Numerical investigation of the autoignition of underexpanded methane jets

Abstract

This work constitutes a numerical investigation of the autoignition of underexpanded methane jets at high-pressure conditions. Injection pressures from 125 to 500bar, back pressures from 40 to 125bar, and pressure ratios between 2.5 and 10 have been targeted. The aim is to identify the effect of the main control variables of gas injection on the autoignition delay and location. To this end, Reynolds-Averaged Navier-Stokes simulations with the k-ωSST model have been carried out for five broad parametric variations. The computational domain represents a Constant Volume Cell with a prototype gas injector. A two-stage workflow enables proper thermodynamic treatment of the conservation equations with real-gas modeling, sufficient resolution for shock structures in the near nozzle area, detailed kinetics described by the San Diego mechanism and treatment of turbulence-chemistry interaction with elliptic Conditional Moment Closure model. The results are interpreted conceptually as an interplay of jet reactivity, effectively described by an exponential dependence on ambient temperature and a power law dependence on ambient pressure, and of jet aerodynamics, empirically described by a quadratic dependence on pressure ratio. Injection temperature is introduced by defining an appropriate characteristic system temperature and a correlation is constructed, whose predictions are juxtaposed against a modified Arrhenius model and measurements from independent experimental studies in literature. The effect of injection variations on ignition location in physical space is also examined.