Week 7 - Mid-term Project - Natural Convection

I. Aim

To design a low voltage control panel, as per the given specifications, and perform thermal analysis of the same.

II. Introduction

In the current project, the following tasks are performed:

1. The model for a low-voltage control panel is designed in ANSYS Icepak using the following given information:

1.1. The images attached below show the single-line diagram of the network and schematic of the switchboard/control panel.

1.2. The control panel dimensions are given as: height 2000 mm, width 1600 mm and depth 700 mm.

1.3. The total incoming current to the control panel is 1200 A. The images attached below show the circuit breaker losses, current distribution and dimensions of busbars and cables.

1.4. The ambient temperature is 20 deg. C.

2. The computational domain is meshed using non-conformal meshing.

3. The following physical phenomena are incorporated in the model:

3.1. Joule heating in the current carrying busbars and cables

3.2. Natural convection (buoyancy-induced flow)

3.3. Turbulence in the flow field

3.4. Radiation heat transfer between the control panel and the walls in the model

4. Steady-state simulation of the model is performed to determine the flow and thermal fields within the computational domain.

A low voltage control panel is a component in the electrical distribution system that allocates the incoming current feed to separate circuits in a facility. It houses circuit breakers for every branch circuit to protect them against electrical overloads and short circuits. Thermal simulation of control panels, similar to the current simulation, is often performed by the thermal engineers working in the electrical industry to determine whether any component or wiring within the control panel is exceeding its thermal limit.

III. Geometry

The complete model of the low-voltage control panel is designed in ANSYS Icepak using the following steps:

1. 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.

i. Cable for circuit breaker IG

ii. Busbar-A

iii. 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.

i. Cable for circuit breaker IG

ii. Busbar-A

iii. Circuit breaker IG

The image attached below shows all the cables, busbars and circuit breakers that are located within the control panel.

2. 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

ii. Properties

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.

4. Walls: The control panel is placed in a room which is designed using six walls and two grilles. Each wall is created using the "Blocks" object in Icepak and is assigned a thickness of 10 mm. "Mica-brick-red or white" and "Paint-non-metallic" are used as solid material and surface material respectively, for each wall. Radiation heat transfer modeling is enabled for the walls. The images attached below show the settings applied in the "Geometry" and "Properties" tabs, to define the walls.

i. Geometry

ii. Properties

Two grilles are defined on a pair of opposite vertical walls to allow: the ambient air to enter into the room and the air inside the room to leave it. The image attached below showsthe room.

IV. Meshing

The following meshing techniques are utilized in the current project to discretize the computational domain:

1. 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.

2. 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 922328 elements and 1019162 nodes. The images attached below show the computational mesh on three mutually perpendicular planes, each passing through the center of the control panel.

i. X-plane

ii. Y-plane

iii. 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. For the current mesh, the minimum value of face alignment is 0.348563 which indicates that the mesh elements aren't severely distorted.

2. Volume

For a double precision solver, the minimum cell-volume shouldn't be less than 1e-15 m3 otherwise the solver may face issues. The minimum cell-volume in the current mesh is 3.86997e-11 m3 and hence the mesh elements aren't small enough to cause problems in the solver.

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). For the current mesh, the minimum value of skewness is 0.205486 which indicates that the mesh elements aren't severely skewed.

V. 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 deg. 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: 5000

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

VI. Results

In the current section, the simulation results are discussed.

1. 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 4500-5000 iterations thereby indicating that the solution has reached a steady state. In the current project, the simulation results after 5000 iterations are considered as steady-state results.

2. Monitor plots

Monitor points are probes set up at various key locations within the computational domain to determine the solution variables (such as pressure, velocity, temperature etc.). Data obtained from monitor points can also be used to ascertain whether the solution has reached a steady state. In the current project, the following four monitor points are created:

i. IG: Temperature monitor point

ii. I5: Temperature monitor point

iii. grille.1: Velocity monitor pont

iv. grille.2: Velocity monitor point

The images attached below show the data from the four monitor points, plotted against the number of iterations.

i. Temperature monitor point

ii. Velocity monitor point

The plots show that the monitored variables don't change appreciably between 4500-5000 iterations. This further confirms that the simulation results after 5000 iterations are steady-state results. The following steady-state values are predicted by the plots:

i. IG: 83.90 deg. C

ii. I5: 47.60 deg. C

iii. grille.1: 0.16 m/s

iv. grille.2: 0.16 m/s

3. 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.

4. 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 at 5000 points (for figure i.) and 2500 points (for figure ii.) 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.

5. 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 deg. C.

6. 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 deg. C.

7. Particle traces

The steady-state particle traces (colored by temperature) of 30 air particles originating at the lower grille of the control panel are plotted within the computational domain, as shown in the animation attached below.

The animation clearly shows the steady-state pathlines traced by the air particles (originating at the lower grille) within the computational domain. The color of a particle trace at a particular location indicates the temperature of the air particle that traces the pathline, at that location.

VII. 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:

i. Joule heating

ii. Natural convection

iii. Turbulence

iv. 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:

i. Both the residual plot and the monitor plot show that the simulation results after 5000 iterations can be considered as steady-state results.

ii. According to the steady-state velocity contours, the maximum air-velocity within the computational domain is 0.41 m/s.

iii. The highest steady-state temperature of 93.79 deg. C is attained by the circuit breaker I3.

iv. The following steady-state temperatures are attained by the circuit breakers:

(a) IG: 83.90 deg. C

(b) I1: 88.75 deg. C

(c) I2: 92.61 deg. C

(d) I3: 93.79 deg. C

(e) I4: 81.65 deg. C

(f) I5: 47.60 deg. C

(g) I6: 21.53 deg. C

(h) I7: 21.20 deg. C