Study On Fuel Injection And Flow Characteristics Under Supercritical Conditions For Engine

In order to improve thermal efficiency and reduce pollutant emissions of engines,many technologies,such as high injection pressure,high boost pressure and high compression ratio,have been used widely for advanced combustion technologies,which further increase the temperature and pressure of fuel atomization in the combustion chamber.As a result,the pressure and temperature in the cylinder exceed the critical point of fuel in the fuel injection atomization process.Many experimental results show that the fuel injection mixing mode under supercritical conditions is quite different from the jet breakup evaporation mode under subcritical conditions.The mechanism and characteristics of supercritical injection need to be further explored and studied.In this paper,theoretical analysis and numerical simulations are performed to study the jet mixing characteristics for the supercritical injection of n-heptane.The characteristics of supercritical injection and the influence of key factors on the supercritical injection are revealed.The research work is of great significance for deepening the theoretical research of fuel injection atomization and extending the fuel spray mixing model,and will provide theoretical basis for the design and development of new clean and efficient engines.The thermodynamic properties of n-heptane and n-heptane-nitrogen mixture under supercritical pressure were calculated and analyzed.The high pressure gas-liquid equilibrium calculation method combining phase stability determination with phase separation calculation was studied,and the gas-liquid equilibrium characteristics for the isobaric adiabatic mixing of n-heptane and nitrogen under supercritical pressure were analyzed.A calculation model of gas-liquid interface characteristics was established based on the nonlinear density gradient theory,and the characteristics of interfacial thickness and interfacial tension of mixture at gas-liquid phase equilibrium state with different temperatures and pressures were analyzed.The results show that the thickness of gas-liquid interface increases exponentially with increasing temperature and increases linearly with increasing pressure,and the interfacial tension decreases exponentially with increasing temperature and decreases linearly with increasing pressure when liquid n-heptane is injected into nitrogen environment and the mixture is at gas-liquid equilibrium state.A numerical simulation model of supercritical injection based on gas-liquid equilibrium was established.The determination method of thermodynamic state region and the calculation method of thermodynamic parameters for binary mixture in gas-liquid equilibrium state were proposed.The research on numerical solution method of supercritical injection model was carried out.The Taylor expansion was used to linearize the thermodynamic relationship between density and pressure,and the pressure Poisson equation was derived.The thermodynamic parameter updating calculation was coupled in the pressure solving cycle,and the PISO algorithm for supercritical jet flow was established.An adaptive discrete scheme combining TVD scheme and KT scheme was constructed with mesh interface Mach number as adaptive coefficient.The simulation model and numerical method of supercritical injection were verified by the experimental data of liquid n-heptane injecting into supercritical environment and supercritical n-heptane injecting into subcritical environment.The numerical simulation of the jet characteristics for liquid n-heptane injecting into supercritical environment was carried out.The distribution characteristics of component concentration in the jet and the thermodynamic characteristics of the gas-liquid mixing layer in the jet were analyzed.The macroscopic characteristics of the jet such as the gas-phase penetration distance,the liquid-phase penetration distance,the liquid core length and the spread cone angle were analyzed,and the influences of the environmental parameters,the injection parameters and the nozzle structure parameters on the jet characteristics were studied.The results show that large isobaric heat capacity and low sound velocity appear in the gas-liquid mixing layer of n-heptane jet.The concentration of n-heptane in the gas-liquid mixing zone and the fully mixing zone is Gaussian along the radial direction,and the relationship between the component and temperature is approximately consistent with isobaric adiabatic mixing.The gas-phase penetration,liquid-phase penetration and spread cone angle are suppressed,and the liquid core length is constant when the ambient pressure increases.The gas-phase penetration and spread cone angle are promoted,the liquid-phase penetration is suppressed,and liquid core length is constant when the ambient temperature increases but the ambient pressure is constant.The gas-phase penetration and liquid core length are promoted,but the liquid-phase penetration and spread cone angle are suppressed when injection pressure increases.The liquid-phase penetration is suppressed,the liquid core length is promoted,and the gas-phase penetration and spread cone angle are basically unchanged when the injection temperature increases.With the increasing of nozzle K-factor,the gas-phase penetration,liquid-phase penetration and liquid core length are promoted,but the spread cone angle is suppressed.With the increasing of nozzle diameter,the gas-phase penetration,liquid-phase penetration,spread cone angle and liquid core length are promoted.Isentropic expansion analysis and numerical simulation were carried out to study the flow characteristics of under-expanded injection of the supercritical n-heptane.The variations of nozzle exit parameters with injection pressure and temperature were analyzed.The influences of isentropic coefficient on nozzle exit parameters were revealed.The effects of injection temperature,injection pressure and nozzle diameter on the structure parameters of Mach disk were quantitatively analyzed.A fitting correlation of Mach disk distance for the n-heptane under-expanded jet was obtained,and the influence of nozzle exit pressure on Mach disk diameter was revealed.There are 78 figure,3 tables and 128 references in this dissertation.