Evaluation of trans-critical transition of single- and multi-component sprays under diesel engine-like conditions

The trans-critical transition of multi-component sprays under the ECN Spray-A conditions has attracted extensive interest. However, the drastic change of critical points of multi-components makes this controversial. In this study, the trans-critical transition of sprays before and after the end of injection is evaluated by numerical methods, and new findings have been obtained. First, the robust and accurate thermodynamic equilibrium solver and multi-component droplet evaporation model were developed and implemented into the OpenFOAM. These models have been validated against the measured phase change diagram and isolated droplet evaporation rate. Then, the mixture critical temperatures of n-dodecane/nitrogen and multi-component diesel/nitrogen were calculated under a wide range of pressures based on the thermodynamic equilibrium solver. The mixture critical temperature decreases almost linearly with the increase of the pressure. Following that, extensive sprays of ndodecane and multi-component diesel under high temperature and pressure conditions were simulated, and the predicted spray liquid penetrations were validated against the experimental data. The results found that the dilute sprays after the end of injection are more prone to transition to the supercritical mixing regime, while the trans-critical possibility of dense sprays before the end of spray decreases due to the cooling effect of extensive evaporation. Finally, the trans-critical transitions of multi-component diesel sprays before and after the end of injection were evaluated and compared with those of n-dodecane sprays. Due to the low mixture critical temperature and evaporation rate of diesel, its sprays are more likely to transition to the supercritical regime. Therefore, it can conclude that the dilute sprays after the end of injection cannot represent the thermodynamic state of dense sprays. Moreover, the multi-component diesel and n-dodecane sprays follow a different transcritical transition pathway.

Automated summary

The transitional behavior of multi-component sprays under ECN Spray-A conditions has been investigated using numerical methods in this study. The development and implementation of robust thermodynamic equilibrium solvers and multi-component droplet evaporation models in OpenFOAM have led to new insights. The results indicate that dilute sprays are more prone to transitioning to the supercritical mixing regime, while dense sprays experience a decrease in trans-critical possibility before the end of injection due to the cooling effect of extensive evaporation. Additionally, it was found that multi-component diesel sprays are more likely to transition to the supercritical regime than n-dodecane sprays due to their low mixture critical temperature and evaporation rate.