Theoretical And Experimental Study On The Breakup Mechanisms Of Power Law Liquid Jets

Liquid jets are a widespread phenomenon in the nature and the engineering applications. The breakup and atomization of liquid jets are of theoretical and practical importance. The fluids in the nature can be divided into Newtonian fluids and non-Newtonian fluids, and the most is the non-Newtonian fluids. For the breakup of Newtonian fluids, the researchers have carried out extensive and in-depth research. But for the non-Newtonian fluids especially for the power law fluids, due to its complexity and diversity, there are a lot of problems that have been understood. In this paper, the breakup of power law fluid jets is investigated theoretically and experimentally in order to reveal the mechanisms and characteristics of jet breakup.First, based on the linear instability theory, a cylindrical jet and a thin sheet jet of power law fluids are investigated, respectively. The corresponding dispersion relations between the growth rate and the wavenumber of disturbed waves are derived after considering the rheological relation of power law fluids. They are available for both the low-speed jets and high-speed jets, also for both the shear-thinning and shear-thickening fluids. By solving the dispersion relations using temporal mode, the effects of several jet parameters on the instability characteristics of the jet, including the maximum growth rate, the dominant and cut-off wavenumbers, are studied.The results reveal that the breakup of power law liquid jets is the result of interaction between the inertia force, viscous force, surface tension and gas-liquid interaction force. Under different jet conditions, the action of different force is different. For the cylindrical jets, in the low-speed jets i.e. Rayleigh mode, surface tension promotes the breakup. The liquid viscosity and power law exponent prevent the jet from breaking up. In the high-speed jets, i.e. Taylor mode, surface tension, viscosity and the power law exponent prevent the jet from breaking up, while the gas-liquid interaction significantly promotes the breakup process.For the sheet jets, there are two kinds of disturbance modes: varicose mode and sinuous mode. In varicose mode, the viscosity, surface tension and power law exponent of the liquid prevent the sheet from breaking up while the gas-liquid interaction promotes the breakup process. In sinuous mode, surface tension prevents the liquid sheet breakup while the gas-liquid interaction promotes the breakup. The effect of viscosity and power law exponent on the breakup is small. Usually, the maximum growth rate of sinuous mode is greater than that of varicose mode. So sinuous mode is dominant. Moreover, for both cylindrical and sheet jet, the instability of Taylor mode is stronger than that of Rayleigh mode. The jet under Taylor mode is easier to disintegrate. And a liquid jet with a smaller power law exponent is easier to break up.Secondly, based on the above work, according to the instability theory, the prediction modes of breakup characteristics of power law fluids for both cylindrical jet and sheet jet are established. They can be used to predict the breakup length and breakup scale, so that it is capable to control the jet more effectively. It is of importance for the practical applications of the power law fluid jets.Finally, a jet experimental system that is made up of liquid transporting and jetting subsystem, environmental parameters controlling subsystem, appearance characteristics measuring subsystem and droplet diameters measuring subsystem, has been established and three kinds of shear-thinning fluids are studied in the experiment. By applying high-speed imaging technique, spray process and characteristics of both the cylindrical jet and sheet jet are obtained. The effect of several parameters on the spray characteristics has been investigated. By using the phase Doppler analyzer, the droplet diameter and distribution in the open environment are measured.The experimental results show that, increasing the jet velocity will reduce the jet breakup length and increase the spray cone angle, and promote the breakup of jet. Increasing the backpressure or using SF6 as background environment will increase the gas-liquid density ratio, which will promote the breakup. The fluid with smaller consistency coefficient and power law exponent is easier to disintegrate. For the cyclindrical or sheet jets, the SMD will decrease with the increase of the injection pressure. At the same axial position, SMD is the symmetrical distribution along the radial direction, and the greater the radial displacement is, the bigger SMD is. The experimental results accord well with the theoretical prediction results.