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Study on the flow distribution in prismatic VHTR core with a multi-block experiment and CFD analysis
Nuclear Engineering and Design 241 (2011) 5174–5182
Contents lists available at SciVerse ScienceDirect
Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes
Study on the flow distribution in prismatic VHTR core with a multi-block experiment and CFD analysis Su-Jong Yoon a,∗ , Chang-Yong Jin a , Jeong-Hun Lee a , Min-Hwan Kim b , Goon-Cherl Park a a b
Department of Nuclear Engineering, Seoul National University, 130 Dong, San 56-1, Daehak-Dong, Kwanak-gu, Seoul 151-742, Republic of Korea Korea Atomic Energy Research Institute, 150-1 Deokjin-Dong, 1045 Daedeokdaero, Yuseong, Daejeon 305-353, Republic of Korea
a r t i c l e
i n f o
Article history: Received 20 April 2011 Received in revised form 2 September 2011 Accepted 7 September 2011
a b s t r a c t VHTR is a gas-cooled, graphite-moderated reactor that was developed for the production of hydrogen and process heat. In the prismatic VHTR core with hexagonal graphite blocks, the bypass gap, which is an interstitial gap between the hexagonal blocks, inevitably forms due to the installation tolerance and irradiation shrinkage of the core blocks. Core bypass flow is the ineffective coolant flow that does not pass through the coolant holes within the fuel block. The core-bypass flow distribution varies according to the fuel cycle because the graphite blocks are deformed by irradiation and heating. Since the bypass flow reduces the core thermal margin, it should be evaluated precisely to guarantee reactor safety. In this regard, experimental and computational studies were carried out to evaluate the core-bypass flow distribution. A multi-block air test facility was designed and constructed in Seoul National University to measure the flow and pressure distributions according to the block combination and cross-flow gap size. The effects of the cross-flow phenomenon on the flow and pressure distribution in the core were investigated. The experimental data were used to validate a CFD model for the analysis of bypass flow characteristics in detail. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In recent years, demand for alternatives to fossil fuels has increased in part because of global warming concerns (NAE, 2004). Hydrogen has been highlighted a promising future energy source due to its high-energy density, clean, abundant and storable nature. Since nuclear energy is not only a resource with high energy density, it is also an economic, sustainable and technologyled energy source. Hydrogen production using nuclear energy is considered the most practical method. In this respect, the Very High Temperature Reactor (VHTR) was developed to produce massive amounts of hydrogen in a clean, safe and economical manner as well as to apply its high temperature for process heat. VHTR is a gas-cooled, graphite-moderated reactor. Two core designs of the VHTR exist: One is a Prismatic Modular Reactor (PMR) core and the other a Pebble Bed Reactor (PBR) core, as shown in Fig. 1. Typical PMR and PBR are the 600 MWth GT-MHR (Shenoy, 1996) and 400 MWth PBMR (Koster et al., 2003), respectively. This study examined the core bypass flow phenomenon that occurs in a prismatic core. The prismatic core is composed
∗ Corresponding author. Tel.: +82 2 880 7439; fax: +82 2 878 6745. E-mail address: [email protected] (S.-J. Yoon). 0029-5493/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nucengdes.2011.09.008
of hexagonal graphite blocks stacked axially in the core. There are interstitial gaps between the core blocks due to the installation tolerance of the graphite blocks, the sizes of which vary during the reactor operation due to irradiation shrinkage of the graphite blocks. Some portion of the coolant, which is defined as core bypass flow, flows through these gaps, which reduces the amount of the effective coolant to cool the reactor core. Therefore, core bypass flow has an adverse influence in the core thermal margin (INEEL, 2003). Despite the importance of core bypass flow, few studies have reported an accurate evaluation of the core bypass flow. In this regard, the aims of this study were to quantify the core bypass flow and examine its characteristics. For these purposes, multi-block core bypass flow experiments and CFD analysis were carried out. Commercial CFD code, CFX-12, was used to complement the experimental data and investigate the flow distribution in detail.
2. Bypass flow in prismatic core In general, the core bypass flow in a prismatic core is defined as the coolant flow not passing through the coolant holes within the fuel blocks. Fig. 2 shows the coolant paths in the prismatic core. The bypass gap is the interstitial gap between the block columns and the cross-flow gap is the interfacial gap between the block layers. The core bypass-flow distribution varies according to the
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Nomenclature BG CH CG Dh f F L R Re P Uin Uavg
bypass gap coolant hole cross-flow gap hydraulic diameter (m) friction factor fuel block length (m) reflector block Reynolds number pressure (Pa) inlet velocity (m s−1 ) average velocity (m s−1 )
Greek letters density (kg m−3 ) Subscripts avg average CR corner region CT central region F fuel side h hydraulic in inlet R reflector side
dimensional changes in the graphite block and the interchange of flow with the coolant holes through the cross-flow gap. Therefore, an understanding and accurate prediction of the cross-flow phenomena are also important for evaluating the flow distribution in a prismatic core. Since the VHTR adopts a graphite moderator, the core elements are made of graphite. According to the nature of graphite, the dimensions of the core graphite blocks vary due to the fast neutron irradiation and heating. In particular, the bypass flow and bypass gap area increase whereas the graphite blocks shrink due to fast neutron irradiation (Burchell et al., 1994). Bypass flow varies from 10% to 25% or more of the total core coolant flow (INEEL, 2005). A previous study on the effect of core bypass flow reported that the core bypass flow affects the reactor integrity and the achievement of the target temperature of the outlet coolant (i.e., increase in the bypass flow leads to an increase in the local maximum temperature of the core block (INEEL, 2003)). Therefore, an evaluation and reduction of the core bypass flow are very important issues in VHTR core development. 3. SNU multi-block air test experiment 3.1. Objectives A unit-cell experiment (Yoon et al., 2007) was carried out to examine the core bypass flow of the VHTR. The unit-cell experiment focused on the effects of the inlet mass flow rate, block combination and bypass gap size. From this experiment, the bypass gap size was found to be a major factor that determines the amount of the bypass flow and block combination affects the local flow distribution. The test section of the unit-cell experiment consisted of a single layer and the bypass gap of this experimental apparatus was restricted by the boundary walls of the test-section. Therefore, multi-block effects, such as the cross-flow phenomenon or lateral flow around the block periphery, could not be examined in the unit-cell experiment. To supplement the shortages of the unitcell experiment, the Seoul National University (SNU) multi-block
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experiment was carried out to examine the flow distribution in the multi-column and multi-layer structure. In this experiment, the effect of the cross-flow gap was also investigated. Another objective of the experiment was to produce reliable experimental data to validate the thermal-hydraulic analysis codes, such as GAMMA+ (Oh et al., 2009), GAS-NET (Vilim, 2007). These codes have been developed to evaluate the flow and temperature distribution of the VHTR core. 3.2. Configuration of the multi-block air test experimental apparatus The multi-block air test experimental apparatus consists of a blower, wind tunnel, test section including test blocks, measuring devices and data acquisition system. This experimental apparatus adopts an open-loop system. Fig. 3 shows a schematic diagram of the experimental facility. Figs. 4 and 5 show a cross-sectional view of the test section according to the block combination and a photograph of the block installation in a test section, respectively. The test section and test blocks were made of acryl. The test section is a rectangular section, 277 mm × 366 mm in size, with a total length of 812 mm. In total, 33 test blocks, which are 11 test block columns in a layer and 3 layers in a column, were installed in the test section. Two types of test blocks were used: the fuel and reflector type blocks. The dimensions of the test block were scaled down to one-third in length of the reference reactor, PMR200 (Jun, 2009). The flat-to-flat width and height of the hexagonal test block were 120 mm and 264 mm, respectively. Since the dimensions of the block were scaled down, the diameter of the coolant hole became too small to manufacture. Therefore, 108 coolant holes were reduced to 6 large holes while preserving the flow area, which is a major factor for the flow distribution. As a consequence of the simplification of the fuel block design, the flow characteristics, such as the Reynolds number and Euler number of the coolant hole was distorted to the actual core. Although the coolant hole of the experimental model was simplified, the cross-flow phenomenon could be identified with this experimental facility design more clearly. As shown in Fig. 2, the flow path of the prismatic VHTR core can be simplified to a parallel pipe system. In this case, if the flow area of the bypass gap is not varied, the magnitude of the cross-flow will be determined by the pressure difference between the coolant hole and the bypass gap. As the flow area difference between two channels increases, the pressure difference also increases, therefore the cross-flow phenomenon can be examined more effectively. The inlet mass flow rate of the test section and outlet mass flow rate of each block column were measured to evaluate the flow distribution and bypass flow ratio. Additional pipes, 1.41 m in length, were installed at the end of fuel block. Bi-Directional Flow Tube (BDFT) (KAERI, 2005) was installed in the additional pipe to measure the mass flow rate. Furthermore, a total of 14 pressure taps were installed on the both side walls of the test section along the central bypass gap to measure the local pressure distribution, as shown in Fig. 5. 3.3. Experimental conditions and test matrix The working fluid was air at normal temperature and pressure instead of helium gas because helium is unsuitable for an open-loop system. The experimental cases were determined by block arrangements and the cross-flow gap size. In this experiment, the bypass gap size was fixed to 2.0 mm. The effect of the block arrangement was estimated by installing three block combinations composed of the fuel and reflector blocks. The experimental cases were classified by the number of fuel block rows in the test section, as shown in Fig. 4. The experimental cases were classified
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Fig. 1. Features of the VHTR.
as F–F–F (F3 case), F–F–R (F2 case) and F–R–R (F1 case) by the block arrangement. The capital letters F and R mean the fuel block and reflector block, respectively. The cross-flow gap (CG) was 0 mm, 1 mm or 2 mm in each case. Table 1 lists the test matrix of the experiment.
Table 1 Test matrix of the multi-block experiment. Case F3CG0 F3CG1 F3CG2 F2CG0 F2CG1 F2CG2 F1CG0 F1CG1 F1CG2
Block combination
F–F–F (F3 case)
F–F–R (F2 case)
F–R–R (F1 case)
Cross-flow gap size (mm) 0 1 2 0 1 2 0 1 2
Bypass gap size (mm)
2
4. CFD analysis for the multi-block experiment 4.1. CFD benchmark and validation Since the flow path of the PMR core is very narrow and complicated, it is very difficult to analyze the temperature and flow distribution in the reactor core experimentally. In particular, it is not easy to measure the local data in the core experimentally. In this respect, CFD code can be used to analyze the local phenomena to supplement the experiment data because CFD analysis has no such restriction. Nevertheless, the reliability of CFD code needs to be validated because the wrong prediction could be obtained due to the numerical errors of CFD codes. Furthermore, the influence of a more fined mesh structure on the simulation result increases as the geometry of the computational domain becomes more complicated. Therefore, CFD validation has been implemented by a comparison with CFD analysis and the multi-block experiments. Commercial CFD code, CFX-12 (ANSYS, 2010), was used to analyze the local velocity and pressure distributions in the experiment. In the present study, CFD simulations for a cross-flow gap size of 2 mm
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Table 2 Mesh statistics of the CFD simulation for the multi-block experiment. Case
Total number of nodes
F1CG2 F2CG2 F3CG2
1,351,160 1,490,480 1,598,840
Number of wedge type mesh 331,632 331,632 331,632
Number of hexahedral mesh 1,019,528 1,158,848 1,267,208
case. Table 2 lists the mesh information of each simulation. A prism mesh was adopted on the walls to improve the accuracy of the simulation. For the multiple connected flow path of the prismatic core, the effect of the mesh structure on the simulation result was critical. When the prism mesh on the wall was not adopted, the CFD simulation result was discrepant from the experimental result in predicting the pressure distribution. In this simulation, the Shear Stress Transport (SST) model based on the Reynolds Averaged Navier–Stokes (RANS) equation with a standard wall function was employed for turbulence closure. The 2nd order upwind scheme was used to calculate the advection terms in the discrete finite volume equations. The inlet boundary conditions were specified according to the experimental conditions of each case. The pressure outlet boundary on the bypass gap outlet and the end of the flow measuring pipes were applied. 5. Results and discussions 5.1. Experimental result Table 3 lists the experimental results of the bypass flow ratio according to the inlet mass flow rate, the block combination and the size of the cross-flow gap. Two flow conditions for the inlet mass flow rate were tested in the experiment. The Reynolds numbers of the coolant hole and bypass gap were 6000 and 24,000 Table 3 Experimental and computational results for the bypass flow distribution. Fig. 2. Coolant flow paths in the prismatic VHTR core.
were performed to investigate the local phenomena caused by the cross-flow gap. 4.2. CFD modeling and boundary conditions The generation of the geometry and mesh structure was conducted using GAMBIT 2.3.16 code (FLUENT, 2006). A hybrid mesh using tetrahedral and hexahedral mesh was used for the efficient simulation. Fig. 6 shows the computational domain of the F3CG2
Case
Inlet flow rate (kg/s)
F3CG0 F3CG1 F3CG2 F2CG0 F2CG1 F2CG2 F1CG0 F1CG1 F1CG2
0.5876 0.5828 0.5828 0.4085 0.4123 0.4104 0.2489 0.2435 0.2502
Bypass flow ratio (%)
Experiment
Fig. 3. Schematic diagram of the experimental facility.
14.175 14.781 14.504 20.939 21.101 21.028 28.672 28.752 29.754
CFD analysis – – 14.508 – – 20.395 – – 28.994
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Fig. 4. Cross-sectional views of the test section according to the block combination.
Figs. 7–9 show the pressure distribution of each case. Linear pressure drops were observed in the CG0 cases of a no cross-flow gap. The linearity of the pressure drop curve was disturbed by the cross-flow gap. However, unlike the F2 and F3 cases, the pressure drop of the F1 case was not affected strongly by the cross-flow. Since the one fuel block column was installed in the F1 case, the effect of the cross-flow was relatively smaller than the other cases. For a similar reason, the cross-flow has a low impact on the pressure of the reflector side of the bypass gap because the bypass gaps at the periphery of the central blocks played a role as a buffer. Regarding the F2 and F3 cases, the pressure distribution of the two cases was similar. A sudden pressure drop across the crossflow gaps occurred. In particular, a strong variation of the pressure curve at the second cross-flow gap was observed. The reason why this phenomenon occurred at the second cross-flow gap is that the pressure difference between the two channels was larger at the second cross-flow gap than at the first gap. The pressure drop of each channel can be calculated by Eq. (1).
Fig. 5. Photograph of the test section.
for the low mass flow rate condition and 8000 and 32,000 for the high mass flow condition, respectively. In general, when the flow is clearly in the fully developed turbulent regime, the flow distribution is not strongly dependent on the Reynolds number (Malek and Hausermann, 1968). As a result, the bypass flow ratio, which is defined as the bypass flow divided by the total inlet flow, was not affected by the inlet mass flow rate conditions in this experiment. Since the flow regimes of the coolant hole and bypass gap were not changed, the flow resistance in these channels were not altered. Consequently, the flow distribution was not changed. To examine the effect of the variation in the flow regime, an additional experiment was carried out for the F3CG0 case by reducing the inlet mass flow rate. In this case, the flow regime of the bypass gap was changed from turbulent flow to laminar flow. As shown in Table 4, the bypass flow ratio was changed because the flow resistance of each flow path was changed by the change in flow regime. The bypass ratio was varied from 14% to 29% according to the block combination and was inversely proportional to the number of fuel blocks. The bypass gap area was not changed because the bypass gap size was fixed to 2 mm in this experiment. However, the coolant hole area increased with increasing number of fuel blocks. Consequently, the area ratio of the bypass gap to the total flow area decreased with increasing number of fuel blocks. This means that the flow distribution in the core can be varied locally by the block arrangement. Table 4 Bypass flow ratio according to the inlet mass flow rate in F3CG0 case. Inlet mass flow rate (kg/s) Bypass flow ratio (%)
0.569 14.50
0.418 13.35
0.098 10.39
P = f
L 2 0.5Uavg Dh
(1)
As for the friction factor, Blasius’s equation (Incropera and DeWitt, 1996) is used most widely for turbulent flow in smooth tubes and is calculated by Eq. (2). f = 0.316Re−0.25 for Re ≤ 2 × 104 f = 0.184Re−0.2 for Re ≥ 2 × 104
(2)
Since the channel shapes of the coolant hole and bypass gap as well as the hydraulic diameters of two channels are different, the Euler number of each channel is also different from each other. Eu =
P 2 0.5Uin
(3)
In this experiment, the pressure coefficient of the coolant hole is smaller than that of the bypass gap. Therefore, the pressure difference between the coolant hole and bypass gap increases so that the intensity of the cross-flow gap is stronger at the second cross-flow gap than at the first cross-flow gap. Although the bypass flow ratio for each block combination is different, the bypass flow ratio was not changed noticeably by the variation of the cross-flow gap size. As mentioned above, the crossflow phenomena affected the pressure distribution, but not the amount of the bypass flow ratio in Table 3. Since the flow length was relatively short according to the scaled-down test section and the outlet of the bypass gap was opened, the absolute magnitude of the pressure difference between the coolant hole and bypass gap was small making the effect of cross-flow on the bypass flow ratio insignificant.
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Fig. 6. Computational domain of the experimental facility (F3 case).
5.2. CFD simulation results CFD analysis for the SNU multi-block experiment was carried out to validate the accuracy of the code and obtain detailed information on the flow field in the test section inside. As mentioned above, a commercial CFD code, CFX-12, was used for the numerical analysis. The CG2 case was selected for CFD analysis because the flow distribution is expected to be more complicated than the others from the pressure distributions in the experiment. Fig. 6 shows the selected points for analyzing the CFD results. Points CHCR and CHCT indicate the data extraction locations of the coolant holes in the corner fuel block and central fuel block, respectively. Points BGF and BGR indicate those of the bypass gaps in the fuel side and reflector side, respectively. In the F3 case, the pressure of BGF is equal to BGR because of the symmetric geometry. The characteristics of the flow field in the test section could be analyzed by the pressure
distribution. When the flow area decreases or the flow velocity increases suddenly, the pressure of the channel also decreases suddenly (Idelchik, 1996). In this respect, when cross-flow flows in a flow channel, the flow velocity of that channel increases resulting in a decrease in channel pressure. Fig. 10 shows the pressure distribution for the F1CG2 case by comparing the CFD analysis results and experimental result. In this figure, the pressures of CHCR decreased suddenly at the first crossflow gap, whereas that of the CHCT increased inversely. At the first cross-flow gap, the pressure of the CHCT was higher than that of CHCR . Therefore, cross-flow occurred from CHCT to CHCR at the first cross-flow gap. The pressures of BGF and BGR were also higher than that of CHCR . Despite the cross-flow needing to occur from the BG to CHCR , the pressures of BGF and BGR decreased linearly. This is caused by the cross-flow from the central fuel blocks to the BGF , which attenuated the effect of the cross-flow, as shown in Fig. 11.
Fig. 7. Experimental data for the pressure distribution in the bypass gap (F1 case).
Fig. 8. Experimental data for the pressure distribution in the bypass gap (F2 case).
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Fig. 9. Experimental data for the pressure distribution in the bypass gap (F3 case).
For BGR , the intensity of the cross-flow was very small because it was relatively far from the coolant hole. As shown in Fig. 12, the intensity of cross-flow was stronger at the second cross-flow gap than at the first cross-flow gap because the pressure difference between CHCT and CHCR increased. At the second cross-flow gap, the pressures of BGF and BGR were also lower than that of CHCT . Consequently, cross-flow flowed from CHCT to BGF and BGR so that sudden pressure drops at BGF and BGR occurred. Since BGF was close to the coolant hole than BGR , the magnitude of the pressure drop of BGF was higher than that of BGR , which is similar to the first cross-flow gap. In contrast to BGR , the pressure of BGF increased slightly before it decreased across the second cross-flow gap. This is due to the lateral flow that inclined toward the central reflector block column, as shown in Fig. 13. Since this lateral flow reduced the mass flow rate through BGF , the pressure of BGF recovered slightly as the velocity of that channel decreased. In contrast to the F1 case, the pressure curve of the bypass gap in the F2 and F3 cases increased slightly at the first cross-flow gap. These flow patterns are similar to the sudden expansion flow. When
Fig. 10. Pressure comparisons with CFD analysis and experiment (F1CG2 case).
Fig. 11. Velocity vectors at the 1st cross-flow gap (F1CG2 case).
a cross-flow gap is formed due to deformation of the core block, the flow path of the cross-flow gap is added to the bypass gap, which has an analogous effect on the bypass gap as the flow area of the bypass gap increases. Unless the cross-flow flows to the bypass gap, the pressure curve of the bypass gap should be the pattern of the sudden expansion flow. In these cases, because the pressures of the coolant holes were smaller than those of the bypass gaps at the first cross-flow gap, as shown in Figs. 14 and 15, the crossflow flowed from the bypass gaps to the coolant hole, as shown in Figs. 16 and 17. In addition, the pressure curve of the bypass gap
Fig. 12. Velocity vectors at the 2nd cross-flow gap (F1CG2 case).
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Fig. 13. Vertical streamlines and velocity vectors at the 2nd cross-flow gap in the F1CG2 case.
Fig. 16. Velocity vectors at the 1st cross-flow gap (F2CG2 case).
increased slightly at the first cross-flow gap. In the F1 case, since the pressure difference between the coolant hole and bypass gap was small, the effect of the interaction between CHCT and CHCR was dominant. On the other hand, in the F2 and F3 cases, since the pressure of the bypass gap was higher than that of the coolant hole, the direction of the cross-flow at the first cross-flow gap was from the bypass gap to the coolant hole. On the other hand, the pressure of the bypass gap decreased suddenly at the second cross-flow gap, whereas that of the coolant hole increased slightly. The reason for the sudden pressure drop of the bypass gap is the effect of the cross-flow, so that the velocity vectors
Fig. 14. Pressure comparisons with CFD analysis and the experiment (F2CG2 case).
Fig. 15. Pressure comparisons with CFD analysis and the experiment (F3CG2 case).
Fig. 17. Velocity vectors at the 1st cross-flow gap (F3CG2 case).
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Therefore, CFD codes have sufficient capability to analyze the flow characteristics of a piece with the complicated flow path in the prismatic core. In addition, the reliability of the experimental and computational results was validated by comparative analysis. 6. Conclusion
Fig. 18. Velocity vectors at the 2nd cross-flow gap (F2CG2 case).
A multi-block air test experimental facility was designed and constructed to supplement the unit-cell experiment. The bypass flow ratio ranged from 14% to 21% according to the block combinations. The flow distribution can be changed by the variation of the flow resistance of each flow path. When both flow regimes of the coolant hole and bypass gap are kept in turbulent flow, the bypass flow ratio is practically independent of the inlet mass flow condition. Multi-block effects, such as the cross-flow phenomenon, were examined by measuring the flow and pressure distribution. Although the effect of the cross-flow was not observed in the flow distribution, the cross-flow had a strong influence on the pressure distribution. The flow direction inside the core can be conjectured by the pressure distribution. In addition, commercial CFD code, CFX-12, was carried out to validate its capability and explain the flow field in detail by a comparison with the experimental result. The CFD simulation showed good agreement with the experimental data. With CFD analysis, the detailed flow field could be investigated locally and the relationship between the pressure and flow distribution could be identified. In conclusion, the reliability of the experimental and computational result was confirmed. The results of this study will be useful for validating the thermal-hydraulic analysis code for VHTR. Acknowledgement This study was supported by Nuclear Research & Development Program of the National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (Grant code: 20082005919). References
Fig. 19. Velocity vectors at the 2nd cross-flow gap (F3CG2 case).
at the cross-sectional plane at the second cross-flow gap were from the coolant holes to the bypass gaps, as shown in Figs. 18 and 19. Since the flow area of the coolant hole is much larger than that of the bypass gap, the effect of the cross-flow is different in each channel. The simulation result of the CFX-12 showed good agreement with the experimental results. The pressure distribution and flow distribution were simulated accurately, as shown in Table 3.
ANSYS Canada Ltd., 2010. CFX-12 Solver Theory. ANSYS Canada Ltd., Ontario, Canada. Burchell, T.D., et al., 1994. The effect of neutron irradiation on the structure and properties of carbon–carbon composite materials. Journal of Nuclear Materials 191–194, 295–299. FLUENT Inc., 2006. GAMBIT 2.3 User’s Guide. FLUENT Inc., Lebanon, NH. Idelchik, I.E., 1996. Handbook of Hydraulic Resistance, third ed. Begell House, New York. Incropera, F.P., DeWitt, D.P., 1996. Introduction to Heat Transfer, third ed. John Wiley & Sons, New York. INEEL, NGNP Preliminary Point Design – Results of the Initial Neutronics and Thermal-Hydraulic Assessment, INEEL/EXT-03-00870, 2003. INEEL, Next Generation Nuclear Plant Research and Development Program Plan, INEEL/EXT-05-02581, 2005. J.S. Jun, Thermal-hydraulic Analysis of PMR 200 MWth Demonstration Reactor Candidate Core Using GAMMA+ Code, NHDD-RD-CA-09-004, KAERI, 2009. KAERI, Development of an Advanced Flow Meter using the Averaging Bi-Directional Flow Tube, KAERI/RR-2621/2005, 2005. Koster, A., Matzner, H., Nicholsi, D., 2003. PBMR design for the future. Nuclear Engineering and Design 222, 231–245. G.J. Malek, R. Hausermann, Analysis of the Multicolumn Flow Distribution Test Data, Gulf General Atomics. Project no. 901.5223, 1968. National Academy of Engineering, 2004. The Hydrogen Economy – Opportunities, Costs, Barriers, and R&D Needs. The National Academy Press, Washington, DC, USA. Oh, C.H., Lim, H.S., Kim, E.S., 2009. Development of GAMMA code as an integrated code analyzing a coupled very high temperature gascooled reactor and hydrogen production plant. Nuclear Technology 166, 101–112. A. Shenoy, et al., Gas Turbine-Modular Helium Reactor (GT-MHR) Conceptual Design Description. Report 910720, Revision 1, General Atomics, 1996. Vilim, R.B., 2007. GAS-NET: a two-dimensional network code for prediction of core flow and temperature distribution in the prismatic gas reactor. In: ICAPP-07, Nice, France, May 13–18. Yoon, S.J., Cho, Y.J., Kim, K.Y., Kim, M.H., Park, G.C., 2007. Experimental evaluation of the bypass flow in the VHTR core. In: SMiRT-19, Toronto, Canada, August 12–17.
Contents lists available at SciVerse ScienceDirect
Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes
Study on the flow distribution in prismatic VHTR core with a multi-block experiment and CFD analysis Su-Jong Yoon a,∗ , Chang-Yong Jin a , Jeong-Hun Lee a , Min-Hwan Kim b , Goon-Cherl Park a a b
Department of Nuclear Engineering, Seoul National University, 130 Dong, San 56-1, Daehak-Dong, Kwanak-gu, Seoul 151-742, Republic of Korea Korea Atomic Energy Research Institute, 150-1 Deokjin-Dong, 1045 Daedeokdaero, Yuseong, Daejeon 305-353, Republic of Korea
a r t i c l e
i n f o
Article history: Received 20 April 2011 Received in revised form 2 September 2011 Accepted 7 September 2011
a b s t r a c t VHTR is a gas-cooled, graphite-moderated reactor that was developed for the production of hydrogen and process heat. In the prismatic VHTR core with hexagonal graphite blocks, the bypass gap, which is an interstitial gap between the hexagonal blocks, inevitably forms due to the installation tolerance and irradiation shrinkage of the core blocks. Core bypass flow is the ineffective coolant flow that does not pass through the coolant holes within the fuel block. The core-bypass flow distribution varies according to the fuel cycle because the graphite blocks are deformed by irradiation and heating. Since the bypass flow reduces the core thermal margin, it should be evaluated precisely to guarantee reactor safety. In this regard, experimental and computational studies were carried out to evaluate the core-bypass flow distribution. A multi-block air test facility was designed and constructed in Seoul National University to measure the flow and pressure distributions according to the block combination and cross-flow gap size. The effects of the cross-flow phenomenon on the flow and pressure distribution in the core were investigated. The experimental data were used to validate a CFD model for the analysis of bypass flow characteristics in detail. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In recent years, demand for alternatives to fossil fuels has increased in part because of global warming concerns (NAE, 2004). Hydrogen has been highlighted a promising future energy source due to its high-energy density, clean, abundant and storable nature. Since nuclear energy is not only a resource with high energy density, it is also an economic, sustainable and technologyled energy source. Hydrogen production using nuclear energy is considered the most practical method. In this respect, the Very High Temperature Reactor (VHTR) was developed to produce massive amounts of hydrogen in a clean, safe and economical manner as well as to apply its high temperature for process heat. VHTR is a gas-cooled, graphite-moderated reactor. Two core designs of the VHTR exist: One is a Prismatic Modular Reactor (PMR) core and the other a Pebble Bed Reactor (PBR) core, as shown in Fig. 1. Typical PMR and PBR are the 600 MWth GT-MHR (Shenoy, 1996) and 400 MWth PBMR (Koster et al., 2003), respectively. This study examined the core bypass flow phenomenon that occurs in a prismatic core. The prismatic core is composed
∗ Corresponding author. Tel.: +82 2 880 7439; fax: +82 2 878 6745. E-mail address: [email protected] (S.-J. Yoon). 0029-5493/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nucengdes.2011.09.008
of hexagonal graphite blocks stacked axially in the core. There are interstitial gaps between the core blocks due to the installation tolerance of the graphite blocks, the sizes of which vary during the reactor operation due to irradiation shrinkage of the graphite blocks. Some portion of the coolant, which is defined as core bypass flow, flows through these gaps, which reduces the amount of the effective coolant to cool the reactor core. Therefore, core bypass flow has an adverse influence in the core thermal margin (INEEL, 2003). Despite the importance of core bypass flow, few studies have reported an accurate evaluation of the core bypass flow. In this regard, the aims of this study were to quantify the core bypass flow and examine its characteristics. For these purposes, multi-block core bypass flow experiments and CFD analysis were carried out. Commercial CFD code, CFX-12, was used to complement the experimental data and investigate the flow distribution in detail.
2. Bypass flow in prismatic core In general, the core bypass flow in a prismatic core is defined as the coolant flow not passing through the coolant holes within the fuel blocks. Fig. 2 shows the coolant paths in the prismatic core. The bypass gap is the interstitial gap between the block columns and the cross-flow gap is the interfacial gap between the block layers. The core bypass-flow distribution varies according to the
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Nomenclature BG CH CG Dh f F L R Re P Uin Uavg
bypass gap coolant hole cross-flow gap hydraulic diameter (m) friction factor fuel block length (m) reflector block Reynolds number pressure (Pa) inlet velocity (m s−1 ) average velocity (m s−1 )
Greek letters density (kg m−3 ) Subscripts avg average CR corner region CT central region F fuel side h hydraulic in inlet R reflector side
dimensional changes in the graphite block and the interchange of flow with the coolant holes through the cross-flow gap. Therefore, an understanding and accurate prediction of the cross-flow phenomena are also important for evaluating the flow distribution in a prismatic core. Since the VHTR adopts a graphite moderator, the core elements are made of graphite. According to the nature of graphite, the dimensions of the core graphite blocks vary due to the fast neutron irradiation and heating. In particular, the bypass flow and bypass gap area increase whereas the graphite blocks shrink due to fast neutron irradiation (Burchell et al., 1994). Bypass flow varies from 10% to 25% or more of the total core coolant flow (INEEL, 2005). A previous study on the effect of core bypass flow reported that the core bypass flow affects the reactor integrity and the achievement of the target temperature of the outlet coolant (i.e., increase in the bypass flow leads to an increase in the local maximum temperature of the core block (INEEL, 2003)). Therefore, an evaluation and reduction of the core bypass flow are very important issues in VHTR core development. 3. SNU multi-block air test experiment 3.1. Objectives A unit-cell experiment (Yoon et al., 2007) was carried out to examine the core bypass flow of the VHTR. The unit-cell experiment focused on the effects of the inlet mass flow rate, block combination and bypass gap size. From this experiment, the bypass gap size was found to be a major factor that determines the amount of the bypass flow and block combination affects the local flow distribution. The test section of the unit-cell experiment consisted of a single layer and the bypass gap of this experimental apparatus was restricted by the boundary walls of the test-section. Therefore, multi-block effects, such as the cross-flow phenomenon or lateral flow around the block periphery, could not be examined in the unit-cell experiment. To supplement the shortages of the unitcell experiment, the Seoul National University (SNU) multi-block
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experiment was carried out to examine the flow distribution in the multi-column and multi-layer structure. In this experiment, the effect of the cross-flow gap was also investigated. Another objective of the experiment was to produce reliable experimental data to validate the thermal-hydraulic analysis codes, such as GAMMA+ (Oh et al., 2009), GAS-NET (Vilim, 2007). These codes have been developed to evaluate the flow and temperature distribution of the VHTR core. 3.2. Configuration of the multi-block air test experimental apparatus The multi-block air test experimental apparatus consists of a blower, wind tunnel, test section including test blocks, measuring devices and data acquisition system. This experimental apparatus adopts an open-loop system. Fig. 3 shows a schematic diagram of the experimental facility. Figs. 4 and 5 show a cross-sectional view of the test section according to the block combination and a photograph of the block installation in a test section, respectively. The test section and test blocks were made of acryl. The test section is a rectangular section, 277 mm × 366 mm in size, with a total length of 812 mm. In total, 33 test blocks, which are 11 test block columns in a layer and 3 layers in a column, were installed in the test section. Two types of test blocks were used: the fuel and reflector type blocks. The dimensions of the test block were scaled down to one-third in length of the reference reactor, PMR200 (Jun, 2009). The flat-to-flat width and height of the hexagonal test block were 120 mm and 264 mm, respectively. Since the dimensions of the block were scaled down, the diameter of the coolant hole became too small to manufacture. Therefore, 108 coolant holes were reduced to 6 large holes while preserving the flow area, which is a major factor for the flow distribution. As a consequence of the simplification of the fuel block design, the flow characteristics, such as the Reynolds number and Euler number of the coolant hole was distorted to the actual core. Although the coolant hole of the experimental model was simplified, the cross-flow phenomenon could be identified with this experimental facility design more clearly. As shown in Fig. 2, the flow path of the prismatic VHTR core can be simplified to a parallel pipe system. In this case, if the flow area of the bypass gap is not varied, the magnitude of the cross-flow will be determined by the pressure difference between the coolant hole and the bypass gap. As the flow area difference between two channels increases, the pressure difference also increases, therefore the cross-flow phenomenon can be examined more effectively. The inlet mass flow rate of the test section and outlet mass flow rate of each block column were measured to evaluate the flow distribution and bypass flow ratio. Additional pipes, 1.41 m in length, were installed at the end of fuel block. Bi-Directional Flow Tube (BDFT) (KAERI, 2005) was installed in the additional pipe to measure the mass flow rate. Furthermore, a total of 14 pressure taps were installed on the both side walls of the test section along the central bypass gap to measure the local pressure distribution, as shown in Fig. 5. 3.3. Experimental conditions and test matrix The working fluid was air at normal temperature and pressure instead of helium gas because helium is unsuitable for an open-loop system. The experimental cases were determined by block arrangements and the cross-flow gap size. In this experiment, the bypass gap size was fixed to 2.0 mm. The effect of the block arrangement was estimated by installing three block combinations composed of the fuel and reflector blocks. The experimental cases were classified by the number of fuel block rows in the test section, as shown in Fig. 4. The experimental cases were classified
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Fig. 1. Features of the VHTR.
as F–F–F (F3 case), F–F–R (F2 case) and F–R–R (F1 case) by the block arrangement. The capital letters F and R mean the fuel block and reflector block, respectively. The cross-flow gap (CG) was 0 mm, 1 mm or 2 mm in each case. Table 1 lists the test matrix of the experiment.
Table 1 Test matrix of the multi-block experiment. Case F3CG0 F3CG1 F3CG2 F2CG0 F2CG1 F2CG2 F1CG0 F1CG1 F1CG2
Block combination
F–F–F (F3 case)
F–F–R (F2 case)
F–R–R (F1 case)
Cross-flow gap size (mm) 0 1 2 0 1 2 0 1 2
Bypass gap size (mm)
2
4. CFD analysis for the multi-block experiment 4.1. CFD benchmark and validation Since the flow path of the PMR core is very narrow and complicated, it is very difficult to analyze the temperature and flow distribution in the reactor core experimentally. In particular, it is not easy to measure the local data in the core experimentally. In this respect, CFD code can be used to analyze the local phenomena to supplement the experiment data because CFD analysis has no such restriction. Nevertheless, the reliability of CFD code needs to be validated because the wrong prediction could be obtained due to the numerical errors of CFD codes. Furthermore, the influence of a more fined mesh structure on the simulation result increases as the geometry of the computational domain becomes more complicated. Therefore, CFD validation has been implemented by a comparison with CFD analysis and the multi-block experiments. Commercial CFD code, CFX-12 (ANSYS, 2010), was used to analyze the local velocity and pressure distributions in the experiment. In the present study, CFD simulations for a cross-flow gap size of 2 mm
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Table 2 Mesh statistics of the CFD simulation for the multi-block experiment. Case
Total number of nodes
F1CG2 F2CG2 F3CG2
1,351,160 1,490,480 1,598,840
Number of wedge type mesh 331,632 331,632 331,632
Number of hexahedral mesh 1,019,528 1,158,848 1,267,208
case. Table 2 lists the mesh information of each simulation. A prism mesh was adopted on the walls to improve the accuracy of the simulation. For the multiple connected flow path of the prismatic core, the effect of the mesh structure on the simulation result was critical. When the prism mesh on the wall was not adopted, the CFD simulation result was discrepant from the experimental result in predicting the pressure distribution. In this simulation, the Shear Stress Transport (SST) model based on the Reynolds Averaged Navier–Stokes (RANS) equation with a standard wall function was employed for turbulence closure. The 2nd order upwind scheme was used to calculate the advection terms in the discrete finite volume equations. The inlet boundary conditions were specified according to the experimental conditions of each case. The pressure outlet boundary on the bypass gap outlet and the end of the flow measuring pipes were applied. 5. Results and discussions 5.1. Experimental result Table 3 lists the experimental results of the bypass flow ratio according to the inlet mass flow rate, the block combination and the size of the cross-flow gap. Two flow conditions for the inlet mass flow rate were tested in the experiment. The Reynolds numbers of the coolant hole and bypass gap were 6000 and 24,000 Table 3 Experimental and computational results for the bypass flow distribution. Fig. 2. Coolant flow paths in the prismatic VHTR core.
were performed to investigate the local phenomena caused by the cross-flow gap. 4.2. CFD modeling and boundary conditions The generation of the geometry and mesh structure was conducted using GAMBIT 2.3.16 code (FLUENT, 2006). A hybrid mesh using tetrahedral and hexahedral mesh was used for the efficient simulation. Fig. 6 shows the computational domain of the F3CG2
Case
Inlet flow rate (kg/s)
F3CG0 F3CG1 F3CG2 F2CG0 F2CG1 F2CG2 F1CG0 F1CG1 F1CG2
0.5876 0.5828 0.5828 0.4085 0.4123 0.4104 0.2489 0.2435 0.2502
Bypass flow ratio (%)
Experiment
Fig. 3. Schematic diagram of the experimental facility.
14.175 14.781 14.504 20.939 21.101 21.028 28.672 28.752 29.754
CFD analysis – – 14.508 – – 20.395 – – 28.994
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Fig. 4. Cross-sectional views of the test section according to the block combination.
Figs. 7–9 show the pressure distribution of each case. Linear pressure drops were observed in the CG0 cases of a no cross-flow gap. The linearity of the pressure drop curve was disturbed by the cross-flow gap. However, unlike the F2 and F3 cases, the pressure drop of the F1 case was not affected strongly by the cross-flow. Since the one fuel block column was installed in the F1 case, the effect of the cross-flow was relatively smaller than the other cases. For a similar reason, the cross-flow has a low impact on the pressure of the reflector side of the bypass gap because the bypass gaps at the periphery of the central blocks played a role as a buffer. Regarding the F2 and F3 cases, the pressure distribution of the two cases was similar. A sudden pressure drop across the crossflow gaps occurred. In particular, a strong variation of the pressure curve at the second cross-flow gap was observed. The reason why this phenomenon occurred at the second cross-flow gap is that the pressure difference between the two channels was larger at the second cross-flow gap than at the first gap. The pressure drop of each channel can be calculated by Eq. (1).
Fig. 5. Photograph of the test section.
for the low mass flow rate condition and 8000 and 32,000 for the high mass flow condition, respectively. In general, when the flow is clearly in the fully developed turbulent regime, the flow distribution is not strongly dependent on the Reynolds number (Malek and Hausermann, 1968). As a result, the bypass flow ratio, which is defined as the bypass flow divided by the total inlet flow, was not affected by the inlet mass flow rate conditions in this experiment. Since the flow regimes of the coolant hole and bypass gap were not changed, the flow resistance in these channels were not altered. Consequently, the flow distribution was not changed. To examine the effect of the variation in the flow regime, an additional experiment was carried out for the F3CG0 case by reducing the inlet mass flow rate. In this case, the flow regime of the bypass gap was changed from turbulent flow to laminar flow. As shown in Table 4, the bypass flow ratio was changed because the flow resistance of each flow path was changed by the change in flow regime. The bypass ratio was varied from 14% to 29% according to the block combination and was inversely proportional to the number of fuel blocks. The bypass gap area was not changed because the bypass gap size was fixed to 2 mm in this experiment. However, the coolant hole area increased with increasing number of fuel blocks. Consequently, the area ratio of the bypass gap to the total flow area decreased with increasing number of fuel blocks. This means that the flow distribution in the core can be varied locally by the block arrangement. Table 4 Bypass flow ratio according to the inlet mass flow rate in F3CG0 case. Inlet mass flow rate (kg/s) Bypass flow ratio (%)
0.569 14.50
0.418 13.35
0.098 10.39
P = f
L 2 0.5Uavg Dh
(1)
As for the friction factor, Blasius’s equation (Incropera and DeWitt, 1996) is used most widely for turbulent flow in smooth tubes and is calculated by Eq. (2). f = 0.316Re−0.25 for Re ≤ 2 × 104 f = 0.184Re−0.2 for Re ≥ 2 × 104
(2)
Since the channel shapes of the coolant hole and bypass gap as well as the hydraulic diameters of two channels are different, the Euler number of each channel is also different from each other. Eu =
P 2 0.5Uin
(3)
In this experiment, the pressure coefficient of the coolant hole is smaller than that of the bypass gap. Therefore, the pressure difference between the coolant hole and bypass gap increases so that the intensity of the cross-flow gap is stronger at the second cross-flow gap than at the first cross-flow gap. Although the bypass flow ratio for each block combination is different, the bypass flow ratio was not changed noticeably by the variation of the cross-flow gap size. As mentioned above, the crossflow phenomena affected the pressure distribution, but not the amount of the bypass flow ratio in Table 3. Since the flow length was relatively short according to the scaled-down test section and the outlet of the bypass gap was opened, the absolute magnitude of the pressure difference between the coolant hole and bypass gap was small making the effect of cross-flow on the bypass flow ratio insignificant.
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Fig. 6. Computational domain of the experimental facility (F3 case).
5.2. CFD simulation results CFD analysis for the SNU multi-block experiment was carried out to validate the accuracy of the code and obtain detailed information on the flow field in the test section inside. As mentioned above, a commercial CFD code, CFX-12, was used for the numerical analysis. The CG2 case was selected for CFD analysis because the flow distribution is expected to be more complicated than the others from the pressure distributions in the experiment. Fig. 6 shows the selected points for analyzing the CFD results. Points CHCR and CHCT indicate the data extraction locations of the coolant holes in the corner fuel block and central fuel block, respectively. Points BGF and BGR indicate those of the bypass gaps in the fuel side and reflector side, respectively. In the F3 case, the pressure of BGF is equal to BGR because of the symmetric geometry. The characteristics of the flow field in the test section could be analyzed by the pressure
distribution. When the flow area decreases or the flow velocity increases suddenly, the pressure of the channel also decreases suddenly (Idelchik, 1996). In this respect, when cross-flow flows in a flow channel, the flow velocity of that channel increases resulting in a decrease in channel pressure. Fig. 10 shows the pressure distribution for the F1CG2 case by comparing the CFD analysis results and experimental result. In this figure, the pressures of CHCR decreased suddenly at the first crossflow gap, whereas that of the CHCT increased inversely. At the first cross-flow gap, the pressure of the CHCT was higher than that of CHCR . Therefore, cross-flow occurred from CHCT to CHCR at the first cross-flow gap. The pressures of BGF and BGR were also higher than that of CHCR . Despite the cross-flow needing to occur from the BG to CHCR , the pressures of BGF and BGR decreased linearly. This is caused by the cross-flow from the central fuel blocks to the BGF , which attenuated the effect of the cross-flow, as shown in Fig. 11.
Fig. 7. Experimental data for the pressure distribution in the bypass gap (F1 case).
Fig. 8. Experimental data for the pressure distribution in the bypass gap (F2 case).
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Fig. 9. Experimental data for the pressure distribution in the bypass gap (F3 case).
For BGR , the intensity of the cross-flow was very small because it was relatively far from the coolant hole. As shown in Fig. 12, the intensity of cross-flow was stronger at the second cross-flow gap than at the first cross-flow gap because the pressure difference between CHCT and CHCR increased. At the second cross-flow gap, the pressures of BGF and BGR were also lower than that of CHCT . Consequently, cross-flow flowed from CHCT to BGF and BGR so that sudden pressure drops at BGF and BGR occurred. Since BGF was close to the coolant hole than BGR , the magnitude of the pressure drop of BGF was higher than that of BGR , which is similar to the first cross-flow gap. In contrast to BGR , the pressure of BGF increased slightly before it decreased across the second cross-flow gap. This is due to the lateral flow that inclined toward the central reflector block column, as shown in Fig. 13. Since this lateral flow reduced the mass flow rate through BGF , the pressure of BGF recovered slightly as the velocity of that channel decreased. In contrast to the F1 case, the pressure curve of the bypass gap in the F2 and F3 cases increased slightly at the first cross-flow gap. These flow patterns are similar to the sudden expansion flow. When
Fig. 10. Pressure comparisons with CFD analysis and experiment (F1CG2 case).
Fig. 11. Velocity vectors at the 1st cross-flow gap (F1CG2 case).
a cross-flow gap is formed due to deformation of the core block, the flow path of the cross-flow gap is added to the bypass gap, which has an analogous effect on the bypass gap as the flow area of the bypass gap increases. Unless the cross-flow flows to the bypass gap, the pressure curve of the bypass gap should be the pattern of the sudden expansion flow. In these cases, because the pressures of the coolant holes were smaller than those of the bypass gaps at the first cross-flow gap, as shown in Figs. 14 and 15, the crossflow flowed from the bypass gaps to the coolant hole, as shown in Figs. 16 and 17. In addition, the pressure curve of the bypass gap
Fig. 12. Velocity vectors at the 2nd cross-flow gap (F1CG2 case).
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Fig. 13. Vertical streamlines and velocity vectors at the 2nd cross-flow gap in the F1CG2 case.
Fig. 16. Velocity vectors at the 1st cross-flow gap (F2CG2 case).
increased slightly at the first cross-flow gap. In the F1 case, since the pressure difference between the coolant hole and bypass gap was small, the effect of the interaction between CHCT and CHCR was dominant. On the other hand, in the F2 and F3 cases, since the pressure of the bypass gap was higher than that of the coolant hole, the direction of the cross-flow at the first cross-flow gap was from the bypass gap to the coolant hole. On the other hand, the pressure of the bypass gap decreased suddenly at the second cross-flow gap, whereas that of the coolant hole increased slightly. The reason for the sudden pressure drop of the bypass gap is the effect of the cross-flow, so that the velocity vectors
Fig. 14. Pressure comparisons with CFD analysis and the experiment (F2CG2 case).
Fig. 15. Pressure comparisons with CFD analysis and the experiment (F3CG2 case).
Fig. 17. Velocity vectors at the 1st cross-flow gap (F3CG2 case).
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Therefore, CFD codes have sufficient capability to analyze the flow characteristics of a piece with the complicated flow path in the prismatic core. In addition, the reliability of the experimental and computational results was validated by comparative analysis. 6. Conclusion
Fig. 18. Velocity vectors at the 2nd cross-flow gap (F2CG2 case).
A multi-block air test experimental facility was designed and constructed to supplement the unit-cell experiment. The bypass flow ratio ranged from 14% to 21% according to the block combinations. The flow distribution can be changed by the variation of the flow resistance of each flow path. When both flow regimes of the coolant hole and bypass gap are kept in turbulent flow, the bypass flow ratio is practically independent of the inlet mass flow condition. Multi-block effects, such as the cross-flow phenomenon, were examined by measuring the flow and pressure distribution. Although the effect of the cross-flow was not observed in the flow distribution, the cross-flow had a strong influence on the pressure distribution. The flow direction inside the core can be conjectured by the pressure distribution. In addition, commercial CFD code, CFX-12, was carried out to validate its capability and explain the flow field in detail by a comparison with the experimental result. The CFD simulation showed good agreement with the experimental data. With CFD analysis, the detailed flow field could be investigated locally and the relationship between the pressure and flow distribution could be identified. In conclusion, the reliability of the experimental and computational result was confirmed. The results of this study will be useful for validating the thermal-hydraulic analysis code for VHTR. Acknowledgement This study was supported by Nuclear Research & Development Program of the National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (Grant code: 20082005919). References
Fig. 19. Velocity vectors at the 2nd cross-flow gap (F3CG2 case).
at the cross-sectional plane at the second cross-flow gap were from the coolant holes to the bypass gaps, as shown in Figs. 18 and 19. Since the flow area of the coolant hole is much larger than that of the bypass gap, the effect of the cross-flow is different in each channel. The simulation result of the CFX-12 showed good agreement with the experimental results. The pressure distribution and flow distribution were simulated accurately, as shown in Table 3.
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