Heat transfer coefficient

In thermodynamics, the heat transfer coefficient or film coefficient, or film effectiveness, is the proportionality constant between the heat flux and the thermodynamic driving force for the flow of heat (i.e., the temperature difference, ΔT ). It is used to calculate heat transfer between components of a system; such as by convection between a fluid and a solid. The heat transfer coefficient has SI units in watts per square meter per kelvin (W/(m²K)).

The overall heat transfer rate for combined modes is usually expressed in terms of an overall conductance or heat transfer coefficient, U. Upon reaching a steady state of flow, the heat transfer rate is:

${\displaystyle {\dot {Q}}=hA(T_{2}-T_{1})}$

where (in SI units):

${\displaystyle {\dot {Q}}}$: Heat transfer rate (W)

${\displaystyle h}$: Heat transfer coefficient (W/m²K)

${\displaystyle A}$: surface area where the heat transfer takes place (m²)

${\displaystyle T_{2}}$: temperature of the surrounding fluid (K)

${\displaystyle T_{1}}$: temperature of the solid surface (K)

The general definition of the heat transfer coefficient is:

${\displaystyle h={\frac {q}{\Delta T}}}$

where:

${\displaystyle q}$: heat flux (W/m²); i.e., thermal power per unit area, ${\displaystyle q=d{\dot {Q}}/dA}$

${\displaystyle \Delta T}$: difference in temperature between the solid surface and surrounding fluid area (K)

The heat transfer coefficient is the reciprocal of thermal insulance. This is used for building materials (R-value) and for clothing insulation.

There are numerous methods for calculating the heat transfer coefficient in different heat transfer modes, different fluids, flow regimes, and under different thermohydraulic conditions. Often it can be estimated by dividing the thermal conductivity of the convection fluid by a length scale. The heat transfer coefficient is often calculated from the Nusselt number (a dimensionless number). There are also online calculators available specifically for Heat-transfer fluid applications. Experimental assessment of the heat transfer coefficient poses some challenges especially when small fluxes are to be measured (e.g. < 0.2 W/cm²).^[1][2]