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Forced convection uses external means of producing fluid movement. Forced
convection is what makes a windy, winter day feel much colder than a calm day with

same temperature. The heat loss from your body is
increased due to the constant replenishment of cold air by

the wind. Natural wind and fans are the two most
common sources of forced convection.

Energy transfer is involved by fluid movement and

molecular conduction is described by the Newton’s

formula:

I
Q  q F    F (t t - ), W
S f (1.11)

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where q –heat transfer rate per unit area, W/m

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Fig. 9 – Convection heat α – convectional heat transfer coefficient, W/(m K);
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transfer F – surface of the wall, m ;
t s – temperature of the surface, K or °C;

t f – temperature of the fluid, K or °C.

Convectional coefficient α, is the measure of how effectively a fluid transfers heat

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by convection. It is measured in W/(m K), and is determined by factors such as the fluid
density, viscosity, and velocity. Wind blowing at 5 mph has a lower α than wind at the

same temperature blowing at 30 mph.

If in the flow (Fig. 9) the Reynolds number is large enough, three different flow

regions exist:

At the wall:

- laminar sublayer (boundary layer) → thermal conduction

Outside the laminar layer:

- buffer layer → eddy mixing and conduction

Beyond the buffer layer:

turbulent region → eddy mixing

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