Numerical Study On The Evaporation Process Of Lubricating Oil Droplet

The distribution of lubricating oil in cylinder is one of main causes of abnormal combustion of natural gas engines.During the stage of the compression stroke,lubricating oil droplets are heated by cylinder charge and produce oil vapor.Under high load working conditions,the lubricating oil is likely to auto-ignite and cause pre-ignition and knock,which are extremely threatening abnormal combustions for gas engines.The evaporation process is one of the key process of auto-ignition of lubricating oil droplets.In order to understand the mechanism of evaporation process,this thesis investigated the evaporation characteristic of lubricating oil droplets under gas engine conditions by numerical simulation.The following research work has been carried out:(1)The existing evaporation model assumes that the diffusion constant is the same,while the physical properties of each component are quite different.Therefore,the establishment of gas phase multi-component diffusion is helpful to improve the model calculation accuracy.First,the effect of multi-component diffusion in the gas phase on gas phase mass transfer is analyzed.The velocity of Stefan flow was modified to guarantee the conservation of component transport equations.Ideal gas model and real gas model were used respectively to describe the vapor-liquid equilibrium of light components and heavy components in the alkane droplets under different ambient temperature,ambient pressure and ambient gas compositions.In addition,the physical parameters such as liquid density,thermal conductivity,viscosity,saturated vapor pressure,diffusion coefficient and specific heat capacity of alkane droplets(including light components and heavy components)were calculated according to different environmental conditions.By comparing the prediction results of ideal gas model with that of actual gas model,the applicable pressure conditions of the models were discussed.(2)The ambient conditions in the cylinder of gas engine are complicated.Lubricating oil was evaporated under ambient gas containing different concentrations of methane mixture.Most of the existing research only concerned the droplets under pure nitrogen conditions.In order to study the evaporation process of lubricating oil droplets in the cylinder of natural gas engine,the evaporation process of alkane droplets was simulated by the ideal model and the real model under a wide range of methane concentration.By comparing the results of the ideal model with the real model,the criteria for the application of the model environmental gas conditions were established.(3)The actual gas model was used to simulate the evaporation process of alkane droplets in a relatively broad range of different ambient temperature,ambient pressure and methane concentration.The effects of environmental conditions on the evaporation laws of alkane droplets and the evaporation laws of heavy components were summarized by comparing the evaporation characteristic of light components and heavy components in the alkane droplets under cylinder conditions of the natural gas engine.(4)The existing multi-component model calculation efficiency and calculation accuracy cannot achieve a good balance.Therefore,it is very important to establish a model with high computational efficiency and computational accuracy.The analogical single-component model was constructed by using the evaporation laws of heavy components.The analogical single-component model as well as the Sequenced single-component model was used to simulate the evaporation process of diesel fuel droplets at different ambient temperatures.Simulation results are compared with that of the evaporation model for multi-component.It was concluded that the analog evaporation model for multi-component has high computational accuracy and computational efficiency.The analog evaporation model for multi-component was also used to simulate the evaporation process of lubricating oil droplets to verify its reliability.