Natural Circulation Pool Reactor Evaluated by Using FLUENT ... · circulation[3]. The Fluent porous...

Journal of Multidisciplinary Approaches in Science 3, Issue 1 (2019) 10-30

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Journal of Multidisciplinary Approaches

in Science (JMAS)

Journal homepage: https://jmas.biz/index.php

Natural Circulation Pool Reactor Evaluated by Using FLUENT Porous Media Model to Simulate the Reactor Core and Heat Exchanger

Muhammad Shehzad Khan 1,* & Muhammad Alamgeer Khan2

1

2 Department of Physics, City University of Hong Kong, Hong Kong

2 University of Science and Technology of China, China

ARTICLE INFO ABSTRACT

Article history: Received 29 December 2018 Received in revised form 27 January 2019 Accepted 19 February 2019 Available online 10 March 2019

Natural Circulation Systems or Free convection circuits are working in several engineering fields. Natural circulation can also be used as an inactive safety system or as a main heat transfer in nuclear reactors. Several few new fast reactors may be desirable earlier than forecast in the Generation IV Plans. Affording to the Special Report on Emission Scenarios commissioned by the Intergovernmental Panel on Climate Control (IPCC) nuclear powers will increase through a factor of four by 2050. The drivers for this improvement are the increasing energy demand in developing countries, energy security, but also climate distresses. The article introduces the simplified modeling of the simulation problem of the reactor based on some simplification and assumptions and introduces how to carry out grid independence analysis and turbulence. Here the flow field, temperature field, pressure field and core flow distribution characteristics of typical liquid metal cooled natural circulation pool reactor were investigated by using FLUENT porous media model to simulate the reactor core and heat exchanger. Lead-bismuth eutectic provides a low melting point (398 K) limiting problems with freezing in the system and features of low chemical activity with water. An LFR performs air excluding the possibility of fire or explosions to have also an economic advantage and it doesn’t need an intermediate coolant circuit. The number of reactor outages can be limited. The latter feature is also relevant for the remote location of prefabricated LFRs.

Keywords: FLUENT ,Neutronics analysis, Pb-Bi , Natural Circulation, Copyright © 2019 JMAS - All rights reserved

1. Introduction In January 2017 new act is passed by U.S. House of Representatives which is called the Advanced Nuclear Technology Act of 2017 (ANTA). At the same time, another act was also introduced which named as Interim Consolidated Storage Act (ICSA). These bills address two of the most vital approvals

* Corresponding author. E-mail address: [email protected] (Muhammad Shehzad Khan)

Journal of Multidisciplinary Approaches in Science Volume 3, Issue 1 (2019) 10-30

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made in 2011 by the President Obama’s Blue Ribbon Commission on America’s Nuclear Future (the BRC)[1]. In near future nuclear industry has also been in waiting for law to break the deadlock of bureaucratic red-tape that has constrained building and developing new type of reactors as well as to generate a vital place for used fuel that can be burned later. ANTA would enhance the permitting and progress of new reactors, presently the most arduous of all regulatory processes [2]. In this research work we introduce some assumptions and simplification to simplified modeling of the simulation problem of the reactors, and present how to convey out grid independence investigation and turbulence the flow field, pressure field, temperature field and core flow distribution characteristics of typical liquid metal cooled natural circulation pool reactor were analyzed. The investigation of natural circulation characteristics of bismuth and lead reactors in typical natural circulation[3]. The Fluent porous media is used to simulate the reactor core and for heat exchange[4]. In 2001, 4th Generation of the International Nuclear Energy Forum (GIF) designated highly developed potential of four generations of nuclear energy system, most of them are running on lead and are integrated pool design. The liquid metal cooling pool reactor has good intrinsic safety and economy, and has good forecasts for improvement[5]. In 2011, for nuclear waste transmutation the Chinese Academy of Sciences launched an engineering project to advance an accelerator driven subcritical system (ADS)[6, 7]. The Institute of Nuclear Energy Safety Technology proposed The and they selected this reactor as a reference for technology development as well as for ADS development of the generation IV lead-cooled fast reactor. Clear-1 with the conceptual design of 10 MW thermal powers has been completed. KYLIN series lead-bismuth eutectic experimental loops have been fabricated to examine the technologies of the coolant, structural materials, key components, fuel assembly, control and operation. In order to test and legalize the main parts and incorporated operating technology of the lead-based reactor, the lead-based zero-power nuclear reactor CLEAR-0 and the lead alloy-cooled non-nuclear reactor CLEAR-S, are under apprehension[8-10].

The main part of this paper are based on thermophysical properties like pressure, temperature, surface tension, enthalpies, compressibility, density, viscosity, heat capacity, electrical and thermal conductivity etc. of molten lead-bismuth eutectic, lead and bismuth (as the component of LBE). In some cases, significant inconsistencies occur among the values assumed by different sources. Lyon , Kutateladze and Nikol’skii were assembled the first compilations of the main thermophysical properties of LBE and Pb[11]. After this most the date was replicated without any changes by Mantell, Lyon Bonilla,Crean, [12]. Only add some of the new results by Friedland, Hultgren, Kubaschewski, Iida, Gurvich, Cheynet, Schenkel, R [13]. During the last decade several reviews have been available in which former data ware reanalysed by Cevolani,Imbeni, Kirillov,Bogoslovskaya,Sobolev [14]. The dependability of data mainly depends on used method for their fabrication and the attention with which the method is used. In general, data regarding metals in the liquid or vapour state show a substantial distribution, with the exception of the melting points. Only limited authors of compilations have paid suitable consideration to dispersion and standard deviation of the reported values e.g. Hultgren and Gurvich[15, 16]

The Chinese ADS development program passes through three important steps. In first step an ADS research capability, which comprise with CLEAR-I, spallation target, and proton accelerator (250 MeV/10 mA) will be built up. In 2nd step predicted the manufacture of an ADS demonstration facility, which contains of a 100MWth nuclear system (China lead-based engineering demonstration reactor, named CLEAR-II) attached with a proton accelerator (_600 MeV/_10 mA). In 3rd step the ADS


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commercial prototype facility will be developed, which comprises of a 1000MWth magnitude reactor (China lead-based commercial prototype reactor, named CLEAR-III) attached with a proton accelerator (_1500 MeV/_10 mA). As pre- testing facility, a multifunctional lead-based zero power reactor (CLEAR-0) will be run to acquire the neutronics experiment data for smearing series license of CLEAR reactors. CLEAR-I was planned to be functioned in a two ways: subcritical operation mode for ADS operation technology validation and critical operation mode for technology test of lead cooled fast reactor (LFR)[17, 18].

CLEAR-II will be considered as an engineering demonstration reactor for nuclear waste transformation using ADS, as pilotplant of lead-alloy cooled small modular reactor (SMR), and it also could be used as high neutron flux irradiation capability for nuclear fuel and fusion reactor materials. The reactor technology will also use for the hydrogen generation based on direct connection of gaseous hydrocarbons and evaporated water[9]. CLEAR-III is the profitable prototype reactor of nuclear unwanted transmutation system. The technologies of CLEAR series reactor design and progress will benefit not only ADS and lead-based fast reactors, but also lead–lithium blanket development of fusion reactor and fusion driven subcritical system, especially in the design and analysis codes, the structural materials, the transmutation fuel element technologies, and the safety assessment technologies[18-20].

1.1 Theory Natural circulation is a state of the fluid in the system continue to flow without the aid of a

pump. One of the causes of natural circulation is the driving head caused by density differences in the fluid caused by temperature differences in the fluid. Reactor safety regulation requires understanding the thermal hydraulic. Thermal hydraulic include process of conduction, convection and process cooling flow out of the reactor core[21]. Pin fuel in the fuel assembly for fast reactor can be arranged in triangular lattice geometry and the coolant flow through the channel in the reactor. Flow channels are designated as the spacer between the fuel pin. Flow channels are non-circular, whereas for thermal hydraulic analysis, much easier when the flow channels are cylinder. This can be overcome by defining effective hydraulic diameter, Dh.

We need to evaluate the heat transfer coefficient h. Value of heat transfer coefficient on liquid metal fast breeder reactor design is calculated by using quantities related to hydraulic flow. There are some heat transfer correlations for liquid metal cooled like Pr (Prandtl number), Re (Reynolds number), Pe (Peclet number) and Nu (Nusseltnumber). The effects of Reynolds and Prandtl numbers on the heat transfer characteristics of an isolated cylinder having the range of conditions 1 ≤ Re ≤ 45 and 0.7 ≤ Pr ≤ 4,000 (the maximum value of Peclet number being 4,000) in crossflow. In addition, the variation of local Nusselt number on each face of the obstacle and representative isotherm plots to elucidate the role of Prandtl number on heat transfer in the steady flow regime. Energy is transferred from the fuel pins by convection to the coolant according to expression as foundational energy engineering equation

ℎ(𝑇%&' − 𝑇%') = 2𝜋𝑟%&'𝑑𝑧 = 𝑞1𝑑𝑧 = 𝑚𝑐𝑝𝑑𝑇 (1) Where 𝑚𝑐𝑝𝑑𝑇 ,heat = mass flow * specific heat capacity * temperature difference 𝑞1= near power density, W/m, kW = kg/s * kJ/kg/°C * °C


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The temperature distribution along the channel is given by ，

T(z) = 𝑇78&9: + <𝑞1=

>

(𝑍)𝑑𝑧/𝑚𝑐A (2)

A quantity that indicates the pressure difference before and after the coolant through the coolant channel is called pressure drop. Pressure on the channel can be lost or drop due to [I] the friction factor by wall along the coolant channel. The pressure drop across channel is， ∆P |fric =f L/Dh ×ρV2/2 (3)

[II] The form factors, K by the grip fasteners two fuel elements caused the loss of pressure. Form pressure drop is given by the equation, ∆P |form = ∆P |grid =∆P |inlet-exit = KρV2/2 (4)

[III] The elevation factor is caused by gravitational acceleration, ∆P |elevation = ρgh (5) 1.2 General Specifications Reactor Design

Research design study of liquid metal Pb-Bi cooled fast reactors and capability of natural circulation Is accomplished using the SRAC for the neutronics analysis. Thermal hydraulic and natural circulation analysis are performed by developing computational code based Borland Delphi 7.0. General specifications for simulation capability of natural circulation are presented in Table 1 as given below.

Table 1 General Specification’s reactor design

Parameters Specifications Reactors FBR

Power 150-250 MWth Coolant Pb-Bi

Shielding Stainless Steel fuel pelle UN-PuN

pitch 1.4 cm Fraction of fuel, cladding, coolant 45%, 10%, 45%

Geometry Cylinder 2-D (R-Z) Enrichment coolant 44.5 % Pb – 55.5 % Bi Size of active core 60 cm x 75 cm

width reflector 30 cm

Inlet temperature 300oC


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1.2.1. Design Pin Fuel and Core This simulation uses a cylindrical cell one dimension divided into 3 regions: fuel, cladding, and

coolant. Calculation diffusion multi group equation uses module in SRAC for the cylindrical 2D (R-Z) geometry of the core which is divided into 4 regions of radial and 7 regions of axial. The fuel composition at radial direction is set different, so that core is heterogeneous. The coolant must have a low neutron absorption cross section, good heat transfer capability, low pump power, low freezing point, high melting points, non-corrosive. Based on these considerations, the coolant Lead-Bismuth (Pb-Bi) with a composition of 44.5% Pb and 55.5% Bi is chosen as a coolant.

Fig. 1 is a sketch of the systems of coolant flow. The system is designed with a core should be at the bottom is lower than the steam generators. The system is simplified by assuming after passing through the steam generator, a temperature inlet equal to the initial inlet temperature.

Fig.1. Circulation of the coolant in the reactor core

Figure 2 shows that the mass flux coolant for each channel in the radial. Mass flux of the channel near the radial center larger than the reflector, it is due to central radial more heat and thus require more coolant to remove the heat.

Fig.2. Mass flux for each channel in the radial


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The thermal hydraulic parameters for variations mass flow rate is given in the table 2 given below. The table indicate that the increase of mass flow rate across the core induces the decrease of coolant outlet temperature, although the higher need the chimney height to achieve a natural circulation.

Table 2 Thermal hydraulic parameters for variations mass flow rate

Parameters 4500 kg/s 5000 kg/s 5500 kg/s maximum outlet temperature (C) 598.607 568.939 544.696 chimney height (m) 4.783 6.690 8.978

The design of the reactor with a total mass flow of 5000kg/sis the optimal design for the inlet temperature of 300 °C produces outlet temperature524.843 °C and chimney 6.690 meters. 2. Explanation

A study of future energy requirements and the potential of various energy sources Such as (coal, oil, gas, nuclear power, hydropower and solar energy) indicates that the contribution to be made by nuclear fission. Now competitive with conventional energy sources, will grow steadily over the next few decades. The potential capability of nuclear power in its present state of development to satisfy the world demand for electricity would depend, to a large extent, on world reserves of natural uranium and the possibility of obtaining uranium-235 as nuclear fuel at reasonable prices. The relatively small percentage of this isotope in natural uranium is however a limitation on the development of nuclear power if it were to be based only on existing light-water reactors which, with their low conversion ratio, cannot utilize more than 2% of the energy potentially available in natural uranium. Thus, the advantages of nuclear power in the long term may not be fully realized unless further sizeable reserves of natural uranium are found or until significant progress is made in more effectively utilizing uranium.

Based on the analysis of the natural circulation characteristics of lead and bismuth reactors in typical natural circulation, this paper introduces the simplified modeling of the simulation problem of the reactor based on some simplification and assumptions, and introduces how to carry out grid independence analysis and turbulence. The flow field, temperature field, pressure field and core flow distribution characteristics of typical liquid metal cooled natural circulation pool reactor were analyzed by using FLUENT porous media model to simulate the reactor core and heat exchanger.

The fourth generation of the International Nuclear Energy Forum (GIF) in 2001 selected the most development potential of the four generations of nuclear energy system. Most of which lead cold reactor and sodium-cooled reactor are integrated pool design, liquid metal cooling pool. The reactor has good inherent safety and economy and has good visions for development. Figures 3-5 is a schematic diagram of a typical liquid metal cooling tank reactor system.


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Fig.3. Schematic diagram of a typical liquid metal cooling pool reactor system. Adopted Wydler, Chimia 59 (2005) 970–976 ( and references therein).

Fig.4. A schematic diagram of a typical lead-cooled reactor. Adopted from Wydler, Chimia 59 (2005) 970–976.


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Fig.5. Typical Modeling of Typical Liquid Metal Cooling Natural Circulation Reactor

Fig.6. core layout Fig.7. core simplified diagram


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Figure 6 shows a typical liquid metal cooling natural circulation reactor core assembly layout diagram, the core layout diagram of the corresponding simplified treatment. Figure 7 shows the layers of components to simplify the radial outward superimposed ring components.

The core is divided into nine layers, and the radial thickness of each layer is the same. Axial, the core is divided into three parts, the first and the third part of the core inlet and outlet flow channel, the second part of the core active area. The core active region is loaded with a heat source, and the heat exchanger zone is set at a fixed temperature of 240 ° C. The porous media method is used to simulate the active region of the core and the heat exchanger region. The core thermal baffle system contains a liquid metal cooling runner. For simplified simulation, the equivalent treatment is a steel with a thermal conductivity of 0.2.

Table 3. Components of the core components

Component Layer Power (KW/m3) 1 0 2 12000 3 10000 4 5500 5 8500 6 3500 7 120 8 90 9 50

Table 4. Core, heat exchanger porous media simulation parameter

Project Viscosity parameter (1 / α) inertial parameter (C 2) Core 1237770 55

Heat exchanger 495108 22

Table 5. Liquid metal-related properties

Project parameter Cp 159-2.72×10-2T+7.12×10-6T2

P (density) 11367-1.1944T

(Thermal conductivity) 9.2+0.011T

（4.56-7.03×10-3T+3.61×10-6T2）×10-3

Here the porous media model was used to simulate the heat transfer of the reactor core and

the heat transfer between the reactor and the heat exchanger. Through a certain simplification, the reactor was a two-dimensional structure model. (Hence the power density can be defined directly in the boundary conditions of the fluent solver).

lh


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Due to the naturality of the core, the core of the core is divided into nine layers, and then the core is given a different volume of heat release. The calculation of the cycle is due to the numerical calculation of the error caused by the core cooling is not ideal but easily lead to divergence. The model was established with Gambit software, the grid was defined, the boundary conditions were defined, the calculation was carried out with Fluent, and different models (Laminar, turbulence K-model model and RNG model, etc.), according to the theory of simple analysis and summary, improve the software simulation capabilities.

3. Methodology 3.1 Model establishment two-dimensional plan

In the Gambit program, the two-dimensional model in the XOY plane is established in the order of the dotted line and the coordinates of each point are set in units of m, and finally the closed plane pattern is formed. Heaters, partitions and cores 9 layers of different volumes of heat dissipation rate of the two-dimensional plane pattern, the final 2D figure 8.

Fig.8. Model plan 3.2 The establishment of surface

Because of the complexity of the geometry, combined with the division of the grid. First select the most outside the line to establish the surface. And then use the Gambit software in the surface of the cutting tool to create different faces. Get the final surface diagram, as shown in Figure 9.


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Fig.9. shows the final diagram

3.3 The division of the grid

Here considering the complexity of geometric, using a hybrid grid. The grid uses a non-structured grid area yellow color, as shown in Figure 10.

Fig.10. irregular part of the grid division

And then use the structure of the grid in other regions, as shown in Figure 11.


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Fig.11. grid diagram

3.4 Definition of boundary

After complete the modeling, we need to define the boundary type. Depending on the conditions given in the case. The different types of grid boundaries are set because the use of the porous media model in this case requires attention in the setup. The specific settings are as follows.

According to Rayleigh criteria.

(6)

As , , , ，Characteristic temperature, .

The Rayleigh is less than the magnitude of the laminar flow, greater than the order of turbulence and the middle of the transition interval.

As the temperature of the thermal wall in the Rayleigh criterion such as, the core temperature is required. The laminar flow model is estimated on the grid with a length of 20 mm. The results show that the maximum temperature of the core is about 640K.This temperature as the temperature of the hot wall. The heat exchanger temperature of about 500K as the temperature of the cold wall. The center of the core and the central level of the heat exchanger vertical height as the characteristic length by getting the following features.

Feature Length:

Characteristic temperature:

3

2vg TRa Gr Pr

aa d nn

× ×D ×= × = ×

1v T

rar

¶= - ×

¶ h cT T TD = - h cT T、P

aClr

=2

h cT T+

810 1010

=1.98md

5702

h cT T K+=


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The temperature is used as the reference temperature for other temperature-related physical parameters.

Body mass coefficient:

Kinematic viscosity:

Thermal diffusivity:

The final estimate is calculated for the Rayleigh number,

So to determine the turbulence, and further should be used to calculate the turbulence model.

The rest of the settings will be described here, in the gambit Operation selected in the third icon, Zones. First select the first icon for the definition of surface boundary type. The final full set is shown in Figure 12.

Fig.12. Boundary type setting

In the Operation selected in the third icon Zones. First select the second icon for body boundary type definition. Define the heat exchanger name, the core of the import and export of New-core-in-out, 9 layers of different volume of heat release fuel block fuel-1 - fuel-9, the boundary type is FLUID. The complete setup as shown in Figure 13.

41 =1.1 10 /v KTra

r-¶

= - × ´¶

-7 2= / 1.6 10 /m sn µ r = ´

-6 2=9.9 10 /P

a m sClr

= ´

11=7.42 10Ra ´


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Fig.13. Body border type setting

4. Results & Discussion

Open the Fluent program, select the 2-dimensional single-precision calculation. Now start the calculation. The calculation steps are as follows.

Import the grid file, File-Read-Case, select the path of the target mesh file saved in Gambit. Now check the grid quality and read the details of the grid file. The view of the grid graph, Display-Grid, select the items to be displayed, as Nodes- nodes, Edges-border, etc. Display the grid image, right click on the grid somewhere, the fluent main panel to display the characteristics of the information.

Modify the unit; Gambit modeling the default unit is the international unit system, but the actual model can be scaled, and then modify the unit in Fluent; in this case the default unit m.

Set the calculation format, Define-Models-Solver. In the open panel selection, Solver select Pressure Based (under pressure, that is, non-coupling solution), Formulation under the choice of Implicit, Space under Axisymmetric, Time under Steady (steady state). Open the Energy Equation dialog box in Define-Models-Energy and tick it. Because in this case the reactor fuel releases energy, there is energy flow, so to open the energy equation.

To set the operating conditions, select Define-Operating-conditions in the fluent interface, tick the gravity option, since the model gravity direction is left to the left. So -9.81 m / s2 in the X-axis direction. Set the fluid material. Select Define-materials in the Fluent interface, select fluid in the Material Type, set coolant in the Name, and set the physical properties of the liquid metal coolant. The liquid metal parameters given in the title are set because the parameters of the coolant are temperature dependent and require special settings here. In the Properties dialog box, Edit and set the parameters.


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In the same order of operation, select Solid in the type, set two different metal materials steel, steel-1 as the partition material. Set the thermal conductivity of 0.2, steel-2 as the reactor vessel, heat exchanger, core and other walls, Now Select Fluent default metal rigidity parameters.

In the Define-Boundary conditions, set the boundary conditions of different regions, axis, core-in-out, core and other boundaries for the axis, interior, wall, and select the corresponding metal materials. The special setting is the boundary condition of the heat exchanger, type fluid, because it is a porous medium, need to set a fixed temperature of 240 degrees in the heat exchanger area boundary, and tick the porous model option. Another very important setting is the reactor core 9 layers of different heat source fuel porous media model settings. Click on each layer of fuel, type fluid, in the porous options. The other eight layers of fuel are set in the order shown in the first layer, except that the volume dissipation rate of the internal heat source is different, and the setting of the viscosity parameter. The porous media model setting of the heat exchanger is the same as the core setting, except that the values of the inertial and viscous parameters are different. Set the solution model and select the solution equation in the Solve in the fluent interface.

The setting of the residuals and the setting of the monitoring window. Select the Residue and Monitor window in the Solve option to set the result. Initialize and set the number of iterations.

In the Solve option, select Initialization and Iterate, respectively, to initialize and set the number of iterations. After the convergence condition is reached, the data is saved for analysis.

Here the porous media model was used to simulate the natural circulation closed loop of the core reactor and the heat exchanger of the pool reactor. Through this experiment, we studied the setting of the porous media model and understood some basic aspects of the theoretical thermal and hydraulic analysis of the reactor Simple situation.

4.1. Modeling and Meshing

Here the modeling should choose the lowest end of the pool reactor for the X axis, the axis of symmetry for the X axis, in the first modeling.

The modeling process should first establish a maximum face, and then split the operation. Cut off the different facets, so easy to divide the grid in the initial modeling. Just to create each part for the surface, resulting in the results of the division of the grid. There are several blocks up and down the number of nodes inconsistent and cannot be divided into grid or grid is not uniform. The grid quality is poor. The blue part shows the number of upper and lower nodes is not the same, cannot divide the grid.


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Fig.14. Mesh Considerations

4.1.1 Porous media model setting

Porous media is a common space occupied by heterogeneous material, but also a multi-phase material coexistence of a combination of solid skeleton that part of the space called the pore, by the liquid or gas or gas-liquid two-phase possession, relative to one Phase, the other phases are dispersed in the solid phase as a solid skeleton, constitute the void space of some empty interconnected. The porous media simulation method considers the effect of the solid structure in the flow region as a distribution resistance attached to the fluid. The role of porous media is to calculate a fluid dynamics equation to add a source term to simulate. The source term consists of two parts, a viscous loss item and an inertia loss item.

The porosity is set to 0.31. Since the axis of symmetry is selected as the X axis and the direction of the fluid flow is the X axis direction. The X direction is 1 and the Y direction is 0 in the fluent setting. In the case of the setting of the viscous parameter and the inertia parameter, the inertial and viscous parameters in the X-axis direction are taken into account, so that the inertial and viscous parameters in the Y direction are set to 100 times in the X-axis direction. The area of the porous media model is chosen as the heat exchanger and the reactor fuel. The boundary type of the two regions is set to fluid in the Gambit, and the corresponding setting is made in the porous-type of fluent boundary condition type.

4.1.2. Union or convergence judgment

In this case, the laminar flow model and the turbulence k-ε model are selected and the RNG model is compared. It is found that the turbulence model cannot reach the convergence requirement. The laminar flow model can meet the convergence requirement. The residual can be controlled at 10-3 and 10-4 or less. If the first layer stream is used to iterate on the basis of the turbulence standard model, the iterative effect is the best.

From the residual convergence point of view, the turbulence model residual has been in a state of shock, there is no stability, and in addition to the energy residue is more stable. Other


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continuity and other parameters are unstable. The laminar flow model is more stable and better analyzes the experimental results. However, from the actual situation, the energy transfer in the reactor is not a simple laminar flow state. It should be more complex turbulence.

The use of a uniform structure of the grid or unstructured grid, through the grid inspection, it is found that only in the core of the core area above the arc area of the grid when the grid appeared sharp of the wedge-shaped area. In this case the boundary conditions are relatively simple. We can turn on some areas of turbulence, and further improve the calculation model and physical field compatibility.

Fig.15. Temperature profile

Fig.16. velocity distribution


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As can be seen from the above figure 16, the area where the velocity of the reactor is the largest is in the upper part of the core, since this simulates the thermal water pressure of the coolant of the natural circulation circuit of the pool reactor.

The core pressure distribution cloud can be seen in the core of the long and short media area has a greater pressure drop, so the pressure is low. At the upper part of the core and the lower part of the Core, the pressure is increased due to the decrease in the flow rate and a high pressure zone appears in the area where the upper geometry is sharp.

Fig.17. Pressure distribution

Fig.18. Local velocity vector


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Fig.19. Local velocity vector

From the distribution of Reynolds number, the Reynolds number of the outlet area of the heat exchanger reaches 105, which belongs to turbulence. The Reynolds number in the upper part of the core, the heat exchanger belongs to laminar flow below 2000. We cannot correctly reflect the flow of heat exchanger outlet, if you choose the turbulence model, in the core and other areas did not reach the turbulence be a correct simulation. So you can draw a suitable method is to select the initial laminar flow model to simulate. After a certain degree of convergence on the basis of turbulence simulation can be a good reflection of the core flow.

From the temperature distribution, the reactor core fuel zone temperature up to 633 degrees. The minimum temperature of about 512 degrees. The temperature vector to further enlarge the view, near the central axis of the second layer of fuel temperature distribution is the highest. Thermal analysis of the heat pipe where, and then compare the subject given in the heat source conditions. The second layer of the volume of the maximum heat dissipation in line with. The temperature distribution in the fifth layer is second only to the second layer. Which is constant with the given heat release volume of the fifth heat source. As shown in the figure 20 given below.


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Fig.20. Temperature distribution and Position

5. Conclusions

The aim of this article is the better understanding of the FLUENT turbulence model. In the operation of dividing the grid, in order to facilitate the calculation of the grid more rules, learned to split the overall graphics. By using a uniform structure of the grid or unstructured grid, through the grid inspection, we can easily found that only in the core of the core area above the arc area of the grid when the grid appeared sharp of the wedge-shaped area. In this case the boundary conditions are relatively simple. We can turn on some areas of turbulence, and further improve the calculation model and physical field compatibility. In the process of adding boundary conditions, through repeated experiments, the boundary conditions are added more completely and rationally. When calculating in FLUENT, it is learned that by judging some feature amounts to determine the convergence of the calculations, and to understand the scope of application of various turbulence models. Here a lot of useful things, such as for the surface of the cut, non-uniform grid and uniform grid division, porous media model of a simulation and so have a certain degree of familiarity and simulation basis.

Acknowledgement The authors wish to acknowledgement and thanks to the Thermal hydraulics research group

of USTC, School of Nuclear Science and Technology for their help and constructive suggestions. References

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