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Nonequilibrium Plasma-Assisted Combustion-Efficiency Control in Vitiated Air

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OBJECTIVE: To investigate electrical control of selective energy deposition for significant modification of combustion kinetics through nonequilibrium thermochemistry and induced flow. DESCRIPTION: Combustion efficiency, reignition and flame holding are important issues for both very high-altitude and high-speed flights. High-altitude jet engine operation is limited by the overall combustion efficiency and the lean flame blow out (LBO) limit. Similarly, the need to improve fuel efficiency and reduce emissions from hydrocarbon combustors has also reinvigorated interest in active monitoring and improving control [1] of LBO. New augmentor designs need to reduce exhaust gas temperatures to improve mission capability, leading to high air mass flow through the augmentor. Short residence time of the gas flow and reactant mixing time leads to two types of combustion instability in augmentors; low-frequency rumble for high-altitude, low-Mach number flights, and high-frequency pressure fluctuation during low-altitude, high-Mach number flights [2]. Combustion instability of a lean flame arises from the balance between the heat generation and the heat loss rate through conduction, convective, and radiative losses. The heat generation rate in a binary reaction is given by HQ=r2c1c2Qexp[-e/RT], where r is the density, c1 and c2 are mole fractions of the reactants, Q is the reaction heat release, e is the activation energy, R is the gas constant, and T is the average temperature in K. For hydrocarbon-air oxidation reactions, the typical value of e>>RTf, where Tf is the adiabatic flame temperature, so most heat release reactions are confined to a thin reaction sheet, where the gas temperature reaches its maximum value. The maximum gas temperature in a combustion zone is dependent on the fuel-air ratio. The lean flame burning condition is, therefore, more susceptible to the combustion instability because of a narrower operating window and strong feedback between the heat generation and the heat loss rate. Either preheating the upstream gas or radical injection can increase flame speed [3,4] by reducing the activation barrier of hydrocarbon oxidation reactions, improving the heat release rate and flame stability. Zero-dimensional chemical kinetic model calculations predict that nearly 1% fuel dissociation is required to enhance combustion [3,4]. These calculations are consistent with the H atom radical depletion reaction with the fuel. In a premixed flame, the cross-over temperature is defined by the condition where radical production is balanced by radical depletion. Therefore, for flame propagation it is necessary for fuel to be depleted, which usually defines the location of the reaction zone. The use of electrical energy input to modify combustion kinetics has been pursued since the 1970's. Those early research results are described by Bradley [6]. Recent advances in non-equilibrium high E/n plasma generation, where E is the electric field and n is the gas density, at high pressure have prompted research on the application of energy-efficient radical production to enhance hydrocarbon-air combustion [7-9]. The direct electron impact dissociation of hydrocarbon fuel and/or O2 molecules produces radicals with very low reaction activation barriers for fuel oxidation, e.g., O2+e -->2O+e, leading to CnHm+O -->CnHm-1+OH and CnHm+OH -->CnHm-1+H2O (and other similar sets of reactions with partially dissociated fuel) initiating heat release below the typical cross-over temperature required for thermal dissociation of fuel and oxidizer. Similar low activation energy reactions are also possible by electron impact dissociation of hydrocarbon CnHm+e -->CnHm-1+H+e, where a chain propagation reaction H+ O2 -->OH+O can be initiated at lower gas temperature than under thermal equilibrium kinetics. A properly designed nonequilibrium plasma source could, therefore, possibly enhance combustion kinetics and reignition under normally adverse condition. The investigation of the scaling laws of nonequilibrium plasmas for energy efficient radical production is relevant for improving combustor operating conditions over a wider range of operating conditions than currently feasible. The contractor shall quantify the effects of nonequilibrium plasma chemistry on both flame speed increase and flame holding improvement, especially in vitiated air. Quantitative optical spectroscopic measurements should be performed to measure heat release rate or gas temperature modification by nonequilibrium plasma kinetics. As discussed above, quantitative measurements of super-thermal H atom, O atom, and OH radicals are essential for combining plasma kinetic models with combustion kinetic models to develop a predictive capability for non-thermal plasma-assisted combustion efficiency improvement in vitiated air and in high-altitude operation of turbine engines. Also, volume and pressure scaling properties of high reduced electric field plasmas produced by high-voltage short time duration pulse excitation will be quantified by accurate measurements of energy deposition efficiency in various hydrocarbonair gas mixture plasmas. PHASE I: Demonstrate quenching-corrected measurements in a laboratory-scale non-equilibrium plasma-assisted combustion rig in 2D imaging capability, by key plasma-chemical radical and intermediate species concentrations (O, H, & OH). The approaches should for high precision and accuracy, including mitigation of undesirable photolytic or other interfering processes, and strategies for absolute calibration. PHASE II: Develop and demonstrate quenching-free measurements of heat-release rate in a turbulent reacting-flow environment assisted by plasma ignition, to be performed along with the key radical species described in the Phase I activities. Use experimental data to validate a plasma-assisted combustion kinetic model. Solve laser optical and molecular with mass spectroscopic for reactive species obtianed during initiating of hydrocarbon chain branching reactions at gas temperatures below 800 K. PHASE III: The developed experimental methods and the validated model can be used for the development of next generation war fighters capable of flying at higher altitudes or significantly higher speeds. Capabilities are useful to the engine manufacturer for development of high-altitude propulsion vehicles. REFERENCES: 1. N. Docquier and S. Candel, Progress in Energy and Combustion Science 28, 107 (2002). 2. W. Krebs, P. Flohr, B. Prade, and S. Hoffmann, Combust. Sci. Tech.174, 99 (2002). 3. K. Takita, G. Masuya, T. Sato, and Y. Ju, AIAA Journal 39, 742 (2001). 4. Y. Ya. Buriko, V. A. Vinigradov, V. F. Goltsev, and P. J. Waltrup, J., Propulsion and Power 16, 1049 (2002). 5. F.A. Williams, Progress in Energy and Combustion Science 26, 657 (2000). 6. D. Bradley in: F. J. Weinberg, (ed.) Advanced Combustion Methods, Academic, New York, 1986. 7. W. Kim, M. G. Mungal and M.A. Cappelli, Combust. Flame 157, 374 (2010). 8. A. Starikovskiy, AIAA 2012-0828, 50th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition, 9-12 January 2012, Nashville, TN. 9. I. V. Adamovich, I Choi, N Jiang, J-H Kim, S Keshav, W R Lempert, E Mintusov, M Nishihara, M Samimy and M Uddi, Plasma Sources Sci. Technol. 18 034018 (2009).
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