Modeling And Experimental Study Of Unsteady Adhered Wall Film Evaporation In Internal Combustion Engines

Spray-wall interaction inevitably results in wall film formation,which has a detrimental effect on engine performance and leads to the pollutant formation.Because of the many assumptions and low accuracy in the current wall film evaporation model,as well as the lack of experimental research on high temperature conditions and multi-component fuel film,a new wall film evaporation model and a multi-component diffusion model were developed from the heat and mass transfer to investigate the affecting factors on wall film evaporation mechanism in the internal combustion engines.The wall film model was simplified as a one-dimensional problem based on the feature that the fuel film thickness is much smaller than its spreading length.In this dissertation,two typical fuels n-decane and n-dodecane have been selected as working substances.Meanwhile,the Shadowgraphy method and Refractive Index Matching(RIM)Method were employed to provide experimental validation data for the models,and to further reveal the mechanism of fuel film evaporation process and explore the effective ways to promote the evaporation of wall film.(1)A new analytical model for the wall film heating and evaporation with consideration of the wall film thickness variation and temperature distribution was developed based on the one-dimentional unsteady thermal conductivity differential equations.First,a simplified model for predicting the evaporation rate of the fuel film was derived by analogy between mass and heat transfer,based on which one-dimensional unsteady wall film evaporation models were developed for the fixed wall temperature condition and the given heat flux conditions,respectively.Second,the effects of wall temperature,initial wall film thickness,initial wall film temperature,ambient temperature,ambient pressure and convective heat transfer coefficient on the wall film heating and evaporation mechanism were investigated respectively using the analytical results of the models.The results show that under the first boundary condition,the heating and evaporation process of the wall film can be roughly divided into two stages:rapid surface heating stage and slow cooling stage.Under the second boundary condition,the evolution behavior of the wall film evaporation can be divided into three stages,i.e.,the initial rapid heating stage,the slow heating stage and the final rapid heating stage.In general,the wall temperature is one of the most significant factors affecting the lifetime of the wall film.Especially when the wall temperature is low,increasing the wall temperature can distinctly enhance the temperature and evaporation rate of the wall film,thus shortening the lifetime of the wall film.(2)Considering the influence of the temporal and spatial inhomogeneity of the concentrations of species in the practical fuel on the evaporation of the wall film,a onedimensional unsteady multi-component fuel diffusion model was developed and the form of its analytical solution was suggested.Then,the model was coupled with the wall film evaporation model under the first boundary condition to compare the differences between the zerodimensional diffusion model and the one-dimensional diffusion model in predicting multicomponent fuel film evaporation.Finally,the effects of species ratios of n-decane and ndodecane on the evaporation characteristics of the wall film was investigated.The results show that the zero-dimensional diffusion model will overestimate the fuel evaporation rate and underestimate the fuel film lifetime.While the one-dimensional diffusion model will provide a more precise prediction.(3)In order to provide reliable experimental validation data for the wall film evaporation model and the species diffusion model,the Shadowgraphy method,which can better capture the variation of thicker wall film thickness was employed to experimentally study the singlecomponent and bi-component wall film of n-decane and n-dodecane fuels.Also,the Shadowgraphy experiment provided further investigations on the influence of wall temperature and species ratio on the evaporation mechanism of the wall film.In the Shadowgraphy experiment,the wall film evaporation process can be divided into three stages.Firstly,the fuel film thickness gradually decreases while its bottom area hardly changes.Secondly,both the fuel film thickness and bottom area shrink inwards the disc.Finally,the wall film volume decreases at a relatively slow rate until the fuel film totally evaporated.It has also been found that the edge of the disc,which was used to deposit the film,has an unignoreable effect on the evaporation of the bi-component film.Namely,under the relatively high wall temperature conditions,the mixed fuel which containing a small proportion of heavy component shows an anomaly,in which the film life is shorter than that of the pure light component film under the relatively high wall temperature conditions.The verification results show that the model developed in this dissertation can offer rather well predictions for the evaporation rate and thickness of the wall film.(4)Based on the RIM method,the spreading behaviour and evaporation characteristics of a thin wall fuel film formed by a high-pressure spray impinging on a high temperature wall were investigated.The RIM experimental conditions are closer to those of an internal combustion engine.Also,the results can provide qualitative mechanistic analysis and quantitative experimental data for the evaporation process of fuel film for the practical engine conditions.The film spreading and evaporation characteristics were investigated for n-decane and n-dodecane fuels at different wall temperatures,injection pressures,injection durations and bi-component fuel mixing ratios.The results show that increasing the wall temperature can significantly reduce the fuel attachment rate and shorten the evaporation time of the film.Also,the fuel adhesion ratio will show an increasing trend first,and then decrease with the increase of injection pressure.Meanwhile,increasing the injection duration will increase the fuel adhesion amount,but the trend is decreasing as the injection duration increases.Finally,the higher proportion of n-dodecane in the bi-component fuel,the longer lifetime required for the wall film evaporation process.