Thermal effusivity

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In thermodynamics, a material's thermal effusivity, also known as thermal responsivity, is a measure of its ability to exchange energy with its surroundings. It is an intensive quantity defined as the square root of the product of the material's thermal conductivity ( λ {\displaystyle \lambda } ) and its volumetric heat capacity ( ρ c p {\displaystyle \rho c_{p}} ) or as the ratio of thermal conductivity to the square root of thermal diffusivity ( α {\displaystyle \alpha } ). e = λ α = λ ρ c p . {\displaystyle e={\frac {\lambda }{\sqrt {\alpha }}}={\sqrt {\lambda \rho c_{p}}}.} Some authors use the symbol r {\displaystyle r} to denote the thermal effusivity. The SI units for thermal effusivity are W s / ( m 2 K ) {\displaystyle {\rm {W}}{\sqrt {\rm {s}}}/({\rm {m^{2}K}})} or, equivalently, J / ( m 2 K s ) {\displaystyle {\rm {J}}/({\rm {m^{2}K}}{\sqrt {\rm {s}}})} . Thermal effusivity can also be a measure of a solid or rigid material's thermal inertia. Thermal effusivity is a parameter that emerges upon applying solutions of the heat equation to heat flow through a thin surface-like region. It becomes particularly useful when the region is selected adjacent to a material's actual surface. Knowing the effusivity and equilibrium temperature of each of two material bodies then enables an estimate of their interface temperature T m {\displaystyle T_{m}} when placed into thermal contact. If T 1 {\displaystyle T_{1}} and T 2 {\displaystyle T_{2}} are the temperature of the two bodies, then upon contact, the temperature of the contact interface (assumed to be a smooth surface) becomes T m = e 1 T 1 + e 2 T 2 e 1 + e 2 {\displaystyle T_{m}={\frac {e_{1}T_{1}+e_{2}T_{2}}{e_{1}+e_{2}}}} Specialty sensors have also been developed based on this relationship to measure effusivity. Thermal effusivity and...