Experimental and numerical investigations of thermal transport mechanisms in porous materials for thermal protection systems

Abstract

Thermal protection systems (TPS) are essential for atmospheric reentry vehicles. They protect the vehicles from the extreme heat generated by its propulsion systems and the high aerodynamic drag encountered during planetary entry. Porous materials, among others, have been extensively used to build TPS due to their advantageous characteristics such as low thermal conductivity, lightweight design, high reliability, and cost-effectiveness. Central to the design and application of porous TPS materials is a comprehensive understanding of the underlying heat transfer mechanisms within them, as well as our ability to accurately model and predict their thermal transport properties. Heat transfer through a porous medium includes combined contributions from conduction along solid matrix, radiation across voids (pores), and conduction through gases filling the voids. Although this qualitative picture of the heat transfer mechanisms is well accepted, a quantitative understanding of the contribution of each individual mechanism is still lacking, primarily due to the complex morphology of the porous materials and the intrinsically coupling nature of different mechanisms across scales. To that end, this study aims to experimentally quantify thermal transport mechanisms in porous materials by creatively isolating individual mechanisms. Model porous media were fabricated from silicon and glass wafers employing microfabrication techniques, which were then tested with a customized thermal test apparatus. The results indicate that solid conduction dominates at lower temperatures, reaching a peak contribution of 70% at a hot side temperature of 101.1°C. In contrast, at higher temperatures, gas conduction and radiation become more prominent, with their peak contributions rising to 72% and 10.3%, respectively, at a hot side temperature of 295°C. Moreover, the experimental results are validated by numerical simulation using StarCCM+.