The application of coherent anti-Stokes Raman scattering to temperature measurements in a pulsed high enthalpy supersonic flow

Abstract

Abstract. Broadband single pulse coherent anti-Stokes Raman scattering (CARS) experiments employing a folded box phase matching geometry in a shock tunnel flow are presented. Rovibrational spectra of molecular nitrogen, produced at the exit of a pulsed supersonic nozzle for a range of flow enthalpies, are examined. Difficulties peculiar to the application of the optical technique to a high enthalpy pulsed flow facility are discussed and measurements of flow temperatures are presented. Theoretically calculated values for temperatures based upon algorithms used to determine shock tunnel flow conditions agree well with experimental measurements using the CARS technique.

Coherent anti-Stokes Raman scattering and spontaneous Raman scattering diagnostics of nonequilibrium plasmas and flows

“…Grisch et al [56] used narrowband dualline N 2 CARS in folded BoxCARS geometry to measure 2D distributions of N 2 rotational temperature and number density in Mach 10 nitrogen flow, in a shock wave-boundary layer interaction region behind compression corner. Pulford et al [57] and Boyce et al [58] used single-shot broadband N 2 CARS spectra, also in folded BoxCARS geometry, for measurements of N 2 rotational and vibrational temperatures in a pulsed free piston supersonic shock tunnel flow facility, both in freestream and in a bow shock layer in front of a blunt body. In these experiments, significant vibrational nonequilibrium was detected in freestream, T rot = 850 ± 100 K, T vib(0,1) = 1985 ± 200 K, while the flow in the shock layer was near equilibrium, T rot = 3730 ± 400 K, T vib(0,1) = 4000 ± 200 K [58].…”

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

“…Boyce et al [10] measured vibrational/ rotational temperatures in the freestream and spherical shock layer at pressures below 3 bar and attributed accuracy and precision of single-pulse CARS up to 10% to be limited by pulse-to-pulse amplitude and phase fluctuations of the dye laser profile's mode structure. Pulford et al [12] measured shock tunnel freestream temperature of up to 3000 K at a scramjet model inlet at pressures around 1 bar with a singleshot precision up to 11.4%. According to the authors' best knowledge, broadband CARS has been successfully applied at the overall highest pressure of 6.5 MPa so far: Grisch et al [13] measured temperatures up to 2000 K, reporting pressure line broadening to become increasingly difficult to capture/ model at simultaneously high pressure and temperature.…”

“…As for spontaneous Raman scattering, a principle strength of LIF is its potential to acquire two-dimensional (2-D) spatial distributions of temperature and species concentration which is yet not deemed to provide much additional information in shock tube flows of uniform centerline development. Quantitative analysis of spectrally resolved LIF signals is significantly affected by radiative decay rates of excited states due to predissociation, photoionization, and collisional quenching which are difficult to account for-the latter being particularly relevant to shock tube flow due to high pressure and density [12,14]. Furthermore, rotational and vibrational energy transfer processes are to be modelled by adequate rate models [11].…”

“…where T denotes the temperature. Coherent signal beam nature is most beneficial to yield high detection efficiency even in the harsh high-enthalpy environment of shock tunnels, further aggravated by spurious extrinsic light interference from background radiation and metallic impurities encountered [12].…”