Particle-Laden Flow Characterization in High Enthalpy Ground Testing Facilities

Kurzfassung

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 studies: it is indeed of main interest to characterize wind tunnels flows, for multiple purposes, such as to get more information about the flow field, to obtain meaningful results from test campaigns, and to gain knowledge of critical aspects about high temperature fluid-dynamics effects, since flow characterization provides meaningful interpretation tools to experimental results and flow phenomena. The experimental technique based on Tunable Diode Laser Absorption Spectroscopy (TDLAS) was recently used to determine the L2K’s flow core’s velocity, and a study demonstrated why it cannot be used to determine flow’s temperature – contrarily to what was theorized in the past – in wind tunnels similar to L2K. The study highlighted the power of non-intrusive techniques based on spectroscopy, which are capable of measuring flow quantities without the necessity of intrusive probes, providing a big leap in the possibility of understanding such complex environments. A renewed scientific interest towards particle-laden flow modelling for Martian atmospheric entry, also 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 this year 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’ average velocity field. The presentation is about the two mentioned recent advances in the topic of “High Enthalpy Flow Characterization”, and it brings a discussion about the experimental challenges that must be faced to provide more-and-more accurate and controlled testing environments, aimed at the development of efficient and reliable spacecrafts, satellites, and interplanetary systems.