Ans:
Gases h = 25 to 250 W/m^2.K
Liquids h = 100 to 20000 W/m^2.K
Why it is better to choose liquid for convection heat transfer over gases?
As it can be seen from the above values of heat transfer coefficients liquids transfer more amount of heat compared to gases. For example, if the engine needs to be cooled just using gas it would need a very large surface area(fins). On the other hand, if liquid like water is used, it can cool the engine with a small requirement of surface area to be in contact with liquid.
