Study On Air-Assisted Injection Atomization Characteristics Of Two-Stroke Aviation Heavy Oil Engine

The two-stroke piston heavy oil engine has high power-to-weight ratio,simple structure and high reliability.It is the preferred power for small and medium-sized unmanned aero-engines.However,heavy oil has high viscosity and is difficult to evaporate.How to realize the atomization of heavy oil has become a key issue in the development of two-stroke aviation.Air-assisted injection is a low-pressure atomization technology,which can achieve good atomization of high-viscosity fuel at lower injection pressure by means of air flow movement inside the nozzle.Moreover,the air-assisted spray penetration distance is short,which can weaken the "wet wall" problem caused by high-pressure direct injection of fuel,so it is widely used in heavy oil two-stroke aeroengines.Air-assisted injection used in two-stroke aircraft includes two sequences: fuel injection and air injection.The atomization quality of fuel is significantly affected by two injection sequences.In this thesis,the spray characteristics of an air-assisted ejector launched by two-stroke heavy oil are studied by numerical simulation and experimental verification.The effects of fuel injection,air injection and two injection intervals on the interaction of heavy oil/air two-phase flow and air-assisted spray are analyzed.The main contents and conclusions are as follows:(1)The constant volume bomb test platform was built,and the spray image and particle size data of the air-assisted ejector were obtained by high-speed camera and Malvern laser particle size analyzer.The macroscopic and microscopic characteristics of the spray were analyzed.According to the basic structure and working principle of the air-assisted ejector,the simulation model of the air-assisted ejector system is established by using the Fluent numerical simulation platform.Then,the reliability and accuracy of the simulation model are verified by comparing the results of the test and simulation.(2)Fuel metering is the most important function of fuel injection.The fuel injection process has three fuel supply stages: initial fluctuation,medium-term stability and later rapid cut-off.Among them,the medium-term stable oil supply stage has a large oil speed and a long time,which directly affects the fuel injection volume of a single cycle.With the increase of injection pressure,the fluctuation time of fuel supply is shortened,and the mass flow rate of fuel injection is obviously increased.The fuel injection quantity can be quantitatively expressed as a function of injection pressure difference,injection pipe diameter and fuel injection time.(3)Different air injection pressures produce two forms of spray.Under the condition of high jet pressure,the spindle-shaped spray is produced,and the fuel droplets are broken well and the penetration distance is long,but the spray transverse development distance is short and the spray cone angle is small.The mixture of fuel and air is mainly distributed near the axis of the spray.Under the condition of low jet pressure,the anchorshaped spray is produced,which has good atomization effect,short penetration distance,long lateral diffusion,more uniform distribution in the upper part of the constant volume bomb,and the droplets are not easy to aggregate in the later stage of spray development.It is more suitable for small cylinder diameter and short stroke two-stroke piston heavy oil engine.(4)The length of the interval between two injections determines the breakage and main distribution position of the fuel in the premixed chamber,thus affecting the fuel ejection characteristics after the air injection is turned on.If the interval time is too long,a large amount of oil will be deposited near the air nozzle valve,so that a large amount of fuel will be ejected when the air nozzle is started,which will affect the early spray quality.If the interval time is too short,the air injection duration will be prolonged accordingly.The ambient back pressure has a great influence on the spray pattern.The higher ambient back pressure will increase the density of air in the constant volume bomb and inhibit the longitudinal development of the spray,but strengthen the lateral diffusion of the spray.The injection time determines the amount of single cycle fuel supply.The more the amount of fuel,the greater the spray density,the lower the nozzle speed,the weakening of the front-end vortex movement,which is not conducive to the atomization of fuel.