Reducing Thermal Contact Resistance for Rigid Insulation Thermal Measurements

Abstract

H EAT transfer through fibrous insulation has been the subject of great interest in the aerospace community because of the use of fibrous insulation in thermal protection systems. Measurement and modeling of heat transfer in rigid and flexible fibrous insulation has been the subject of various studies [1–6]. Heat transfer through a high-porosity rigid insulation is composed of combined radiation and conduction heat transfer. Conduction consists of both solid and gaseous conduction. Solid conduction is a significant component of heat transfer in rigid insulations, with its relative significance increasing with increasing insulation density, but generally decreasing with increasing temperature. Radiation’s significance increases with increasing temperature and decreases with increasing insulation density. Gas conduction is negligible in a vacuum and increases with increasing static pressure and temperature. A simple measurement and modeling technique for heat transfer in a rigid fibrous insulation was investigated. As part of this study, the thermal contact resistance for thermal measurements on rigid fibrous insulation was also investigated. For flexible fibrous insulation, the flexibility of the insulation and negligible solid-conduction mode of heat transfer in the insulation result in negligible thermal contact resistance at the boundaries of test sample [7]. For rigid insulation, the solid-conduction mode of heat transfer is significant and obtaining perfect flatness on sample and test-setup surfaces is not easily achievable. Even a sample with a flat surface may bow from thermal deformations due to temperature gradients across the sample thickness during testing; therefore, thermal contact resistance at the boundaries of the test sample may be significant. For the thermal test setup used in the present study with large temperature differences maintained across the sample thickness, thermal contact resistance on the sample hot side is ignored because on the sample hot side the main mode of heat transfer is radiation. Conversely, on the sample cold side, the main mode of heat transfer is solid conduction; therefore, thermal contact resistance can be significant, which manifests itself as radiation and gas conduction in the void spaces between the bottom of the test sample and the top of the test-setup cold-side surface. Therefore, one objective of the present study is to investigate whether thermal contact resistance is significant for thermal measurements on rigid-insulation samples and, if so, to investigate a technique for eliminating it.