Understanding the boiling characteristics of bi-component droplets with improved bubble nucleation and break-up mechanisms

Abstract

With the increasing use of blended fuels, the vaporization process of mixed droplets imposes new requirements on classical droplet vaporization models, highlighting the urgent need for the development of a broadly applicable multi-component fuel droplet vaporization model. In this study, the boiling characteristics of n-heptane/n-hexadecane two-component mixed droplets with different physical properties were experimentally studied under wide ambient temperatures and initial mass fractions. The droplet boiling mainly involves two phenomena, i.e., puffing and micro-explosion. Based on the experimental observations, the puffing phenomenon is further refined and categorized into weak puffing, foam puffing, and strong puffing. Then, the analysis of the influence of initial conditions on the boiling modes was conducted. Furthermore, a droplet vaporization model was constructed according to the experimental data, which includes droplet evaporation, droplet boiling, and bubble point sub-models. The new model considers the effects of fuel viscosity and surface tension and makes improvements to the heterogeneous nucleation and bubble break-up sub-models, reducing the dependence on empirical parameters. The resistance terms were introduced in the heterogeneous nucleation sub-model to prevent the nucleation rate from reaching its upper limit. The relationship between the initial disturbance and the Ohnesorge number in the break-up sub-model was summarized. Based on the above improvements, an updated method for calculating the boiling mass rate was proposed. The results indicate that the vaporization model can effectively replicate droplet lifetime and diameter evolution under different mixing fractions and wide ambient temperatures.