Methodology for Reliable Emissivity Measurements at High Temperatures to Support NASA Free-Flight Experiments

At high temperatures, surface radiation becomes an important, often even dominating mechanism for the thermal household of the body under consideration. In addition, the requirement of remote temperature measurements through pyrometry arises if temperatures exceed the application range of contact thermometers or if access to the object is impossible (for example objects in a closed environment, in motion, or small or fragile structures). Both for the heat balance and pyrometry, accurate knowledge of spectral directional surface emissivity is of significant importance to increase accuracy and decrease margins. The emissivity depends on the material itself and may change with the surface condition, its temperature, and the direction into which the radiation is emitted. In addition, it may show significant changes with wavelength. The intent of this work is to establish a reliable methodology to measure spectral directional emissivity at elevated temperatures up to 1500K which was pre-developed in the frame of a master's thesis. The Graduate Fellowship will provide the student to finalize his work and tie it to current NASA projects and requirements, offering an opportunity for fundamental research, usually not covered through agency activities, with the benefits of flexibility and low cost research opportunities in an academic environment. Once successfully demonstrated, this methodology is anticipated to provide excellent education opportunities for future graduate and undergraduate students in the engineering and aerospace field, tied into the educational and research interests of the college. The measurement of spectral directional surface emissivity will be accomplished by using an appropriately designed test specimen in an existing heating facility through a comparison of blackbody radiation to the emission of the surface of concern. This methodology will be applied to samples used in free-flight experiments in the Ballistic Range Facility at NASA Ames Research Center. Here, the transition from laminar to turbulent flow and the resulting heat flux augmentation are investigated with particular respect to isolated and distributed surface roughness. Transition to turbulence is a key parameter to predict surface heating during hypersonic flight and to interpret experimental data such as temperature distributions on a heat shield during atmospheric entry e.g. during the entry of NASA's Mars Science Laboratory (MSL) in 2012. In the ballistic range, temperature distributions on models accelerated to high velocities are measured at different axial locations during the free flight phase in the facility through infra-red imaging. From these temperature distributions, the surface heat flux is inferred which shows a significant increase in the turbulent heating regime. The temperature dependent surface emissivity was identified as major error source in these measurements. Within the proposed project, spectral directional emissivity of the stainless steel models will be measured at different temperatures within the relevant range up to 1200K for a selection of surface conditions and provide to NASA. Beyond the described measurements to support the Ballistic Range measurements, the application to other research and industrial processes seems a promising application wherever contact-less temperature measurements are performed or radiative heat exchange is important. Heat shield materials such as TUFROC rely on radiative cooling only and are candidate materials for future commercial thermal protection systems of service vehicles for the International Space Station (ISS). A typical industrial application is given by temperature measurements of electrodes made of tungsten. Through a successful demonstration, the proposed activity will establish the UK Radiation Sciences Lab as a qualified source for emissivity data at high temperatures, which in anticipated to trigger future industrial collaborations to build infrastructures outside the SpaceGrant domain.