Development of a tunable diode laser probe for measurements in hypervelocity flows.

摘要

This thesis describes the development and demonstration of tunable-diode laser absorption spectroscopy (TDLAS) probes for ground-based hypervelocity flowfield measurements. Flight testing of hypervelocity systems exacts high risks and costs, thereby requiring ground-based testing in facilities such as reflected shock tunnels and shock/expansion tunnels. Diagnostics deployed in ground-based facilities measure pertinent performance parameters for flight-system evaluation. Therefore, an accurate assessment of the freestream thermodynamic-state is required for worthwhile data interpretation and CFD model validation.; The tunable diode-laser absorption technique's quantum and species-specific nature leads to direct flowfield measurements of velocity, temperature and target-species concentration. The probes are fiber-coupled to spectrally tuning lasers (at kilohertz rates) across tracer-absorption transitions. The probe pitches and catches laser beams that non-intrusively interrogate the flowfield. Analysis of the recorded laser-transmitted intensities from each scan yields parameter time-histories.; During gas-driven reflected shock tunnel operation, driver gas permeates the test gas. When testing in air with a hydrogen driver, the resultant water is exploited as a spectroscopic target. Temperature, concentration, and velocity are obtained from water absorption (near 1.4 mum). The measurements agreed well with the computationally predicted values of velocity and temperature (4500 m/s and 600 K). However, later in each test an increase in water vapor concentration correlated with a velocity increase and a varying temperature. In a separate study, a miniaturized probe was developed to measure velocity while targeting naturally present chemically-frozen, freestream potassium (near 0.77 mum). Velocity measurements were consistent with facility models and simultaneous water-vapor measurements.; Another miniature probe was developed to measure velocity at 30.3 kHz rates in shock/expansion tubes with water-seeded nitrogen. The average of the velocity measurements agreed well with traditional time-of-flight measurements. However, an initial unsteadiness, believed to be caused by facility boundary layer growth and/or the presence of a reflected shock, was observed.; This work successfully demonstrates that TDLAS probes are capable of making repeatable measurements in hypervelocity flowfields. Their value is highlighted by their ability to directly measure the flow composition time-history and thermodynamic state in high-enthalpy, ground-based flow facilities.