A Study On The Theory And Experiments Of Dimethyl Ether Engine Spray & Atomization

The property of dimethyl ether (DME), which is used as a substitute for diesel, is obviously different from diesel. And fuel spray, which is as the source of in-cylinder working process, has the most important influence on the subsequent combustion and exhaust of engines. Unfortunately, the mechanism of liquid jet breakup is not understood completely, and the process of fuel wallinteraction is complicated. So, after reading a great number of literature on relevant research field, and basing on the theory, the author take a thorough study on the spray characteristics of DME by combining experiments with multi-dimensional numerical simulation.According to the formation mechanism of cavitations inside a nozzle, a nozzle flow model is established and first applied to DME that possesses higher saturation vapor pressure in this thesis, which can simulation more truly the flow status of DME and provides accurately initial parameters such as the velocity and diameter of a liquid jet for in-cylinder atomization. It is obvious that the nozzle flow model links the flow process inside a nozzle with the in-cylinder spray process.A dispersion equation of three-dimension unstable disturb wave on the surface of a viscous liquid jet with evaporating is first derived. And based on the dispersion equation which has been simplified, analysis analyses in detail the effects of surface tension, liquid viscosity, gas density and liquid jet velocity upon the surface instability of DME liquid jet. Form the analysis, the deeply understand about the spray characteristics of DME liquid jet and the aerodynamic atomization mechanism of surface unstable disturb wave is obtained.Based on the existing zero-dimensional evaporation model, a one-dimensional one (it is also called limited heat conductivity evaporation model) is constructed, which takes into account the effects of droplet movement, liquid property, high pressure and droplets group upon the evaporating process. And a solving method of TDMA(Tri-diagonal Matrix Algorithm) and the treatment method of moving boundary for the model are given. Droplets evaporation during spray process can be simulated more accurately by using the present model.Based on the detailed comparisons and analysis to many kinds of spray impingement models, two representative spray impingement models, wall spread and splashing, are introduced in this thesis. In the model, the exchange of heat and momentum between the wall, droplets and the gas above liquid film are modeled. The spray impingement of DME is simulated and compared with that of diesel. Due to DME quickly evaporating, its film thickness is much smaller than the one of diesel, which show that the accumulation of DME on the wall is not serious to be of advantage to combustion process.The stagger grid system, solving process, governing equations and numerical approaches, gas state equations, the coupling of gas-liquid, the calculating of time step, and boundary conditions and so on are emphatically introduced in the form of tensor, according to the KIVA3. During the process of numerical approximations, the difference formula of time transient terms, and the central/upwind difference expressions for acquiring boundary value are chosen. And the turbulence models, especially the RNG model, are depicted here. Through the above mentioned, a ground for in-cylinder spray numerical simulation is made.Using the high speed camera, the liquid spray experiments of DME and other fuels in pressure constant volume chamber are carried out, of which the spray wall impingement tests of DME is the first time in domestic researches. The effects are tested of inject pressures, ambient pressures, the diameters of nozzle hole, impinging angles and distances, etc. upon the spray characteristics such as penetration, spray cone angle, wall spread, the thickness of spray body and liquid film on the wall, air entrainment, and so on. A great deal of data is obtained from experiments and is treated, and many valuable conclusions are derived, which provide reliable origin...