Analysis of Thin Film and Slug Calorimeters for Hypersonic Applications

Thin-film and slug calorimeters are essential for accurate heat flux measurements in hypersonic environments. While thin films are comparatively fragile and well suited for measuring high-frequency phenomena, slug calorimeters are rugged and appropriate for measuring large integral heat loads. A primary challenge in designing thin-film sensors lies in selecting an appropriate film thickness and accounting for heat loss from the film into the substrate. For slug calorimeters, heat loss into the surrounding insulator and through the back face introduces measurement inaccuracies. Simulations of the unsteady heat diffusion inside thin-film gauges and a novel bi-metal slug calorimeter were carried out. For the former, a laser heat source was assumed. For the latter, the heat flux was obtained from a thermo-chemical non-equilibrium simulation of Mach 18 air flow over a hemisphere. The frequency response of the thin-film to a box function and square wave heat flux input is inversely related to the film thickness. Minimal lateral contact of the thin film with the insulator can have a substantial impact on the frequency response. The slug calorimeter results reveal a 20% percent variation of the heat flux prediction depending on the wall catalycity. Inverse theory estimates of the heat flux from temperature measurements inside the slug are in good agreement with the known input data.