Andrew D. Cutler

Coherent Anti-Stokes Raman Spectroscopic Thermometry in a Supersonic Combustor

An experiment has been conducted to acquire data for the validation of computational fluid dynamics codes used in the design of supersonic combustors. The flow in a supersonic combustor, consisting of a diverging duct with a single downstream-angled wall injector, is studied. Combustor entrance Mach number is 2 and enthalpy nominally corresponds to Mach 7 flight. The primary measurement technique is coherent anti-Stokes Raman spectroscopy, but surface pressures and temperatures have also been acquired. Modern design of experiment techniques have been used to maximize the quality of the data set (for the given level of effort) and to minimize systematic errors. Temperature maps are obtained at several planes in the flow for a case in which the combustor is piloted by injecting fuel upstream of the main injector and one case in which it is not piloted. Boundary conditions and uncertainties are characterized.

Dual-Pump Coherent Anti-Stokes Raman Scattering Measurements in a Supersonic Combustor

The dual-pump coherent anti-Stokes Raman scattering (CARS) method was used to measure temperature and the mole fractions of N 2 and O 2 in a supersonic combustor. Experiments were conducted in NASA Langley Research Centers Direct-Connect Supersonic Combustion Test Facility. In this facility, H 2 - and oxygen-enriched air burn to increase the enthalpy of the simulated air test gas. This gas is expanded through a Mach 2 nozzle and into a combustor model consisting of a short constant-area section followed by a small rearward-facing step and another constant-area section. At the end of this straight section, H 2 fuel is injected at Mach 2 and at a 30-deg angle with respect to the freestream. One wall of the duct then expands at a 3-deg angle for over 1 m. The ensuing combustion is probed optically through ports in the side of the combustor. Dual-pump CARS measurements were performed at the facility nozzle exit and at four planes downstream of fuel injection. Maps are presented of the mean temperature, as well as N 2 and O 2 mean mole-fraction fields. Correlations between fluctuations of the different measured parameters are also presented.

Mixing of swirling jets in a supersonic duct flow

Hydrogen fuel injected into a scramjet combustor must mix rapidly if complete combustion is to occur within a reasonable stream wise distance. An experiment has been conducted to determine whether the addition of swirl will improve the mixing of a supersonic jet of fuel simulant (helium or air) injected at 30 deg to the wall into a confined Mach 2 airflow. The swirling jets were created by injecting the fuel simulant tangentially into a cylindrical chamber and accelerating it through a convergent-div er gent nozzle. The flow was visualized by imaging Rayleigh scattering from a laser light sheet, and the plume penetration and cross-sectional area were obtained. The plumes from the swirling and nonswirling jets had comparable penetration and area, but the swirling jets contained substantially less mass flow, suggesting better mixing efficiency. Interaction of streamwise vorticity within the plumes of the swirling jets with their images in the duct wall caused the plumes to be inclined laterally to the freestream.

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 ...

The relaxation of a turbulent boundary layer in an adverse pressure gradient

Reattached turbulent boundary layer relaxation downstream of a wall fence is investigated. An adverse pressure gradient is imposed upon it which is adjusted to bring the boundary layer into equilibrium. The pressure gradient is adjusted so as to bring the Clauser parameter G down to a value of about 11.4 and then maintain it constant. In the region from the reattachment point to 2 or 3 reattachment lengths downstream, the boundary layer recovers from the initial major effects of reattachment. Farther downstream, where G is constant, the pressure-gradient parameter changes very slowly and profiles of non-dimensionalized eddy viscosity appear self-similar. However, pressure gradient and eddy viscosity are both roughly twice as large as expected on the basis of previous equilibrium turbulent boundary layer studies.

A crossed hot-wire technique for complex turbulent flows

This paper describes a crossed hot-wire technique for the measurement of all components of mean velocity, Reynolds stresses, and triple products in a complex turbulent flow. The accuracy of various assumptions usually implicit in the use of crossed hot-wire anemometers is examined. It is shown that significant errors can result in flow with gradients in mean velocity or Reynolds stress, but that a first order correction for these errors can be made using available data. It is also shown how corrections can be made for high turbulence levels using available data.

Large-Eddy / Reynolds-Averaged Navier-Stokes Simulations of a Dual-Mode Scramjet Combustor

Numerical simulations of reacting and non-reacting flows within a scramjet combustor configuration experimentally mapped at the University of Virginia s Scramjet Combustion Facility (operating with Configuration A ) are described in this paper. Reynolds-Averaged Navier-Stokes (RANS) and hybrid Large Eddy Simulation / Reynolds-Averaged Navier-Stokes (LES / RANS) methods are utilized, with the intent of comparing essentially blind predictions with results from non-intrusive flow-field measurement methods including coherent anti-Stokes Raman spectroscopy (CARS), hydroxyl radical planar laser-induced fluorescence (OH-PLIF), stereoscopic particle image velocimetry (SPIV), wavelength modulation spectroscopy (WMS), and focusing Schlieren. NC States REACTMB solver was used both for RANS and LES / RANS, along with a 9-species, 19- reaction H2-air kinetics mechanism by Jachimowski. Inviscid fluxes were evaluated using Edwards LDFSS flux-splitting scheme, and the Menter BSL turbulence model was utilized in both full-domain RANS simulations and as the unsteady RANS portion of the LES / RANS closure. Simulations were executed and compared with experiment at two equivalence ratios, PHI = 0.17 and PHI = 0.34. Results show that the PHI = 0.17 flame is hotter near the injector while the PHI = 0.34 flame is displaced further downstream in the combustor, though it is still anchored to the injector. Reactant mixing was predicted to be much better at the lower equivalence ratio. The LES / RANS model appears to predict lower overall heat release compared to RANS (at least for PHI = 0.17), and its capability to capture the direct effects of larger turbulent eddies leads to much better predictions of reactant mixing and combustion in the flame stabilization region downstream of the fuel injector. Numerical results from the LES/RANS model also show very good agreement with OH-PLIF and SPIV measurements. An un-damped long-wave oscillation of the pre-combustion shock train, which caused convergence problems in some RANS simulations, was also captured in LES / RANS simulations, which were able to accommodate its effects accurately.

Width-increased dual-pump enhanced coherent anti-Stokes Raman spectroscopy

Width-increased dual-pump enhanced coherent anti-Stokes Raman spectroscopy (WIDECARS) is a technique that is capable of simultaneously measuring temperature and species mole fractions of N(2), O(2), H(2), C(2)H(4), CO, and CO(2). WIDECARS is designed for measurements of all the major species (except water) in supersonic combustion flows fueled with hydrogen and hydrogen/ethylene mixtures. The two lowest rotational energy levels of hydrogen detectable by WIDECARS are H(2) S(3) and H(2) S(4). The detection of these lines gives the system the capability to measure temperature and species concentrations in regions of flow containing pure hydrogen fuel at room temperature.

Near-field flow of supersonic swirling jets

The addition of swirl to scramjet fuel jets has been proposed as a method of enhancing fuel mixing, but little of a fundamental nature is known about supersonic swirling flows. Several jets with different amounts of swirl were created by tangential injection and acceleration through a convergent-divergent nozzle. The flowfields near the nozzle exit were investigated using pitot, cone, and total-temperature probes, and Rayleigh scattering from a laser light sheet. The results show that tangential injection is an efficient method for generating swirling jets, that the swirling jets mix much more rapidly with the stagnant air than comparable straight jets, and that, when overexpanded, turbulence is created in the jet core as a result of vortex breakdown. Mixing layer growth rates are shown to correlate with Richardson number

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...