Supersonic mixing and combustion of a hydrogen jet in a coaxial high-temperature test gas.
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Beach, H. L., Jr.↗
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Supersonic mixing and combustion in scramjets
Experimental and theoretical studies are being conducted to explore techniques to enhance mixing in scramjet combustors using parallel fuel injection from the base of swept and unswept wall-mounted ramps. The experiments reported herein were conducted using Mach 2 and 3 combustor inlet conditions. Supporting computational and cold flow studies indicated that the observed enhanced mixing for the swept ramp configuration is primarily due to the substantially higher degree of vorticity and entrainment generated by the swept trailing edges.
Northam, G. B.↗
Pretest Simulations of a Supersonic Mixing and Combustion Validation Experiment to Assess Sensitivities
The reliance on CFD simulations to develop, design, and optimize scramjet systems (or components) has become commonplace. This reliance inevitably hinges on the ability of the computational analyst to quantify the level of confidence in their computational results. Unfortunately, nearly all the measured data available for this assessment comes from antiquated experimental datasets, or from tests that focused on the extraction of scramjet system (or component) performance. The objective of a CFD validation experiment is to quantify the predictive accuracy of one or more of the CFD physics submodels, implying that other uncertainties related to replicating the facility flow environment (e.g., knowledge of boundary conditions) must be minimized to the extent possible. This inevitably places stringent requirements on the quality and quantity of measurements taken to accurately specify inflow, outflow, and surface conditions for the CFD simulations; in addition to the measurements taken for the validation of physics submodels. This places additional demands on the experimental process above and beyond those for test article performance assessment. A recent high speed code credibility workshop series sponsored by AFRL identified a gap in existing validation data for fundamental assessments of turbulent mixing and combustion CFD closure models at scramjet engine relevant conditions. To address this gap, engineers at AFRL have designed a coaxial jet flame configuration that will be tested at two facilities (Research Cell 19 at the Air Force Research Lab, and at Purdue University). The effort described here documents pretest simulations of this validation experiment with the goal of fleshing out the extent of the facility flowpath that must be included to adequately reproduce the facility test section flow environment. The findings indicate that the flow around the support structure for the fuel injection centerbody upstream of the facility nozzle generates disturbances that persist throughout the nozzle expansion process; corrupting the azimuthal symmetry that was desired in the fuel/air mixing region of the test section. Simulations without this support structure maintained a high degree of azimuthal symmetry up until the fuel injection plane. However, even in this scenario the azimuthal symmetry was not maintained once the centerbody boundary layer transitioned to a wake flow downstream of the fuel injection plane.
CFD↗
Pretest Simulations of a Supersonic Mixing and Combustion Validation Experiment to Assess Sensitivities
The reliance on CFD simulations to develop, design, and optimize scramjet systems (or components) has become commonplace. This reliance inevitably hinges on the ability of the computational analyst to quantify the level of confidence in their computational results. Unfortunately, nearly all the measured data available for this assessment comes from antiquated experimental datasets, or from tests that focused on the extraction of scramjet system (or component) performance. The objective of a CFD validation experiment is to quantify the predictive accuracy of one or more of the CFD physics submodels, implying that other uncertainties related to replicating the facility flow environment (e.g., knowledge of boundary conditions) must be minimized to the extent possible. This inevitably places stringent requirements on the quality and quantity of measurements taken to accurately specify inflow, outflow, and surface conditions for the CFD simulations; in addition to the measurements taken for the validation of physics submodels. This places additional demands on the experimental process above and beyond those for test article performance assessment. A recent high speed code credibility workshop series sponsored by AFRL identified a gap in existing validation data for fundamental assessments of turbulent mixing and combustion CFD closure models at scramjet engine relevant conditions. To address this gap, engineers at AFRL have designed a coaxial jet flame configuration that will be tested at two facilities (Research Cell 19 at the Air Force Research Lab, and at Purdue University). The effort described here documents pretest simulations of this validation experiment with the goal of fleshing out the extent of the facility flowpath that must be included to adequately reproduce the facility test section flow environment. The findings indicate that the flow around the support structure for the fuel injection centerbody upstream of the facility nozzle generates disturbances that persist throughout the nozzle expansion process; corrupting the azimuthal symmetry that was desired in the fuel/air mixing region of the test section. Simulations without this support structure maintained a high degree of azimuthal symmetry up until the fuel injection plane. However, even in this scenario the azimuthal symmetry was not maintained once the centerbody boundary layer transitioned to a wake flow downstream of the fuel injection plane.
CFD↗
Modeling of turbulent supersonic H2-air combustion with a multivariate beta PDF
Recent calculations of turbulent supersonic reacting shear flows using an assumed multivariate beta PDF (probability density function) resulted in reduced production rates and a delay in the onset of combustion. This result is not consistent with available measurements. The present research explores two possible reasons for this behavior: use of PDF's that do not yield Favre averaged quantities, and the gradient diffusion assumption. A new multivariate beta PDF involving species densities is introduced which makes it possible to compute Favre averaged mass fractions. However, using this PDF did not improve comparisons with experiment. A countergradient diffusion model is then introduced. Preliminary calculations suggest this to be the cause of the discrepancy.
Baurle, R. A.↗
The 17th JANNAF Combustion Meeting, Volume 1
The combustion of solid rocket propellants and combustion in ramjets is addressed. Subjects discussed include metal burning, steady-state combustion of composite propellants, velocity coupling and nonlinear instability, vortex shedding and flow effects on combustion instability, combustion instability in solid rocket motors, combustion diagnostics, subsonic and supersonic ramjet combustion, characterization of ramburner flowfields, and injection and combustion of ramjet fuels.
Eggleston, D. S.↗
Combustion of Highly Reactive Fuels in Supersonic Airstreams
The feasibility of adding heat to supersonic airstreams by combustion was studied in a small wind tunnel. Aluminum borohydride, pentaborane, hydrocarbon-aluminum borohydride mixtures, trimethylaluminum, diethylaluminum hydride, alkylboranes, alkylboron hydrides and vinylsilane were tested. The first three ignited easily and burned well. The others either failed to ignite or burned only in the diffuser. Trimethylaluminum and diethylaluminum hydride produced light and other evidence of heat evolution when water was simultaneously injected into the tunnel. Gross effects on flow were studied by observation of shock patterns and water sprays injected from the tunnel walls. Examples are given which illustrate the use of these techniques in aerodynamic studies in larger wind tunnels.
Fletcher, Edward A.↗
Thermochemical nonequilibrium and radiative interactions in supersonic hydrogen-air combustion
The two-dimensional, elliptic Navier-Stokes equations are used to investigate supersonic flows with nonequilibrium chemistry and thermodynamics, coupled with radiation, for hydrogen-air systems. The chemistry source term in the species equation is treated implicitly to alleviate the stiffness associated with fast reactions. The explicit, unsplit MacCormack finite-difference scheme is used to advance the governing equations in time, until convergence is achieved. The specific problem considered is the premixed, expanding flow in a supersonic nozzle. The reacting flow consists of seven species, one of which is the inert N2 molecule. The thermal state of the gas is modeled with one translational-rotational temperature and five vibrational temperatures. The harmonic oscillator model is used in the formulation for vibrational relaxation. The tangent slab approximation is used in the radiative flux formulation. A pseudo-gray model is used to represent the absorption-emission characteristics of the participating species. Results obtained for specific conditions indicate the presence of nonequilibrium in the expansion region. This reduces the radiative interactions and can have a significant influence on the flowfield.
Chandrasekhar, R.↗
A Study of the Combustion of Aluminum Borohydride in a Small Supersonic Wind Tunnel
The combustion of aluminum borohydride in the Mach 2 airstream of a 3.84- by 10-inch wind tunnel was studied by analyzing gas samples taken from the airstream. Gas mixture composition was determined with the aid of a vacuum apparatus and gas chromatograph. The overall combustion efficiency of the fuel was assumed to be good because the combustion efficiency of its hydrogen component was found to be high. The increase of the lateral surface area of the combustion region was dependent upon the degree of mixing of fuel and its combustion products with air. In the flame zone where there was insufficient oxygen for complete combustion, the metal components of the fuel appeared to burn preferentially to the hydrogen component.
Allen, Harrison, Jr.↗
Review of the PDWA Concept for Combustion Enhancement in a Supersonic Air-Breathing Combustor Environment
This paper reviews the design of the Pulsed Detonation Wave Augmentor (PDWA) concept and the preliminary computational fluid dynamics studies that supported it. The PDWA relies on the rapid generation of detonation waves in a small tube, which are then injected into the supersonic stream of the main combustor. The blast waves thus generated are used to stimulate the mixing and combustion inside the main combustor. The mixing enhancement relies on various forms of the baroclinic interaction, where misaligned pressure and density gradients combine to produce vortical flow. By using unsteady shock waves, the concept also uses the Richtmyer-Meshkov effect to further increase the rate of mixing. By carefully designing the respective configurations of the combustor and the detonation tubes, one can also increase the penetration of the fuel into the supersonic air stream. The unsteady shocks produce lower stagnation pressure losses than steady shocks. Combustion enhancement can also be obtained through the transient shock-heating of the fuel-air interface, and the lowering of the ignition delay in these regions. The numerical simulations identify these processes, and show which configurations give the best results. Engineering considerations are also presented, and discuss the feasibility of the concept. Of primary importance are the enhancements in performance, the design simplicity, the minimization of the power, cost, and weight, and the methods to achieve very rapid cycling.
Canbier, Jean-Luc↗
Viscous characteristics analysis
Program considers combustion and diffusive effects in analysis of supersonic, combustion-flow fields with imbedded subsonic regions. Effects of finite-rate chemistry, mixing, and wave propagation are linked together. Program handles up to 20 simultaneous shock waves. Some chemistry terms are computed for seven-species, eight-mechanism, hydrogen-and-air reaction scheme. Program is aid for supersonic-combustor development studies and is written in FORTRAN IV for batch execution on CYBER 175.
Jenkins, R. V.↗
Dual-Mode Free-Jet Combustor
The dual-mode free-jet combustor concept, pictured in figure 1, is described. It was introduced in 2010 as a wide- operating-range propulsion device using a novel supersonic free-jet combustion process. The unique feature of the free-jet combustor pictured in figure 1a, is supersonic combustion in an unconfined free-jet that traverses a larger subsonic combustion chamber to a variable nozzle. During this mode of operation, the propulsive stream is not in contact with the combustor walls, and equilibrates to the combustion chamber pressure. To a first order, thermodynamic efficiency is similar to that of a traditional scramjet under the assumption of constant-pressure combustion. Qualitatively, a number of possible benefits to this approach are obvious.
supersonic combustion↗
Dual-Mode Free-Jet Combustor
The dual-mode free-jet combustor concept, pictured in figure 1, is described. It was introduced in 2010 as a wide- operating-range propulsion device using a novel supersonic free-jet combustion process. The unique feature of the free-jet combustor pictured in figure 1a, is supersonic combustion in an unconfined free-jet that traverses a larger subsonic combustion chamber to a variable nozzle. During this mode of operation, the propulsive stream is not in contact with the combustor walls, and equilibrates to the combustion chamber pressure. To a first order, thermodynamic efficiency is similar to that of a traditional scramjet under the assumption of constant-pressure combustion. Qualitatively, a number of possible benefits to this approach are obvious.
hypersonic propulsion↗
Modeling of turbulent supersonic H2-air combustion with an improved joint beta PDF
Attempts at modeling recent experiments of Cheng et al. indicated that discrepancies between theory and experiment can be a result of the form of assumed probability density function (PDF) and/or the turbulence model employed. Improvements in both the form of the assumed PDF and the turbulence model are presented. The results are again used to compare with measurements. Initial comparisons are encouraging.
Baurle, R. A.↗
Photo-sensitized ignition of hydrogen/oxygen mixtures for hypersonic flight vehicles
An approach toward improving supersonic ignition and combustion efficiency was investigated under a feasibility program with NASA-Ames. The idea is to selectively irradiate targetted species in the combustion zone. The ensuing photodissociation reactions produce highly reactive radicals which modify the gas phase kinetics. The overall objective of this research was to demonstrate photosensitized ignition of H2/O2 mixtures, and to gain a preliminary understanding of the ignition mechanism, especially as it pertains to the concept of supersonic combustion. To this end, a series of experiments were performed with a model sensitizer, NO2. At subatmospheric pressures and moderate temperatures (above 100 F), mixtures of H2/O2/NO2 are ignitable by a pulsed eximer laser at 308 nm, 115 to 200 mJ. Ignitability was found to be a function of NO2 pressure, temperature, laser energy per pulse, and laser lens focal length (fluence at focal volume). Mixtures could not be ignited in the absence of the sensitizer. While increasing temperature improved sensitized ignition in the lower temperature range, a retarding effect was observed at the highest temperatures. To more thoroughly understand these thermal effects, the UV-Vis and IR spectra of NO2 and H2/O2/NO2 mixtures were studied over the temperature range 70 to 900 F. Initially, increasing temperature shifts the N2O4 in a reversible reaction to 2 NO2 equilibrium toward NO2, favoring photochemical ignition. However, at still higher temperature, NO2 converts to non-absorbing NO + 1/2 O2, thus preventing photosensitized ignition.
Lavid, Moshe↗
An analysis of internal supersonic flows with diffusion, dissipation and hydrogen-air combustion
Numerical analysis of internal supersonic flows with diffusion, dissipation, and hydrogen-air combustion
Dash, S.↗
Simultaneous Temperature and Velocity Measurements in a Large-Scale, Supersonic, Heated Jet
Two laser-based measurement techniques have been used to characterize an axisymmetric, combustion-heated supersonic jet issuing into static room air. The dual-pump coherent anti-Stokes Raman spectroscopy (CARS) measurement technique measured temperature and concentration while the interferometric Rayleigh scattering (IRS) method simultaneously measured two components of velocity. This paper reports a preliminary analysis of CARS-IRS temperature and velocity measurements from selected measurement locations. The temperature measurements show that the temperature along the jet axis remains constant while dropping off radially. The velocity measurements show that the nozzle exit velocity fluctuations are about 3% of the maximum velocity in the flow.
Danehy, P. M.↗
Transforming Energy through Computational Excellence. Exascale Computing: Combustion; Simulating Effects of Fuel Injection Location in Supersonic Jet Engines
Computational tractable simulations using an adaptive-mesh-refinement solverfor compressible reacting flows help researchers understand how variations in fuel injection location within the supersonic flow cavity impacts combustion efficiency. By identifying the important physical determinants of the combustion processes, this study shows a promising pathway to improving flame stability and combustion efficiency, as well as reducing emissions.
adaptive mesh refinement↗