Development Status of the Helium-CooledPorous Tungsten Heat Exchanger Concept

Shahram Sharafata, Aaron Aoyamaa, Manmeet Narulaa, Jaafar El-Awadya, Nasr Ghoniema, Brian Williamsb,Dennis Youchisonc

aDigital Materials Solutions Inc., Granada Hills, CA, U.S.A.b Ultramet Inc., Pacoima, CA, U.S.A.

c Sandia National Laboratories, Sandia, NM, U.S.A.

Abstract-The development status of a helium cooled refractorymetal heat exchanger (HX) concept using tungsten foam forenhanced heat transfer is presented. The HX design is based onazimuthal flow of helium through the foam sandwiched betweentwo concentric tungsten tubes. This concept holds the promise foran efficient and low pressure-drop HX concept for plasma facingcomponents, such as divertors. A prototypical flat-top HX-tube isbeing manufactured for testing at the high heat flux testingfacility at SNL. Concept design optimization requires knowledgeof the enhanced heat transfer coefficients due to the foamstructure. Solid models of representative metal foams weredeveloped for use in CFD analysis. Initial CFD results showimproved heat transfer between the heated wall to the coolant.For a 1-mm thick foam with a specific density of 12% and a poredensity of 65 PPI an average heat transfer coefficients of 40 000W/m2-K was estimated, along with a pressure drop of -60 kPa.For a 10 MIW/M2 surface heat load and an inlet heliumtemperature of 600 °C at a pressure of 4 MPa, maximumstructural temperatures were estimated to be 1060 °C Thispreliminary design has a maximum combined primary plussecondary von Mises stress of less than 600 MPa.

Keywords: metal foam, porous, heat exchanger, helium-cooledPFC

I. INTRODUCTION

Divertor target plates of fusion power plants have to handlehigh heat fluxes (> 10 MW/M2), deliver heat at elevatedtemperatures for high efficiency, and use coolants that arecompatible with the blanket power conversion systems. Thus, ahelium-cooled refractory metal heat exchanger (HX) conceptwould be desirable. Furthermore, the coolant pressure dropshould be kept to a minimum to help with the overall plantpower balance and coolant exit temperatures should be as highas possible for efficient power conversion. To meet theserequirements, an advanced heat exchanger (HX) concept isunder development using refractory foam to enhance heattransfer. The concept consists of refractory foam bondedbetween two concentric channels, as described previously [1].The advantages of this concept are (1) all-refractory metal HXtube (no dissimilar materials to join), (2) high heat loadcapacity due to increased heat transfer coefficient (htc), and (3)low pressure drop because of azimuthal flow through foamsections.

In this work we report recent concept design- and analysisactivities along with the fabrication status of a prototypical flat-top advanced HX-tube.

II. ADVANCED CONCEPT

Starting with a "Foam-In-Tube" concept [1], an advancedHX-design was developed based on bonding the foam betweentwo concentric tubes. The inner tube has a centrally locatedslot, which runs the length of the tube. The helium coolantinitially flows radially through the slot then splits into twostreams and then flows azimuthally through the foam. Thefoam extends only as far as the heated top surface, to minimizethe helium flow-path length through the porous media. Thecoolant enters the foam at the center of the foam and then splitsinto two streams, which reduces the pressure drop by about 12.Figure 1 shows CAD drawings of a round- and a flat topadvanced concept.

Figure 1. CAD models of (left) round- and (right) flat-top 0.30 m longadvanced HX-concept.

III. CFD ANALYSIS

Design optimization requires reliable knowledge of theenhanced heat transfer coefficients due to the foam structure.The use of correlations developed for packed beds and granularmaterials to model the behavior of low density foams is highlyspeculative. The reasons are (a) granular materials or packedbeds have porosities in the range 0.4 - 0.6 while foam densitiesrange between 0.8 and 0.1, and (b) the assumption of localthermal equilibrium between the two phases is invoked. Theassumption that the solid and fluid phases have the sametemperature field results in using a single homogeneousequation to describe energy transport between the two phases.

This work was supported by the US Department of Energy, Office ofScience - Fusion Energy Sciences Program SBIR Grant with Ultramet Inc.

1-4244-1194-7/07/$25.00 02007 IEEE.

Thus, an effective stagnant thermal conductivity, ke, of a porousmedium is used to account for the solid and fluid phaseconductivities. The assumption of local thermal equilibriumdoes not hold if the stagnant conductivities of the solid andfluid are significantly different, which is the case when usinghelium as a fluid and tungsten as the low density foamstructure.

We present here CFD based modeling for estimatingenhanced solid-fluid interface heat transfer coefficients andpressure drops in low density foam structure. We reportpreliminary findings, which require greater refinement beforeconclusive thermo-fluid parameters can be gleaned.

A. Solid Modeling ofMetal Foam StructuresManufacturing of open-cell metal foams starts by chemical

vapor deposition (CVD) of a metal on a RVC (ReticulatedVitreous Carbon) foam skeleton. The RVC foam structureconsists of Tetracaidecahedra cells, which have 14 faces, 36edges, and 24 vertices. Depending on desired foam densities,ligament thickness typically range between 30 to 200 ptm withpore densities ofup to several hundred PPI.

An in-house program was developed to calculate thevertices (nodes) coordinates for joined tetracaidecahedra cells.The coordinates where then imported into a graphics packageto produce a solid model of the foam. Figure 2 shows atetracaidecahedra cell-based solid model of foam. The depictedfoam model has a pore density of about 65 PPI (pores perinch), relative density of about 11.7%, a specific surface area of8788 m2/m3 and ligament diameter of about 50 ptm. The foamis modeled in such a way as not to be mathematically regular.Instead, vertices are displaced from their original coordinatesrandomly to within ±5O of original location. Using the CADgenerated foam structure a solid model of the HX-tube sectionwas made for CFD analysis (Figure 3).

Figure 2. Solid model of foam (left); typical metal foam micrograph (right).

B. CFD ModelComputing resources needed to model an entire section of

the advanced HX tube are prohibitively large. A complete sliceof the HX would contain an 11x2xl mm3 (lxhxw) foamsection. Instead, exploratory CFD analysis is performed bymodeling a sub-section of the HX channel. Figure 3 shows theCFD model, which contains a 2x 1 xl(hxlxw) mm3 foamsection sandwiched between two 1-mm thick solid walls.

Reynolds number models for accurate simulation of near wallregions along with models for fluid solid conjugate heattransfer analysis. Turbulent heat transfer enhancement at thesolid/fluid interface and phase change heat transfer are butsome of the additional capabilities. The code uses varyingfluid properties, which change with time, temperature, andflow conditions. It can also handle diffusion of species andradiation heat transfer. An incompressible flow model is usedfor helium flow with the RNG k-E model [3] to calculate thetransfer coefficients.

The model was meshed and a total of 15 million elementswere generated. The computation time was of the order of 5hours on a dual core 64-bit machine. Flow conditions andmaterial properties used for CFD analyses are listed in Table I.

q1 =10 MW/Mr

1.62 mm

1rh1mm

Helium Flow:T = 600 OCp=4 MPaV= 100 MS

Figure 3. Solid model of foam (left); typical metal foam micrograph (right).

The initial temperature of the tungsten structure, foam, andHe-coolant is set at 600 °C and the surface heat flux is 10MW/M2. All other faces were assumed to be adiabatic. Theinlet flow velocity of 100 m/s was based on earlier He-flowtests performed at the SNL EB1200 facility [4]. The averageflow velocity is 134 m/s through the 2-mm wide slot in a 0.06m long HX-channel (Figure 1) with 4 MPa helium at 900 Kand a total flow rate of 27X 10-3 kg/s.

TABLE I. PARAMETERS USED IN THE CFD ANALYSIS.

PropertyHelium Inlet Flow VelocityHelium Inlet Temperature

Helium ViscosityHelium DensityThermal conductivity (helium)Tungsten Specific Heat

Tungsten Thermal ConductivitySurface Heat Flux

Value100 m/s

600 °C4.195 x 10-5 Pa-s

1.6 kg/m30.328 W/m-K

135 J/kg-K110 W/m-K

10 MW/m2

Using the CFD code SC-Tetrag [2] 3-D velocity,temperature, pressure drop profiles, as well as the solid-fluidinterface heat transfer coefficient were estimated. The SC-Tetrag code has several turbulence models with various low

C. CFD Analysis ResultsFigure 4 shows the CFD-based structural temperature

results. The effectiveness of the foam is illustrated. Although,

only 20% of the heated wall is covered by foam, the maximumtungsten surface temperature stays below 1080 °C. In theabsence of foam, a CFD analysis shows that the maximumstructure temperature is -150 °C higher.

CRADLETemperature Distribution (K)

873 987 1112 1226 1350k A a l peu

presuredrp,idepeauretonhihtouresestof thefoacur(ma (6 PPI°).

Incmaigue5shon,sN mheaueapressure dropacofs98e kPam forna

3.81X10-1 m long foam section, which has a density of 20%oand a 10 PPI pore density.

CRADLEPressure Distribution (Pa)

-30000 -1200 30000 58800 90000

Figure 6 shows the solid-fluid interface HTC contoursalong the inside of the heated wall and on foam ligaments.Two observations are made. First, the wall experiences arather constant htc of about 40000 W/m2-K with somelocalized maximum values of about 60000 W/m2-K, whichoccur in the vicinity of ligaments-wall contacts. Second thefoam ligaments themselves provide large areas with very highhtc values of about 40000 W/m2-K. These results imply thatthe foam enhances heat transfer by three mechanisms: (1)increased turbulence and mixing of helium neat the heatedwall; and (2) conduction of heat away from the wall throughligaments; and (3) convection from the ligament surfaces tothe fluid.

C.RAD ESurface Heat Transfer Coefficient (W/m2 K)

20000 29600 40000 49600 60000

Figure 6. Solid-fluid interface heat transfer coefficients (40 to 60 kWrm2-K)

Figure 7 shows the coolant temperature of about 100 °C.Except for a narrow region close to the heated wall, thecoolant temperature remains close to the inlet value. Thus,coolant mixing perpendicular to the heated wall is minimal.For a 10-mm long foam section more perpendicular mixingwill occur. However, it is speculated that a foam height of- 2mm is too large. In the near future, a channel height of lessthan 1 mm will be investigated.

Figure 5. Absolute coolant pressures (APfoam - 60 kPa). Figure 7. Coolant temperatures (AT = 100 OC)

IV. THERMO-MECHANICAL ANALYSIS

Using temperature dependant thermo-physical properties athermo-mechanical analysis of a prototypical 20-cm long flat-top single channel HX Design was conducted. The underlyingassumption for the thermal analysis is that the foam does notradiate and does not contribute to convective heat transfer. Theassumptions made for the structural analysis are that the tubeends are not connected to a manifold, and thus are free todeform. Two attachment rods are mounted underneath the tube,having a sliding boundary condition along the tube axis.Analysis conditions are listed in Table II. Figure 8 showsresulting temperature- and thermal stress contours. Maximumsurface temperatures are near 1050 °C and maximum thermalstresses are around 300 MPa. The primary stress due to 4 MPahelium coolant pressure increases the combined stress to amaximum of about 600 MPa, which is slightly above the yieldstress for > 80% hot worked and stress relieved tungsten at1000 C [5]. Design optimizations based on CFD-based HTCs,foam geometry, flow rates, and overall HX channel dimensionswill be performed at a later time to reduce maximum combinedstresses and temperatures.

(a) (b)

(c)~~~~~~~~~~d

Figure 8. (a) Solid model of flat-top HX concept (dimensions in Fig. 9), (b)temperatures (Tmax =1056 'C); (c) thermal stress (cGina 300 MPa); (d)

combined primary plus secondary stress (cina 600 MP).

TABLE II. PARAMETERS USED IN THE THERMO-MECHANICAL ANALYSIS.

Property

q

Helium Ti,Helium AT

Pressure

Massflow (mlAfo m)

h,oef (max)h,oef (min)

k, p, c,E, V, 7t, 7

Value

10 MWIm2

600 °C

100 OC4 MPa

0.23 gs/mm2

50,000 W1m2K

3,500 W1m2K

f(T)

f(T)

V. PROTOTYPE FABRICATION

Fabrication of an advanced flat-top HX-tube is underwayusing pure CVD tungsten. Both the outer- and slotted innertubes have successfully been fabricated. Figure 9 shows themanufactured CVD tungsten outer- and inner tubes as well asthe "CVD" welded HX-tube with foam (the coarse W-foam atthe bottom of the tube serves only as a "flexible" standoff andsupport for the inner tube).

Figure 9. Manufactured CVD tungsten flat top HX-channel showing (top)outer tube and (middle) slotted insert tube; (bottom) CVD-welded HX tube.

ACKNOWLEDGMENT

The authors wish to thank Dr. Siegfried Malang (FZK,Germany) for the many helpful and encouraging discussionsregarding the development of advanced helium-cooled HX-concepts.

REFERENCES

[1] S. Sharafat, A. Mills, D. Youchison, R. Nygren, B. Williams, N.Ghoniem. "Ultra Low Pressure-Drop Helium-Cooled Porous-TungstenPFC." Fusion Science and Technology, 2007: in press

[2] SC-Tetra CFD, Code. Vers. 6.0. Cradle Software Co. Ltd. 2006.http://www.cradle.co.jp/eindex.htm (accessed 2006).

[3] V. Yakhot, S. A. Orszag, "Renormalization Group Theory." J. Sci.Comput. 1 (1986) 3 - 51.

[4] D. L. Youchison, T.J. Lutz, B. Williams, R.E. "High Heat Flux Testingof a Helium-Cooled Tungsten Tube with Porous Foam," 24th SOFTConference, Warsaw, Poland, 2006.

[5] Dafferner, Michael Rieth and Bernhard. "Limitations of W and W-1%La203 for use as structural materials." J. of Nucl. Mater., Volume342, Issues 1-3, 30 June 2005, Pages 20-25. 342, no. 1- 3 (2005).

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