Abstract
Experimental determination of test gas caloric quantities in high-enthalpy ground testing is impeded by excessive pressure and temperature levels as well as minimum test timescales of short-duration facilities. Yet, accurate knowledge of test gas conditions and stagnation enthalpy prior to nozzle expansion is crucial for a valid comparison of experimental data with numerical results. To contribute to a more accurate quantification of nozzle inlet conditions, an experimental study on non-intrusive in situ measurements of the post-reflected shock wave stagnation temperature in a large-scale free-piston reflected shock tunnel is carried out. A series of 20 single-shot temperature measurements by resonant homodyne laser-induced grating spectroscopy (LIGS) is presented for three low-/medium-enthalpy conditions (1.2–2.1 MJ/kg) at stagnation temperatures 1100–1900 K behind the reflected shock wave. Prior limiting factors resulting from impulse facility recoil and restricted optical access to the high-pressure nozzle reservoir are solved, and advancement of the optical set-up is detailed. Measurements in air agree with theoretical calculations to within 1–15%, by trend reflecting greater temperatures than full thermo-chemical equilibrium and lesser temperatures than predicted by ideal gas shock jump relations. For stagnation pressures in the range 9–22 MPa, limited influence due to finite-rate vibrational excitation is conceivable. LIGS is demonstrated to facilitate in situ measurements of stagnation temperature within full-range ground test facilities by superior robustness under high-pressure conditions and to be a useful complement of established optical diagnostics for hypersonic flows.
Highlights
Evaluation of experimental test data in high-enthalpy and hypersonic ground testing requires complementary flowfield computations in order to obtain flow quantities of interest that are not available to direct measurement [1]
By varying the shock tube (ST) pressure of 0.5 MPa, 0.25 MPa, and 0.15 MPa (C1, C2, and C3), the condition captured for run 1 (C1) is seen to be strongly undertailored, reflecting a post-shock pressure p5(exp) that steadily decreases due to the expansion fan originating from contact surface interaction with the endwall-reflected shock wave
Whereas a priori filtering of the raw signal before signal analysis can be advantageous for discrete fast Fourier transform (DFT) frequency analysis, the time-domain nonlinear least-squares curve fitting procedure is found to be very robust against background noise and to be entirely unaffected by signal filtering
Summary
Evaluation of experimental test data in high-enthalpy and hypersonic ground testing requires complementary flowfield computations in order to obtain flow quantities of interest that are not available to direct measurement [1]. For this purpose, the accurate definition of boundary conditions is vital. Total flow enthalpy is determined by quasi one-dimensional (1-D) numerical codes such as L1d, ESTCj [3], or Kasimir [4] based on incident shock Mach number and assuming normal shock relations [5] as well as quasi-steady conditions thereafter, as long as test conditions are properly tailored. The time interval of constant post-shock temperature (and species concentration) can be smaller than quasi-steady pressure duration, i.e., test time, even in tailored interface mode due to premature driver gas
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