Emanuela C. A. Gallo

Dual-Pump Coherent Anti-Stokes Raman Spectroscopy Measurements in a Dual-Mode Scramjet

In this paper, the authors describe dual-pump coherent anti-Stokes Raman spectroscopy (CARS) measurements of mixing and combustion in a direct-connect scramjet combustor operating at equivalent flight Mach typical of the ramjet–scramjet transition, in the scram mode. Measurements were performed in the University of Virginia’s scramjet test facility in which the air is heated by electrical resistance heaters. The CARS technique is used to acquire temporally and spatially resolved measurements of temperature and species mole fraction. Measurements were at four planes: one upstream of an H2 fuel injector and three downstream. Contour plots of mean flow and standard deviation statistics are presented for cases with and without reaction of the fuel. The vibrational temperature at the exit of the M=2 facility nozzle, and in the freestream of the scramjet combustor, is elevated compared with the rotational temperature; the air–N2 vibrational temperature is the same as the facility stagnation temperature. There a...

Dual-Pump CARS Measurements in the University of Virginia's Dual-Mode Scramjet: Configuration "C"

Measurements have been conducted at the University of Virginia Supersonic Combustion Facility in configuration C of the dual-mode scramjet. This is a continuation of previously published works on configuration A. The scramjet is hydrogen fueled and operated at two equivalence ratios, one representative of the scram mode and the other of the ram mode. Dual-pump CARS was used to acquire the mole fractions of the major species as well as the rotational and vibrational temperatures of N2. Developments in methods and uncertainties in fitting CARS spectra for vibrational temperature are discussed. Mean quantities and the standard deviation of the turbulent fluctuations at multiple planes in the flow path are presented. In the scram case the combustion of fuel is completed before the end of the measurement domain, while for the ram case the measurement domain extends into the region where the flow is accelerating and combustion is almost completed. Higher vibrational than rotational temperature is observed in those parts of the hot combustion plume where there is substantial H2 (and hence chemical reaction) present.

OH PLIF Visualization of the UVa Supersonic Combustion Experiment: Configuration A

Hydroxyl radical (OH) planar laser-induced fluorescence (PLIF) measurements were performed in the University of Virginia supersonic combustion experiment. The test section was set up in configuration A, which includes a Mach 2 nozzle, combustor, and extender section. Hydrogen fuel was injected through an unswept compression ramp at two different equivalence ratios. Through the translation of the optical system and the use of two separate camera views, the entire optically accessible range of the combustor was imaged. Single-shot, average, and standard deviation images of the OH PLIF signal are presented at several streamwise locations. The results show the development of a highly turbulent flame structure and provide an experimental database to be used for numerical model assessment.

OH PLIF Visualization of a Premixed Ethylene-fueled Dual-Mode Scramjet Combustor

Hydroxyl radical (OH) planar induced laser fluorescence (PLIF) measurements have been performed in a small-scale scramjet combustor at the University of Virginia Aerospace Research Laboratory at nominal simulated Mach 5 enthalpy. OH lines were carefully chosen to have fluorescent signal that is independent of pressure and temperature but linear with mole fraction. The OH PLIF signal was imaged in planes orthogonal to and parallel to the freestream flow at different equivalence ratios. Flameout limits were tested and identified. Instantaneous planar images were recorded and analyzed to compare the results with width increased dual-pump enhanced coherent anti-Stokes Raman spectroscopy (WIDECARS) measurements in the same facility and large eddy simulation/Reynolds average Navier-Stokes (LES/RANS) numerical simulation. The flame angle was found to be approximately 10 degrees for several different conditions, which is in agreement with numerical predictions and measurements using WIDECARS. Finally, a comparison between NO PLIF non-combustion cases and OH PLIF combustion cases is provided: the comparison reveals that the dominant effect of flame propagation is freestream turbulence rather than heat release and concentration gradients.

Visualization of Simulated Fuel–Air Mixing in a Dual-Mode Scramjet

Nitric oxide planar laser-induced fluorescence measurements have been performed in a small-scale scramjet combustor at the University of Virginia Aerospace Research Laboratory at nominal simulated Mach 5 flight. A mixture of nitric oxide and nitrogen was injected at the upstream end of the inlet isolator as a surrogate for ethylene fuel, and the mixing of this fuel simulant was studied with and without a shock train. The shock train was produced by an air throttle, which simulated the blockage effects of combustion downstream of the cavity flameholder. Nitric oxide planar laser-induced fluorescence signal was imaged in a plane orthogonal to the freestream at the leading edge of the cavity. Instantaneous planar images were recorded and analyzed to identify the most uniform cases, which were achieved by varying the location of the fuel injection and shock train. This method was used to screen different possible fueling configurations to provide optimized test conditions for follow-on combustion measurements...

OH PLIF Visualization of the UVa Supersonic Combustion Experiment: Configuration C

Non-intrusive hydroxyl radical (OH) planar laser-induced fluorescence (PLIF) measurements were obtained in configuration C of the University of Virginia supersonic combustion experiment. The combustion of hydrogen fuel injected through an unswept compression ramp into a supersonic cross-flow was imaged over a range of streamwise positions. Images were corrected for optical distortion, variations in the laser sheet profile, and different camera views. Results indicate an effect of fuel equivalence ratio on combustion zone shape and local turbulence length scale. The streamwise location of the reaction zone relative to the fuel injector was also found to be sensitive to the fuel equivalence ratio. The flow boundary conditions in the combustor section, which are sensitive to the fuel flow rate, are believed to have caused this effect. A combination of laser absorption and radiative trapping effects are proposed to have caused asymmetry observed in the images. The results complement previously published OH PLIF data obtained for configuration A along with other non-intrusive measurements to form a database for computational fluid dynamics (CFD) model validation.

Development of a Premixed Combustion Capability for Dual-Mode Scramjet Experiments

Hypersonic airbreathing engines rely on scramjet combustion processes, which involve high-speed, compressible, and highly turbulent reacting flows. The combustion environment and the turbulent flam...

Nitric Oxide PLIF Visualization of Simulated Fuel-Air Mixing in a Dual-Mode Scramjet

Nitric oxide (NO) planar induced laser fluorescence (PLIF) measurements have been performed in a small scale scramjet combustor at the University of Virginia Aerospace Research Laboratory at nominal simulated Mach 5 flight. A mixture of NO and N2 was injected at the upstream end of the inlet isolator as a surrogate for ethylene fuel, and the mixing of this fuel simulant was studied with and without a shock train. The shock train was produced by an air throttle, which simulated the blockage effects of combustion downstream of the cavity flame holder. NO PLIF signal was imaged in a plane orthogonal to the freestream at the leading edge of the cavity. Instantaneous planar images were recorded and analyzed to identify the most uniform cases, which were achieved by varying the location of the fuel injection and shock train. This method was used to screen different possible fueling configurations to provide optimized test conditions for follow-on combustion measurements using ethylene fuel. A theoretical study of the selected NO rotational transitions was performed to obtain a LIF signal that is linear with NO mole fraction and approximately independent of pressure and temperature.

WIDECARS Measurements of Major Species Concentration and Temperature in an Air-Ethylene Flame

Width increased dual-pump enhanced coherent anti-Stokes Raman spectroscopy (WIDECARS) measurements were conducted in a McKenna air-ethylene premixed burner, at NASA Langley Research Center, at nominal equivalence ratio range between 0.55 and 2.50 to provide quantitative measurements of six major combustion species (C2H4, N2, O2, H2, CO, CO2) concentration and temperature simultaneously. The purpose of this test was to investigate the uncertainties in the experimental and spectral modeling methods in preparation for an upcoming scramjet C2H4/air combustion test at the University of VirginiaAerospace Research Laboratory. A broadband Pyrromethene (PM) PM597 and PM650 dye laser mixture was optimized to excite the Raman shift of all the target species. Two hundred single shot recorded spectra were processed, theoretically fitted and then compared to computational models, to verify where chemical equilibrium or adiabatic condition occurred, providing experimental flame location and formation, species concentrations, temperature, and heat losses inputs to computational kinetic models. The Stark effect, temperature, and concentration errors are discussed.

Image Analysis of Hydroxyl-Radical Planar Laser-Induced Fluorescence in Turbulent Supersonic Combustion

Flow visualization and statistical analysis of a supersonic, turbulent reacting flow from nonintrusive hydroxyl radical planar laser-induced fluorescence images are presented. The OH-PLIF images show instantaneous structures of turbulence in two combustor configurations at different fuel-equivalence ratios. Proper orthogonal decomposition and autocorrelations are performed on the OH-PLIF data to extract quantitative information about turbulent fluctuations and length-scale correlations. Changes in correlation length scales, representative of the integral length scales of the flow, and turbulent energy flow patterns are observed as a function of position in the combustor and fuel-equivalence ratio. Correlation length scales were found to increase with streamwise distance for all configurations. Dual-mode operation with supersonic and subsonic combustion is demonstrated in the facility. Correlation length scales and the growth rate of these length scales were found to be smaller in the subsonic combustion mode.