Optical Measurements of Particle Size and Trajectory in an Arc Jet

Arc-heated high-enthalpy test facilities at NASA are used for evaluation of reentry vehicle thermal protection system (TPS) materials. The use of newer ablative TPS materials in the high-enthalpy flow result in complex processes such as decomposition of products, surface ablation, and spallation. Optical measurement techniques that can size and track particles in the flow would be of value for validation of facility operation and refinement of numerical models. Unfortunately, optical measurements of particles in an arc jet are challenging due to the strong emission from the test article and reacting flow which can saturate the camera. ISSI proposes employing an optical measurement technique known as Particle Shadow Velocimetry (PSV) that has been used to measure particle size, velocity, and acceleration a solid rocket motor at rates of over 10-kHz. PSV is accomplished by projecting light from an LED through the measurement volume onto a camera which allows the shadows created by the particles to be recorded. The individual particle shadow can be sized or tracked in time to compute particle velocity, acceleration, or trajectory. Utilizing an LED-based PSV system for measurements of the size and trajectory of spallation particles in an arc jet offers several unique advantages. First, the combination of a relatively narrow wavelength LED and a matching narrow-band filter on the imaging system can be used to suppress the broadband emission from the flow. This allows acquisition of distinct particle images, even in the presence of strong emission from the reacting flow. Second, the in-line illumination that is required for PSV measurements allows particles to be imaged very close to surfaces, within 50-mm of the ablating TPS surface. PSV is a high TRL tool that has already been demonstrated for acquiring particle size and trajectory measurements in a high enthalpy flow. This proposal offers a low-risk opportunity to acquire similar data on test articles in an arc jet.