HIGH TEMPERATURE THERMAL CONDUCTIVITY MEASUREMENT APPARATUS

Abstract

Reliable dimensioning of highly efficient thermal protection for entry or re-entry applications depends on an accurate knowledge of the thermal conductivity of the considered materials. The fibre and foam insulations, as well as ablator char, used in such thermal protection systems are porous materials, where the thermal conductivity varies with both, applied temperature and ambient pressure. Precise thermal conductivity measurements in the lower temperature range and for isotropic insulations are state-of-the art. The transfer of well-established lower temperature measurement princi-ples to very high temperatures and to non-isotropic insulations is a challenge. The German aerospace center (DLR) owns a facility of adequate size which enables thermal conductivity measurements at high temperatures up to 1600°C and for controllable ambient pressures between 1 mbar and ambient pressure. This facility has been used in previous projects, however, the quality of steady state thermal conductivity measurements was not satisfactorily in particular for reduced pressure levels. This is caused by uncontrolled lateral heat exchanges at the test specimen surface. Control of these lateral heat exchanges is therefore the key for improvement of measurement accuracy. With this in mind, a test setup for measurements under varying temperatures up to 1600°C and for pressures between 1 mbar and ambient pressure has been developed as part of ESA’s Innovation Triangle Initiative. This test setup is not only applicable to today’s most advanced insulations, i.e. to nanoporous insulations and to anisotropic insulation sys-tems like internal multiscreen insulation (IMI), but also to the direct measurement of ablator char thermal conductivity. At first, the measurement of thermal conductivity and the therefore developed test setup HitCon are described. Next, the results from a measurement campaign, conducted to verify the functionality of the design, are shown, followed by sen-sitivity analysis for further improvement of the test setup.