New Type Tube-insert Design and Heat Transfer Enhancement Characteristics

XUE Xiuli1, HUANG Haizhen2, ZHOU Ming1, LI Lin1

1Yunnan Vocational College 1of Mechanical & Electrical Technology of Automotive technical engineering

department, Kunming, Yunnan 650203,China;

2Jinlin University Department of Thermal engineering, Changchun 130012,China

Abstract: In most cases fluid flow is constrained in the channel, if insert objects with different

structures, through changing the fluid flow pattern, it can improve the heat-transfer capability of

the fluid. According to this, a new type tube-insert component of heat transfer enhancement was

innovative designed through using the Field Synergy Principle and the Principles of Enhanced

Heat Transfer in the core flow; Took numerical simulation on the new design component by the

Orthogonal Text, it not only identified the primary and secondary influence factors of the new

type component on heat-transfer capability and obtained the optimal level combination of Nu, f

and PEC, but also proved the feasibility of the design. Meanwhile, in this article analyzed on the

heat-transfer capability of the specific structure enhancement component under the turbulent state

in the water, and researched on the enhancement heat transfer mechanism from the microscopic

view of the temperature filed, pressure field and the flow filed .

Key words: single-phase convection; insert; heat transfer; orthogonal test; numerical simulation; product design

Introduction

In recent years, with the rapid development of society, the needs of natural resources such as oil,

natural gas, coal is bigger and bigger. Due to the limited number of non-renewable resources and

the associated environmental pollution, energy wastage and other negative effects when it’s

exploited and utilized, it becomes negative factors of restricting the social healthy development,

while improve the efficient usage of energy is a feasible means for building a conservation-minded

and environment-friendly society.

The enhancement heat transfer of the energy is widespread in the thermal power, chemical

industry, metallurgy and other traditional industrial departments. Higher requirement for further

study on enhancement heat transfer technology is putting forward[1]. 1xuexiuli611@163.com

The heat exchanger is the main industrial equipment for heat transfer and conversion, thus it

can be seen, research on enhancement convection heat transfer has an important practical

significance. If insert the components, it can improve the heat-transfer capability of the fluid, So

tube-insert is becoming a hot field for researching enhancement heat transfer.

At present, the tube-insert has many species, the ideal effect among them for heat transfer

including porous media, spiral coil, helical spring, conical ring, longitudinal vortex generators,

static mixer, and twisted tape [2-17]. Visible, on the basis of researching on all kinds of enhancement

heat transfer theory and technology, to design a reasonable structure of tube-insert has a great

theoretical and practical significance.

1 The design idea of new tube-insert, structure characteristics and

research methods

1.1 The design idea of new tube-insert

When the fluid flow in the tube, flow boundary layer and thermal boundary layer will be formed

near the inner wall, while the variation of the velocity and temperature away from the wall is not

obvious. As the result of the existence of the boundary layer, it limits the mass and energy

exchange of the fluid in normal direction of the wall. The general idea of enhancement heat

transfer is to thin or damage the boundary layer, and increase the mixing effect of the fluid to

improve the mass and energy exchange of the fluid. But the measure leads to the resistance

increases. The Field Synergy Principle embodied in the synergistic effect of the flow field and

temperature field. A new enhancement heat transfer idea [18-21] was put forward from the

perspective of both average temperatures and drag reduction by the Principles of Enhanced Heat

Transfer in the core flow.

Through studying on the Field Synergy Principle and the Principles of Enhanced Teat Transfer

in the core flow, combining the advantages and disadvantages of the technology of tube-insert.

This article envisaged insert a disturbing objects into the mainstream area to increase the mass and

energy exchange of the fluid in the core area and the boundary layer, at the same time make the

fluid temperature homogenization, compressing the boundary layer thickness, increasing

temperature gradient near the wall to strengthen the heat transfer; Meanwhile, increase periodic

helical objects in the channel to make the fluid produces rotating centrifugal movement and

disturbance to thin the boundary layer, and then to increase the temperature gradient, ultimately

achieving the goal of enhancement heat transfer.

1.2 Structure characteristics, parameter characterization and design purpose of

the new tube-insert

According to the design idea of the new tube-insert, it is made of supporting bar which has

mixing effect, oval segment with vortex flow and the disturbance effect, and the locating shaft

with the effect of fixing. Base on the structure characteristics of the new tube-insert; it is named

Rotary Offset Enhancement Heat Exchanger, the structure diagram as shown in figure 1 and

figure 2.

Figure 1 3D left sketch

Figure 2 3D sketch

Spiral coil is a kind of enhancement heat transfer device which can make the fluid near the

pipe wall appear swirling flow, at the same time disturb the boundary layer of the fluid to prevent the

development of the boundary layer. It causes very small resistance which mainly comes from the

local resistance in the backwash of the coil. The ellipse segment of the rotary offset enhancement

heat transfer component is cut periodically in a certain length between arc lengths of the spiral coil.

The aim is to retain the characteristics of swirling flow, meanwhile, considering the vortex has a

certain continuity along the process, so it can achieve the effect of vortex and turbulence by only

cutting a section.

When the effect of vortex and turbulence was weak along the flow, the ellipse segment set

periodically along the axial can make the effect of enhancement the vortex and turbulence again,

finally achieve the purpose of enhancement heat transfer. The structure parameters of ellipse

segment contains: The semi-major axis a, semi minor-axis b, the central angle β used to characterize

the control factors of the elliptic arc length, the slope angle α used to characterize the inclined

degree(Shaft angle between the plane and the position that the axis of ellipse segment in),and the

ellipse segment thickness radius R1,the structure diagram as shown in Fig.3.

Figure 3 Ellipse segment positioning structure

After introducing the design of the elliptic segment, the support bar is introduced in order to

install and locate the segment, its structural installation method as shown in figure 4.The design of

the support bar not only plays the role of supporting the ellipse segment, the more important point is

its special effect on the function. On the basis of the Principles of Enhanced Heat Transfer in the

core flow, it mainly plays the effect of uniform temperature and mixing to the fluid, compressing the

thickness of the boundary layer to expand the temperature gradient. Through alternating rotation in a

certain distance with the support bar, this design can not only ensure the continuity of swirling flow

and the periodic disturbed flow, also the support bar plays the effect of uniform temperature and

mixing, by which not to cause too much resistance.

Figure 4 The structure of the support bar installation diagram

The corresponding structure and the installation parameters of the support bar and the

locating shaft including radius R2 of the support bar, highly H (height between the axis of locating

shaft and the axis of ellipse segment),the rotation angle γ（ the angle between the two adjacent

support bar along the axial direction）, rotary offset L(axial distance between two adjacent support

bar）,and radius R of the locating shaft.

1.3 The optimization analysis of the component

The factors that affect the heat-transfer capability of the insert are a lot, including central angle

β, slope angle α, etc. At first, the article determines the meaning and function of the design,

secondly, considers the combined influence on the convection heat transfer by this component and

other factors. Therefore, in previous studies, take the diameter of ellipse segment, support bar and

locating shaft to 5mm, the height of support bar to 17mm, and the semi-major axis and semi minor

axis of the ellipse segment should meet the requirements of a = H/cos(α) and b = H, the rest

structural parameters of the component can be taken different level parameter, the following table 1

is the level value.

Table 1 The structure parameter level table of the component

a/° b/mm β/° α/° H/mm γ/° L/mm

1 17.60 17 75 15 17 30 20

2 18.76 17 85 25 17 45 40

3 20.75 17 95 35 17 60 604 24.04 17 105 45 17 90 80

The orthogonal experiment of this component is under the condition of without considering the

interaction effect between the various factors, using the orthogonal table L16(45), analyzing 16

representative level groups selected from all the level combination in the experimental factors. Table

2 lists the combination relationship in the experiment between different levels of various factors.

Table 2 Orthogonal parameter table of the component

β/° α/° γ/° L/mm1 75 15 30 202 75 25 45 403 75 35 60 604 75 45 90 805 85 15 60 806 85 25 90 607 85 35 30 408 85 45 45 209 95 15 90 4010 95 25 60 2011 95 35 45 8012 95 45 30 6013 105 15 45 6014 105 25 30 8015 105 35 90 2016 105 45 60 40

1.4 Numerical simulation research method

1.4.1 The physical parameters and boundary conditions of the numerical simulation

Smooth pipe and this component material are set for steel, the wall is set to no slip conditions,

the temperature of the tube is the constant wall temperature 323K, the component is in the condition

of thermal insulation; Water as flow medium, not considering the influence of the fluid temperature

on the physical parameters, the physical parameter is a fixed value which is set to be the velocity-

inlet, temperature of the inlet fluid is 293 k, the turbulence degree of inlet is determined by the

formula I=0.016RE-1/8Error: Reference source not found,the hydraulic diameter is 40 mm; the exit is the pressure-

outlet, re-flow temperature is 293 k, the conditions for outlet turbulence degree and hydraulic

diameter is the same as inlet. If no special emphasis in this article, the simulation processes are set

on the above terms and conditions.

1.4.3 Geometric model of numerical simulation

The component length is 2000mm, diameter is 40mm. In order to determine the optimal

structure characteristics of the component, first of all, get the optimal structure characteristics from

the test of the orthogonal simulation which took in the condition of a certain fluid velocity, and then

to simulate and analyze under different flow velocity.

The smooth pipe as the benchmark before and after enhancement heat transfer, the thickness is

set to 0 mm; set the diameter of different structural parameters of the component, locating shaft,

ellipse segment and support bar to 5 mm, the length of the locating shaft is set to a constant value of

2000 mm; The length between the first support bar and the inlet end is 40 mm, the other parameters

of the support bar and ellipse segment are determined according to the orthogonal table 1.

2 The simulation process and discussion results

2.1 The numerical simulation of the smooth pipe

To get the numerical solution of high accuracy, first of all, made the numerical solutions of the

Nu and f when the smooth tube in different flow velocity as shown in table 3, and then compared

with calculated value from the experimental correlations.

Table 3 The flow rate of the water and the corresponding Re

Re 10000 13000 16000 20000 30000

uin（m/s） 0.25 0.32 0.4 0.5 0.75

The numerical solution of Nu is larger than the theoretical solution, the maximum is 10% and

the minimum deviation is 6%, the whole change trend and experimental correlations are identical;

The numerical solution of f is also lager than the theoretical solution, the maximum is 8%, the

minimum deviation is 3%,the whole change trend and experimental correlations are identical; it can

be seen that this numerical calculation method is reliable in the range of allowable error.

2.2. Water as the flow medium for the numerical simulation of the component

2.2.1 The conditions of the orthogonal numerical simulation

In order to determine the optimal level combination structure of the component, first of all,

made the numerical simulations on different structural combination by 16 groups under the certain

flow velocity. In the process of the numerical simulation, the inlet velocity of the water is 0.4 m/s,

the rest conditions set according to above requirements.

2.2.2 The orthogonal simulation analysis and verification simulation of Nu, f and PEC

The orthogonal simulation results of Nu, f and PEC as shown in table 4. In 16 group simulation

results, the maximum value of Nu is 331, increased by 106% compared with the simulation results of

160 in the smooth pipe; The minimum value is 195,increased by 21.9%.The data of 16 groups

reflects the fact that the component has different degree capabilities of enhancement heat transfer,

different structure combination shows different heat-transfer capability.

Table 4 The simulation results and analysis on different parameters of Nu, f and PECNo. β/° α/° γ/° L/mm Nu f PEC1 75 15 30 20 290 0.256 1.102 75 25 45 40 234 0.093 1.253 75 35 60 60 252 0.104 1.304 75 45 90 80 249 0.092 1.345 85 15 60 80 195 0.112 0.986 85 25 90 60 279 0.173 1.217 85 35 30 40 197 0.110 1.008 85 45 45 20 284 0.201 1.179 95 15 90 40 331 0.249 1.2710 95 25 60 20 286 0.307 1.0311 95 35 45 80 257 0.088 1.4012 95 45 30 60 238 0.083 1.3213 105 15 45 60 231 0.120 1.1314 105 25 30 80 257 0.104 1.3215 105 35 90 20 317 0.316 1.1216 105 45 60 40 257 0.129 1.23

Analysisk1 256 262 245 294k2 239 264 251 255k3 278 256 248 250k4 265 257 294 239

MAX 278 264 294 294MIN 239 256 245 239

Range 39 8 49 55k2 0.149 0.169 0.125 0.145k3 0.181 0.154 0.163 0.120k4 0.167 0.126 0.208 0.099

MAX 0.181 0.184 0.208 0.270MIN 0.136 0.126 0.125 0.099

Range 0.045 0.058 0.083 0.171k1 1.25 1.12 1.19 1.11k2 1.09 1.20 1.238 1.19k3 1.26 1.20 1.13 1.24k4 1.20 1.27 1.236 1.26

MAX 1.26 1.27 1.238 1.26

MIN 1.09 1.12 1.13 1.11Range 0.17 0.14 0.11 0.15

The simulation result shows that the primary and secondary factors affect Nu are L >γ>β >α, the

optimal level combination that affect Nu is β=95°, α=25°, γ=90°, and L=20 mm; the primary and

secondary factors effect on the f are L >γ > α >β, the optimal level combination that affect f is β=75

°, α=45°, γ=45° and L=80 mm; the primary and secondary factors affect PEC is not obvious ,it can

be thought of as β> L >α >γ, the optimal level combination that affect PEC is β=95°, α=45°, γ=45°

and L=80mm.

This combination (β=95°, α=25°, γ=90°, and L=20 mm) has the optimal effect on the Nu .The

value of Nu should be greater than 331 in theory, but it is not in 16 sets of orthogonal numerical

simulation experiments. In order to confirm the reliability of the simulation and the correctness of

the analysis results, only made simulation calculation, got Nu for 333 which was larger than 331.In

the same way, from the results of the simulation on the optimal combination of f and PEC, they all

proved that the reliability of the numerical simulation.

2.3The influence of the L on Nu, f ,PEC

In order to determine the influence rule of L with different structure combination on Nu，f and

PEC, respectively made adjustment to L of the optimal structure level of Nu， f and PEC to

20mm，40mm，60mm and 80mm under the same condition as the orthogonal simulation.

Figure 5 The influence of the optimal level combination of Nu, f and PEC with different L on

Nu

As Fig 5 shows, the smaller of L, the greater of the influence on Nu, which is especially obvious

on the optimal level combination of Nu. At the same time with the increase of L, the influence on

Nu of the three kinds of structure tends to be close to the same value which indicate that the L is the

primary factors affect Nu. The influence is limited when L is increased to be a certain distance, that

is because L has determined the uniform temperature and migmatization. So the smaller of L, the

stronger of the uniform temperature and migmatization, then the heat-transfer capability is improved.

Figure 6 The influence of the optimal level combination of Nu, f and PEC with different L on f

As Fig 6 shows, the value of f is gradually decreasing with the increase of L, but its value is still

less than the other two. The effect of L on the optimal level combination of Nu is obvious, but the

trend of the influence on the optimal combination of PEC and f is almost unanimously, so the

determined L in the orthogonal simulation is the main influence factors on f and is the important

parameter that affect the resistance and the heat-transfer capability.

Figure 7 The influence of the optimal level combination of Nu, f and PEC with different rotary offset on PEC

As Fig 7 shows, the value of PEC of the three doesn’t change obviously along with the L, from

which can be seen that the influence of L on PEC is limited, the main influence factors on PEC is

central angle which has been identified in the orthogonal analysis..

2.4 The heat-transfer capability of the component in the state of turbulent flow

In this article the heat-transfer capability of the component with different level combination

structure was respectively researched when the flow velocity was 0.25m/s, 0.32m/s， 0.40m/s,

0.50m/s and 0.75m/s.

（1） The influence analysis of Nu

As shown in Fig 8 is the comparison chart of Nu among the three kinds of optimal combination.

Nu has increased after inserting the three kinds of components, but the greatest influence on Nu is

the optimal structure combination of Nu. The heat-transfer capability increased from 85 to 12.While

the strengthened heat-transfer capability has been further improved as the increasing of the flow

velocity. When the flow velocity is 0.75m/s, Nu reached to 530 which was the 2.38 times in the

smooth pipe.

Figure 8 The influence of Re on Nu of the optimal level combination

（2） The influence analysis of f

From the Fig 9 can be seen that the f of the three structure showed a trend of decreasing along

with the increase of the flow velocity in the state of turbulent. But the f of the optimal level

combination(β=95°, α=25°, γ=90°, and L=20 mm) is the largest, that’s because the increasing of Nu

is under the condition of increasing the resistance loss and partial loss. At the same time, the f of the

others are almost unanimously small.

Figure 9 The influence of Re on f of the optimal level combination

（3） The influence of PEC

The Fig 10 shows when the Re is 10000, the PEC of the three is all less than 1, which indicates

that the comprehensive enhancement ability is weak. When the Re number increases to about 13000,

the PEC of the three is increased significantly, especially the optimal combination of f and PEC

retains at around 1.3 with the increase of Re. The PEC of the optimal level structure of Nu is just

larger than 1,without playing the rule of comprehensive enhancement heat transfer.

Figure 10 The influence of Re on PEC of the optimal level combination

3 Analysis of the component

In order to further recognize the heat transfer reasons of the component from the heat-

transfer mechanism, take the optimal level component of Nu as a example, analysis the

similarities and differences of the velocity and temperature fields under the condition of its inlet

velocity is of 0.4m/s and when it’s in the Smooth pipe, respectively.

3.1 Temperature field analysis of the optimal level combination of Nu

The Fig 11 shows that after insert the component, except the temperature along the axial is rising

and the radial temperature inward is dropping which is similar to the smooth pipe, its main

difference is that in the same cross-section enhancement temperature is higher than that in smooth

pipe, the increase of fluid temperature is faster along the axial than in smooth pipe. Meanwhile, at

the same cross-section along the radial direction from introversion to outside, the temperature in the

center flow area is more uniform, that shows the structure have played an important role in

enhancement the temperature uniformity of the fluid in core area, which is just the basic principles of

Principles of Enhanced Heat Transfer in the core flow. From the Fig 11, it can be seen that the

temperature in the area behind the support bar is significantly higher than that in other areas, it will

be inevitable to destroy the development of the temperature layer, that is the reason why the

temperature along the axial rises so faster.

Figure 11 The temperature cloud picture comparison of the five section along the axial when insert the strengthened tube of rotary offset structure

3.2 Velocity field analysis of the optimal level combination of Nu

From Fig 12 and 13 we can see the fluid velocity behind the support bar is extremely low, that

is due to the disturbed flow effect of the support bar, behind which formed the whirl vortex and then

lead to the decrease of the flow velocity; The flow velocity on both two sides of the support bar

gradually increases, that is because when the fluid flow is blocked, the fluid is bound to flow to the

two sides. Meanwhile, We can see the distributions of velocity is significantly different with that in

the smooth tube, starting from the inlet it has no obvious development processed of flowing, the flow

velocity is close to everywhere, but overall it is larger when close to the center area; And, on the

radial the change of velocity is not obvious which is distinctly different with that in the smooth tube,

it also illustrates the flow mixes sufficiently and reflects its temperature uniformity which has been

indirect proofed from its temperature cloud picture.

Figure 12 The velocity cloud picture comparison of the five section along the axial when insert the strengthened tube of rotary offset structure

Fig 13 The local velocity cloud picture of the strengthened tube along the inlet section，middle section and outlet section of the axial

The Fig 14 and 15 is the velocity vector diagram and the partial enlarged drawing of the section

which is 1000mm distance away from the inlet. From the picture we can see the smooth tube in the

whole are is mainly axial flow after inserting the component, while except the center area of the

strengthened tube is mainly axial flow but also appears circumferential swirl flow. By the centrifugal

movement, the circumferential swirl flow must wash the wall of the tube, it can not only strengthen

the mixing effect of the fluid in radial direction by which plays a role of uniform temperature.

Meanwhile, it can thin the boundary layer by washing the wall, through which improves the

temperature gradient near the wall and then achieves the purpose of enhancement heat transfer, that

is also the important reason why the component can improve Nu.

Fig 14 The velocity vector diagram of the strengthened tube and the smooth tube along the axial for 1000mm distance

Figure 15 The partial enlarged drawing of the strengthened tube and the smooth tube velocity and vectors along the axial 1000mm distance

4 Conclusion

In this article, the tube-insert design concept was put forward innovatively, and accordingly

designed a kind of rotary offset enhancement heat transfer component, through the orthogonal

numerical simulation experiment and other simulation experiments verified the feasibility of the

design, meanwhile obtained the mechanism of the heat transfer enhancement.

In a word, in this article only four parameters and one kind of fluid were selected to analysis,

some other factors were not studied, so it still has a lot of problems need to be analyzed and

researched on the component.

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