Thermal impedance

Starting from a brief review on the concepts of electrical resistance and impedance and the definition of thermal resistance, it is shown how thermal impedance can be defined from a simple situation of an ideal semi-infinite solid sample subjected to uniform sinusoidal heating at its surface. Using the thermal impedance definition, the non-stationary component of the temperature field perpendicular to the surface of the sample can be rewritten in a more compact form than as usual. It is shown that a dimensionless number dependent on thermal impedance can be defined for non-stationary phenomena to compare the efficiency between the different modes of heat transfer. The thermal impedance concept for the little-known for most people (but not least important) case of non-Fourier heat conduction is also discussed here. We show that the thermal impedance for diffusive Fourier heat transfer is a complex valued parameter with the same real and imaginary parts. However, this behavior is affected when approaching non-Fourier conditions, under which the imaginary component can become a pure imaginary parameter. These topics are not treated in heat transfer and physics textbooks. However, the subject can be of interest not only for students and teachers at undergraduate and graduate levels but also for scientists dealing with heat transfer in the presence of time-varying periodical heat sources.