Investigation On The Breakup Mechanism Of A Power-Law Liquid Sheet Into Confined Gas Streams

With the increasing crisis of fossil fuels,more and more non-Newtonian powerlaw fluids,are always involved in engineering applications,such as gelled propellant,coal water slurry,and some alternative fuels(sunflower oil).With the effect of nonNewtonian features,it shows many new breakup characteristics different from that of Newtonian fluids.Accordingly,it is of great importance to investigate the breakup mechanism and improve the atomization quantity of power-law fluids.The gasatomizer spray is extensively employed in engineering applications,such as internal combustion engines,gas turbines and swirl burners.In the current work,the theoretical and experimental investigation has been combined to reveal the breakup characteristic of the power law fluid ejected from a gas atomizer nozzle.This will help to the design and investigation of combustion systems and fuel injection systems in internalcombustion engines taking non-Newtonian fluids as fuels.Based on the jet perturbation theory,a physical model was deduced to characterize the instability of power-law annular jets into bounded compressible gas streams.The effect of various gaseous feature parameters on jet instability with two disturbance modes,i.e.para-sinuous mode and para-varicose mode was investigated.Results show that the gas-liquid velocity difference is a key parameter to control the breakup of power-law fluids.With the increase of the gas-liquid velocity difference,the interface wave is gradually dominated by a shear wave,also named as Kelvin-Helmholtz originated from the relative motion between both phases.This wave is more unstable and the wave length is shorter.The gas confinement has an unstable effect on the annular jet,while the promotion will become weak with the gas velocity increasing.The gas compressibility can more significantly destabilize the power-law annular jet with a larger gas-liquid velocity difference.With the influence of flow conditions and rheological features,two disturbance modes will compete with each other to dominate the disintegration of power-law fluids.Then,the effect of gas confinement and asymmetry on the instability of powerlaw plane jets was analyzed in detail.The gas asymmetry is caused by the asymmetric distribution between two gas boundaries or densities,called as the boundary asymmetry and aerodynamic asymmetry,respectively.It is found that a limited gas environment will strengthen the interaction degree between gas-liquid phases and destabilize the power-law plane jet.The power-law fluid is easier to disintegrate into droplets in the asymmetric gas medium than in symmetric case.With the increment of the total disturbance energy of two media on both sides of a plane jet,the enhancement effect of the aerodynamic asymmetry will become more obvious.For a large Weber number,the aerodynamic asymmetry is a more effective and unstable factor than the boundary asymmetry.Besides,the effect of the aerodynamic asymmetry on the unstable range was studied.Finally,the breakup regime and characteristic of power-law fluids ejected from a gas-atomizer nozzle was investigated experimentally by using High speed photograph with shadow image method.Results show that with the increase of velocity difference of gas-liquid phases,the breakup regimes could be classified into four species,i.e.cluster-bobble,closed bobble,open bobble(annular liquid sheet)and complete breakup.Importantly,the gas-liquid velocity difference is a key parameter to differentiate the breakup regimes and control the breakup process of power-law fluids,which agrees well with the analysis results.Besides,it is found experimentally that the internal flow of injection system,shear-thinning feature and relative motion of gas-liquid phases are the three leading factors to influence the breakup of power-law sheets.The aerodynamic force could effectively improve the atomization quantity of power-law fluids.