How much effect does wind have on cooling an object?

I was reading a question on how to cool frying oil on the cooking SE, and in a comment I suggested that having moving air would speed up the cooling as the heat transfer from the oil to the air would be quicker as already heated air us removed by wind. Some protests were raised, claiming that convection would remove heat from the air above the container holding the oil pretty efficiently anyway. So my question is; how large an effect does wind have on cooling a given object? And what other factors influence what effect the wind has, e.g. size of the object getting cooled, air temperature, etc?

Wind, or more generally air flow, has a positive effect on the cooling rate of objects, provided the air is cooler than the object itself. By the Laws of Thermodynamics heat always flows from hot to cold, so the hot object will transfer heat to the cooler air, which it then carries away.

Mathematically we can use Newton's Cooling law to show the effect:

$$\frac{dQ}{dt}=hA\Delta T(t),$$

where $\frac{dQ}{dt}$ is the cooling rate (in $\mathrm{W/s}$), $h$ is the overall heat transfer coefficient, $A$ the surface area of the object and $\Delta T(t)$ the difference in temperature between the hot object and the flowing air (here as a function of time $t$).

Blowing cool air over a hot object has the effect of increasing $h$. We can also see that when $\Delta T(t)$ is large, which means using cool air, the rate of cooling is also increased. Increasing surface area $A$ also increases cooling rate: all other factors being equal, the object with the largest surface area will cool down fastest.

Air cooling is widely used in various technologies such as cooling motorbike engines but also some car engines (think e.g. the 'beetle' car).

• And various small airplane engines. – Mike Dunlavey Dec 2 '15 at 1:28
• @MikeDunlavey - there's almost no end to air cooled systems, really. Think PCs, for example... – Gert Dec 2 '15 at 2:30