Week 7 - Mid-term Project - Natural Convection

Aim: Simulate cooling of Low Voltage Panel using natural convection heat transfer.

Objective of this challenge is to authenticate the increase in temperature for the low voltage control panel for the configuration in a statement. Perform a simulation in accordance with the regulations provided for in the challenge.

• Build the model as per the provided information.
• Mesh the model with non-conformal meshing.
• Solve the problem of natural convection.

Introduction

Low voltage panels are used to distribute power from the main power source to different branches. The main components of a typical electrical panel are listed below:

1. Housing / Enclosure: It houses all the components of the panel. Generally, it is made up of steel material. Grills are used to circulating air through the enclosure. Heat transfer from the enclosure takes place by radiation and convection.
2. Busbars: Busbars are used to distribute power between different circuit breakers. It is generally made up of material with high electrical conductivity. Also, a cross-section of the busbar is made larger to reduce the electrical resistance.
3. Circuit breakers: Circuit breakers are used to distribute power to different branches at the desired time and of the required amount.
4. Cables: Cables are used to supply power to the different components from circuit breakers. It is also made up of high electrical conductivity material to reduce power loss.

Model

The schematic of the low-voltage cabinet as provided is shown in Fig. 1. The cabinet consists of 8 circuit breakers (CBs) connected through busbars of the specified dimensions. Liberty has been taken to dimension the components for which no data has been provided.

Geometry Modeling:

Circuit breakers are created with dimensions as shown below. Default material Aluminum Extruded is used. Radiation is turned on for all the components as one can not neglect it with natural convection heat transfer. The following table lists power dissipation for all the circuit breakers.

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.

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.

3. Walls: The control panel is placed in a room that 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.

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 a "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.

4. 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.

i. Cable for circuit breaker IG

ii. Busbar-A

iii. Circuit breaker IG

Mesh

• To obtain the non-conformal meshing, we have toggled the mesh assemblies separately option in the mesh control.
• A non-conformal mesh is when there are partial or zero matchings of nodes at the interface. This means that one, a few, or every node on one side of the interface doesn’t have a match on the other side of the interface.
• Non-conformal interface calculations are handled using a virtual polygon approach, which stores the area vector and centroid of the polygon faces.
• This approach does not involve node movement and cells are not necessarily water-tight cells. Hence gradients are corrected to take into account the missing face area.
• When the Mesh separately options for an assembly is turned on, the offset added to Assembly in the respective direction is called Slack Values.
• By default, the slack values are zero in all directions. However, in order to use non-conformal mesh, the slack values should always be a positive number except it can be zero when the non-conformal boundary touches a hollow object or cabinet boundary.
• The Slack values should be specified in such a way that cells on both sides of the interface are either fluid or solid, with the same material, and it does not violate any of the "Non-Conformal Assembly Rules" given in the attachment.

• Here we generate the non-conformal mesh with the slack values, the generated elements are 3435154, and the nodes are 3506940.
• A slack value of 30 mm is used for the assemblies.

Mesh Quality

• Skewness is defined as the difference between the shape of the cell and the shape of an equilateral cell of equivalent volume.
• A general rule is that the maximum skewness for a triangular/tetrahedral mesh in most flows should be kept below 0.95, with an average value that is less than 0.33.
• In this case, we achieved a good skewness value regarding the mesh quality.

The qualitative and quantitative analysis of the mesh indicates the quality of the mesh is good to capture the physics of the flow.

Now as the mesh is set up, the problem setup has to be provided. For this, the problem setup wizard is used to indicate the nature of the problem. As the flow is buoyancy-driven, natural convection is the method of heat transfer analysis. The operating density and the acceleration due to gravity are set. A zero equation turbulence model is used to model turbulence and radiation heat transfer is included. The model is set to solve for a steady state solution. Monitor points are set up on the CBs and busbars to monitor convergence. BCGSTAB stabilization method is provided for pressure and temperature in case of natural convection problems. The model can be saved now and then the solution is started.

Methodology 

The created geometries of the CBs and busbars have to be provided with material properties.

The busbars carry currents of different magnitudes. The heating of these objects depends on their length, hence Joule heating is turned on for busbars as seen in Figure. A current magnitude is provided for Joule heating for each of the busbars according to the table provided in Fig. 2. The longest length through which the current flows has also been provided for power calculation.

An assembly is created to combine all the block objects that include the CBs and busbars with the intention to mesh them separately. This is required to perform non-conformal meshing to reduce the cell count and eliminate mesh bleeding. Slack values are provided for the assembly in each direction. To check if the created assembly would run into a conflict during meshing, an automatic case check tool option provided under the Macros toolbar is used. As can be seen in Fig. 6, a case check for the current assembly does not result in any conflict.

Solver setting:

The three-dimensional steady-state Navier-Stokes equations for the model are solved within the computational domain for flow and temperature fields, using the Fluent solver available in ANSYS Icepak. The following settings are applied to the solver:

• Variables solved: Flow (velocity, pressure) and Temperature
• Radiation: On
• Flow regime: Turbulent (Zero equation)
• Time variation: Steady
• Solution initialization
• X velocity: 0
• Y velocity: 0
• Z velocity: 0
• Temperature: ambient (20 deg. C)
• Number of iterations: 200
• Convergence criteria
• Flow: 1e-6
• Energy: 1e-7
• Joule heating: 1e-7
• Configuration: Parallel
• Number of processors: 4
• GPU computing: Enabled
• Number of GPUs: 2
• Precision: Double

Basic Parameters:-

• In the basic parameters where we have given whether the solution runs for temperature, flow, and setting up for the initial temperature as well as radiation temperature, etc.
• To capture the natural convection phenomena in the problem, it is necessary to toggle the radiation option and then toggle the surface-to-surface radiation model.
• A turbulent flow regime has to be calculated with the gravity vector of -9.81 m/s in Y-direction in the natural convection space.
• The initial temperature for the case is set to 25 deg centigrade.
• Here in the basic settings panel, we need to give the number of iterations to 100.
• The convergence criteria of Flow are set to 0.0001 and energy is set to 1e-7, turbulent kinetic energy to 0.001, and joule heating is set to 1e-7.
• Here I have toggled the parallel option and given the four processors because my PC supports them at this point. We can increase or decrease the number of processors as per they are situated in the PC.

Advanced Setting

• Here in this panel, we give the inputs as the discretization schemes for the pressure, momentum, and temperature are of the first order.
• The double precision is given. Single precision means that the floating-point numbers will be represented in a 32-bit system whereas double-precision means that they will be represented in a 64-bit system. So Calculation in double precision will be more accurate.

Residual

From the below convergence plot, it can be observed that the solution converged after 180 iterations as there is not any change in the residual pattern.

• From the above residual plot, we have seen that the solution gets converged at nearly iteration no.60.
• After iteration no.60, the solution remains the same and we observe no instability in the residual plot.
• At the same time the velocity, continuity, and energy equation are well below the ranges we applied in the basic setting of iterations. This means the solution gets very well converged.

Monitor Points

The following figure shows the temperature variation of all the circuit breakers. It can be observed that the solution has been converged as the temperature of monitor points is not changing with iterations.

Results and Discussion:

A. Temperature contour on object faces

The temperature contour is plotted on the face of circuit breakers, cables, and busbars as shown below. It can be observed that a maximum temperature of ${85}^{o}C$ is observed at the main cable.

B. Temperature contour on the cut plane

The temperature contour is plotted on the midplane of the domain as shown below. From the figure below the temperature of the air can be observed along with the temperature of all the components. It can be observed that air entering from the bottom grille is at a lower temperature. As it comes in contact with components that generates heat, its temperature increases and rises as its density decreases. The air that comes out from the top grille is at a higher temperature.

C. Velocity vector on the cut plane

From the velocity vector on the cut plane, it can be seen that air enters from the bottom grille and comes out from the top grille.

D. Velocity contour on the cut plane

Velocity contour is plotted on the mid plane of the panel as shown below. It can be observed that air velocity increases as it gets heated. Lower velocities are observed near the walls of the housing.

Summary report:

• A summary report is a short, written communication that may have a variety of purposes, such as: To brief the reader on the details of a particular event. To analyze a particular issue, draw conclusions, and make recommendations. 
• Go to report -> summary report -> Select the object -> select the value (temp, speed, heaty flow, etc.) -> Write.

•  It gives us the max and min. temperatures of the objects we have selected collectively. 
• Here we can also get the mean temperature, standard deviation, and Area/Volume ratio data that can be used to compare the results.

Velocity:

• Above in the velocity section, we have discussed that the max. the velocity of the enclosure is 11.45 m/s. We can conclude that by seeing here mean. temp.

Heat Flow:

• Heat can flow from one place to another, like all kinds of energy. Heat flows in solids by conduction, where two objects in contact with each other transfer heat between them.
• That happens because the molecules hit each other, and the faster-moving molecules in the hot object spread that energy into the cooler object.
• Here if we see the total heat loss or heat flow of the circuit breakers, it is 375.7 W manually (means the input), coming to the simulation it is 374.324 W. Which is matched up to 99%.
• This can be another proof that our simulation gets converged and throws the correct values as outputs. 

Heat transfer coefficient:

• It is used in calculating heat transfer, typically by convection. The heat transfer coefficient has SI units in watts per squared meter kelvin: W/(m²K).
• The heat transfer coefficient is the heat transferred per unit area per kelvin.
• Thus the area is included in the equation as it represents the area over which the transfer of heat takes place. The areas for each flow will be different as they represent the contact area for each fluid side.
• The negative gradient in min. heat transfer coefficient of circuit breakers represents: In the case of constant wall temperature, using adiabatic wall temperature as reference temperature can result in a negative heat transfer coefficient, which means the heat flux has a different direction from the defined driving temperature difference.

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:

1. Both the residual plot and the monitor plot show that the simulation results after 300 iterations can be considered steady-state results.
2. According to the steady-state velocity contours, the maximum air velocity within the computational domain is 0.41 m/s.
3. The highest steady-state temperature of 93.79 °C is attained by the circuit breaker I3.

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