Breakup And Atomization Mechanism Of Liquid Jet In Supersonic Crossflows

Based on the liquid hydrocarbon fuel injection in the combustion chamber of scamjet,the research of liquid jet in supersonic crossflows(LJISC)was presented in this thesis.By combining experimental study and theoretical analysis method,the atomization process of LJISC was studied,aiming to reveal the jet column deformation and breakup mechanism,and grasp the inherent regulation of breakup and atomization.Firstly,the gas flow field and liquid flow field of LJISC are recognized from the macro structures.The clear bow shock,separation shock,and separation zone were captured by high speed schlieren method.The standoff distance,angle,and oscillation characteristics of bow shock,and separation zone size were analysed quantitatively.PIV method was used to ‘frozen' image of liquid jet/spray on the center symmetry plane in a supersonic crossflow.The typical structures,such as surface waves,spray block,coherent vortex structures and so on,were observed.Thus the liquid flow field was clearly proposed to be divided into three regions according to the structure characteristics: breakup zone dominated by surface waves,quick atomization zone and uniform mixing zone.Then,the liquid flow field was researched as a key point.The thesis developed the pulsed laser background imaging technique based on PIV equipment.Due to the Tyndall scattering effect of the sol medium,a uniform pulsed light source is formed by eliminating the interference effect of laser.The clear image of supersonic jet/spray was captured with high spatial and temporal resolution.The surface wave structure in LJISC was confirmed existence experimentally,The spray block structure,droplet peeling phenomenon and wiredrawing phenomenon were also discovered.The time evolution characteristics of the surface waves and spray block structures were analysed.Based on the image feature matching method,the velocity distribution and acceleration characteristics along the path were studied.The internal reasons of the formation and development of the flow structure in near-nozzle area were revealed by force analysis.The droplet size distribution and velocity distribution in the spray field after primary breakup were measured using the Phase Doppler Analyzer(PDA)and PIV technology.the time evolution of coherent structures in uniform mixing zone was analysed,and the jet / spray velocity field was also studied.In detail,the spray droplet size distribution was measured,the statistical distribution rules on a single spatial piont,and the change rule of Sauter mean diameter(SMD)along the spatial variation was analysed under different working conditions.The competition mechanism of small droplet coalescence and large droplet breakup in the atomization process of LJISC were explored according to the spatial distribution of the velocity and the particle size.Furthermore,the author Proposed the concept of ‘boundary band',and defined a new nondimensional parameters,spray fraction(?),to quantitatively describing the oscillation distribution and concentration distribution.of the jet/spray.Based on results of imaging experimentation,the jet/spray longitudinal spatial distribution was researched.The spray fraction distributions under various operation parameters were calculated.Thus a vertical boundary band model was established to quantitatively describe the jet/spray boundary oscillation process and the concentration distribution.combined the conception of spray fraction,the cross-sectional distribution was investigated using PIV method.It was found that the cross-sectional distribution was displayed as the shape of ‘?'.The mathematical method was firstly used to describe the cross-sectional distribution,and the three-dimensional model of LJISC was also established.