Electronic cooling Analysis of a Finned surface heat sink using Ansys Icepak
Objective:
To create a finned surface heat sink and simulation the heat sink using Ansys ice-pak and keeping the design objective, the base of the devices should not exceed 65°C when air sweeps the fins at an ambient temperature of 20°C.
Introduction:
A heat sink is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant, where it is dissipated away from the device, thereby allowing regulation of the device's temperature. In computers, heat sinks are used to cool CPUs, GPUs, and some chipsets and RAM modules. Heat sinks are used with high-power semiconductor devices such as power transistors and optoelectronics such as lasers and light-emitting diodes (LEDs), where the heat dissipation ability of the component itself is insufficient to moderate its temperature.
A heat sink is designed to maximize its surface area in contact with the cooling medium surrounding it, such as the air. Air velocity, choice of material, protrusion design and surface treatment are factors that affect the performance of a heat sink. Heat sink attachment methods and thermal interface materials also affect the die temperature of the integrated circuit. Thermal adhesive or Thermal grease improves the heat sink's performance by filling air gaps between the heat sink and the heat spreader on the device. A heat sink is usually made out of aluminum or copper
A heat sink transfers thermal energy from a higher temperature device to a lower temperature fluid medium. The fluid medium is frequently air, but can also be water, refrigerants, or oil. If the fluid medium is water, the heat sink is frequently called a cold plate.
In thermodynamics, a heat sink is a heat reservoir that can absorb an arbitrary amount of heat without significantly changing temperature. Practical heat sinks for electronic devices must have a temperature higher than the surroundings to transfer heat by convection, radiation, and conduction. The power supplies of electronics are not 100% efficient, so the extra heat is produced that may be detrimental to the function of the device. As such, a heat sink is included in the design to disperse heat.
Geometry creation:
First, we need to create the cabinet with the dimension of 75 mm X 250 mm X 356 mm and the cabinet acts as an enclosure for the total geometry
Backing-plate:
The heat sink backing plate is a plate that helps secure a heat sink to a motherboard, improving the heat removal process and making a more secure mount for the heat sink. The backing plate mounts to the underside of the motherboard.
A Backing plate with a thick of 6mm was used in this simulation process.
The use of a backplate with the heatsink is a great way to reduce stress on a chip. Backplates are sometimes called a backup plate or bolster plate. Thermal systems engineers know that their designs must not only cool the electronics but must also be mechanically reliable to avoid costly field failures.
Today’s high-performance electronic systems tend to employ very high-powered electronic devices that demand sophisticated thermal solutions utilizing high performance such as Radian’s high-performance line of heatsinks that include heat pipes or vapor chambers. In addition, thermal system engineers generally need to include high-performance thermal interface materials (TIMs) at the interface between the heatsink (or cold plate) and the device to get the best thermal performance.
Cabinet Opening:
Cabinet opening acts as the outlet for the airflow from the total cabinet box and it is considered as the outflow boundary condition for the simulation process.
the shape of the opening was considered to be in rectangular and location was set in the X-Y direction.
Fans:
There are no specific intake or exhaust fans, it is just which way round you install the fan, so you would, for example, fit a fan to the front of your case which would pull air in from the outside and exhaust it to the inside of your case in this orientation it would be called an intake fan.
We have considered that the Fan-1 center is at Z min (40mm, 47.5 mm, 0mm) having an external radius of 30 mm and an internal radius of 10 mm. the fan has fixed volumetric flow 18 CFM and Fan-2 77.5 mm from Fan1, Fan-3 77.5 mm from Fan 2
All ventilation fan systems should contain intake and an exhaust variable, which can be fans or ventilation holes. In an ideal set up, there should be a fan located near the top of the cabinet configured to exhaust out air, and a fan located near the bottom to push in air.
Electronics such as receivers and computers within a cabinet will cause air to heat up. Due to natural convection, warmer air which is less dense than colder air will rise on its own. An efficient fan system helps accelerate natural convection by rapidly exhausting warm air out the top of the cabinet while pushing colder air in at the bottom.
Heat source:
A heat sink is a thermal energy reservoir that can receive huge amounts of energy without a significant change in its temperature.
For example, A heat source is a thermal energy reservoir that can supply huge amounts of energy without undergoing a change in its temperature.
Fins:
Fins are surfaces that extend from an object to increase the rate of heat transfer to or from the environment by increasing convection.
Total 10 fins of thickness 2.5 mm having width 69 mm in X dir and length 281 mm in Z dir and all fins are 25 mm apart (Fins are aluminum extruded) were used in the geometry.
In the study of heat transfer, fins are surfaces that extend from an object to increase the rate of heat transfer to or from the environment by increasing convection. The amount of conduction, convection, or radiation of an object determines the amount of heat it transfers. Increasing the temperature gradient between the object and the environment, increasing the convection heat transfer coefficient, or increasing the surface area of the object increases the heat transfer. Sometimes it is not feasible or economical to change the first two options. Thus, adding a fin to an object increases the surface area and can sometimes be an economical solution to heat transfer problems.
Final Geometry:
After alignment of the total components inside the cabinet, we will end up with the following geometry which has been shown in the picture below.
1.Generate the Mesh (Coarse, and Fine mesh)
Coarse mesh:
The coarse mesh was generated with elements of 47948 and nodes of 52306 for the total geometry object.
Remember that every time you discretize a continuum you are adding a term called truncation term. It means that you are not solving the same equation as the continuum expression. Thus coarse meshes indicate that your truncation term is very large
A low-quality mesh will not only result in inaccurate simulation results but might even cause the solver to produce an error due to instability.
From the above picture, we can clearly see that the mesh has not to been generated in between the fins and by which the data generated in between the fins cannot be plotted.
Fine Mesh:
The finer mesh was generated with elements of 85777 and nodes of 92644 for the total geometry object.
A high-quality mesh also means that there is an optimal balance between the computational cost and the level of fineness achieved.
From the above picture, we can clearly see that the mesh has been generated between the fins and by which the data generated between the fins can be plotted.
Physical and Numerical Settings
1.Basic settings panel: set no of iterations and convergence criteria
The Reynolds and Peclet numbers are approximately 13, 000 and 9, 000, respectively, so the flow is turbulent. ANSYS Icepak will recommend setting the flow regime to turbulent.These values are only estimates, based on the current model setup. Actual values may vary and may need to be verified, depending on your design.
The total number of iteration was set to be 200 iterations and coming to the convergence criteria the flow was set to 0.001 and energy was 1e-7, and the turbulent kinetic energy and turbulent dissipation rate were 0.001
Parallel setting:
Parallel exchanging messages between multiple computers running a parallel program across distributed memory. In parallel computing, multiple computers or even multiple processor cores within the same computer are called nodes.
Advanced solver setup:
In applied mathematics, discretization is the process of transferring continuous functions, models, variables, and equations into discrete counterparts. This process is usually carried out as a first step toward making them suitable for numerical evaluation and implementation on digital computers.
The discretization scheme properties were set accordingly as shown below
2.Basic parameters panel:
The chosen variable to solve was the velocity and pressure, the temperature in the cabinet domain.
The chosen turbulence model was Zero equation model
The heat-transfer due to the radiation was Ignore for this simulation.
Solve for Flow and Temperature and Radiation was off and The turbulence model is the Zero equation models were set accordingly as shown above.
convergence criteria:
Residual plot:
From the residual plot, it is clear that the simulation has reached the converged stage and also obtained the converge, and they are no fluctuations in the plot.
3.Set up monitor points in the model:
We have considered two monitor points in the model in which we have placed one on the fan inlet side and the other monitor point at the outlet opening of the geometry.
The temperature monitor point:
From the above graph, we can clearly see that the atmospheric air ambient temperature that enters the cabinet was at 20°C. and the max temperature in the domain that has been obtained by the air was near 23.5°C and the outlet temperature of the air was at nearly 22.2°C.
4. Save the model.
ANSYS Icepak will save the model for you automatically before it starts the calculation, but it is a good idea to save the model (including the mesh) as well. If we exit ANSYS Icepak before you start the calculation, we will be able to open the job that we saved and continue with our analysis. If we start the calculation in the current ANSYS Icepak session, ANSYS Icepak will simply overwrite your job file when it saves the model.
5. Solve the model.
There are no universal metrics for judging convergence, a good indicator is when the solution no longer changes with more iterations, and when the residuals have decreased to a certain degree. The default criterion is that each residual will be reduced to a value of less than 10^-3 except the energy residual, for which the default criterion is 10^-7. It is a good idea to judge convergence not only by examining residuals levels but also by monitoring relevant integrated quantities.
ANSYS Icepak will begin to calculate a solution for the model, and a separate window will open where the solver will print the numerical values of the residuals. ANSYS Icepak will also open the Solution residuals graphics display and control window, where it will display the convergence history for the calculation.
Examine and post-process the results
1.Plane cut
The cut plane was generated with respect to the required plane and the set position was in x plane through the center for the geometry
2.Vector plot
The vector plot was created by taking the velocity magnitude for the plane with 2D arrowheads
The initial velocity starts at 0.424 m/sec and enters the intake fans by this more circular zones are formed at the fan portions and the intake fan forces the airflow into the cabinet and the air max velocity reaches nearly 3.401 m/sec and the outlet velocity can be considered to reach at 2.125 m/sec.
Animation for the Velocity Vector profile on the Cut plane in the x-axis
The vector animation which represents the flow of the velocity magnitude over the finned heat sink surface was shown in the above animation.
3. Cut plane contours
The Temperature contours were created by taking the Temperature magnitude for the plane by a contour level of fixed spacing.
Temperature contour
The air inlet was set at a temperature of 20°C, the maximum temperature obtained was nearly59.9°C at the heat source and the outlet temperature has reached nearly to 40°C due to the fins are surfaces that extend from an object to increase the rate of heat transfer or from the environment by increasing convection.
We can clearly see that the fins have increased the rate of heat transfer from the environment by increasing convection.
Animation for the temperature profile on the Cut plane in the x-axis
The Temperature animation which represents the flow of the Temperature magnitude over the finned heat sink surface was shown in the above animation.
Velocity contour
The initial velocity starts at 0.425 m/sec and enters the intake fans by this more circular zones are formed at the fan portions and the intake fan forces the airflow into the cabinet and the air max velocity reaches nearly 3.401 m/sec and the outlet velocity can be considered to reach at 1.700 m/sec.
Animation for the Velocity profile on the Cut plane in the x-axis
The Velocity animation which represents the flow of the Velocity magnitude over the finned heat sink surface was shown in the above animation.
Pressure Contour:
The initial pressure starts at -7.20 N/m^2 and enters the intake fans by this more circular zones are formed at the fan portions and the intake fan forces the airflow into the cabinet and the max pressure reaches nearly 1.179 N/m^2 and the outlet pressure can be considered to reach -0.921N/m^2
Negative pressure would mean that air is being sucked into your case from all the tiny gaps you can't control and don't have filters on, which means less efficient cooling over time. Aim for slightly positive-pressure, with slightly higher intake CFM than exhaust CFM.
If we attain positive pressure which means there is more air entering the case than exiting then air will escape the case via those gaps. Whereas if we achieve negative pressure more air exiting the case than entering then the air would be drawn in through those gaps.
Negative pressure generally refers to a place where pressure is smaller in one place relative to another place. You will often hear about negative room pressure. That means the air pressure inside the room is lower than the pressure outside the room and air will flow into the room from outside
Animation for the Pressure profile on the Cut plane in the x-axis
The Pressure animation which represents the flow of the Pressure magnitude over the finned heat sink surface was shown in the above animation.
4.Object face
Object face:
An object-face view allows you to examine the distribution of a solution variable on one or more faces of an object in the model. To generate an object-face view, you must select the object and specify both the variable to be displayed (e.g., temperature, Velocity, Pressure) and the attributes of the view (e.g., shading type).
Object face to display on all blocks:
We will use the Object face panel to create a solid-band object-face view of temperature on all five high-power devices and on the backing plate. To open the Object face panel, select Object faces in the Post menu.
Object face to display temperature contour on overall cabinet:
The overall temperature distribution throughout the cabinet was shown in the above Figure.
Object face to display Velocity contour on overall cabinet:
The overall Velocity distribution throughout the cabinet was shown in the above Figure.
Object face to display Pressure contour on overall cabinet:
The overall Pressure distribution throughout the cabinet was shown in the above Figure.
Conclusion:
we have performed a simulation analysis on the finned heat sink and According to the design objective, the base of the devices has not exceeded 65°C when air sweeps the fins at an ambient temperature of 20°C and a detailed view for the simulation output.
Parameters like temperature, pressure, and velocity were plotted. And monitor points were considered at the inflow and outflow boundary conditions to obtain the temperature that has generated in the cabinet.