DROPLET SIZE AND VELOCITY MEASUREMENTS IN A CRYOGENIC JET FLAME OF A ROCKET-TYPE COMBUSTOR USING HIGH-SPEED IMAGING

摘要

Injectors of cryogenic liquid rocket engines produce large polydisperse and dense sprays due to the pressure and mass flow conditions. Atomization is the dominating process that drives the flame behavior in cryogenic jet flames, when the propellants are injected in subcritical conditions. The main objective of this study is the characterization of a liquid oxygen (LOX) spray in gaseous hydrogen (GH2), in reacting conditions. In the breakup region where liquid particles are not spherical, laser based drop-size techniques suffer from a low validation data rate; thus imaging techniques can be better suited to characterize the spray. High-speed shadowgraphs were used to provide the spray characteristics such as sizes and velocities of the LOX dispersed phase atomized by a GH2 co-flow injected by a shear coaxial injector in a 1 MPa combustion chamber. Cryogenic combustion investigations presented in this paper were carried out on the Mascotte test bench, at Onera. The reacting case was compared qualitatively to a cold flow test, with gaseous He instead of GH2, for which LOX spray shadowgraphs were also recorded. The cold LOX jet was constituted by an envelope of small droplets around the LOX core whereas in the reacting case, those small droplets were not present and bigger liquid structures were revealed, due to vaporization by the flame. Moreover, the difference in terms of gas density between the hot fire and the cold case led to a reduction of the aerodynamic forces which can also explain that droplet velocities, at the injector exit, were higher in the cold case. Velocities of the dispersed phase in reacting conditions were obtained with two different imaging methods, which were applied to the same shadowgraphs: a PTV algorithm and a PIV software, developed at Onera. Both methods agreed well as soon as the droplet density is high enough and they showed that droplet velocities decreased by a factor of 3 in the area investigated. Droplet sizes were measured by image processing in the atomization zone and results were compared with Sauter mean diameters obtained with phase Doppler measurements from the literature. The droplet size is combined with PTV to obtain droplet size/velocity correlations which show that velocity of the smallest droplets decreased more rapidly than velocity of the biggest one, as the axial distance from the injector increased.