Why is air a good convector?

Answer 1

Let us first define "good convector." We assume the question is discussing the transfer of heat. We assume one defines "good" as transferring more heat and that air meets some "good" criteria in terms of its effectiveness.

As a short answer, air has a modest heat capacity and a low viscosity and high degree of thermal expansion which combine to make it a "good" material for transferring heat through convection. One should interpret "good" as being a very modest way.

Basic convection occurs when the density of a fluid material changes when its temperature changes, usually density goes up as temperature goes down and conversely usually density decreases when temperature increases. (Mostly, but not always true for all materials. There are exceptions and water between the temperatures of 0 and 4 degrees centigrade is the most famous example.) With an uneven temperature distribution throughout the volume of a fluid, the resulting density difference causes the denser part of the fluid to tend to sink and the less dense part tend to rise. This is a simple consequence of buoyancy since anything less dense than its surrounding fluid will tend to rise and denser things tend to sink. The speed at which this occurs depends on the viscosity of the fluid. A smaller viscosity means less force is required (less buoyancy in the present discussion) to move through the fluid.

So, qualitatively, air has a low viscosity and moves easily when there is a force and the change in density of air with change in temperature is relatively large, both factors work towards increasing the rate of convection. (Normally, heat capacity is given in terms of the heat that can be absorbed per mole of a material and gasses at the same volume will typically have similar number of moles. When comparing by mass, materials have large variations in the heat that can be absorbed. Comparing convection in gasses and convection in liquids requires comparing materials with huge differences in density and viscosity and thermal conductivity and so does not lead to a simple analysis.)

Qualitatively, for good convection one would like a large thermal expansion, large heat capacity and small viscosity, but that is merely the simplest qualitative assessment. There is also thermal conductivity to be considered and air is not a terribly good thermal conductor and that enhances the tendency to for convection upon heating or cooling.

Quantitatively, convection is a very complex process in fluid dynamics and doing calculations with reliable accuracy involves many other factors, particularly the shape of the volume containing the fluid and the rate and geometry of the heat input or extraction process. This issue is of the highest important when considering matters such as insulating something as common as a house where convection can be the major method of heat loss in winter. There are many more applications and it is a matter of extensive technical and scientific interest.

Finally, it should be mentioned that heat transfer can occur in several ways and convection is only one way. It can be a very effective (or "good) form a heat transfer when compared to conduction. Conduction is usually considered to be a slow form of heat transfer, so if speed is a consideration in "good" then convection is faster. Radiative heat transfer is, of course, fast but may not be able to transfer a large amount of heat, so if transferring a lot of heat over small distances with modest temperature differences is important, then convection again wins out as "good." All of these considerations are simplified in this answer to normal human scale temperatures and distances and times. Astronomical distances or atomic scale distances or picoseconds or centuries will lead to different meaning of "good" evaluations for heat transfer.