What is Convection?
Convection is the process of heat transfer in which transfer of heat energy occurs by the mass movement of molecules of the fluids like gases and liquids. Gases and liquids are not a good conductor of heat under normal conditions, but they can easily transfer heat.
Heat transfer through convection occurs other through diffusion or advection or both. Convection does not take place in solids, because no movement of its constituent particles occurs. The diffusion of heat occurs in solids, and it is called thermal conduction.
The process in which heat transfer occurs between a surface and a liquid or a fluid that is in contact with the surface is called convection heat transfer. Convection plays a major role while transferring heat from one liquid to another liquid through a barrier. Heat transfer by convection either occurs due to thermal diffusion (motion of fluid molecule) or advection, in which heat is transferred by the bulk motion of heat currents in the fluid.
Newton’s Law of Cooling
Under normal conditions, the heat transfer due to convection is directly proportional to the temperature difference between the parts. Newton’s law of cooling has expressed this phenomenon, and the law states that:
The heat transfer rate of a body due to convection is directly proportional to the temperature difference between the body and its surroundings. The temperature difference should be small, and the nature of the radiating surface remains the same. This is the convection heat transfer equation:
P = [frac{{dq }}{dt}] = hA(T – T0)
Where
P = [frac{{dq }}{dt}] is the rate of heat transfer
h is the coefficient of the convection heat transfer
A is the surface area of the body
T is the temperature of the object
T0 is the temperature of the fluid or liquid which is subjected to convection
Heat-transfer coefficient h depends on the following factors:
Density, Thermal conductivity, Viscosity, coefficient of convective heat transfer, and specific heat capacity
The constant of proportionality mentioned in the above formula is an important parameter and is called a coefficient of convective heat transfer ‘h’. It is defined as:
The rate at which heat transfer occurs between a solid and a fluid part per unit surface area and for the unit temperature difference:
𝒉 = [frac{{q}}{ΔT}]
Where
q is the local heat flux density [W.m-2]
H is the heat transfer coefficient [W.m-2.K]
ΔT is the temperature difference [K]