<title>Theoretical and experimental studies of laser-initiated detonation waves for supersonic combustion</title>

Abstract

ABSTRACT Spherical detonations of C2H/OdN2 mixtures in an open flow system (initially at 1 atmosphere)and planar detonations of C2HdO2 and HdOdC2H2 mixtures in an enclosed tube are successfully initiatedby use of an ArF laser at 193 nm. The required critical energy for the initiation of spherical detonationsis found to be relatively low .l2 mJ for a 40% C2H2 in C2H,J02 mixtures. This small critical energymay be attributed to a relatively strong absorption of C2H2 at 193 nm, and possible enhancement by thephotodissociation products of C2H and H. The initiation appears to be accomplished without overdrivingthe mixtures through a blast wave. The critical energy, delay time, detonation velocity and pressures aremeasured as functions of stoichiometric mixture ratio, initial pressure and incident laser energy, for bothspherical and planar detonations. 1 . INTRODUCTIONHigh-speed air-breathing combustion systems necessitate the release of chemical energy duringthe relatively brief residence time of reactants within combustors of practical length. A supersonic-combustor concept based on the nonintrusive stabilization of a conical-detonation wave by means ofrapidly-repeated laser pulses is proposed1, in order to achieve rapid combustion. Each pulse, non-intrusively, directly initiates, at a fixed site relative to the combustor, a spherical detonation wave in thecombustible mixture flowing at a supersonic flow speed. The interaction of a train of such sphericalwaves results in the stabilized conical wave. This combustor concept differs from other oblique-detonation-wave-engine (ODWE) concepts proposed previously. Other ODWE concepts rely on theintroduction of an intrusive body into the supersonically flowing stream of combustible mixture toinitiate shock-induced combustion. This disadvantage associated with the introduction of a solid body isthe potentially greater entropy rise, which is a measure of the energy unavailable to do useful work.A goal of a supersonic combustor concept is to minimize the entropy rise associated with mixingand burning to generate net thrust. Our theoretical analysis1 indicates that, as the fixed finite frequencyof spherical-detonation-wave initiation is increased from one value to an indefinitely large value, theentropy increase associated with reflected shocks from the interaction of neighboring spherical detona-tions becomes progressively smaller. Furthermore, our analysis shows that, within a relatively shortstreamwise distance, the detonated mixture can be uniformized for exhaust at ambient pressure. Thissuggests that a supersonic combustor based on a relatively weak (oblique) detonation wave incurs rela-tively little entropy rise. If the combustor is enveloped with a streamlined sheath, a favorable thrust-to-drag ratio seems attainable.To demonstrate the proof of principle for the laser-initiated-oblique-detonation-wave-engineconcept, we have investigated the use of an AF excimer laser at 193 nm for the direct initiation ofdetonation waves. Most reports of the direct initiation of detonation by laser involve the use of either aruby laser at 0.694 nm2' or a neodymium-Yag laser at 1.06 Since these wavelengths are not inresonance with the reactive-mixture absorption, relatively high laser energy is required to generate astrong blast wave, which is then transformed into a detonation wave. In some cases, a metallic needlewas placed near the focal point to enhance, via surface breakdown, the coupling of the laser energy into