Week 7 - Mid-term Project - Natural Convection
Aim:
To design a low voltage control panel, as per the given specifications, and perform thermal analysis of the same.
Geometry:
The complete model of the low-voltage control panel is designed in ANSYS Icepak using the following steps
Cables, busbars and circuit breakers: They form the internal components of the control panel and are created using multiple "Blocks" objects in Icepak, as per the given dimensions. The dimensions and locations of the circuit breakers are assumed appropriately. The image attached below shows the geometry settings used to create one of each of the following: cable, busbar and circuit breaker.
Cable for circuit breaker IG:
Busbar-A:
Circuit breaker IG:
All the internal components (cables, busbars and circuit breakers) are assigned the same solid material ("Cu-Pure") and surface material ("Cu-polished-surface"). Joule heating with "Constant" power type setting is enabled for the busbars and cables and they are assigned currents based on the given specifications. The circuit breakers are assigned constant thermal dissipation values, as per the given specifications. The images attached below show the "Properties" tab of each one of the following: cable, busbar and circuit breaker.
Circuit breaker IG:
The image attached below shows all the cables, busbars and circuit breakers that are located within the control panel.
control panel: The control panel encloses the current carrying components (cables, busbars and circuit breakers) and is created using the "Enclosure" object in Icepak, as per the given outer dimensions. "Steel-stainless-400" and "Steel-Oxidized-surface" are used as the solid material and the surface material respectively, for the control panel. Each of the control panel-walls is assigned a thin type boundary condition and a thickness of 2 mm. Radiation heat transfer is enabled for all the walls except the "MinZ" wall, as directed by the given specifications. The images attached below show the settings applied in the "Geometry" and "Properties" tabs, to define the control panel.
i. Geometry
Two grilles are defined on a pair of opposite walls of the control panel to allow: the ambient air to move into the control panel and the hot air inside the control panel to move out of the control panel. The grille for the intake of ambient air is intuitively placed near the base of the control panel while the grille for the output of hot air is placed near the top of the control panel. Hence the two grilles allow air-cooling of the heat generating components located inside the control panel, due to natural convection. The image attached below shows the control panel.
Meshing:
The following meshing techniques are utilized in the current project to discretize the computational domain
Non-conformal meshing allows the user to selectively refine the mesh within a sub-region around an assembly by selecting the "Mesh separately" option available in the "Meshing" tab of the assembly settings. A comparatively coarse mesh can be used for the rest of the model. Multiple assemblies, that employ non-conformal meshing, are defined in the model. The image attached below shows the settings applied in the "Meshing" tab to define one such assembly.
The "Object params" option available in the "Mesh control" window allows the user to refine the mesh locally near one or more objects/assemblies defined in the model. "Per-object meshing parameters" are specified for the assembly defined around the control panel in the current model. The image attached below shows the settings applied in the "Per-object meshing parameters" window for the assembly.
After completing the above steps, the computational mesh is generated. The image attached below shows the "Mesh control" window.
The image shows that the generated mesh yields 714753 elements and 811333 nodes. The images attached below show the computational mesh on three mutually perpendicular planes, each passing through the center of the control panel.
X-plane:
Y-plane:
Z-plane:
The images attached below show the volume mesh on the components located inside the control panel: cables, busbars and circuit breakers.
Visually the mesh appears to be fine. The images attached below show the histograms with the number of elements plotted along the X-axis and the quality measures plotted along the Y-axis.
1. Face alignment
Values less than 0.05 indicate the presence of severely distorted elements in the mesh.
2. Volume
3. Skewness
A skewness of 0 indicates that the mesh element is degenerate whereas a skewness of 1 indicates that the mesh element is ideal (equilateral/equiangular).
Solver:
The three-dimensional steady-state Navier-Stokes equations for the model are solved within the computational domain using the Fluent solver available in ANSYS Icepak. The following settings are applied to the solver:
1. Variables solved: Flow (velocity, pressure) and Temperature
2. Radiation: On
2.1. Model: Surface to surface radiation model
2.2. Include objects: Cabinet, the six room-walls and the control panel
2.2. Participating objects: The six room-walls and the control panel
3. Flow regime: Turbulent
3.1. Model: Zero equation model
4. Time variation: Steady
5. Solution initialiation
5.1. X velocity: 0
5.2. Y velocity: 0.000980665 m/s
5.3. Z velocity: 0
5.4. Temperature: Ambient (20 °C)
6. Natural convection
6.1. Fluid density: Boussinesq approximation
6.2. Operating pressure: 101325 N/m^2
6.3. Gravity vector: 9.80665 m/s^2 directed along the negative Y-axis
7. Number of iterations: 600
8. Convergence criteria
8.1. Flow: 1e-7
8.2. Energy: 1e-7
8.3. Joule heating: 1e-7
9. Configuration: Parallel
10. Number of processors: 4
11. GPU computing: Enabled
12. Number of GPUs: 1
13. Discretization scheme
13.1. Pressure: Body Force
13.2. Momentum: First
13.3. Temperature: First
14. Under-relaxation factors
14.1. Pressure: 0.3
14.2. Momentum: 0.3
14.3. Temperature: 1
14.4. Viscosity: 1
14.5. Body forces: 1
14.6. Joule heating potential: 1
15. Linear solver settings
15.1. Pressure - Type: V, Termination criterion: 0.1, Residual reduction tolerance: 0.7, Stabilization: None
15.2. Momentum - Type: flex, Termination criterion: 0.1, Residual reduction tolerance: 0.7
15.3. Temperature - Type: W, Termination criterion: 0.1, Residual reduction tolerance: 0.7, Stabilization: BCGSTAB
15.4. Joule heating potential - Type: F, Termination criterion: 0.1, Residual reduction tolerance: 0.7, Stabilization: BCGSTAB
16. Precision: Double
Results:
In the current section, the simulation results are discussed.
Residuals:
The residuals of the following equations are plotted against the number of iterations:
i. Continuity equation
ii. X-Momentum equation
iii. Y-Momentum equation
iv. Z-Momentum equation
v. Energy equation
The image attached below shows the residuals plotted against the number of iterations.
The plot shows that the residuals don't change appreciably between 200-300 iterations thereby indicating that the solution has reached a steady state.
Velocity contours:
The steady-state velocity contours are plotted on the Z-plane passing through the center of the control panel, as shown in the images attached below.
i. Inside the cabinet
ii. Inside the control panel
The contours show that the air inside the room and the control panel are in motion. This can be attributed to natural convection which drives the air upwards, under the gravitational field, due to the difference in density created as a consequence of temperature difference. According to the contour-legend the maximum air-velocity on the Z-plane passing through the center of the control panel is 0.41 m/s.
Velocity vectors:
The steady-state velocity vectors are plotted on the Z-plane passing through the center of the control panel, as shown in the images attached below.
i. Inside the cabinet
ii. Inside the control panel
The vector plots show the velocity vectors for air is distributed uniformly throughout the Z-plane passing through the center of the control panel. Figure i. clearly shows that the air within the room enters into the control panel through the lower grille and the air inside the control panel leaves through the upper grille. The velocity vectors in figure ii. clearly show the upward motion of air inside the control panel, which can be attributed to natural convection.
Temperature contours:
The steady-state temperature contours are plotted on the Z-plane passing through the center of the control panel, as shown in the images attached below.
i. Inside the cabinet
ii. Inside the control panel
The contour plots clearly show that the air within the computational domain is cooler than any other object. It can be seen that the temperature of air, both inside the room and the control panel, increases as we move up (against gravity). This can be explained in the following manner: the air that is in direct contact with the comparatively hotter objects takes up their heat due to which its temperature increases and its density decreases. Under the influence of the downward gravitational field, this lighter (hotter) air rises up and the surrounding heavier (cooler) air rushes in to fill up the evacuated space. According to the contour-legend, the maximum temperature on the Z-plane passing through the center of the control panel is 93.79 °C.
Object face temperature contours:
The steady-state temperature contours are plotted over all the objects that are located inside the control panel, as shown in the image attached below.
The contours show that the circuit breakers (that are assigned a constant power generation) attain the highest steady-state temperatures, followed by the busbars and the cables (that have internal heat generation due to Joule heating). According to the contour-legend, the highest temperature attained by the objects inside the control panel is 93.79 °C.
Conclusion:
In the current project, the model of a low-voltage control panel was designed in ANSYS Icepak using the given design-specifications. The model was meshed using the following features available in the Icepak mesher: non-conformal meshing and per-object meshing parameters. The solver was set up using suitable settings to incorporate the following physical phenomena:
1. Joule heating
2. Natural convection
3. Turbulence
4. Radiation heat transfer
The three-dimensional steady-state governing equations for the model were solved for flow and thermal fields within the computational domain using the Fluent solver available in ANSYS Icepak. The following conclusions can be drawn based on the simulation results:
a. Both the residual plot and the monitor plot show that the simulation results after 300 iterations can be considered as steady-state results.
b. According to the steady-state velocity contours, the maximum air-velocity within the computational domain is 0.41 m/s.
c. The highest steady-state temperature of 93.79 °C is attained by the circuit breaker I3.
d. The following steady-state temperatures are attained by the circuit breakers:
1. IG_CRB: 83.90 °C
2. I1_CRB: 88.75 °C
3. I2_CRB: 92.61 °C
4. I3_CRB: 93.79 °C
5. I4_CRB: 81.65 °C
6. I5_CRB: 47.60 °C
7. I6_CRB: 21.53 °C
8. I7_CRB: 21.20 °C