Study On The Jet Breakup In Two-dimensional Shear Crossflow

In the aero-engine Combustor,liquid fuel is broken into liquid droplets under the combined action of aerodynamic forces and jet instability.The atomization process of liquid fuels is very complicated,especially the initial atomization process;and the accurate primary atomization mode has not proposed yet.Therefore,it is studied intensively of liquid cross jet breakup mechanism and process theoretically and experimentally.The linear instability analysis method is applied in the theoretical analysis.The dispersion equations of the jet are derived under the conditions of neglecting the viscosity of the jet?water?and considering the viscosity?water,alcohol,and glycerin-water mixture?.It is predicted of the jet breakup according to the growth rate of surface wave and the unstable wave number range.In the experimental study,the high-speed camera is used to analyze the characteristics of jet breakup by observing the dynamic process of breakup.The fluid is water and the jet diameter is 1mm.The research results are as follows:The growth rate of surface wave of inviscid liquid jet is dominated by jet velocity shear,transverse aerodynamic force and liquid surface tension.When the Weber number of air or liquid increases,the growth rate of jet surface wave increases significantly,while the optimal wavelength decreases significantly.The critical liquid to air momentum ratio indicates which instability of Rayleigh-Taylor?R-T?and Kelvin-Helmholtz?K-H?is the dominant factor in jet breakup.When the liquid to air momentum ratio is larger than the critical value,K-H instability is dominant;on the contrary,R-T instability is dominant.The two-dimensional shear flow has a velocity gradient in the jet direction.Under two-dimensional shear flow,the effect of air or liquid Weber number on jet breaking is similar.Compared with uniform airflow,shear flow only changes the effect of transverse aerodynamic force on jet surface wave.With the flow rate constant,the negative gradient shear flow can accelerate the jet breaking.Four kinds of jet breakup are observed and verified:cylinder breakup,bag breakup,composite breakup and shear breakup.In the uniform airflow,the jet trajectory presents a quadratic power relationship,which is only related to the liquid and air momentum ratio to the resistance coefficient;in the shear airflow,the jet trajectory presents a quartic power relationship,which is also related to the shear airflow velocity gradient.With the increase of air Weber number,the diameter of liquid drop formed by the breakup of liquid jet decreases.Compared with the uniform airflow,the diameter of the droplet formed under the shear airflow is smaller and the atomization effect is better.The growth rate decreases due to the consideration of viscosity of liquid.For water,alcohol and other liquids with less viscosity,the effect of viscosity on the growth rate is less,which can be considered as non viscous liquids.The viscosity of glycerin water mixture is equivalent to the inhibition of surface tension,and the effect of viscosity cannot be ignored.When the ratio of surface tension to viscosity reaches 10^-1,the effect of viscosity cannot be ignored.In two-dimensional shear flow,the effect of viscosity on jet breaking is similar to that of uniform flow.Viscosity can inhibit the K-H and R-T instability of cross jet breakup.With the increase of air velocity,the inhibition of viscosity to both increases.For different liquids,viscosity has a stronger inhibitory effect on K-H instability.