Flow-heat coupling numerical simulation method for air gap bubbling flow blocking working condition based on dynamic grid technology

Abstract

The invention relates to a flow-heat coupling numerical simulation method for an air gap bubbling and flow blocking working condition based on a dynamic grid technology, which comprises the following steps of: establishing a geometric model of a fuel structure and an adjacent coolant flow channel model for a plate-shaped fuel element, setting boundary conditions and initial conditions, and performing preliminary simulation; when initial simulation converges, setting a dynamic grid, a boundary condition and gas pressure, and then performing accurate simulation. According to the invention, bubble generation is simulated by using a dynamic grid technology, and the flow heat transfer behavior of the plate-shaped fuel assembly under specific working conditions such as bubbling and the like is simulated.

Description

Technical Field

[0001] The 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 plugging conditions based on a dynamic grid technology. Background Art

[0002] The coolant channels of plate-type fuel assemblies are closed and arranged in parallel, and their gap size is relatively small, usually in the range of a few millimeters. Under long-term core irradiation, the fuel plate will release a large amount of fission gas. The accumulation of these gases will cause blistering in the cladding. When the gap of the coolant channel is small, even a small geometric change can cause the fuel plate temperature and mass flow to deviate from their nominal values, which may lead to fuel plate rupture and core damage. Summary of the invention

[0003] In view of the above-mentioned deficiencies in the prior art, the present invention proposes a flow-heat coupling numerical simulation method for air gap bubbling blockage conditions based on dynamic grid technology, which utilizes dynamic grid technology 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 plugging conditions based on dynamic grid technology. A geometric model of a fuel structure and an adjacent coolant flow channel model are established for a plate-shaped fuel element. A preliminary simulation is performed after boundary conditions and initial conditions are set. 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 It is a flow chart of the present invention;

[0007] Figure 2 It is a 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-heat coupling numerical simulation method for air gap bubbling plugging 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 layers of 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 boundary conditions and initial conditions.

[0015] 2.1. Use unstructured tetrahedral mesh for meshing to ensure that the mesh nodes on 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 a suitable 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-heat coupling numerical simulation solutions for the flow equation, turbulence equation, and energy equation.

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

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

[0020] 3.2. Change the structural options of the two surfaces in the wall boundary conditions to Intrinsic FSI in 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 boundary deformation of the fluid domain and the solid domain occurs simultaneously. Resolve to obtain the flow field and temperature field information under the slowly changing bubble. Moreover, when the time t1 passes, the pressure reaches the preset value and remains constant and no longer changes. Then, after the time t2 passes, 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, and the deformation is mainly concentrated in the center.

[0023] like Figure 4 The figure shows the temperature distribution cloud diagram of the bubbling fuel plate and the control group and its rectangular flow channel. It can be seen from the figure that the fuel plate with gas bubbling has a local high temperature in the air gap. And the fluid temperature on the bubbling side is lower than the fluid temperature 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 1. At the air gap, the heat flux is almost 0, 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] The above-mentioned specific implementation can be partially adjusted in different ways by those skilled in the art without departing from the principle and purpose of the present invention. The protection scope of the present invention shall be based on the claims and shall not be limited by the above-mentioned specific implementation. Each implementation scheme within its scope shall be subject to the constraints of the present invention.

Claims

1. A fluid-heat coupling numerical simulation method for air gap bubbling plugging conditions based on dynamic grid technology, characterized in that: A geometric model of the fuel structure and its adjacent coolant flow channel model are established for the 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 a precise simulation.

2. The fluid-heat coupling numerical simulation method for air gap bubbling and plugging flow conditions based on dynamic grid technology according to claim 1 is characterized in that: The coolant flow channel model includes: 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.

3. The fluid-heat coupling numerical simulation method for air gap bubbling and plugging flow conditions based on dynamic grid technology according to claim 1 is characterized in that: The boundary conditions and initial conditions are specifically set as follows: 2.

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

3. Select a suitable 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-heat coupling numerical simulation solutions for the flow equation, turbulence equation, and energy equation.

4. The fluid-heat coupling numerical simulation method of air gap bubbling and plugging flow condition based on dynamic grid technology according to claim 1 is characterized in that: The simulation refers to: setting the dynamic grid, boundary conditions and gas pressure to perform accurate simulation, specifically including: 3.

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

2. Change the structural 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 growing 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 grid will move toward the fluid domain. At this time, the boundaries of the fluid domain and the solid domain are deformed at the same time. The solution is re-solved to obtain the flow field and temperature field information under the slowly changing bubble. When the time t1 passes, the pressure reaches the preset value and remains constant. After the time t2 passes, 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.

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