Thermal wave reflections of a pulsed stripe heat source from a plane boundary

Abstract

An experimental and theoretical study is carried out of thermal wave reflections from a plane boundary, such as the back wall of a solid slab, following pulsed heating of the front surface of the slab. The heating pattern is a long, uniform stripe source of finite width. Imaging is carried out using a high spatial resolution, high frame rate focal plane array infrared camera to monitor the surface temperature, through its emission of IR radiation in the 3–5 μm spectral band. We consider the spatial temperature distribution as a function of time, not only on the back surface of the slab, but also on the front surface. The theory is based on the assumption of thermal wave reflections from the two insulating (solid/air) boundaries of the slab, and its predictions are in excellent agreement with experimental data for both surfaces of a pure isotropic slab (Cu) and a highly anisotropic slab (uniaxial carbon-fiber-reinforced polymer composite). The unheated region of the front surface shows two peaks as a function of time — one resulting from the direct propagation of the thermal pulse along the surface from the edge of the source to the observation point, the other resulting from the highly damped reflections from the back surface of the slab. Because of the fact that the first peak depends only on the in-plane diffusivity, and the second peak is strongly dependent on the through-slab diffusivity, the technique can be used to make single-sided measurements of all three principal components of the thermal diffusivity.