Convection

The symbol H is once again the rate of heat transfer, nothing new there. The symbol h is new though - it stands for the convection heat-transfer coefficient. The "A" and "T"s are the same again, area and temperature respectively. Since more surface area is key to releasing heat, it's only natural to want to have more and more parts of the heatsink exposed to open air. If you take a look at your heatsink, there will be all sorts of fins and pins all over it. In doing this, the overall surface area increases, but the size of the object does not. You generally want to look for heatsinks that have a great deal of surface area, although in some cases this might not help. If the fins on the heatsink are sufficiently close enough together, efforts at trying to get rid of heat are going to be thwarted.

Thermal convection is a measure of how well a heatsink will transfer heat to its surroundings, be it air, water or anything else. For most of us, the end place for the heat to go is the air; and with our terrible luck, air is a terrible conductor of heat. This is one of the main reasons we have fans on our heatsinks. Stagnant air won't help the cause for cooler heatsinks. The second a fan is added to the equation, performance is increased greatly. When hot air is being actively moved out of the heatsink, the equation gets put to work completely. By moving air in that is lower in temperature, the temperature difference becomes greater, thereby increasing the rate of heat transfer.

If you've been looking at heatsink/fan combos, you've probably heard of the infamous Delta that runs at nearly 7,000 RPMs. However, you may be surprised to know that there are methods you can use to get the same high performance, but without the astonishing 46 dB sound levels. For instance, some stores are now selling fan adaptors, giving you the ability to mount 80mm and 92mm fans onto heatsinks that would never have the room to fit a fan of that size. The benefit of a larger fan does quite a bit. With the larger blade area, you get increased airflow at a lower RPM. Now if you could find large fans with high RPMs…

Air Direction

After you've got your heatsink, the next logical step is to put a fan on it. But which way should it blow? There really isn't a set answer for this question. It all depends on the situations that this fan and heatsink are in. Let's take into account heatsink design; some coolers come with shrouds to go over them. In cases such as these, it is beneficial to have the fan suck the air out of the heatsink, as the shroud causes air from the bottom of the heatsink to be pulled up. Most shrouds have a small opening at the bottom, near the base to allow the influx of cooler air, to replace the heated air that was pulled out. If you are trying to pull air out of a heatsink without a shroud, the results can be mixed. Depending on the design of the heatsink and the airflow patterns that result from it, the only thing you can do is try both ways to see which works better. We're betting that air blowing onto the heatsink will be the better solution in this case. If the fan is going to be pulling air out of the heatsink, the airflow patterns will take the path of least resistance, and as these fans aren't terrible powerful, chances are that the hottest air (the air closest at the very bottom) will not be moved effectively.