ILASS-Europe 2002 Zaragoza 9 - 11 September 2002 INFLUENCE OF J AND WE NUMBER ON GH 2 /LOX IGNITION PROCESS V. Schmidt * , O. Gurliat † , M. Oschwald * and O. J. Haidn * email: Volker.Schmidt@dlr.de * German Aerospace Center (DLR), Space Propulsion Institute, D-74239 Hardthausen, Germany † Snecma Cooperant at German Aerospace Center (DLR), Space Propulsion Institute, Snecma Rocket Engine Division, Forˆ et de Vernon - BP 802, F-27208 Vernon Cedex, France Abstract First results obtained from investigations of the transient ignition process of a cryogenic GH 2 /LOX spray in a model rocket combustion chamber are reported. Ignition of the propellants has been initiated with a pulsed laser and ignition and flame stabilization process have been analyzed using high-speed visualization methods. Local flame velocities as well as local convection velocities at the point of ignition are determined by image displacement velocimetry methods. During the ignition process four distinct phases could be observed. 1 Introduction Although the problem of ignition in rocket engines has been of interest for years, detailed insight into the ignition transient is still missing. High speed visualization techniques have been successfully applied for combustion of hydrocarbon sprays [1] but has not being used for H 2 /LOX spray ignition due to high data rates required for. Recent investigations of high-pressure cryogenic combustion have mainly been focused on stationary conditions up to now [2, 3, 4, 5]. Hence, only very little experimental data are available on transient ignition phenomena in cryogenic rocket combustors. No systematic investigation of the influence of the injection conditions on the evolution of the flame, the interaction of the flame with the atomization process of the liquid oxygen, and the flame stabilization process is known. The objectives of this study were to visualize the entire ignition process, identify the relevant physical pheno- mena, and determine the effect of injection conditions on the ignition behaviour. At stationary cold flow conditions the LOX/GH 2 -spray has been ignited by laser-induced gas breakdown. Simultaneous imaging of the OH-emission and Schlieren photography during the ignition process delivers information on the evolution of the flame and the flow field. From high-speed visualization of the flame front local burning velocities as well as convection velocities can be obtained. Furthermore, the temporal evolution of the distance of the upstream flame edge from the injector is the main feature to characterize the flame anchoring and flame stabilization process. The use of high speed video recording sets a synchronization problem. The reduced observation time allowed by this diagnostic method requires an exact synchronization between ignition and recording so that the classical ignition methods such as pilot flame are not possible [6]. With laser ignition it is possible to exactly synchronize the time of ignition with high speed data acquisition. Furthermore, as compared to electric spark ignition which has been used by Quintilla et al. [7] laser ignition gives high flexibility in choosing the location of energy release. Additionally, temporal evolution of combustion chamber pressure and flame emission intensity are applied for a further characterization of the different phases of the process. 2 Experimental set-up 2.1 The micro-combustion chamber M3 The M3 micro combustor is a cryogenic test bench for optical investigations on LOX/GH 2 spay combustion. Two fast opening valves of 5 ms opening time guarantee a short injection transient. The injector used is a single shear coax injector without recess and tapering. The geometry of the LOX post is fixed while the outer diameter on the H 2 side can be varied in order to vary the flow parameters at the injector exit (J , We, Re). Nozzle diameter, and propellant supply pressures are then adjusted to keep the chamber pressure constant. For more details about the test bench, see [3, 8]. The figure 1 shows a schematic of the M3 combustion chamber with a single injector arrangement. Planar quartz glass windows on both sides of the combustor are used for flame observation while a small upper slit quartz window allows an optical access for the igniter laser.