Natural convection : This arise due to difference of densities at two places on account of gravity.
(i) Natural convection takes place from bottom to top while forced convection in any direction.
(ii) Natural convection is not possible in a gravity free region such as a free falling lift or an orbiting satellite.
Forced convection : If a fluid is forced to move then it is called forced convection. In this case Newton’s law of cooling holds good. According to which rate of loss of heat from a hot body is directly proportional to the surface area of body and excess temperature of body over its surroundings.
where T = Temperature of body and T0 = Temperature of surrounding
