Effect of hybrid breakup modelling on flame lift-off length and soot predictions

Abstract

An improved droplet breakup model coupled with the effect of turbulence flow within the nozzle was implemented into the general transport equation analysis code to describe the flame lift-off length and predict the soot distribution. This model was first validated by the non-evaporating and evaporating spray experimental data. The computational results demonstrate that the breakup model is capable of predicted spray penetration and liquid length with reasonable accuracy. The inclusion of turbulence enhanced the breakup model, increased the droplet breakup rate, decreased spray penetration for about 6–12% compared to the results of Kelvin-Helmholtz Rayleigh-Taylor (KH-RT) breakup model. Then, the model was applied to investigate the influence of ambient density, temperature, oxygen concentration and injection pressure on the flame lift-off length under typical diesel combustion conditions. The predictions showed good agreement with the experimental data. The result also indicated that the turbulence inside the nozzle strengthen the rate of breakup, resulting in more smaller droplets, leading to high evaporation rate and smaller vapour penetration lengths, thus decreases the lift-off length about 8%. Finally, the model was used to explore the soot distribution. The overall trend of soot with the variations in injection pressure was well reproduced by the breakup model. It was found that the droplet with faster velocity under high injection pressure, this could lead to larger lift-off length, which will play a significant role for the fuel–air mixing process and thus cause a decrease in soot in the fuel jet. Results further indicated that the turbulence term can decrease the soot mass about 5–9% by improved the droplet breakup process.