FLOTHERM Analysis of a Desktop Computer

This is a typical FLOTHERM analysis of a desktop computer system. The processor is a 66MHz Pentium?in a CPGA (Ceramic Pin Grid Array) package mounted in a socket on the motherboard. The heat sink is a 9x12 pin-fin heatsink typical of those found in PC applications. A system fan, in this case a PAPST 612L, is used to draw air in through the vents in the front panel and to blow air directly across the heatsink. This case is being run for full processor power (16W) at a room temperature of 35癈.

The FLOTHERM models of the processor, socket, heatsink and fan are all existing FLOTHERM library parts so that creating this model took about 2 hours.

Results

Here are some result from the analysis. The peak temperature in the heat sink is about 85癈 and the temperature of the processor die is about 104癈 (see below). The cooling system seems to be working as planned. Or is it?

This view shows the air flow vectors and the temperature field on a plane through the center of the package. The most noticeable effect is that the component and the socket are so high that they block all the flow from the lower part of the fan annulus and force much of the cooling air up to the roof of the enclosure where it can do little good.

Now take a closer look at the velocity vectors in the plane of the processor center line. These show a curious effect. Because the fan generated cooling flow on the center-line of the package is being deflected upwards, the flow through the heatsink on this plane is actually going backward! Fortunately, the flow from the sides of the fan penetrates the heat sink fins and is sufficient to cool the processor, so things are still under control.

Are we getting good performance from the fan? This figure shows the paths of some "particles" released into the fan air flow - red on one side of the fan and blue on the other. These clearly show how the flow re-circulates around on one side (the blue side) meaning that much of the fan work is wasted!
 

If this designer had taken the time to analyze the design before building it, it might have been possible to:

• position the fan to provide the processor with optimum cooling; and
  • develop a simple baffle arrangement to prevent re-circulation through the fan.

  This might then have enabled a smaller, quieter and cheaper fan to be used as well as running the processor at a lower temperature.