An Investigation On Collision Dynamicsand Transition Boundaries Of Non-Coalescence Laminar Fluid Jets

Collision of two fluid masses is a common natural and industrial phenomenon that occurs,for example,in cloud and raindrop formation,as well as the spray and mixing processes within liquid-fuel combustors such as diesel and rocket engines.It has been found that two collisional liquid masses could present non-coalescence or even bouncing phenomena other than merging as normally expected.Jet collision,as an efficient method to spray and mix liquid fuel and oxidizer,is widely used in the liquid and hypergolic propellant rocket/missile engines.The possible non-coalescence behavior between collisional jets might prevent the mixing between fuel and oxidizer,and hence could decrease the ignition reliability and propellant efficiency.Therefore,it is crucial to quantitatively understand the dependence of the collision outcome on the collision parameters.Furthermore,jet bouncing is a stable,continuous and controllable non-coalescence phenomenon,providing an ideal platform for the study of the gas layer evolution and gas-liquid interface interaction in various non-coalescence behaviors.The existing studies on dynamics of collisional jets are mainly focused on the geometrical properties of the merged jets.On the contrary,the non-coalescence behavior of colliding jets,although was firstly discovered in more than one century ago,has not received much attention since then,our knowledge about which is consequently very limited.In this dissertation,the collision output,the collision dynamics and the corresponding dependence on the control parameters of the colliding jets are systematically investigated by combining experimental observations,theoretical analysis and numerical simulations.A unique experimental system with the precise jet control and pressure adjusting capability is designed and built up,and a set of experimental methods are developed to quantitatively study the collision dynamics of colliding jets.Technical grade water,ethanol and n-alkanes,namely,n-decane,n-dodecane,n-tetradecane,and n-hexadecane,are used as the working fluids to investigate the regime diagram of the collision phenomena.The mechanism and occurrence conditions of jet bouncing are uncovered,and the morphological evolution of the jets during and after collision is also theoretically analysed.Three non-monotonic coalescence and non-coalescence regimes,namely soft collision merging,bouncing and hard collision merging,are discovered.Thereby,the entire suite of possible outcomes of jet collision and the corresponding dependence on collision angle and impact velocity are uncovered and characterized.Based on the pressure balance and jet deformation analysis,a theoretical model is developed to understand the thickness evolution of the gas layer during collision.It is found that the high velocity part of the transition(hard transition)between jet bouncing and hard merging is dominated by the dynamics of the gas layer and then can be described by a dimensionless gas layer thickness KCr.By comparing the relative importance of the jet surface instability and gas layer instability,it's found that the former one,i.e.Rayleigh-Plateau instability,dominates the transition(soft transition)between jet bouncing and soft merging.This soft transition can be described by the ratio of the jet collision velocity and the characteristic transition velocity of the Rayleigh-Plateau instability.Furthermore,a non-monotonic response of the jet collision caused by pressure variation is also measured and analyzed by considering the joint effects of the gas entrainment and gas layer properties.This non-monotonic response of the collision output on pressure leads to a critical pressure PCr,at which the occurrence of the jet bouncing can be optimal.With the open source code,Gerris,which uses the joint method of N-S equation,VOF/PLIC interface capture and adaptive mesh method,the numerical simulations on bouncing jets are conducted.The detailed internal motion of the flow in the jets,the shape variation of the jets and the pressure distribution inside the jets are numerically studied.