How does heat sinks work ?

When the air flow through the heat sink decreases, this results in an increase in the average air temperature. This in turn increases the heat sink base temperature. And additionally, the thermal resistance of the heat sink will also increase. The net result is a higher heat sink base temperature.

The increase in heat sink thermal resistance with decrease in flow rate will be shown in later in this article.

The inlet air temperature relates strongly with the heat sink base temperature. For example, if there is recirculation of air in a product, the inlet air temperature is not the ambient air temperature. The inlet air temperature of the heat sink is therefore higher, which also results in a higher heat sink base temperature.

Therefore, if there is no air or fluid flow around the heat sink, the energy dissipated to the air can not be transferred to the ambient air. Therefore, the heat sink functions poorly.

Furthermore, a heat sink is not a device with the "magical ability to absorb heat like a sponge and send it off to a parallel universe".

Other examples of situations in which a heat sink has impaired efficiency:

Pin fins have a lot of surface area, but the pins are so close together that air has a hard time flowing though them. Aligning a heat sink so that the fins are not in the direction of flow. Aligning the fins horizontally for a natural convection heat sink. Whilst a heat sink is stationary and there are no centrifugal forces and artificial gravity, air that is warmer than the ambient temperature always flows upward, given essentially-still-air surroundings; this is convective cooling.