Mixing and Spray Formation in Coaxial Jets

The e ow regimes and e ne-scale structure of the mixture resulting from the destabilization of a dense, slow jet by a fast light annular jet are discussed. From the primary shear instability between the streams, it is shown that the momentum ratio M = of the annular to the inner stream is the key parameter 2 2 r u /r u 2 2 1 1 on which the inner potential core length and the condition of a recirculation transition depend. The instability analysis shows that the initial wavelength of the disturbances l is proportional to the vorticity thickness d of the fast stream at the nozzle exit l ; d(r1/r 2) 1/2 ; this further provides, by an original scenario, an estimate of the droplets’ size formed by the capillary instability of the sheet’ s rim formed from the initial interface disturbances. Liquid viscosity is not expected to play any signie cant role in practical conditions in liquid rocket propellant engines. These e ndings are put in relation with a selected review of known or yet unexplained results on the subject. HERE is a frequent need to realize a uniform mixture from two initially segregated streams in many practical instances. A signie cant example is the case of liquid rocket propellant engines because the length of the combustion chamber is limited by the ability of the injecting devices to fragment and mix the reactants down to a sufe cient level of homogeneity for evaporation and combustion to completion at the desired distance from the injector outlet. For technological reasons one of the reactants is usually available in the liquid state and the other in the gas phase; for historical reasons the coaxial geometry is commonly used to merge the two streams. 1 The relative e ow rates of the reactants must be adjusted so that the global stoichiometry is at least respected, or such that the gas phase is in excess for all of the liquid to vaporize and burn, this implies that the gas stream is usually much more rapid than the liquid stream at the injector outlet. This is the situation encountered in H 2/O2 engines, where a slow, dense liquid oxygen (LOX) stream in the central jet of the coaxial injector is surrounded by a fast, light, gaseous hydrogen annular stream. At the root of the interpenetration process between the two phases is a strong shear destabilizing the central liquid jet, which further fragments into a more or less uniform spray. This process, which is contrasted with the case of a simple jet issuing in a quiescent environment, 2 is known as airblast atomization. 3