Evaporation and condensation characteristics of n-heptane and multi-component diesel droplets under typical spray relevant conditions

摘要

The evaporation and condensation characteristics of n-heptane and multi-component diesel droplets are comprehensively investigated under typical spray relevant conditions. A multi-component diffusion submodel, in conjunction with the real vapor-liquid thermodynamic equilibrium, is employed to capture the phase change rate on droplet surface. The finite thermal conduction model is taken into account to calculate the temperature distribution inside droplets. All the thermal and transport properties in gas phase depend on the temperature, pressure and composition. This enhanced model is verified extensively against the experimental data for isolated n-heptane and multi-component diesel droplets, and satisfactory agreement has been achieved for both fuels. The influence of ambient composition on the evaporation and condensation characteristics of droplet is elucidated. With the increased vapor concentration, the droplet evaporation rate is reduced and its lifetime is significantly prolonged, owing to the reduced mass transfer rate and increased diffusional enthalpy flux. Two thresholds are defined based on the ambient composition, and they determine the condensation occurrence on droplet surface, as well as the droplet approximately stable state, respectively. The heavy-end fuel vapor around droplets shows much more significant effect on the droplet lifetime than the light-end one. Additionally, it is found that the influence of pressure on the droplet evaporation rate is not only dependent on the ambient temperature, but also determined by the ambient composition.