In situ nozzle reservoir thermometry by laser-induced grating spectroscopy in the HELM free-piston reflected shock tunnel

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.

Simultaneous thermometry and velocimetry for a shock tunnel using homodyne and heterodyne detection

“…between 1.2 and 2.1 MJ/kg for an evaluable signal-to-noise ratio [28]. Only under moderate conditions, such as those present in the cases studied here, seeding of the test gas may be necessary to achieve a high signal-to-noise ratio (SNR), while under higher pressures and temperatures formation of a sufficient amount of NO 2 is expected.…”

“…For non-reacting gases the temperature T then can be calculated using the expression: with the isentropic exponent and the specific gas constant R s . At our institute a homodyne LITGS measurement setup was developed [23] and its operability was proven in a conventional shock tube and at HELM [3,[24][25][26][27][28].…”