Convection

Convection is of two types – natural and forced.

Natural convection is due to buoyancy and the buoyant force acting on the fluid. For natural convection to occur, there has to be a temperature difference across the fluid. Examples of natural convection include sea breeze and land breeze.

Forced convection occurs when an external agent, like a fan or pump, transports the fluid. The effect is similar to natural convection. Examples of forced convection include a convection oven, air conditioner, car radiator, and water heater.

A space heater is an excellent example of understanding natural convection. It heats the air at the bottom of a room. As a result, the kinetic energy of the particles increases, and the air expands. The air becomes less dense and rises to the top. Then, it cools down, reduces its volume, and becomes denser. The cold air sinks and reaches the bottom, where it heats up. This process repeats itself and forms a cycle known as convection currents.

Heat transfer in convection typically occurs between a surface and a fluid, where the fluid carries the heat generated by the surface. The equation for heat flow is \( Q = h_C A (T_S – T_f) \)

Where

Q: Heat transferred per unit time

h_c: Convective heat transfer coefficient

T_S: Temperature of the surface

T_f: Temperature of the fluid

The unit for the convective heat transfer coefficient is W⋅m⁻²⋅K⁻¹ It depends on the physical properties of the surface and the fluid, like surface geometry, bulk fluid velocity, and the nature of fluid motion.