Heat Transfer Enhancement with Mixing Vane Spacers Using

CHINESE JOURNAL OF MECHANICAL ENGINEERING Vol. 30,aNo. 1,a2017

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DOI: 10.3901/CJME.2016.0621.076, available online at www.springerlink.com; www.cjmenet.com

Heat Transfer Enhancement with Mixing Vane Spacers Using the Field Synergy Principle

YANG Lixin1, 2, ZHOU Mengjun1, 2, and TIAN Zihao1, 2

1 School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, China 2 Beijing Key Laboratory of Flow and Heat Transfer of Phase Changing in Micro and Small Scale,

Beijing Jiaotong University, Beijing 100044, China

Received February 25, 2016; revised June 6, 2016; accepted June 21, 2016

Abstract: The single-phase heat transfer characteristics in a PWR fuel assembly are important. Many investigations attempt to obtain

the heat transfer characteristics by studying the flow features in a 5´5 rod bundle with a spacer grid. The field synergy principle is used

to discuss the mechanism of heat transfer enhancement using mixing vanes according to computational fluid dynamics results, including

a spacer grid without mixing vanes, one with a split mixing vane, and one with a separate mixing vane. The results show that the field

synergy principle is feasible to explain the mechanism of heat transfer enhancement in a fuel assembly. The enhancement in subchannels

is more effective than on the rod’s surface. If the pressure loss is ignored, the performance of the split mixing vane is superior to the

separate mixing vane based on the enhanced heat transfer. Increasing the blending angle of the split mixing vane improves heat transfer

enhancement, the maximum of which is 7.1%. Increasing the blending angle of the separate mixing vane did not significantly enhance

heat transfer in the rod bundle, and even prevented heat transfer at a blending angle of 50°. This finding testifies to the feasibility of

predicting heat transfer in a rod bundle with a spacer grid by field synergy, and upon comparison with analyzed flow features only, the

field synergy method may provide more accurate guidance for optimizing the use of mixing vanes.

Keywords: rod bundle, mixing vane, heat transfer enhancement, field synergy

1 Introduction

Considering the operating conditions in a PWR, it is

necessary to understand the importance of the single-phase heat transfer characteristics of a fuel assembly. As the most important component of a fuel assembly, a spacer grid and mixing vanes, which comprise a mixing device attached on top of a spacer grid, are generally adopted to enhance the heat transfer coefficient in a fuel assembly. It is important to investigate the detailed heat transfer characteristics in a fuel assembly with spacer grids for maintaining a safe, high-efficiency PWR.

Over the past several decades, scholars have attached importance to the influence of a spacer grid or mixing vanes on hydraulic characteristics, including velocity, turbulence intensity, pressure and the heat transfer coefficient. They worked with the aims of understanding flow phenomena in a rod bundle with a spacer grid, obtaining heat transfer coefficients along fuel rods, and providing experimental benchmark data for computational fluid dynamics(CFD).

Generally, fluid exchange in subchannels and between

* Corresponding author. E-mail: [email protected] Supported by National Natural Science Foundation of China(Grant No.

51376022) © Chinese Mechanical Engineering Society and Springer-Verlag Berlin Heidelberg 2017

adjacent subchannels is considered important. MCCLUSKEY, et al[1], indicated that swirling flow in a rod bundle with a split mixing vane spacer grid was initially centered in the subchannel, and then migrated away from the center of the subchannel along the flow direction. CHANG, et al[2], showed distinguishing flow features for a split mixing vane spacer grid and a swirl mixing vane spacer grid. Compared with swirl mixing vanes, split mixing vanes generated no remarkable swirling flow within a subchannel and vigorous lateral flow in rod gaps. IKEDA and HOSHI[3] investigated the correlation between the cross-flow in a rod bundle downstream spacer grid and the axial flow inside a spacer grid, which indicates the cross-flow velocity is proportional to the axial velocity hitting mixing vanes. AGBODEMEGBE, et al[4], showed that directed cross-flow is determined by mixing-vane inclination and that cross-flow varied by pressure fluctuations between subchannels is negligibly small. LEE, et al[5], focused on flow mixing and flow pulsation and obtained the mixing coefficient. They found that the time-averaged axial velocity in the center of the subchannel becomes faster than that in the rod gap center, and that the root-mean-square value of the lateral velocity and the lateral turbulence intensity in the rod gap was higher than those in the subchannel center. Furthermore, some scholars, such as CUI and KIM, expressed the flow features in a rod bundle with a mixing vane spacer grid by several

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dimensionless parameters, which include the swirl factor and the cross-flow factor[6]. This describes directly and quantitatively the impact of a mixing vane on the flow characteristics. It was showed that swirl factor and cross-flow factor have positive effects on the heat transfer coefficient in a rod bundle, which means that an increasing swirl factor or cross-flow factor may promote heat transfer in the rod bundle. These works described the flow characteristics in a rod bundle, but they did not consolidate flow features and heat transfer at the same time.

HOLLOWAY, et al[7–9], showed a variation of the heat transfer coefficient in a rod bundle with a split mixing vane spacer grid, which included heat transfer coefficients along the flow direction and around the central rod. While they provided experimental results of heat transfer, they provided no explanations of what resulted in the heat transfer variation. On the other hand, many experimental results have provided references to CFD simulation results. DOMINGUEZ-ONTIVEROS, et al[10–11]. measured velocity and turbulence fields near a spacer grid with an experimental technique called particle image velocimetry (PIV).

From reviewing this research on flow and heat transfer in a rod bundle, we have doubt regarding whether the variation of the heat transfer coefficient can be estimated by the sole factor of the flow feature in a rod bundle. Are there some more accurate criteria to estimate heat transfer in a rod bundle?

From the view of energy, GUO, et al[12–13], regarded the convective heat transfer as heat conduction with an interior heat source and proposed the field synergy principle. This principle dramatically represents the enhanced convective heat transfer. It indicates that the convective heat transfer depends on the flow and temperature fields and on the intersection angle between velocity and temperature gradients. This concept has been extended to other phenomena, such as natural convection[14–15], forced convection[16–18], porous media[19], etc. However, there has been no study of whether it is suitable to apply the field synergy principle to estimate heat transfer in a rod bundle with a spacer grid.

This paper analyzes the enhanced single-phase heat transfer in a rod bundle with a mixing vane spacer grid by the field synergy principle. Based on the validation of CFD simulation with Holloway’s experimental results[7], a separate mixing vane of AFA 3G and one typically shaped split mixing vane was simulated. The impact of these two shapes and six mixing-vane blending angles on field synergy and heat transfer in a rod bundle were compared. This investigation proved the applicability of the field synergy principle on single-phase heat transfer in a rod bundle with a spacer grid, and may provide guidance for optimizing a mixing-vane spacer grid. We can speculate that the differences in heat transfer in rod bundles with spacer grids are due to different mixing-vane configurations.

2 Modeling

2.1 Grid models

This study was based on a 5´5 rod bundle with a spacer grid. In order to compare the impact of mixing vanes on flow and heat transfer, a spacer grid without mixing vanes was simulated. Fig. 1(a) shows the spacer grid without mixing vane made up of straps, springs, and dimples. Fig. 1(b) shows the arrangement of the mixing vanes on the spacer grid. A pair of mixing vanes is attached on each strap intersection, and comparing the pair of mixing vanes, other mixing vanes on adjacent intersections are reserved to it. For the pair of mixing vanes and other mixing vanes on adjacent intersections, straps are perpendicular and the blending directions are contrary. Fig. 2 shows two types of mixing vanes and its blending angle. The two type mixing vanes are separate mixing vanes which are used in AFA-3G fuel assembly and a typical split mixing vane respectively. The blending angle of mixing angle are 25°, 30°, 35°, 40°, 45°, and 50°. The blocking ratio ε, which is caused by mixing vane, refers to the ratio of projected area of mixing vane and the fluid flow area in the flow direction. Both the shape and the blending angle affect the blocking ratio. The blocking ratio of separate mixing vane is 0.53 times as split mixing vane under the same blending angle.

Fig. 1. Spacer grid

Fig. 2. Mixing vanes

2.2 Fluid Domain Geometry Models

CFD simulation results were acquired in the 5´5 rod bundle. The rod bundle, as shown in Fig. 3, is constructed out of 9.5 mm diameter rods that were 1116 mm long and assembled in a spacer grid on a square array having a pitch of 12.6 mm. The spacer grid is located 250 mm from the

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section inlet. The equivalent diameter of the subchannel is 11.78 mm. Based on the same spacer grid without the mixing vane, 13 different CFD simulation domains were established, which includes six domains with the separate mixing vane, six domains with the split mixing vane, and one domain without a mixing vane.

Fig. 3. Computational domain of a 5´5 rod bundle

with a spacer grid

2.3 CFD Models and Verification The analysis was performed using the commercial CFD

code ANSYS CFX14.5. The main factors affecting the single-phase simulation accuracy include mesh and turbulence model.

2.3.1 Mesh

A detailed discussion of the mesh generation method that was used in this paper and its effect on CFD results were given by CHAO, et al[20]. The full domain was divided into three parts. These are the inlet section which is the region upstream of the spacer grid, the spacer grid section which is the region covering the spacer grid and the outlet section which is the region downstream of the spacer grid. Fig. 4 shows meshes on local surfaces and sections of the CFD domain. The meshes of the spacer grid section are full of tetrahedral sites because of the complex structure. The meshes in the region of inlet section and outlet section were obtained by extending the cross-sectional mesh profile at the inlet or outlet of a spacer grid section. This meshing method can ensure the accuracy of simulation results and minimize the number of mesh to save the calculating resources simultaneously. Mesh sensitivity was analyzed to determine a final mesh model that had a total number of about 6.7 million. The same mesh parameters are used in these thirteen domains to eliminate the difference caused by the mesh model between thirteen different CFD domains.

Fig. 4. Mesh plots on local surfaces and sections

2.3.2 Turbulence model

Holloway provided the experiment results of the 5´5 rod bundle with the split mixing vane spacer grid with a blending angle of 30°. The entrance boundary conditions assumed in the simulation correspond to the inlet condition of the experiments performed by Holloway, with the inlet

mass flow rate and the temperature set to 5.99 kg/s and 20 ℃, respectively. This simulation assumed operating at atmospheric pressure. At the outlet of each domain, a relative average pressure of 0 Pa was defined. The surfaces of the rods, housing, and spacer grid were defined as wall and uniform heat flux density 34 530 W/m2 on twenty-five rods was defined. RNG k-ε, k-ε, and SSG turbulence model were used in CFD simulation, respectively.

Table 1 shows the CFD simulation results and Holloway’s corresponding experimental results. It includes pressure loss of spacer grid, pressure loss of rod bundle and Nu. Fig. 5 shows Nu/Nufd on different location downstream the spacer grid. Nufd means the Nusselt number on condition of fully developed fluid flow in rod bundle, which was described in detail by Holloway.

Table 1. CFD result validation with Holloway’s experimental results

Quantity Experiment

results

CFD results and error with Holloway’s experiment

k-ε RNG k-ε SSG

ΔPgrid/Pa 6860 7589/ +10.63%

7163/ +4.42%

6858/ –0.03%

ΔProd/Pa 1720 1953/ +13.55%

1834/ +6.63%

1767/ +2.73%

Nu (1Dh–10Dh)

213.93 234.34/ +9.54%

217.76/ +1.79%

209.45/ –0.46%

Nu (11Dh–40Dh)

206.19 221.50/ +7.43%

207.75/ +0.76%

208.16/ +0.96%

Fig. 5. CFD results and Holloway’s experimental results

Comparing the CFD results of the RNG k-ε, k-ε, and

SSG turbulence model with Holloway’s experimental results shows that the SSG turbulence model is suitable for simulating the single-phase flow and heat transfer in fuel assembly. The error of pressure loss and Nu is about ±3% and ±1% respectively. Based on this situation, SSG turbulence model was chosen to simulate these thirteen domains under the same operation condition described above.

3 Results Analysis

3.1 Field Synergy Principle Many works focused on flow field in rod bundle with


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spacer grid and analyzed the impact of mixing vane on flow characteristics. The turbulence intensity and several mixing factors along flow direction were applied to explain the fact that enhanced heat transfer by mixing vane. From the view of energy, GUO regarded the convective heat transfer as heat conduction with interior heat source. The temperature gradient T , velocity U , and angle between the velocity vector and the temperature gradient or heat flow vector codetermine the vector dot product •U T . It can be expressed as

• • • cosU T U T = , (1)

where β is the intersection angle between velocity vector and temperature gradient. The field of •U T indicates the enhanced convective heat transfer by flow field directly. The convection heat transfer depends on the velocity, the temperature gradient, and on their synergy. The better synergy of velocity and temperature gradient results in a higher convection heat transfer value under the same conditions. That is to say, a smaller synergy angle β enhances convective heat transfer.

The filed synergy principle was extended to analyze the mechanism of the enhanced heat transfer in the rod bundle with the mixing vane spacer grid. The CFD simulation results indicated two ways of enhancing heat transfer in the rod bundle with the spacer grid. One was to enhance the heat transfer on the rods surface, and the other one was to enhance heat transfer in subchannels. Fluid flows through the mixing vane and then up in the spiral type around the rods surface. At the same time, fluid forms a swirl flow in the subchannel and a cross flow between different subchannels. Based on this phenomenon, the synergy angle on the surface of rod βs and the synergy angle in subchannel βv were defined as

s

darc

•cos

d

•U T S

U T S

=

òò

, (2)

v

darc

•cos

d

•U T V

U T V

=

òò

, (3)

respectively. Eqs. (1)–(3) indicate that there is a corresponding relationship between the synergy angle on the surface of rod βs and the vector dot product •U T or the synergy angle in subchannel βv and the vector dot product .•U T The larger the vector dot product

,•U T the smaller the synergy angle. The same boundary conditions and the same structure, which includes the standard spacer grid and rod bundle, but not mixing vane, were applied to all thirteen CFD simulations. It is feasible to compare the synergy angle βs and βv to estimate the enhanced heat transfer caused by different mixing vanes, because the vector dot product •U T was nearly equal in

the inlet section for all thirteen simulations. 3.2 Mechanism of heat transfer enhancement

of the mixing vane The mechanism of the enhanced heat transfer of the

mixing vane in the rod bundle was analyzed by comparing CFD results of the 5´5 rod bundle with separate mixing vanes at a blending angle of 30° with the spacer grid and the results of the 5´5 rod bundle with no mixing vane spacer grid. The relationship of the surface heat transfer coefficient h and the synergy angle βs and βv was discussed. Analysis objects include the surface of the central rod and the corresponding cross section of subchannels.

For situations with separate mixing vane at a blending angle of 30° and without mixing vane, Fig. 6(a) and Fig. 6(b) show contour plots of heat transfer coefficient h, synergy angle βs, and synergy angle βv of subchannels at Z/Dh = 1, Z/Dh = 10, and Z/Dh = 20, respectively. Fig. 6(a) shows that synergy angle βs distribution is similar to the heat transfer coefficient at the surface of the central rod when the central rod synergy angle βs is less than 90°. Consequently, a smaller synergy angle βs caused a higher convection heat transfer. The average heat transfer coefficient of central rod is 12.3 kW/(m2 • K). As for synergy angle βv of subchannels, its value is less than 90° at widespread area. However, the area vanishes gradually along the flow direction and the average synergy angle βv almost equals 90°. Fig. 6(b) shows that the synergy angle βs of central rod nearly equals 90° beyond Z/Dh = 1, that is to say, the fluid velocity and the heat flow are nearly perpendicular to each other at the central rod surface. Synergy angle βv in subchannels of different locations also nearly equals 90° along the flow direction. The average heat transfer coefficient of the surface of the central rod is 11.9 kW/(m2 • K).

Mixing vanes change the fluid flow direction and enhance convection heat transfer in the rod bundle. On the one hand, the synergy angle βs decreased and convection heat transfer enhanced on the surface of the rod. On the other hand, the synergy angle βv decreased and convection heat transfer enhanced in subchannels. The heat transfer coefficient on the surface of a rod is codetermined by synergy angles βs and βv.

There is a phenomenon in which the heat transfer coefficient h becomes higher, while the synergy angle βs increases. This happens on a region downstream of the spacer grid between 01Dh. The flow characteristics in the region are extremely complex because springs and dimples disturb the flow field in the subchannels. The heat transfer on the rod’s surface is enhanced even without the mixing vane. Fig. 7 shows distributions of the average values of βs, βv, and h on the region. The heat transfer enhances within 1.2Dh because of the change of local flow direction , which is induced by the straps and the mixing vane.

The analysis above all shows that it is feasible to reveal the mechanism of the enhanced heat transfer in fuel


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assembly with the synergy principle. A method of estimating the effects of the mixing vane on enhancing the heat transfer is synergy characteristics, especially the synergy angle.

Fig. 6. Effect of synergy angle on the heat transfer

coefficient of the rod surface

Fig. 7. Synergy angle and heat transfer coefficient

of the rod surface

3.3 Effects of mixing vane The ratio parameters of pressure loss, heat transfer, and

synergy angle were defined based on the results of the spacer grid without the mixing vane. Total pressure loss and heat transfer performance were discussed with separate and split mixing vanes for different blending angles. The field synergy principle was applied analyze how different

mixing vanes enhance the heat transfer.

3.3.1 Parameter definition ΔP was defined as pressure difference of the spacer grid

top plane and the outlet plane of the CFD domain. ΔP0 indicates the value of ΔP under condition of spacer grid without mixing vane. Similarly, Nu0 indicates the value of Nusselt number of spacer grid without mixing vane. Eqs. (4) and (5) defined the average Nusselt number and the average heat transfer coefficient of the rod bundle channels downstream spacer grid, respectively. Eqs. (4) and (5) are expressed as

hDhNu

= , (4)

m p f 2 f1

w f

( )

( )

q c t th

A t t

-=

-, (5)

where Dh indicates an equivalent subchannel diameter, λ indicates fluid heat conductivity coefficient, A indicates heat transfer area that is analyzed, qm indicates mass flow rate, cp indicates heat capacity of fluid, tf indicates the average temperature of fluid, and tw indicates the average temperature of rods surface.

3.3.2 Pressure loss and Nu

Fig. 8 shows pressure loss and the average Nusselt number for the separate and split mixing vanes for six different blending angles. The pressure loss increases linearly as the blending angle increases for the separate and the split mixing vanes. The reason is blocking ratio ε increasing with blending angle. Obviously, the additional pressure loss in the separate and split mixing vane present different slope coefficients with the changing blending angle. However, the average Nusselt number had no significant changes with increased blending angle.

Fig. 8. Effect of mixing vane on pressure loss and Nu

For the separate mixing vane, the pressure loss ranges

from 1.2ΔP0 to 1.4ΔP0 with the blending angle increasing from 25° to 50°. The rate of additional pressure loss is 0.8% per degree. However, the average Nusselt number has no significant difference. For the split mixing vane, the pressure loss ranges from 1.3ΔP0 to 1.78ΔP0 with the blending angle increasing from 25° to 50°. The rate of


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additional pressure loss is 3.1 percent per degree. The average Nusselt number has no significant difference when blending angle less than 30°, and both of them increase by 5%. A blending angle of 40° increases by 7%, while the blending angle of 45° declines slightly. Blending angle of 50° reaches a maximum of 7.1%.

Based on the discussion above, we concluded that the heat transfer in the rod bundle with the split mixing vane spacer grid can be enhanced by increasing the blending angle, which causes much more pressure loss. For the separate mixing vane, increasing the blending angle has no apparent effect on heat transfer, while using the same blending angle induces a slight pressure loss.

3.3.3 Velocity and Temperature

The lateral velocity in a rod bundle is similar for a split mixing vane spacer grid and a separate mixing vane spacer grid due to the same arrangement of mixing vanes. Using the lateral velocity on a 4Dh downstream spacer grid as an example, Figs. 9(a) and 9(b) show the lateral velocity with split mixing vanes and separate mixing vanes, which have the same flow features and a difference of velocity magnitude in subchannels. Compared with a separate mixing vane, the lateral velocity is more obvious for a split mixing vane. Figs. 9(c) and 9(d) show the temperature distribution on a 4-Dh downstream spacer grid with a split mixing vane and separate mixing vane. The velocity magnitude for a split mixing vane and separate mixing vane results in differences in temperature; the temperature gradient distribution also exhibits a difference. The relationship between velocity and temperature gradient affect heat transfer in a rod bundle. All velocity and temperature distributions considered, the heat transfer for a split mixing vane may be stronger than that for a separate mixing vane.

Fig. 9 Lateral velocity and temperature

on a 4-Dh downstream spacer grid

3.3.4 Field synergy analysis Subsection 3.2 shows the convection heat transfer in a

rod bundle from the view of combining synergy angle βs on a rod surface and synergy angle βv in subchannels. Based on the field synergy, this subsection discusses the effects of different mixing vane on enhancing heat transfer of rod surface and subchannels. The distribution of average synergy angle βs on central rod surface, average synergy angle βv in four adjacent subchannels around central rod for separate, and split mixing vane for sixing blending angle are described. Similarly, average synergy angle βs on 25 rods and average synergy angle βv in all subchannels are also described.

Figs. 10(a) and 10(b) show the synergy angle βs on central rod surface along flow direction. The synergy angle βs is 90° expect these positions close to the top of spacer grid without the mixing vane. It indicates that there is no enhanced heat transfer, for the reason that fluid flow is vertical to heat flux. The phenomenon, which decreases the synergy angle βs on central rod surface after fluid flow through mixing vane, happened under condition of spacer grid with mixing vane. Spiraling up and around the rod caused the enhanced heat transfer on the rod surface.

In the case of the spacer grid with separate mixing vane, the synergy angle βs on the central rod surface has a small fluctuation between 02Dh, gradually decreases after 2Dh, and reaches the minimum at 8Dh. It indicates that the enhanced heat transfer is obvious before 8Dh and gradually decays after this position. The blending angle of 45° and 25° are effective and worst for enhancement of heat transfer, respectively. It is worth nothing that the synergy angle βs gradually decreases after 20Dh.

In the case of the spacer grid with split mixing vane, the enhanced heat transfer on the surface of rod is more effective than separate mixing vane between 0–16Dh. The synergy angle βs reaches the minimum at 6Dh and gradually increases after this position. Blending angles of 50° and 25° are the most effective and the least effective for enhancing heat transfer, respectively. It is a remarkable fact that the distribution of synergy angle βs at a blending angle of 45° and 50° are different from other. It is also probable that the spiraling flow around rod changed.

Figs. 10(c) and 10(d) show the synergy angle βv in four adjacent subchannels around central rod along flow direction. Mixing vane has an effect of improving the performance of synergy in subchannels. Synergy angle βv in subchannels is smaller than the synergy angle βs on the rod surface. It indicates that the enhanced heat transfer in the subchannel is obvious. The distribution of synergy angle βv for the split and the separate mixing vanes is similar. However, the value of synergy angle βv for the split mixing vane is smaller than the separate mixing vane. It means that enhancing the heat transfer is more effective. The area of synergy angle βv is below 80° between 016Dh for the split mixing vane, while it is only below 80° between 0 and 8 Dh for the separate mixing vane.


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Fig. 10. Distribution of synergy angle along flow direction

Fig. 11 shows the average synergy angle βs and βv of the

5´5 rod bundle for 12 mixing vanes to present the effect of the mixing vane on enhancing heat transfer. It is obvious that the average synergy angles are fully consistent with the average Nusselt number of the 5´5 rod bundle by comparing Figs. 8 and 11. The average synergy angle βv is smaller than the average synergy angle βs. It indicates that the enhanced heat transfer in the subchannels is more effective than on the rod surface. If the pressure loss is ignored, the performance of the split mixing vane is superior to the separate mixing vane based on the enhanced heat transfer. The effect of the split mixing vane on enhancing heat transfer improves with increasing blending angle. However, this trend is not suitable for the separate mixing vane. Increasing the blending angle of the separate mixing vane does not significantly enhance heat transfer in the rod bundle, and can even reduce it at a large blending angle.

4 Conclusions

(1) The pressure loss caused by mixing vane increases almost linearly with the increased blending angle and the rate of the split mixing vane is 1.6 times for the separate mixing vane.

(2) The field synergy principle is feasible to explain the

reason of enhancing heat transfer in a rod bundle with a spacer grid. Enhanced heat transfer can be predicted by the distribution of synergy angle βv and βs. Enhancing the heat transfer in the subchannels is more effective than on the rod’s surface.

(3) If the pressure loss is ignored, the performance of the split mixing vane is superior to the separate mixing vane based on the enhanced heat transfer. The effect of split mixing vane on the enhanced heat transfer improves with increased blending angle. Increasing blending angle of separate mixing vane does not significantly enhance heat transfer in the rod bundle, and can reduce it at large blending angles.

Fig. 11. Effect of the mixing vane blending angle on βs and βv


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Biographical notes YANG Lixin, born in 1969，is an ssociate professor at Beijing Jiaotong University, China. His research direction is complex flow, heat and mass transfer process. Tel: +86-133-11181768; E-mail: [email protected] ZHOU Mengjun, born in 1993, is currently a postgraduate candidate at Beijing Jiaotong University, China. Her research direction is flow and heat transfer characteristics in fuel assembly. Tel: +86-185-13231865; E-mail: [email protected] TIAN Zihao, born in 1992 and major in thermal power engineering, is currently a postgraduate candidate at Beijing Jiaotong University, China. Tel: +86-188-13032554; E-mail: [email protected]

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Heat Transfer Enhancement with Mixing Vane Spacers Using