Air Velocity and Heat Sink Fans

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For case cooling, volumetric flow rate and air temperature are all that really matter. For cooling a heat sink, however, air velocity is also critical. Air velocity is one of the biggest determinants of the convection coefficient. Average air velocity at the fan is defined as flow rate divided by fan area. As air leaves the fan, it will not remain a fixed diameter column of moving air. It will disperse and its average velocity will drop. As it disperses, it will entrain some of the surrounding, static air. This entrainment will raise the flow rate slightly as the velocity drops.

In some instances, raising the fan slightly above the heat sink results in lower temperatures. One reason for this is the extra air that gets pulled in by the moving air stream. The other reason is that all fans have a dead spot in the center of flow immediately following the fan. This dead spot is cause by the central hub of the fan. This dead spot disappears after a short distance. Since the center of the heat sink has the highest temperature, the dead spot can cause elevated temperatures.

As a general rule, a smaller diameter fan of equal flow rate will result in lower temperatures than a larger fan of equal flow. As a general rule, a fan will provide slightly improved performance with a small gap between the fan outlet and top of the fins/pins. As a general rule, raising a fan too far from the heat sink will cause an increase in temperatures.

If you wish to use a larger diameter fan, you should pair it with a funnel to increase the velocity of the air hitting the heat sink.

To compare the air velocity of different fans, divide the rated flow by the fan diameter squared.

General Fan Performance Guide - Cases and Cooling 7

As an example, a 60mm fan pumping 38CFM yields a relative velocity of .0106 while an 80mm fan pumping the same volume yields .0059. The air velocity from the 60mm fan will be 78% higher resulting in better air convection. When the flow rating is equal, you need only divide the large diameter by the small and square the result. The answer will be how many times faster the small fan air velocity is compared to the large fan.

Altering Fan Flow

There are a few things we do that alter fan flow. Some of these things increase airflow; however, most reduce airflow.

Increasing Air Flow

Methods for increasing airflow may depend upon which fan we discuss.

Alternatives for increasing flow include the following:

  1. Choosing a more powerful fan
  2. Stabilizing ambient air at the fan intake
  3. Spacing the fan a small distance above the heat sink (heat sink fan only)
  4. Increasing fan voltage
  5. Removing the fan guard
  6. Increasing open area in the bezel/grate (case intake fans, power supply intake fans only)
Decreasing Air Flow

While it isn’t intentional, we often do things that decrease airflow.

Sometimes we intentionally decrease flow to make things quieter.

  1. Blocking air intake (insufficient guard/bezel openings, clogged filters, high resistance filters, heck any filters, cables, lack of clear flow path)
  2. Blocking air exhaust (insufficient guard/bezel openings, heat sink fins/pins, cables, lack of clear flow path)
  3. Decreasing fan voltage
  4. Turbulence, secondary flow conditions
  5. Choosing a less powerful fan
Most of the above items are self-explanatory. The conditions that may be confusing or require more explanation include clear flow paths and turbulence. In the most basic sense, air has mass and therefore, momentum. Accelerating air requires energy. A fan’s energy is well spent when it is used to accelerate stationary air axially through the body of the fan. When is a fan’s energy not spent well?

When a flow pathway is not clear (read “straight and unobstructed”), the air must flow through a tortuous course. Each time the air stream curves, energy is required to turn the stream. As an example, hold your hand out a car window while moving. When your hand is parallel to the direction of travel, you feel wind resistance dragging on your hand. The air must separate and flow around your hand, rejoining as it leaves your hand. As you turn you hand such that your palm begins to face the direction of travel it is pushed backwards and to the side. The drag increases because your hand is offering more resistance to the airflow and causing a larger disruption of that flow.

The closer the obstruction is to the fan, the greater the impact on the airflow. In some cases, such as with heat sinks, we have no choice but to locate the fan close to the obstruction. In others, such as case fans, we can move or reduce obstructions such as drive cables, bezels, and fan guards. Let’s not forget the overall case placement. If tucked into a cubbyhole of a desk or the corner of a room, air intake and exhaust may also be compromised.

Turbulence and secondary flows have a similar effect. If air is already flowing in one direction, energy is required to get it to flow in another direction. There are two examples that apply directly to heat sink fans. In one instance, a too-powerful exhaust fan or power supply intake fan may cause problems. Either may draw so much air from the case intakes that the cool intake air short-circuits to the exhaust without stopping by to visit the heat sink fan. A relatively warm region of air may stagnate around the heat sink and lead to an increase in CPU temperature. The other instance involves powerful side intake fans. These may create such high air velocities around the heat sink fan intake that the fan’s flow rate drops. The heat sink fan spends too much energy trying to pull air into its intake. If the side fan directs air into the heat sink fan, flow should improve. If the side fan creates a chaotic flow pattern around the heat sink fan, flow will decrease.

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