A fluid-thermal coupling numerical simulation method for air gap bubbling blockage based on dynamic grid technology

Abstract

A coupled flow-heat numerical simulation method for air gap bubbling and blockage conditions based on dynamic mesh technology is proposed. A geometric model of the fuel structure and its adjacent coolant flow channel is established for a plate-shaped fuel element. Boundary and initial conditions are set for a preliminary simulation. Once the preliminary simulation converges, the dynamic mesh, boundary conditions, and gas pressure are set, and a precise simulation is performed. This method utilizes dynamic mesh technology to simulate bubble formation, aiming to simulate the flow and heat transfer behavior of plate-shaped fuel assemblies under specific conditions such as bubbling.

Description

Technical Field

[0001] The present invention relates to a technology in the field of nuclear reactors, in particular to a fluid-heat coupling numerical simulation method for air gap bubbling and flow blocking conditions based on dynamic grid technology. Background Art

[0002] The coolant channels in plate-type fuel assemblies are closed, parallel channels with relatively small gaps, typically within a few millimeters. Under prolonged core irradiation, the fuel plates release significant amounts of fission gases. These gases accumulate and can cause blistering in the cladding. When coolant channel gaps are small, even minor geometric variations can cause the fuel plate temperature and mass flow to deviate from nominal values, potentially leading to fuel plate rupture and core damage. Summary of the Invention

[0003] In response to the above-mentioned shortcomings of the existing technology, the present invention proposes a flow-heat coupling numerical simulation method for air gap bubbling blockage conditions based on dynamic grid technology. The dynamic grid technology is used to simulate bubble generation, aiming to simulate the flow and heat transfer behavior of plate-shaped fuel assemblies under specific conditions such as bubbling.

[0004] The present invention is achieved through the following technical solutions:

[0005] The present invention relates to a fluid-thermal coupling numerical simulation method for air gap bubbling and blockage conditions based on dynamic grid technology. A geometric model of the fuel structure and an adjacent coolant flow channel model are established for a plate-shaped fuel element. A preliminary simulation is performed after setting boundary conditions and initial conditions. When the preliminary simulation converges, a dynamic grid, boundary conditions and gas pressure are set to perform an accurate simulation. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Figure 1 Flowchart of the present invention;

[0007] Figure 2 Schematic diagram of the physical model of the fuel plate and its adjacent channels;

[0008] In the figure: fuel plates 1, 2, internal heat source 3, gas bubbler 4;

[0009] Figure 3 is the deformation diagram of the fuel plate;

[0010] Figure 4 It is the temperature distribution cloud diagram of the fuel plate and rectangular channel;

[0011] Figure 5 This is the heat flux cloud diagram of the fuel plate and rectangular channel. DETAILED DESCRIPTION

[0012] like Figure 1As shown, this embodiment relates to a fluid-thermal coupling numerical simulation method for air gap bubbling and flow blocking conditions based on a dynamic grid technology. The dynamic grid technology is used to simulate the natural formation process of bubbles by injecting helium with an initial pressure into the cavity, specifically including:

[0013] Step 1) For example Figure 2 The plate-shaped fuel element shown establishes a geometric model of the fuel structure and its adjacent coolant flow channel model, including: upper and lower fuel plates 1, 2, an internal heat source 3 located in the upper plate, and a gas bubble 4 located in the middle gap of the fuel plate, wherein: fluid passes through the flow channels on the upper and lower sides of the fuel plates 1, 2, and the fluid flows from right to left.

[0014] Step 2) Perform a preliminary simulation after setting the boundary conditions and initial conditions.

[0015] 2.1. Use unstructured tetrahedral mesh for meshing to ensure that the mesh nodes at the interface between the fluid domain and the solid domain are completely matched.

[0016] 2.2. According to the actual operating conditions of the reactor, boundary conditions and initial conditions are set for the fluid domain and solid domain.

[0017] 2.3. Select an appropriate turbulence model based on the value of Y+ of the surface of interest, use the same time step in the fluid domain and the solid domain, and carry out fluid-thermal coupled numerical simulation solutions of the flow equation, turbulence equation, and energy equation.

[0018] Step 3) Set the dynamic mesh, boundary conditions, and gas pressure to perform accurate simulation, including:

[0019] 3.1. Activate the structural model and dynamic mesh options, set the interfaces between the air gap and the cladding, and between the cladding and the coolant as dynamic mesh regions, set the type to intrinsic FSI, and the mesh deformation mode to smoothing and resurfacing;

[0020] 3.2. Change the structural options of both surfaces in the wall boundary conditions to Intrinsic FSI in all three directions (x, y, z).

[0021] 3.3. Through UDF control, a linearly growing pressure source term is set in the initial small helium cavity. This moment is recorded as t=0, and t is the calculation time. When the pressure of the fluid on the bubble is less than the pressure of the gas on the bubble, the fluid-solid interface grid will move toward the fluid domain. At this time, the boundaries of the fluid domain and the solid domain will deform simultaneously. Re-solve to obtain the flow field and temperature field information under the slowly changing bubble. Moreover, after time t1, the pressure reaches the preset value and remains constant and no longer changes. After time t2, the height of the bubble reaches half of the thickness of the flow channel, the shape is stable and no longer changes, and the calculation converges (the preset pressure value is determined through multiple simulation tests).

[0022] like Figure 3 As shown, this is a cloud diagram of the deformation of the fuel plate caused by the air gap pressure. The deformation is mainly concentrated in the center.

[0023] like Figure 4 Figure 2 shows the temperature distribution contours for the bubbling fuel plate and the control group, along with their rectangular flow channels. The figure shows that the fuel plate with gas bubbles exhibits localized high temperatures in the air gap. Furthermore, the fluid temperature on the bubbling side is lower than that on the non-bubbling side.

[0024] like Figure 5 The heat flux distribution cloud diagram of the bubble fuel plate and the control group is shown in Figure 2. At the air gap, the heat flux is almost zero, while a high heat flux area appears around the air gap.

[0025] As shown in Table 1, the heat transfer coefficients of the simulation results of different methods are compared at the center of the bubble.

[0026] Table 1 Heat transfer coefficients at the center of the fuel plate bubble under three bubbling conditions

[0027]

[0028]

[0029] The above-mentioned specific implementation can be partially adjusted in different ways by those skilled in the art without departing from the principles and purpose of the present invention. The scope of protection of the present invention shall be based on the claims and shall not be limited by the above-mentioned specific implementation. All implementation schemes within its scope shall be subject to the constraints of the present invention.

Claims

1. A fluid-thermal coupling numerical simulation method for air gap bubbling and flow blocking conditions based on dynamic grid technology, characterized in that: For the plate-shaped fuel element, a geometric model of the fuel structure and its adjacent coolant flow channel model are established. After setting boundary conditions and initial conditions, a preliminary simulation is performed. When the preliminary simulation converges, a dynamic mesh, boundary conditions, and gas pressure are set to perform a precise simulation. The setting of boundary conditions and initial conditions specifically includes: 2.

1. Use unstructured tetrahedral mesh for meshing to ensure that the mesh nodes at the interface between the fluid domain and the solid domain are fully matched; 2.

2. Set boundary conditions and initial conditions for the fluid and solid domains according to the actual operating conditions of the reactor; 2.

3. Select an appropriate turbulence model based on the value of Y+ of the surface of interest, use the same time step in the fluid and solid domains, and perform fluid-thermal coupled numerical simulation solutions to the flow equation, turbulence equation, and energy equation; The simulation is to set the dynamic grid, boundary conditions and gas pressure and then perform accurate simulation, including: 3.

1. Activate the structural model and dynamic mesh options, set the interfaces between the air gap and the cladding, and between the cladding and the coolant as dynamic mesh regions, set the type to intrinsic FSI, and the mesh deformation mode to smoothing and resurfacing; 3.

2. Change the structure options of the two surfaces in the wall boundary conditions to built-in fluid-structure coupling in three directions (x, y, z); 3.

3. Through UDF control, a linearly increasing pressure source term is set in the initial small helium cavity. This moment is recorded as t=0, where t is the calculation time. When the pressure of the fluid on the bubble is less than the pressure of the gas on the bubble, the fluid-solid interface mesh will move toward the fluid domain. At this time, the boundaries of the fluid domain and the solid domain undergo simultaneous boundary deformation. The solution is re-solved to obtain the flow field and temperature field information under the slowly changing bubble. After time t1, the pressure reaches the preset value and remains constant. Then, after time t2, the height of the bubble reaches half of the flow channel thickness, the shape stabilizes and no longer changes, and the calculation converges.

2. The fluid-thermal coupling numerical simulation method for air gap bubbling and flow blocking conditions based on dynamic grid technology according to claim 1 is characterized in that: The coolant flow channel model includes: two layers of upper and lower fuel plates, an internal heat source located in the upper plate, and gas bubbles located in the middle gap of the fuel plate, wherein: fluid passes through the flow channels on the upper and lower sides of the fuel plate, and the fluid flows from right to left.

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