Numerical Investigation of Mixing and Combustion Enhancement in Supersonic Combustors by Strut Induced Streamwise Vorticity

This paper describes an experimental effort to characterize the flame-holding process of a hydrogen jet injected into a high total enthalpy supersonic cross flow. An expansion tube is used to provide a correct simulation of true flight combustion chemistry, including ignition delay and reaction times. This approach permitted a number of unique experiments involving acceleration of radical-free air to high total enthalpies. The experiments were designed to map the near-field flow characteristics and autoignition process of an underexpanded transverse hydrogen jet injected into flight-Mach number 10 and 13 total enthalpy flow conditions. Flow visualization techniques included planar laser-induced fluorescence (PLIF) of OH and schlieren imaging applied simultaneously. Schlieren images show the shock structure around the jet and the periodically formed coherent structures in the jet-free-stream interface. Overlaid OH-PLIF and schlieren images allow characterization of the autoignition of a hydrogen jet in air cross flow for different jet-to-free-stream momentum flux ratios at both flow conditions. Transverse jet autoignition and flame-holding characteristics observed in both side view and top view images by OH-PLIF reveal differences with previous results in the literature. In the present experiments, the first OH signals are obtained in the recirculation region upstream of the jet exit and in the bow shock region, while in past experiments with similar geometry but lower total enthalpy conditions, no strong OH signal was observed within the first 10 jet diameters. The OH-PLIF results for Mach 10 conditions also show that the OH signal level decreases significantly as the mixture expands around the jet flow field, indicating a partial quenching of the ignition. This indicates that combustion of hydrogen and air in these high total enthalpy conditions is a mixing-limited process. It is evident from the results that improved injection schemes will be required for practical applications in seramjet engines.

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Experimental investigation of flame-holding capability of hydrogen transverse jet in supersonic cross-flow

The ine uence of different injector geometries on the mixing behavior of planar supersonic jets is investigated. Hydrogenisinjectedinthee owdirectionthroughthebluntendofastrut.Differentlipthicknessesattheinjectorend induce different extents of recirculation zones. Additionally, changes are caused within the shock-wave/expansion fan pattern at the injector exit that have an important ine uence on loss in total pressure. The accuracy of the used numerical scheme is demonstrated for one injector geometry by comparison with experimental data. Based on this cone guration, modie cations in lip thickness and injection Mach number are investigated numerically and are assessed using several calculated performance parameters. Results show that the chosen lip thickness has a much stronger ine uence on loss in total pressure than moderate modie cations in injector length and height. For the very thin hydrogen jets that were investigated, the mixing efe ciency is nearly independent on injector lip thickness.

Numerical Investigation of Hydrogen Strut Injections into Supersonic Airflows

Ignition, flame-holding, and mixing enhancement are fundamental aspects of supersonic combustion and are critical to the development of hypersonic airbreathing propulsion engines. High velocities associated with supersonic/hypersonic flight speeds constrain the performance of propulsion systems because of the limited flow residence time inside the combustor. A useful hypervelocity propulsion system therefore requires enhanced mixing of fuel and air, injection with very low drag penalty, and effective distribution of fuel over the burner cross-section. One of the simplest approaches is the transverse injection of fuel from wall orifices. The interesting but rather complicated flow-field dynamics of transverse jets injected into a supersonic crossflow has been studied by many supersonic combustion researchers since 1960’s, but with limited freestream flow conditions. Most of the previous research was performed in conventional wind tunnels by accelerating cold air into supersonic conditions, namely in low velocity and low total enthalpy flow conditions. However, a real supersonic combustor environment at flight speeds beyond Mach 8 can only be simulated using impulse facilities due to the required high total enthalpies. Among various impulse facilities, expansion tubes are especially useful in providing high total enthalpy flows with the proper chemical composition, namely the absence of dissociated species. This research is focused on studying the near-field mixing and ignition properties of transverse fuel jets injected into realistic supersonic combustor flows. We use advanced flow visualization techniques, namely planar laser-induced fluorescence (PLIF) imaging of the hydroxyl radical (OH) and ultra-fast-framing-rate schlieren imaging. While schlieren indicates the location of shock waves, jet penetration and large scale flow features, OH-PLIF is used to map the regions of ignition. The first objective of the present work is to characterize the expansion tube facility

Experimental investigation of mixing and ignition of transverse jets in supersonic crossflows

This paper offers a detailed review and analysis of more than 100 papers on the physics and chemistry of scramjet ignition and flameholding combustion processes, and the known effects of air vitiation on these processes. The paper attempts to explain vitiation effects in terms of known chemical kinetics and flame propagation phenomena. Scaling methodology is also examined, and a highly simplified Damkoehler scaling technique based on OH radical production/destruction is developed to extrapolate ground test results, affected by vitiation, to flight testing conditions. The long term goal of this effort is to help provide effective means for extrapolating ground test data to flight, and thus to reduce the time and expense of both ground and flight testing.

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Review of Air Vitiation Effects on Scramjet Ignition and Flameholding Combustion Processes

Transverse jet mixing and combustion experiments at a freestream velocity of 5 km/s have been conducted in the T5 free-piston shock tunnel at GALCIT. The experiments were performed in a 50 by 25 mm combustor model using a flush-mounted, Mach 1.7 single injector introducing the fuel at an angle of 15 deg relative to the main flow. The test conditions cover a range of freestream pressures from low values chosen to match previous experiments with the same duct in the HYPULSE expansion tube facility at GASL, to high pressures corresponding to the conditions closer to those in a real scramjet combustor. Tests with cold and hot hydrogen fuel were performed, the latter to better reproduce the fuel conditions of a scramjet propulsion system where the hydrogen will have to be used first as a cooling fluid. A small combustion-driven shock tunnel was used simultaneously with T5 to supply the hot fuel.

Jacques Belanger

Papers

Transverse Jet Mixing and Combustion Experiments in Hypervelocity Flows