Models for automotive fuel droplets heating and evaporation

Abstract

The paper presents recent approaches to the modelling of heating and evaporation of automotive fuel droplets with application to biodiesel, diesel, gasoline, and blended fuels in conditions representative of internal combustion engines. The evolutions of droplet radii and temperatures for gasoline, diesel, and a broad range of biodiesel fuels and their selective diesel fuel blends have been predicted using the Discrete Component model (DCM). These mixtures combine up to 112 components of hydrocarbons and methyl esters. The results are compared with the predictions of the case when blended diesel-biodiesel fuel are represented by pure fossil and biodiesel fuels. In contrast to previous studies, it is shown that droplet evaporation time and surface temperature predicted for 100% biodiesel (B100) are not always close to those predicted for pure diesel fuel. Also, the previously introduced MultiDimensional Quasi-Discrete model and its application to these fuels and their mixtures are discussed. The previous application of this model has resulted in up to 96% reduction in CPU time compared to the case when all fuel components are considered using the DCM. Keywords Biodiesel, Diesel, Fuel droplet, Fuel blends, Gasoline IntroductionThere have been several attempts to simulate fuel droplets heating and evaporation. These have been either based on the analysis of individual components, the discrete component model (DCM) [1][2][3], or on the probabilistic analysis of a large number of components (the continuous thermodynamics [4][5][6] and the distillation curve ([7-9] models]. In most studies, several simplifying assumptions have been made, e.g. species inside droplets mix infinitely quickly (infinite diffusivity (ID) model) or do not mix at all (the single component (SC) model). Also, the temperature gradients inside droplets have been ignored in most cases with the assumption that the liquid thermal conductivity is infinitely large (infinite thermal conductivity (ITC) model). This paper will present our recent approaches to address these gaps in literature. Based on recent research findings (e.g. [3,[10][11][12][13]), the drawbacks in modelling fuel droplets heating and evaporation processes (computationally expensive models, ignoring temperature gradient and transient species diffusion) are partially addressed using the MDQD model. This paper summarises some comparisons between the results, referring to fuel droplet evaporation times and time evolution of droplet surface temperatures and radii, predicted by the previously suggested simplified models, the recently developed version of the DCM and the multidimensional quasi-discrete model (MDQDM) [14][15][16]. The latter two models take into account the recirculation, temperature gradient, and diffusion of species inside the droplets, based on the Effective Thermal Conductivity and Effective Diffusivity (ETC/ED) models. In the following section (Models), the main principles of the DCM and MDQDM are summarised. The results of using these models for the analysis o...

Droplets heating and evaporation: an application to diesel-biodiesel fuel mixtures

“…C 27 H 56 , C 27 H 54 (1, 4), increase with time at the expense of the lighter ones. This result is compatible with the findings presented in [7,11,18]. In some previous studies (e.g.…”

“…The droplets are moving at 10 −1 in still air of pressure and temperature equal to 30bar and 800K, respectively. This assumption is compatible with those used in [17]. The evolutions of droplet surface temperatures ( ) and radii ( ) for three mixtures of diesel-biodiesel fuels (B5, B20, B50) and pure biodiesel fuel (B100) of 19 types of biodiesel fuels are analysed.…”

“…The mixtures are treated as ideal gases. As in previous studies [7,10,11], blended fuel vapour is assumed to diffuse from the surface of the droplet, without changing its composition, based on averaged binary diffusion of diesel fuel vapour into dry air.…”

“…Following [10,11], the analysis of droplets heating and evaporation processes is made in application to several blends of diesel and biodiesel fuels. In contrast to previous approaches, the discrete component model (DCM) (described in [5,12] and validated experimentally in [13]) is used for this analysis; and the model is applied to a broad range of diesel-biodiesel fuel fractions.…”

“…where the diffusion coefficient of liquid species is linked with the liquid diffusion coefficient by the following equation of the Effective Diffusivity (ED) model [6,11]:…”