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Forced and natural convection

In forced convection the fluid is driven by a fan or pump, while natural convection is driven by buoyancy from temperature differences alone. Forced convection usually gives much higher $h$ values because a faster flow makes the boundary layer along the surface thinner, so heat is conducted away into the fluid faster.

Nu = \dfrac{hL}{k}

Nusselt number, the ratio of convective to conductive heat transport

Symbols

$Nu$	Nusselt number
$L$	characteristic length	m

Example

Natural convection in air typically gives $h \approx 2$ – $25$ W/(m²·K), while forced convection in air can give $h \approx 10$ – $200$ W/(m²·K).

A higher speed gives a thinner boundary layer and hence a higher

h

– this applies to forced, not natural, convection.

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Part of Heat Transfer: Convection.