Modeling And Experimental Study Of Flash Boiling Of A Single Droplet

Flash boilingm,which is frequently encountered during fuel injection in the internal combustion(IC)engines,is a highly-transient and violent phase change phenomenon.It is of crucial importance to accurately describe the bubble nucleation and the subsequent growth and breakup mechanisms inside a superheat fuel droplet during the flash boiling process under IC engine operating conditions.In the present paper,a new flash boiling model for a single droplet is developed based on thermodynamics and heat transfer analysis.The model is mainly applied to investigate the microscopic evolution and heat transfer characteristics during the flash boiling process of dimethyl ether(DME)and DME-blended fuels.(1)The homogeneous nucleation of both pure liquid and binary liquid mixture is investigated.For the pure liquid,the change of the chemical potential during boiling phase change and the critical radius of the bubble nucleus are proposed based on the nonequilibrium thermodynamics.For the b inary liquid mixture,a modified sub-model for bubble point pressure is developed on the basis of the Wilson equation.This sub-model is adopted to study the homogeneous nucleation of the liquid mixture and to obtain the gaseous composition of each component at vapor-liquid equilibrium.With a combination with the classical nucleation theory,the critical nucleus radius and the nucleation rate are predicted for DME/n-decane and DME/n-dodecane mixtures.The homogeneous nucleation of DME mixed with n-decane and n-dodecane was calculated.The superheat limit temperature(SLT)of mono-component and bi-component liquid was obtained,the influence of DME mixing ratio on homogeneous nucleation was discussed as well.The model can be used to provide the initial conditions for flash boiling of mono-component and bi-component liquid.(2)Based on the nucleation model,a new flash boiling model for a single one-component droplet is developed.Compared to the previous flash boiling models,the new model is comprised of several sub-models,e.g.the modified homogeneous nucleation sub-model,bubble growth sub-model and the boiling explosion sub-model.The model is validated by the experimental data of superheated water.This model is used to predict the whole process of flash boiling of DME single droplet.The change of droplet average temperature during bubble evolution is predicted,and the influence of droplet temperature on bubble growth of DME is discussed.Based on the present model,the boiling explosion time is quantitatively provided at wide ranges of fuel temperature and ambient pressure which have a vital significance in predicting the secondary breakup of the droplet.Finally,the model is applied to study the bubble growth and evolution characteristics for different fuels under flash boiling conditions.(3)The flash boiling model is then extended to investigate the evaporation and explosion characteristics of a superheated bi-component droplet under IC engine conditions.The model also consists of several sub-models addressing the sub-processes of the flash boiling period,including the homogeneous nucleation.the bubble growth,and the boiling explosion.The modified Wilson equation is also integrated to discuss the bubble point pressure.The temperature-dependent and concentration-dependent thermos-physical properties are considered in predicting the bubble growth and evaporation of the DME/n-pentane droplet with different DME concentrations.Two criteria in parallel are employed to predict the time of boiling explosion.The results show that a larger DME concentration and higher liquid temperature can decrease the duration of the surface tension controlled stage,increase the bubble growth rate,and consequently shorten the boiling explosion time.A higher DME concentration can improve the evaporation rate of DME.Meanwhile,the peak value of the evaporation rate is shifted to an earlier moment at a fixed droplet temperature.Furthermore,the boiling explosion time map illustrates the region with appropriate temperature and the mass fraction of DME for the flash boiling explosion.(4)Experimental observation is also conducted on the single droplet evaporation and explosion characteristics for both pure liquid and binary liquid mixture.With the traditional pendant-drop method,n-butanol,n-hexadecane,and their stable,finely dispersed mixtures are chosen as the test liquid.The experiments are conducted at various ambient temperatures(537 K,555 K,573 K.591 K,and 609 K)to quantitatively analyze the evaporation and explosion characteristics.The results indicate that the one-component droplet evaporation follows the classical d2-law at various ambient temperatures,and the droplet evaporation consists of the transient heating period and steady evaporation period.However,for the binary liquid mixtures,the droplet evaporation is composed of three sub-periods.i.e.the transient heating period,the fluctuant evaporation period,and steady evaporation period.The fluctuant evaporation period is caused by the evaporation inside the droplet induced by the existence of a more volatile component.Under certain conditions,boiling explosion may occur during the fluctuant evaporation period,which depends on the superheat degree.The results show that the boiling explosion is possible when pa/psat<1.The boiling explosion intensity of the binary mixture changes nearly parabolic with the increase of ambient temperature.