| Flow focusing/blurring nozzle is a new type of two-phase flow nozzle,which can be used not only for liquid fuel spray,but also for monodisperse droplet preparation,droplet evaporation and combustion research.It has wide application background and application prospects.Due to the particularity of structure,flow focusing/blurring nozzle have different flow modes and spray characteristics under different structure parameters and flow parameters.The study of the flow mode and the spray characteristics can further deepen the understanding of gas-liquid interaction and spray mechanism.In this paper,the flow modes,the flow morphology and characteristics and the characteristics of jet breakup and spray were studied by combining experimental and numerical simulation methods.The research has important academic significance and engineering value.Based on the structural forms and structural parameters of nozzle at home and abroad,a visualized flow focusing/blurring nozzle design is carried out,and the flow and spray test platform was built.The characterization parameters of gas-liquid flow,gas-liquid interaction and spray were determined,and the experimental image acquisition method and characterization parameters extraction method were given.A numerical model of gas-liquid two-phase flow in a flow focusing/blurring nozzle was established and verified.The flow morphology inside flow focusing/blurring nozzle were studied by large eddy simulation and experiments.Combined with flow pattern diagram,the flow modes of nozzle were studied by experimental method.The results show that the gas-liquid flow inside the nozzle presents different flow modes and flow morphologies with different flow parameters or structure parameters.The gas-liquid interaction is the main reason for the change of flow morphology under different flow modes.The flow focusing mode mainly appears in the condition of small liquid and gas flow rates,and the flow blurring mode mainly appears in the condition of small liquid and large gas flow rates.The regulation of gas fluid,the unsteady fluctuation and breakup of liquid jet inside the nozzle,the influence mechanism of gas fluid on jet breakup and the flow parameter pulsation at the nozzle outlet were studied by visualization experiment and numerical simulation.The results show that the gas flow rate can effectively control the liquid jet shape,liquid jet breakup and gas-liquid mixing.The viscous shear force and the surface pressure of liquid jet are two forces which have important effect on the breakup of the liquid jet and the transformation of the flow mode.The relationship between the unsteady fluctuation of liquid jet and the flow rates of gas and liquid,as well as the relationship between the pulsation characteristics of flow parameters at the nozzle outlet and the flow rates of gas and liquid were determined.The morphology and characteristics of the spray,the breakup characteristics of the spray and the correlation between the spray morphology and the flow mode of the nozzle were studied by visual experiment and Malvern laser test method.The results show that the spray morphology can be divided into droplet,oscillating splitting,oscillating breakup and atomization.In droplet mode,the jet breaks at the valley of unstable surface fluctuation.When the gas flow rate is small,broken droplets with alternating sizes are generated.When the gas flow rate is large,basically the same broken droplets are generated.In the oscillatory splitting mode,the jet breakup to produce liquid filaments,and droplets of different sizes are generated through liquid filaments breaking.Through the study,the variation of gas-liquid mixture consistency,spray half-cone angle,spray core length,and particle size of droplet with gas and liquid flow rates,as well as the spatial variation of particle size were clarified.A spray numerical model of flow focusing/blurring nozzle was established based on the flow pulsation at the nozzle outlet.On this basis,the formation mechanism of spray morphology,the space-time development of spray morphology and its influencing factors,as well as the space-time development characteristics of jet breaking and gas-liquid mixing were studied.The results show that the rebounce-collapse-rebound of gas fluid in spray is the main reason for the expansion-contract of spray.The weakening of axial penetration force at the center of spray is the main reason for the "flat" spray head.The driving effect of gas fluid on liquid fluid is the main reason that the spray penetration distance increases with the decrease of liquid flow rate.Through the study,the variation rule of spray half cone angle and penetration distance with the gas and liquid pulsation at the nozzle outlet and the flow rate of gas and liquid,the variation rule of the gas liquid mixing degree and the droplet breakup degree with the space position and the flow rate of gas and liquid were clarified.There are 128 figures,11 tables and 171 references in the dissertation. |
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