Heat transfer via convection

Heat Transfer via Convection

Convection is a heat transfer mechanism that is driven by the movement of fluids. It can be seen as a combination of conduction and fluid motion. A hot object will transfer heat to the fluid that surrounds it, and that hotter fluid will be carried away either by the buoyancy of the fluid or bulk air movement. Convection can usually only take place in fluids where there is a bulk movement. If there is no bulk movement, conduction is enough to describe the movement of heat. The greater the bulk fluid motion, the greater the rate of heat transfer via convection.

There are two main types of thermal convection: natural and forced convection.

Natural convection occurs when convection is primarily driven by differing buoyancies between warmer fluids and cooler fluids. Warmer fluids are more buoyant than cooler fluids, so they will tend to rise relative to the cooler fluid. Natural convection also occurs during phase changes. When a liquid boils into a gas, it will be more buoyant than the surrounding liquid, and will rise. This can be seen when water boils, the water vapor that forms at the bottom of a pot while it boils rises to the top. Similarly when water condenses, it will sink because it is less buoyant than the water vapor (water will condense on preexisting sites such as dust particles in the atmosphere or on solid/liquid surfaces, in which case there may be water vapor that the liquid water could still sink through, but due to adhesion between the liquid water and the solid, it will remain on the solid).

Forced convection occurs when the fluid is being forced to move by an external source, such as a fan, a pump, or a current. In the example where water is boiling, the water vapor will rise in the liquid water until it reaches the air, whereupon it will continue to rise as the hotter water vapor is more buoyant than the surrounding air. However, if we add a fan that blows air over the top of the boiling water, the water vapor that reaches the air will be carried away the the bulk air movement powered by the fan. Forced convection can increase the rate of heat transfer compared to natural convection by replacing the warm fluid surrounding a heat source with cooler fluid, which increases the temperature difference between the heat source and sink, although this is usually a continuous cycle.

Movement of air in natural vs. forced convection (Çengel 2020).

Newton's Law of cooling can be used to determine the rate of heat transfer in many convective situations.

\overset{•}{Q}=hA({T}_{s}-{T}_{f}) =hAΔT(t)

Where

\overset{•}{Q}

is the rate of heat transfer (SI units Watts), A is the surface area through which heat transfer takes place,

{T}_{s}

is the temperature of the solid at the surface,

{T}_{s}

is the bulk fluid temperature away from the surface, and h is the convection heat transfer coefficient. h must be determined experimentally based on the conditions that will exist during heat transfer. For a general range however, h (in W/m^2*K) will be 5-25 for the natural convection of gases, 50-1,000 for the natural convection of liquids, 25-250 for the forced convection of gases, 50-20,000 for the forced convection of liquids, and 2,500-100,000 for convection in boiling and condensation processes (Çengel 2020).

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