James C. McDaniel

Experimental investigation of a supersonic swept ramp injector using laser-induced iodine fluorescence

Planar measurements of injectant mole fraction and temperature have been conducted in a nonreacting supersonic combustor configured with underexpanded injection in the base of a swept ramp. The temperature measurements were conducted with a Mach 2 test section inlet in streamwise planes perpendicular to the test section wall on which the ramp was mounted. Injection concentration measurements, conducted in cross flow planes with both Mach 2 and Mach 2.9 free stream conditions, dramatically illustrate the domination of the mixing process by streamwise vorticity generated by the ramp. These measurements, conducted using a nonintrusive optical technique (laser-induced iodine fluorescence), provide an accurate and extensive experimental data base for the validation of computation fluid dynamic codes for the calculation of highly three-dimensional supersonic combustor flow fields.

Experimental and Numerical Study of Swept Ramp Injection into a Supersonic Flowfield

Time-averaged measurements of pressure, temperature, velocity, and injectant mole fraction are presented using the planar laser-induced iodine fluorescence technique in the complex three-dimensional compressible flbwfield around a swept ramp fuel injector. Within the range of thermodynamic conditions present in the test case studied, the techniques accuracy is estimated to be 4% for pressure, temperature, and velocity and 3% for injectant mole fraction. Comparisons with numerical simulations using the SPARK three-dimensional NavierStokes computer code with an algebraic turbulence model are made at the centerplane of the flowfield as well as on three crossflow planes downstream of the injector. Calculations and measurements are in good agreement throughout the flowfield, with deviations on the order of 5%; however, in specific regions, such as in the base of the ramp, deviations are larger. A weak asymmetry in the incoming flowfield appears to be amplified by boundary-layer separation occurring when the ramp-generated shock reflects off the tunnel walls. Ramp-generated vortices are weaker in the calculated results due to the effects of numerical viscosity in the vortex cores. This leads to less turning and mixing of the jet plume than observed in the experiments. The rate of decay of the maximum injectant mole fraction with streamwise distance is greater for the present ramp injection scheme than for previously measured transverse injection schemes. In the recirculation region at the base of the injector, laminar calculations show better agreement with the measurements than turbulent calculations.

Experimental Study of a Dual-Mode Scramjet Isolator

A constant-area isolator was fabricated and tested in conjunction with a Mach 2 hydrogen-air combustor operating at a simulated Mach 5 flight enthalpy. Predicted isolator performance was validated through pressure measurements obtained via low-frequency pressure taps. The maximum pressure ratio measured in the combustor approached the design limit of 4.5. Scramjet operability, the range of equivalence ratios over which combustion was sustained without shock-inlet interaction, was improved to 0.06-0.32, as opposed to 0.32-0.37 without the isolator. For a given change in fuel equivalence ratio, the location of the shock train was easier to control with the isolator modification. Shock-train location repeatability was found to vary somewhat with equivalence ratio. Small fluctuations in the time-resolved pressure history indicated that the shock train was relatively temporally steady for a given equivalence ratio. High-frequency pressure measurements were within a 95% confidence interval of low-frequency pressure measurements. High-frequency results indicated that an increase in pressure and large pressure fluctuations occurred near the leading edge of the shock train. Power spectral analyses also indicated that there is significant variation in the frequency content of the pressure signal upstream and downstream of the shock-train leading edge. These results suggest that methods of shock-train leading-edge detection may be developed using pressure-time history characteristics other than the pressure magnitude.

Planar measurement technique for compressible flows using laser-induced iodine fluorescence

A laser-induced fluorescence technique for conducting planar measurements of temperature, pressure, and velocity in nonreacting, compressible flows has been developed, validated, and demonstrated. Planar fluorescence from iodine, seeded into air, was induced by an argon-ion laser and collected using a liquid-nitrogen cooled charge-coupled device camera. The temperature measurement, which has been described earlier, is used in conjunction with a sophisticated model of the fluorescence excitation spectrum to produce accurate pressure measurements. The demonstration velocity measurements represent the first planar velocity mapping using molecular seed in a highly three-dimensional supersonic flow of practical importance

Experimental and Numerical Study of a Dual-mode Scramjet Combustor

A Mach 2, hydrogen-air combustor with an unswept 10-deg ramp fuel injector was experimentally and numerically studied for a simulated flight Mach number near 5. Numerical modeling was performed using the Viscous Upwind Algorithm for Complex Flow Analysis code, and results were compared against experimental wall-pressure distributions, fuel plume images, and fuel plume velocity measurements. The model matched wall-pressure distributions well for the case of fuel-off and fuel-air mixing. For a fuel-air reacting case, pressure was matched well in the upstream third of the duct. Downstream, however, the pressure rise as a result of combustion was underpredicted. Based on the fuel plume imaging and velocity measurements, fuel plume shape was matched well for both the mixing and reacting cases. However, plume size, penetration, and centerplane axial growth were generally underpredicted by the model. The full extent of the velocity reduction caused by thermal choking was also not predicted. Despite these findings, the numerical model performed better than a previous model developed by the investigators. It was proposed that differences between the present numerical model and experiment stemmed from numerical underprediction of fuel-air turbulent mixing, and this resulted in underprediction of heat release.

Quantitative investigation of compressible mixing - Staged transverse injection into Mach 2 flow

Planar measurements of the injectant mole fraction distribution and the velocity field within a supersonic mixing flowfield have been made using laser-induced iodine fluorescence. The flowfield investigated in this work is staged transverse injection of air into a Mach 2 freestream. A complete three-dimensional survey of the injectant mole fraction distribution has been generated, and a single planar velocity measurement has been completed. The measurements reveal the dramatic effect of streamwise vortices on the mixing in the near field of the injectors, as well as the rapid mixing generated by staging two fuel injectors. Analysis of the downstream decay of the maximum injectant mole fraction in this and other supersonic mixing flowfields indicates that the relative rate of injectant mixing well downstream of the injectors is independent of injection geometry, freestream Mach number, and injectant molecular weight. Mixing within this region of the flowfield is dominated by small-scale turbulence within the injectant plume. The transition of the dominant mixing mechanism, from vortex-driven mixing in the near field to small-scale turbulent mixing in the far field, was found to occur in the region about 10 diameters downstream of the injectors.

Injectant mole-fraction imaging in compressible mixing flows using planar laser-induced iodine fluorescence

A technique is described for imaging the injectant mole-fraction distribution in nonreacting compressible mixing flow fields. Planar fluorescence from iodine, seeded into air, is induced by a broadband argon-ion laser and collected using an intensified charge-injection-device array camera. The technique eliminates the thermodynamic dependence of the iodine fluorescence in the compressible flow field by taking the ratio of two images collected with identical thermodynamic flow conditions but different iodine seeding conditions. The resulting images are, to our knowledge, the first quantitative planar measurements of mole-fraction distributions in a nonreacting compressible flow field and allow mixing to be studied directly.

Large-Eddy/Reynolds-Averaged Navier–Stokes Simulations of Reactive Flow in Dual-Mode Scramjet Combustor

Numerical simulations of the turbulent reactive flow within a model scramjet combustor configuration, experimentally mapped at the University of Virginia’s Scramjet Combustion Facility at an equivalence ratio of 0.17, are described in this paper. A hybrid large-eddy simulation/Reynolds-averaged Navier–Stokes method is used, with special attention focused on capturing facility-specific effects, such as asymmetric inflow temperature distributions, on flow development within the combustor. Predictions obtained using two nine-species hydrogen oxidation models are compared with experimental data obtained using coherent anti-Stokes Raman spectroscopy, hydroxyl radical planar laser-induced fluorescence, stereoscopic particle image velocimetry, and focusing schlieren techniques. The large-eddy simulation/Reynolds-averaged Navier–Stokes models accurately capture the mean structure of the fully developed flame but tend to overpredict fluctuation levels toward the outer edge of the reactive plume. Model predictions ...

Experimental Study of Test-Medium Vitiation Effects on Dual-Mode Scramjet Performance

An experimental study was performed to characterize the effects of vitiation due to combustion-air preheating on dual-mode scramjet combustion.Major combustion vitiation species (H2O andCO2)were added to the freestreamof an electrical-resistance-heated, direct-connect facility simulating Mach 5 flight enthalpy. With clean, dry air, the combustor operated in the supersonic mode at fuel equivalence ratios below 0.22, and in the subsonic mode for equivalence ratios above 0.26. Hysteresis was observed in the dual-mode transition region between 0.22 and 0.26, as the mode of combustion was dependent on whether the fuel rate was increasing or decreasing. Adding increasing amounts of water vapor and carbon dioxide to the freestream decreased combustor pressures by 10 to 30% for the same fuel equivalence ratio. Vitiation also caused transition between supersonic and subsonic combustion to occur at a higher fuel equivalence ratio thanwith clean air. This work represents the first direct evaluation of the effect of testmedium vitiation on dual-mode scramjet combustion atMach 5 enthalpy simulation in the same facility. The results indicate the importance of accounting for test-medium vitiation when extrapolating from ground-testing to flight, particularly in the dual-mode transition region between subsonic and supersonic combustion regimes.

Test gas vitiation effects in a dual-mode scramjet combustor

An experimental study was conducted to characterize the influence of combustion air preheater major vitiate species (H 2 O and CO 2 ) on scramjet combustion. These species were added to an initially clean airflow that was supplied by an electrically heated facility. With dry air, the scramjet combustor operated in the supersonic mode at an equivalence ratio in the range of 0.25-0.32 and transitioned to dual mode over an equivalence ratio range of 0.35-0.37. At an equivalence ratio of 0.27, the combustor operated in the supersonic mode for three cases: 1) dry air, 2) air vitiated with 5% H 2 O by mole, and 3) air vitiated with 5% H 2 O and 2.5% CO 2 by mole. In the second case, the combustor pressure distribution decreased 10% relative to dry air and, in the third case, another 2% decrease was measured. At an equivalence ratio of 0.35, the combustor operated in the dual mode with dry air, but in the supersonic mode with 7 % H 2 O. This is the first demonstration of mode transition solely caused by test gas vitiation. It is therefore important to account for such effects when extrapolating from vitiated ground testing to flight.