Abstract
Thermal protection systems (TPS) are crucial to atmospheric reentry vehicles to shield the intense heat generated by propulsion systems as they enter the atmosphere of planets. 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. 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 highlight the dominant contribution of gas conduction to the overall heat transfer in the entire test temperature range. However, as the temperature increases, the contribution from radiation has been observed to increase.
