Gas/Surface Interaction Study of Low-Density Ablators in Sub- and Supersonic Plasmas
Speaker: Bernd Helber, von Karman Institute for Fluid Dynamics
Presentation
Abstract
Selection and sizing of the thermal protection material (TPM) are the two key performance parameters in TPS design, and prediction inaccuracies can be fatal for the crew or the success of robotic missions. Ground testing in plasma wind-tunnels becomes a fundamental requirement for validation of material response codes, TPM qualification, and TPS design. Although present wind-tunnel technology does not allow a complete simulation of the hypersonic flight, specific phenomena can be studied separately in a narrowed environment, where dominant parameters, such as heat flux, pressure and the resulting boundary layer chemistry are close to the reentry environment.
Ablation experiments have been carried out with the carbon-phenolic material AQ61 and a non-pyrolyzing carbon fiber preform in sub- and supersonic air and nitrogen plasmas in the VKI Plasmatron facility. We performed an in-situ recession analysis, including volumetric ablation, and observed the temperature and radiance of the surface for emissivity estimations, as well as spatial molecular radiation profiles in the boundary layer, which were compared to a numerical approach. Surface temperatures as low as 1600K allowed observation of the temperature distribution over the surface using an infrared camera. The carbon preform test samples were varied in shape as well as regarding their fiber direction. Sublimation of the surface was reached in supersonic plasma flow at a cold wall heat flux of 9.5MW/m2. Carbon preform emissivities were found to be in the order of 0.86 - 0.97. A stagnation line description with an ablation boundary condition helped to reproduce experimentally measured boundary layer emission profiles of the CN violet molecule. In spite of the very simplified model, emission intensities were in the same order of magnitude compared to the experimental data. The model was able to reconstruct the location of the maximum emission for two cases.
Biography
Bernd Helber studied Aerospace Engineering at the University of Stuttgart and graduated in 2009 after a final year project at the Centre for Hypersonics in Brisbane, Australia. Following the one-year research master program at the von Karman Institute for Fluid Dynamics, he started his PhD at the VKI in 2011 on material response characterization of ablators in the Plasmatron facility. Since then, he is also affiliated with the Research Group Electrochemical and Surface Engineering at the Vrije Universiteit Brussels.