An experimental and theoretical study, including frictional and heat transfer effects, of pulsed pressure-gain combustion
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AbstractIt has been noted, from experimental results reported in the literature, that a non-optimised. valveless pulsed pressure-gain combustor of 73 mm diameter has produced a stagnation-pressure rise in excess of 5 percent at a combustor stagnation temperature ratio of 3:1, One of the objectives of the present work was to optimise this pressure-gain configuration and hence explore the possibility of achieving an improved pressure-gain, Two fluidic devices, which capture and redirect all outflow (inlet backflow) leaving the inlet section of the pulsed combustor, were designed for the optimisation task, Various secondary duct systems, coupled to the fluidic device, were constructed and optimised experimentally. Theoretical studies available in the literature show the lack of an adequate analytical tool suitable for designing simple inlet, valveless pulsed combustors. A second objective was therefore, to initiate the analytical approach presented in this study in an attempt to alleviate this shortcoming. The theoretical analysis established a new numerical technique to predict the non-steady flow of a compressible fluid, with frictional and heat transfer effects, in a duct with a spatially varying cross-sectional area. The difference equations derived and the numerical technique, are adaptable to a wide range of unsteady flow problems. The numerical procedure, as applied to the pulsed combustor, incorporates a simple heat release scheme to model the combustion process. Quasi-steady boundary conditions were developed as well as expressions for the pressure and temperature, versus time relationships in the combustion chamber. The numerical technique was found to be capable of converging to cyclic operation within five to six pulsed-combustor cycles as modelled by the computational procedure. The theoretical model produced information resulting in the generation of quantitative evaluations of: mass flows, temperatures, pressures, fuel consumption, thrust developed, fuel air ratios and the influence on the foregoing performance parameters of geometrical changes. It was found that the theoretical predictions obtained compared very well with experimental data gathered from various sources. The main conclusion derived from the experimental portion of the study shows that a stagnation-pressure rise of 5 percent, at a combustor stagnation-temperature ratio of 2.7:1, is the ultimate to be expected from the present pressure-gain configuration having a combustion zone diameter of 73 mm. The complexity of the combustion process in the combustion chamber of a pulsed combustor coupled with the current lack of knowledge associated with mixing, kinetics of reaction and non-equilibrium effects makes a detailed theoretical analysis, based entirely on first principles, of an actual pulsed combustor practically impossible at the present state of knowledge. This situation will, presumably, only be relieved when an adequate combustion model, taking into account the previously mentioned, has been developed.
Bibliography: p. 221-226.