# Determining Heat Sink CFD Simulations and What to Examine

May 14, 2020

What You Can Takeaway

• Understanding heat sink CFD simulations for greater clarity

• Learn better about analytical functions like transient thermal FEA analysis

• Optimize  your geometry for heat sink CFD simulations Heat sink CFD simulation results

Anyone that has opened up an old computer has seen the large chunk of metal placed on a CPU, GPU, or other high-power IC. The shapes of different heat sinks can vary wildly and are very unique. These shapes are not chosen by accident, they are carefully examined and designed using a series of heat sink CFD simulations. So how can you determine the best geometry for your heat sink, and why are CFD simulations needed?

With the geometry of a typical heat sink being rather complex, it can’t be accurately examined analytically, so you need a simulation package that solves the related CFD problem numerically. The fundamental equations in CFD cannot be solved analytically in every situation, so numerical techniques are required to understand how airflow removes heat from electronic components and their heat sinks. Here’s what goes into a CFD simulation and what you should examine during one of these simulations.

## What Goes Into a Heat Sink CFD Simulation?

All CFD simulations are meant to solve the Navier-Stokes equation, conservation of fluid momentum equation, and the heat equation in an arbitrary geometry. In some limiting situations, particularly where the geometry is rather simple, you can apply some approximations to these equations and get a steady-state solution. In more complicated geometries, you’re almost never able to get a full time-dependent solution in terms of analytical equations.

Because the geometry of a typical heat sink can be quite complex, and due to transfer heat in the system over time, a numerical CFD simulation needs to be performed to examine how heat is transported away from a component. If you’re designing a heat sink for a high-power IC or other component, you need to consider the following aspects of your system in your heat sink CFD simulation:

• Sources and sinks: Your component is a source of heat, and any forced or natural airflow through the system acts as a sink of heat. The heat source is defined by the current distribution in the component, while airflow is defined by the presence of any forcing from a fan and natural convection.

• System mesh: The mesh in the system needs to be specified in an FDTD simulation (for time-dependent problems) or in an FEA simulation (for steady-state problems). A finer mesh will provide more accurate results, but it will require longer convergence time.

• Boundary conditions: The boundary conditions in the system need to be specified. This is normally specified as a limit as one dimension tends to infinity, or in a specific surface at the edge of the system.

• Initial conditions: The transient behavior in CFD simulations is extremely sensitive to changes in the initial conditions in the system. You could try to iterate through multiple initial conditions to examine this sensitivity.

Although you could simulate the time-dependent behavior of the system, your system will eventually reach a steady state as long as the heat source and airflow source are constant in time. You can read more about the differences between steady-state and transient thermal FEA simulations in this article.

One useful application of a time-dependent simulation is to examine how the temperature of a component changes in response to changes in its power consumption, followed by a change in the airflow rate. This gives you a strategy for determining when a fan should be toggled on or off, or when the speed should be changed in response to a change in component current use. The airflow rate has an associated velocity field, which can show you where heat tends to be transported away from a heatsink and into the larger system. This is shown below using velocity contours, and this should be examined using CFD simulations. Velocity contours showing airflow rate due to a fan in a heat sink CFD simulation: (top) isometric view and (bottom) side view.

The airflow velocity contours, air temperature, and steady state temperature of the heat sink and component will depend on the geometry of the heat sink. Therefore, your goal in a series of heat sink CFD simulations is to determine the geometry that maximizes heat dissipation away from the component of interest.

## Optimizing Heat Sink Geometry in CFD Simulations

Because the geometry of a typical heat sink can be quite odd, the best way to maximize heat transport away from a component is to vary different geometric parameters in the heat sink structure. Each time an aspect of the structure is changed, a new CFD simulation is run to determine how this affects heat dissipation by the heat sink. With this simple procedure, you can generate a set of curves that shows how heat dissipation from the heat sink varies with different heat sink geometric parameters.

Once you’ve determined the heat sink geometry that maximizes heat transfer away from an IC, you should simulate how heat is transported around the entire system. One simple way to do this is to set the temperature of the component with the optimized sink as a boundary condition and define the heat transported away from the component as a heat source. You can then determine the final temperature in the larger system by defining other important heat sources and running a steady-state simulation. 3D results from a thermal CFD simulation for a larger system. This shows how heat is transferred away from the heat sink and into the rest of the system.

This type of simulation can be used to determine whether a more aggressive thermal management strategy should be used in your system. Examples include placing heat sinks on nearby components, strategically placing cooling fans on specific components or on the enclosure, or performing a redesign to the entire system enclosure. Each time you modify the design, you should check that creates a more desirable temperature distribution inside the system.

When you need to manage heat transfer for high-power IC, you need a set of PCB design and analysis features that integrate with a 3D field solver. The Celsius Thermal Solver and SI/PI Analysis Point Tools from Cadence integrate into a complete system for analyzing heat transfer in electronic systems. Once you’ve completed your design, you can generate a documentation package for your manufacturer to produce your PCB and heat sinks. 