Experimental And Numerical Simulation Of The Breakup Process Of Swirling Conical Liquid Sheet

This dissertation aimed at the liquid conical sheet and liquid jet breakup processaffected by coflowing gas. When studying the liquid conical sheet breakup process, thegas-liquid coaxial injector adopted by a type of gas generator was chosen. The baselinegeometry of the injector was selected from the hot tests in which it behaved best.Theorical analysis, experimental study and numerical simulation were combinedtogether to study the formation, the development and the breakup of the swirl conicalsheet. Different geometry parameters were analyzed to investigate their influence on theswirl conical sheet. When studying the liquid column jet breakup process, the actualliquid coaxial shear injector was chosen and the primary breakup characteristics wereinvestigated under lower pressure drop.A transparent injector was designed to study the filling process of the liquid phasein the pressure swirl injector. Interface tracking method Volume of Fluid (VOF) wasused to simulate the flow field inside the injector. The liquid phase enters the tangentialinlets, oppressing the gas phase out of the injector. When the liquid phase flows out ofthe injector, the flow field inside the injector is established. It takes a majority of thetotal filling time to occupy the swirl chamber and the convergent section. The velocityand pressure field were also analyzed. The tangential velocity of the liquid phasedecreases when it passes the orifice section. Most of the total pressure loss exsits in theconvergent section and orifice section.Many cold tests were carried out to study the parameter influence on the injectorperformance, including the discharge coefficient, the spray cone angle and the SauterMean Diameter (SMD). The spray cone angle was measured using the image processingprogramme to analyze images of the spray at the exit of the injector. A non-dimensionalparameter was defined to do a quantitive evaluation of geometry parameters’influence on the spray performance. Finally, new formule of the discharge coefficient,the spray cone angle and the SMD were deduced, which consider the geometryparameter A, the ratio of the diameter of the swirl chamber to that of the central post,and ratio of the length of the central post to the diameter of the central post and othergeometry parameters. The new formule give some refrences in the pressure swirlinjector design and supply the initial condtition of the numerical simulation of the gasgenerator.A spray cutter was designed to improve the back-illumination method, which canexclude the interaction between the liquid on the opposite sides of the conical liquidsheet. The spray cutter behaves better under the high pressure drop operating condition.Different methods, including the high-speed shadowgraph, and the high-speedback-illumination combined with the spray cutter, were used to investigate the breakupprocess of swirl conical liquid film. The surface of the conical liquid sheet breaks upwith the perforation pattern at lower Weber number, and with turbulent fragmentation patten at larger Weber number.Image proceeding codes were programmed to measure the primary breakupcharacteristics of the swirl conical liquid film. A non-dimensional parameter wasdefined to evaluate the liquid fraction at different distance to the exit of the injector. Thebreakup length was achieved through the high-speed shadowgraph images and thewavelength was got through the high-speed photography images with the spray cutter.The relationship between the breakup length and Weber number, also the wavelength atthe breakup point with the Weber number were achieved. Finally, the breakup lengthformula was achieved based on Han’s formula. The coefficient of the breakup lengthhas much to do with the geometry characteristic parameters of the pressure swirlinjector.The laser sheet technique was used to investigate the breakup process of swirlconical sheet with the coaxial gas. Larger droplets lie at the boundary of the liquid sheet,smaller droplets are enrolled by the surrounding gas into the center of the spray. Thecross-correlation algorithm was used to compute the flow field. Inside the primarybreakup region, the velocity at the center of the conical spray is smaller than that at theconical spray boundary. The velocity profile appears as U-patter. The radial velocity iscentrosymmetric, increases as the coordinate increases. Affected by the coflowing gas,the smaller droplets are accelerated more markedly than the larger ones. From the centerof the conical sheet to the spray boundary, the velocity of droplets decreases firstly, thenincreases. The velocity profile appears as W-patter.High speed photography was used to study the breakup process of gas-liquidcoaxial shear injector. Two kinds of breakup mechanism were found. One is the wavebreakup, the other is the peeling mechanism. The relationship between SMD and thewavelength at breakup point was deduced.When gas was added around the liquid, the liquid surface tends to be more instableand the liquid phase breakup earlier. Meanwhile, the turbulent intensity increases, whichaugments the turbulence transport and the SMD minishes correspondingly.