HTHF were trained in the areas of testing and data

collection as per the QA program requirements to ensure that the produced data are

acceptable. Furthermore, the operation pr