Non-Conformal Meshing

Cold-Plate Model with Non-Conformal Meshing

Objective: The model contains two heated plates, cooled by water circulating inside the cold-plate cavity, as well as by air driven by natural convection externally. Separately meshed assemblies will be employed to reduce the overall mesh count in the domain.

Introduction: Cold Plate

Today’s most demanding, high-performance electrical and technological applications were made possible largely due to innovative thermal management techniques. In applications where power output is high and space is limited, cold plates have become vital for their combination of high thermal conductivity and compact design.

Cold plates are typically made from metals, with flow paths machined into them for a heat-transfer fluid to flow through. As the plates absorb electrical waste heat, they dissipate it through the flow paths using liquid cooling.

The flow paths inside the cold-plates can be machined in various shapes and sizes and can allow cold plates to more effectively absorb and transfer large amounts of heat.

The effective heat transfer coefficient of a cold plate depends on the available surface area in the flow paths, the liquid used, the flow rate of the liquid, and the material used to make the cold plate.

Problem Description:

• The model consists of a cold-plate, where the cold-plate fluid is transporting a significant fraction of the heat from two plates mounted on either side of it.
• The natural convection in the external air is also instrumental in some heat transfer. The objective of this exercise is to illustrate the use of two different fluids in ANSYS Icepak.
• The model includes two heated plates, cooled by water circulating inside the cold-plate cavity, as well as by air driven by natural convection externally.
• Separately mesh assemblies will be employed to reduce the overall mesh count in the domain. The model will be constructed using the default metric unit system.

By opening the Ansys – Icepak module, a popup showed to create a new project, open an existing project, and unpack the geometry. Here we need to select the new project.

Geometry:

Cabinet:

• The cabinet is a case that all the other parts have been included and mounted on it. It looks like an enclosure.

• ANSYS Icepak will create a default cabinet with the dimensions 1 m × 1 m × 1 m, and display the cabinet in the graphics window. To build the model, you will first resize the cabinet to its proper size.

• The actual dimensions for the cabinet are 400 mm X 300 mm X 200 mm.

Cold plate solid block:

• The cold plate solid block has been created with the given dimensions and specifications.

• Specifications are: Geometry shape is prism Dimensions are starting at (50 mm, 80mm, 70 mm), Ended at (350mm, 220 mm, 130 mm).
• The material of the block provided is Aluminium – Extruder.

Cold plate fluid block:

• The cold plate fluid block has been created with the given dimensions and specifications.
• Specifications are: Geometry shape is prism Dimensions are starting at (60 mm, 90mm, 80 mm), Ended at (340mm, 210 mm, 120 mm).

• The material of the block provided is Water (@ 280k).

Incoming pipe:

• The incoming pipe is created in a cylindrical shape with the dimensions of 100mm * 0mm * 100mm. The thickness of the pipe is 5mm. The inlet water velocity given is 0.2 m/s. This pipe is connected to the cold plate fluid block.

Outgoing pipe:

• The outgoing pipe is created in a cylindrical shape with the dimensions of 300mm * 0mm * 100mm. Thickness of the pipe is 5mm. The radius of the pipe is 15 mm. This pipe is connected to the cold plate fluid block.

Create Plates:

• The plates are placed beside the solid and fluid block. These two plates are the main heat sources for the model.
• The two plates are created as per the dimensions with the conducting thickness of 0.01m.

Plate1: The plate is located at 60mm having dim (260 mm X 100 mm) in the Z plane. The plate exhibits the power of 200 Watts.

Plate2: The plate is located at 130mm having dim (260 mm X 100 mm) in Z plane. The plate exhibits the power of 200 Watts.

Opening:

• The cabinet opening is not an object or a component, but we consider it as a medium and provide some parameters and dimensions that match with the cabinet.
• The cabinet opening acts as the inlet or outlet for the airflow from the total cabinet chamber. It is an outlet to the airflow that occurs due to the fans provided. Here we have to give the outlet boundary conditions to the problem domain.

• There are a total of four openings in the model. Two for the passage of air for natural convection, and two in the cylinders which water is passed for forced convection.

Complete Geometry:

• This is the whole geometry we are going to simulate.

1. Generate the Meshing:

• A non-conformal mesh, when there are a partial or zero matchings of nodes at the interface. This means that one, 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.

• The mesh is generated in the absence of the per-object parameters. Which means the object parameters.

• Here we generate the non-conformal mesh without the per-object parameters, the generated elements are 50101, and nodes are 54646.

Use object parameters:

• After using the object parameters, the model gets even refined and there will be an increase in the number of elements and nodes. The generated elements are 52685 and nodes are 57834.

Slack Values when we use Mesh assemblies separately:

• When the Mesh separately options for an assembly is turned on, the offset added to Assembly in 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.

• To create a separate meshed assembly, highlight all the objects in the model tree other than the cabinet, opening.3, and opening.4.
• Right mouse clicks on them and chooses Create assembly.
• To enable separate meshing for the assembly, double-click on assembly.1 to edit the assembly.
• Under the Meshing tab, toggle on the Mesh separately button and then enter the slack values as follows.

Mesh Generation:

• As we have seen above, there will be no mismatches in the mesh lines.
• No mesh bleeding happening along the length of the mesh lines.
• The mesh is perfectly a Non-Conformal mesh. and it satisfies all the parameters of a non-conformal mesh that need to be followed.

• The above-shown picture is a solid part of the cold plate assembly and it's meshing.
• By visualizing the mesh separately as a solid medium and fluid medium gives us clarification in many terms.
• The below-shown picture is a fluid medium mesh.

2. Numerical setting:

Use problem set up wizard:

• The problem setup wizard is a 14 step command box, where we need to specify the turbulence equation, implementation of radiation, and gravity relational entries, etc.

set no of iterations and convergence criteria:

• In the basic settings panel, we need to give the number of iterations to 300.

• The convergence criteria of Flow is set to 0.001 and energy is set to 1e-7, turbulent kinetic energy to 0.001 and turbulent dissipation rate is set to 0.001.

Solve for Flow and Temperature and Radiation is off. The turbulence model is the Zero equation model:

• A calculation of the Reynolds number shows that the problem is turbulent.

• To set up a turbulent flow, expand the Problem setup tree (top of the model tree), select Basic parameters, and choose the Zero equation turbulence model for the flow regime in the General setup tab.

Set up the gravity vector:

• Gravity acts in the negative x-direction in this problem.
• To setup, the effects of gravity, toggle on the Gravity vector in the General setup tab.
• . Enter the new values for the gravity vector as x = -9.80665, y = 0 and z = 0.

Setup transient simulation with an initial velocity of 0.005 m /s in antigravity direction:

• Now go to the Transient setup tab and set an initial velocity X velocity = 0.005 m/s in the x-direction. Accept all other defaults in the Basic parameters panel.

Set up monitor points in the model:

• Add three monitor points to the Points folder, one to monitor the velocity at the center of the opening.1 (outlet opening), and two to monitor the temperature at the center of block.2 and plate.2, respectively.

• The easiest way to create them is to select the objects from the Model tree and then drag them to the Points folder of the tree.

• ANSYS Icepak will then automatically monitor values at the centers of these objects.

• The default settings are to monitor Temperature. To change this, double click on the object under the Points folder, and choose which variables to monitor at that location.

Save and Solve the model:

• To furtherly into the simulation, it is a best practice to save the model before we solve the model.
• To solve the go to File > save the project, or just click ctrl+S to save.
• To solve the project click the Run solution option in tools bar, and a box will appear. in that check whether the radiation is disabled or not, and click Start solution.

Residual Plot:

• From the above residual plot, we have seen that the solution gets converged at nearly the iteration no.150.
• After the iteration no.150, 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. Which means the solution gets very well converged.

Monitor Points Graphs:

• The monitor points are set for the opening.1 for velocity measurement and block.2 and plate.2 for the temperature measurement.

• The initial velocity measured is 0.144 m/s. And with the implementation of the velocity of 0.005m/s to the reverse gravity of 9.81 m/s, makes the increase in the velocity.
• The velocity reaches 0.23 m/s and then it shows almost equal velocity throughout the simulation.

• The temperature of plate.2 seems to start at nearly 37-degree centigrade and settled at 33 degrees. And block.2 temperature is raised from 20 degrees to 22 degrees, which means it (fluid medium) absorbs the heat.

Examine and post-process the results:

Plane cut:

• In the plane cut, it is actually an option to post-process the results that are offered by Icepak after the simulation runs successfully.
• In the plane cut, we need to provide the position in terms of axes, in which direction we had to see the results graphically.
• In this plane cut, we can see the results in three forms; such as the contours, vectors, and particle traces.
• After selecting a particular option, we need to select a proper parameter which changes with respect to the given conditions. These are the functions of the plane cut.

Vector plot:

• The vector plot explains, A vector plot is a type of cartesian chart where each point has an X and Y position, a length, and a direction. Vectors are drawn as arrows.
• The vector plots can be obtained by selecting the vector option in the cut plane.

• To represent the velocity results, the vector plot is a best method to represent by using the color legends.
• Because velocity has a position, direction, and magnitude.
• Here in the above vector plot, the max. velocity observed is 0.2362 m/s which is also matched with the value obtained from the monitor point.
• Here in the plot, the direction of the arrows shows the direction of the fluid flow inside and outside the model or cabinet.
• If we want to see the velocity in the filled color legends, then we need to select the speed option in the contour.

• If we observe the picture, at the walls of the solid medium there is zero velocity.

Velocity Animation: https://www.youtube.com/watch?v=u-3nJBEUybU

Speed Animation: https://youtu.be/En4e0BMWP2o

Temperature Contour:

• Filled colors, give a good visualization for the temperature, pressure, and mass flow rate, etc.
• The color legends are used to show the temperature contour.

• The maximum temperature observed as 33.14 degrees Centigrade and the minimum temperature noted as 20-degree Centigrade which is a given temperature.

Temperature Animation: https://youtu.be/1yuzOv-FWWQ

Object Face:

Velocity:

• The object face option gives the results of a particular object in the post-processing.
• The above picture is the object face vector of velocities at all the openings.

Speed:

• The picture shows that at the openings there is difference in the velocity or speed. Which is because of the given initial and boundary conditions.

Temperature:

• Here in this picture, the temperature is visualized on the basis of object faces.
• Here at the left side opening, there is no change in temperature, instead, it is same as the given temperature.
• The other opening has a change in its temperature because of the natural convection.
• The blocks and the plates also have the variations in their temperature due to the given input power of 200 Watts.
• There will also be an effect of conductivity between plates and blocks.

Conclusion:

The simulation is performed for the Cold plate model using the Non-conformal mesh. The model includes two heated plates, cooled by water circulating inside the cold-plate cavity, as well as by air driven by natural convection externally. Separately mesh assemblies will be employed to reduce the overall mesh count and mesh bleeding in the domain. The required numerical and physical settings have been given to the model and taken the required vector plots, temperature, speed, and parametric change in object faces.