Experimental Study And Numerical Simulation On Evaporation Characteristics Of Single-and-bi-component Droplets

The evaporation of droplets is accompanied by heat and mass transfer and flow of gas/liquid,where the droplet exchanges heat with the surrounding high-temperature gas-phase medium,absorbs heat,vaporizes,and mixes with the high-temperature medium,affecting the concentration of gas-phase medium components and diffusion coefficients.Understanding the evaporation characteristics of droplets is of great significance for a clear understanding of various combustion processes in engines.Ndecane,as an important alternative fuel for engines,has advantages such as a single component,high calorific value,and good environmental performance.The production of ethanol fuel is simple and low-cost,and as an additive,it can effectively improve the economy of alternative fuels.Moreover,the addition of ethanol changes the flow and heat transfer inside the n-decane/ethanol bi-component droplet,presenting physical and chemical properties different from those of bi-component droplets,which significantly affects the evaporation characteristics of droplets to achieve optimized combustion and reduced emissions process,making it one of the current research hotspots.Therefore,in this paper,n-decane/ethanol mono-and bi-component droplets are taken as the research objects,and the evaporation characteristics of droplets are studied through theoretical calculations,experimental testing,and numerical simulation.(1)Theoretical model and analysis of the evaporation characteristics of monocomponent droplets are presented.By establishing physical and mathematical models,the distribution of the liquid and gas-phase temperature fields during the transient evaporation stage and the variation of droplet size with time are systematically analyzed,and the relationship between this theoretical model and other classical models is studied.The results show that the internal temperature distribution of the droplet follows the form of the spherical Bessel function;as the evaporation proceeds,the droplet size gradually decreases;the temperature distribution of the gas-phase field outside the droplet follows a Gaussian error function relationship.It is noteworthy that this theoretical model is the core of other classical evaporation models when studying the evaporation characteristics of mono-component droplets and can be simplified to be consistent with other models.(2)Experimental testing,theoretical analysis,and model reliability verification of the evaporation characteristics of mono-component droplets under different operating conditions are presented.An evaporation experimental platform is established based on the hanging droplet method,and the shape change process of the droplet and the droplet area are recorded using a high-speed camera.Subsequently,the relevant data is postprocessed to obtain the square law of the normalized droplet diameter at different ambient temperatures.Considering the effects of convective heat and mass transfer of high-temperature air on droplets and the radial flow of fuel vapor generated by evaporation on the heat and mass transfer process of droplets,different Stefan factors are introduced to correct the droplet evaporation rate constant for different types of mono-component droplets in the transient and isothermal stages.The reliability of the model is verified by experiments.(3)Experimental and numerical investigation of the evaporation characteristics of bi-component droplets of n-decane and ethanol under different operating conditions is presented.The results show that both environmental temperature and ethanol mass fraction have significant effects on the droplet evaporation behavior.During the isothermal stage,the evaporation rate constant increases linearly with the environmental temperature(from 573 K to 873 K).However,it exhibits a "V" shaped trend with the increase of ethanol mass fraction,and the minimum value is reached at40% mass fraction.Using the multiphase flow model and Lee’s model in ANSYS FLUENT,the droplet evaporation process is numerically simulated.The calculated evaporation rate constants are in good agreement with experimental values,especially for mono-component n-decane droplets and low ethanol mass fraction ndecane/ethanol bi-component droplets.The research findings of this article contribute to the understanding of the evaporation characteristics of mono-and bi-component droplets and provide reference for the development of new alternative fuels.