Robert A. Baurle

Fundamental Studies of Cavity-Based Flameholder Concepts for Supersonic Combustors

Experimentalandcomputationalinvestigationsofthee owe eldassociatedwithseveralcavity-basede ameholders in a nonreacting supersonic e ow are described. All cavity e ows were of the open type, that is, length-to-depth ratio L/D<10. Two values of L/D were studied with several offset ratios (OR) and aft ramp angles µ. Results indicate that the aft ramp angle plays an important role in determining the character of the shear layer that spans the cavity. For a rectangular cavity with OR=1 and µ=90 deg, a compression wave forms as the e ow separates from the cavity’ s upstream corner. A strong recompression occurs at the aft wall, and the e ow is visibly unsteady. The pressure on the cavity fore wall decreases steadily and the recompression process occurs more gradually with decreasing aftrampangle.Higherdrag coefe cientsandshorterresidencetimesarefoundin cavitieswithshallower ramp angles.

Hybrid Simulation Approach for Cavity Flows: Blending, Algorithm, and Boundary Treatment Issues

The maturation of high-performance computer architectures and computational algorithms has prompted the development of a new generation of models that attempt to combine the robustness and efficiency offered by the Reynolds averaged Navier-Stokes equations with the higher level of modeling offered by the equations developed for large eddy simulation. The application of a new hybrid approach is discussed, where the transition between these equation sets is controlled by a blending function that depends on local turbulent flow properties, as well as the local mesh spacing. The utilization of local turbulence properties provides added control in specifying the regions of the flow intended for each equation set, removing much of the burden from the grid-generation process. Moreover, the model framework allows for the combination of existing closure model equations, avoiding the difficulty of formulating a single set of closure coefficients that perform well in both Reynolds averaged and large eddy simulation modes. Simple modifications to common second-order accurate Reynolds averaged Navier-Stokes algorithms are proposed to enhance the capturing of large eddy motions

Plasma-Assisted Ignition in Scramjets

This study assesses the prospect of main-fuel ignition with plasma-generating devices in a supersonic flow. Progress from this study has established baseline conditions for operation, such as the required operational time of a device to initiate a combustion shock train as predicted by computational fluid dynamics computations. Two plasma torches were investigated: a direct current constricted-arc design and an alternating current unconstricted-arc design based on a modified spark plug. Both plasma torches are realistic in size and operate within the same current and voltage constraints, although differing substantially in orifice geometry. To compare the potential of each concept, the flow physics of each part of the igniter/fuel-injector/combustor system was studied. To understand the constraints involved with the ignition process of a hydrocarbon fuel jet, an experimental effort to study gaseous and liquid hydrocarbons was conducted, involving the testing of ethylene and JP-7 fuels with nitrogen and air plasmas. Results from individual igniter studies have shown plasma igniters to produce hot pockets of highly excited gas with peak temperatures up to 5000 K at only 2 kW total input power. In addition, ethylene and JP-7 flames with a significant level of the hydroxyl radical, as determined by planar laser-induced fluorescence, were also produced in a Mach 2 supersonic flow with a total temperature and pressure of 590 K and 5.4 atm. Information from these experiments is being applied to the generation of constraints and the development of a configuration with perceived high ignition potential in full scramjet combustor testing.

Inflow boundary conditions for hybrid large eddy/Reynolds averaged Navier-Stokes simulations

Inflow boundary conditions are developed for hybrid large-eddy simulation (LES)/Reynolds-averaged Navier-Stokes approaches. They are based on an extension of the rescaling-reintroducing method developed for LES to a hybrid scheme. A blending function is used to shift the turbulence closure from a κ-ζ model near the wall to a κ-Δ subgrid-scale model away from the wall. The approach was tested for a flat plate and then applied to the study of a 25-deg compression-expansion ramp for a Mach number of 2.88 and a Reynolds number of 3.24 × 10 7 /m. In general, improvements over the κ-ζ model were noted in the recovery region. The significance of this work is that it provides a way for LES methods to address flows at a high Reynolds number

MODELING OF HIGH SPEED REACTING FLOWS: ESTABLISHED PRACTICES AND FUTURE CHALLENGES

Computational fluid dynamics (CFD) has proven to be an invaluable tool for the design and analysis of highspeed propulsion devices. Massively parallel computing, together with the maturation of robust CFD codes, has made it possible to perform simulations of complete engine flowpaths. Steady-state Reynolds-Averaged Navier-Stokes simulations are now routinely used in the scramjet engine development cycle to determine optimal fuel injector arrangements, investigate trends noted during testing, and extract various measures of engine efficiency. Unfortunately, the turbulence and combustion models used in these codes have not changed significantly over the past decade. Hence, the CFD practitioner must often rely heavily on existing measurements (at similar flow conditions) to calibrate model coefficients on a caseby-case basis. This paper provides an overview of the modeled equations typically employed by commercialquality CFD codes for high-speed combustion applications. Careful attention is given to the approximations employed for each of the unclosed terms in the averaged equation set. The salient features (and shortcomings) of common models used to close these terms are covered in detail, and several academic efforts aimed at addressing these shortcomings are discussed.

Newly Developed Direct-Connect High-Enthalpy Supersonic Combustion Research Facility

Anew continuous-e ow,direct-connect,high-enthalpy, supersonic combustion researchfacility isdescribed. This test facility provides combustor inlet e ow conditions corresponding to e ight Mach numbers between 3.5 and 7, at dynamic pressures up to 95.8 kPa. Most of the major components of the new facility are water cooled (including the vitiated heater, the instrumentation and transition sections, and the facility nozzle and isolators ); the current exception is the variable-geometry heat-sink combustor. A variety of conventional and advanced instrumentation, including a steam calorimeter and a thrust stand, exists for accurate documentation of combustor inlet and exit conditions and performance parameters. In a recent calibration effort, pitot pressure surveys, total temperature surveys, and wall static pressure distributions were obtained for a wide range of inlet conditions using Mach 1.8 and 2.2 facility nozzles. In addition, three-dimensional numerical simulations of each test case were completed. Results from thecomputations compare favorably with experimental results for all cases and yield estimates of the integral boundary-layer properties at the isolator exit.

Assumed Joint Probability Density Function Approach for Supersonic Turbulent Combustion

In a recent experiment, Cheng et al. used UV spontaneous vibrational Raman scattering and laser-induced predissociative fluorescence techniques for simultaneous measurements of temperature and concentrations of O2, H2, H2O, OH, and N2 (and the rms of their fluctuations) in supersonic turbulent reacting shear layers. Because present computational techniques are not suited for the prediction of all of the above measurements, a new approach has been developed and is being used to predict all relevant flow properties and the rms of their fluctuations (where appropriate). The approach explores the use of a multivariate Beta PDF for concentrations. In particular, a version developed by Girimaji to model scalar mixing in turbulent flows is employed. Predictions using this model were, in general, satisfactory in regions preceding ignition, but not in regions downstream of ignition. Part of the discrepancy is a result of our current inability to relate Favre and time averages. 10 refs.

Hybrid Large-Eddy / Reynolds-Averaged Navier-Stokes Simulation of Shock-Separated Flows

An assessment of a hybrid large-eddy/Reynolds-averaged simulation (LES/RANS) procedure for high-speed, shock-separated flows is reported. A distance-dependent blending function is used to shift the turbulence closure fromMenters k-w shear-stress-transport model near solid surfaces to a k-Δ subgrid closure away from solid surfaces and in free-shear regions. A modified recycling/rescaling procedure is used to generate time-dependent fluctuation data that are fed into the inflow plane for some calculations, with the goal being to replace the incoming boundary layer with a hybrid LES/RANS boundary layer that maintains nearly the same levels of fluctuation energy. Simulations of Mach 3 flow over a ramped-cavity configuration highlight the effects of grid refinement and choice of hybridization strategy, while simulations of Mach 3 flow over a 20-deg compression corner illustrate the effects of the choice of model constants and the inclusion of boundary-layer recycling on the mean-flow solutions.

Simulations of High -Speed Internal Flows using LES/RANS Models

Computational stud ies of transverse sonic injection of ethylene into a Mach 1.98 cro ss flow and Mach 5 flow of air into a subscale inlet / isolator configuration are presented . A hybrid large -eddy simulation / Reynolds -averaged Navier -Stokes (LES/RANS) turbulence model is used , with the two -equation Menter -BSL closure for the RANS part of the flow and a Smagorinsky -type model for the LES part of the flow . A time -dependent blending function, dependent on modeled turbulence variables, is used to shift the closure from RANS to LES. Turbulent st ructures are sustained through the use of a ‘random -walk’ recycling / rescaling technique . The ethylene injection results using the hybrid model show s very good agreement with the Raman scattering data collected at the Air Force Research Laboratory . The LE S/RANS database is used to examine the validity of the commonly -used assumption of a constant Schmidt number in the intense mixing zone downstream of the injection location . Predictions of Mach 5 flow into the inlet / isolator are compared with particle imaging velocimetry (PIV) and wall pressure data obtained at the University of Texas . Preliminary computational results are presented for two cases involving shock -train propagation within the isolator, one of which leads to inlet unstart. Here, the compu tational method appears to predict more flow separation than indicated in the experiment, leading to stronger shock trains that are not stabilized at the correct position.

Modeling of supersonic turbulent combustion using assumed probability density functions

Recent calculations of turbulent supersonic reacting shear flows using an assumed multivariate beta probability density function (PDF) resulted in reduced production rates and a delay in the onset of combustion. This result is not consistent with available measurements. Earlier work was based on a one-equation turbulence model that required a specification of the length scale, PDFs that did not yield Favre-averaged quantities, and the gradient diffusion assumption. The present work incorporates a two-equation turbulence model based on a kappa-omega formulation, a PDF that yields Favre averages, and relaxes the gradient diffusion assumption. Results suggest that the form of the assumed multivariate PDF and the gradient diffusion assumption are the main causes of the discrepancy. 15 refs.