What generally increases heat transfer when air is propelled across a surface?

When air is propelled across a surface, an increase in velocity enhances heat transfer primarily due to the process of convection. Convection involves the movement of air over the surface, which allows heat to be exchanged more efficiently. As the velocity of the air increases, more air molecules come into contact with the surface in a given time, which helps carry away heat from the surface more effectively.

In practical terms, when air moves swiftly, it disrupts the thermal boundary layer that tends to form just above the surface. This boundary layer can insulate the surface from the surrounding air, slowing down heat transfer. However, faster-moving air reduces the thickness of this layer, leading to improved convective heat transfer rates.

While humidity and temperature can influence heat transfer, their effects are secondary to the velocity of the airflow in this context. An increase in surface area can also impact heat transfer by allowing more area for interaction between the air and the surface but does not directly involve the dynamics of airflow speed as the primary driver of heat exchange.