Recession measurements of cork samples in particle-laden arc-heated wind tunnel

Kurzfassung

A renewed interest towards modelling and characterization of two-phase high-enthalpy flows arose in the frame of past and future missions to Mars, such as ExoMars 2016 and 2028. On Mars, seasonal dust storms can happen that spread dust all over the planet at altitudes of 80 km. Martian landers need to be provided robustness to Martian weather, and they must be able to enter the atmosphere even during dust storms. Atmospheric entry is a severe condition for landers, where the hypersonic speed is converted in thermal energy through friction and compression of the atmosphere: heat shields are needed to protect the spacecraft from the severe heat loads. What can be the effect of particles in such a challenging regime? A collaboration between NASA Ames and DLR was established to increase the modelling accuracy of such condition, by validation with experiments in supersonic wind tunnels and arc-heated wind tunnels. This renewed scientific interest towards particle-laden flow modelling requires the capability of creating particle-laden flows in high enthalpy facilities, to gain a direct insight into the uncertain behavior of particles in such conditions, and to provide experimental validation data. A high-enthalpy particle-laden flow was created in the L2K facility, and Particle Image Velocimetry (PIV) experimental technique was performed – for the first time in a wind tunnel of this kind – with the aim of characterizing particles’ velocity field around several stagnation samples made of P50 cork (from Amorim Cork Composites). Particles velocity was measured in the free stream right upstream the sample, and in the shock layer. Velocity ranges from 0 m/s to approximately 2100 m/s. The phenomenon of particle-induced erosion on 10 cork samples is observed through IR thermography and recession measurements. A picture of a test in the L2K facility is presented in Figure 1. The Supersonic and Hypersonic Technologies Department at the German Aerospace Center (DLR) in Cologne operates the LBK test facility, composed by two arc-heated, high-enthalpy wind tunnels – named L2K and L3K – capable of simulating the heat loads encountered during atmospheric entry. The LBK is one of the European key testing facilities for Thermal Protection Systems (TPS) performances, and for components’ demisability behavior, as it can reproduce high heat flux for long testing times. The experimental characterization of materials’ and components’ during atmospheric entry requires partially significant changes to the nominal operation modes of long-duration high-enthalpy facilities. Therefore, the LBK facility was upgraded in terms of operation and measurement techniques, aimed towards flow characterization, and towards the study of TPS behaviour in particle-laden high-enthalpy flows. Two flow conditions with different enthalpy are studied, and the phenomenon of particle agglomeration due to melting (agglo-melting) is observed in one of the two. To better understand this phenomenon, particle samples are collected and observed with a scanning electron microscope. This study highlights the complexity of the physical environment surrounding the spacecrafts entering a dust-laden atmosphere, and raises the awareness for numerical modellers to consider the agglo-melting phenomenon. This study also brings a discussion about the experimental challenges that must be faced to provide moreand-more accurate and controlled testing environments, aimed at the development of efficient and reliable spacecrafts, satellites, and interplanetary systems.