Large eddy simulations of diesel-fuel injection and auto-ignition at transcritical conditions

Abstract

Large eddy simulations of transcritical injection and auto-ignition of n-dodecane in a combustion chamber are performed. To this end, a diffuse-interface method is employed that solves the compressible multi-species conservation equations, and a cubic state equation together with real-fluid transport properties is employed to describe the transcritical fluid state. The reaction chemistry is represented by a finite-rate chemistry model involving a 33-species reduced mechanism for n-dodecane. Compared to commonly employed two-phase approaches, the method presented in this work does not introduce tunable parameters for spray-breakup. Large eddy simulation calculations are performed by considering the Spray A single-hole injector at non-reacting and reacting conditions at a pressure of 60 bar and temperatures between 800 and 1200 K. Quantitative comparisons with measurements for liquid and vapor penetration lengths are performed for non-reacting conditions, and sensitivity to threshold values on mixture fraction are examined. The analysis of reacting flow simulations focuses on comparisons of the instantaneous temperature and species fields for OH and CH₂O at 800 and 900 K, respectively. Quantitative comparisons with measurements for ignition delay and lift-off heights as a function of ambient temperature are performed. To examine the transient ignition phase, comparisons of radially integrated OH profiles obtained from the simulations with reported measurements for OH* are performed, showing good agreement. These results show that the large eddy simulation modeling framework adequately reproduces the corresponding ignition processes, which are relevant to realistic diesel-fuel injection systems.