Mixing and Autoignition of Underexpanded Methane Jets at High Pressure Conditions

Abstract

Dual-fuel internal combustion engines with high-pressure direct injection of natural gas into the cylinder and ignition assist via a diesel pilot offer reduced emissions of carbon dioxide, particulate matter and nitrogen oxides compared to standard diesel engines, while achieving higher thermal efficiency and power output than natural gas engines operating in other modes, while additionally eliminating problems of incomplete combustion at part load, including substantial methane slip. The fundamentals of high pressure gaseous jets, however, are relatively poorly understood. The objective of this thesis to obtain insight into the development, mixture formation and autoignition of underexpanded methane jets at high pressure conditions. To this end, a multi-dimensional computational framework for consistent treatment of single-phase flow with real-gas thermodynamics is established and verified against high fidelity simulation results from extensively validated solvers. The significance of real-gas corrections is also assessed. Subsequently, Reynolds-Averaged Navier-Stokes simulations with the newly established framework are validated against Schlieren and Tracer Laser Induced Fluorescence measurements in an optically accessible constant volume chamber, equipped with a prototype gas injector. For injection pressures up to 300bar and for supercritical pressure ratios up to 10, non-reactive jet development is examined in terms of global jet characteristics, such as mass flow rates, jet tip penetration and jet volume, and compared to scaling laws from literature. The influence of parametric variations in injection pressure, chamber pressure and pressure ratio on both macroscopic and local mixture formation is investigated and interpreted qualitatively with the aid of those laws. In a third step, the autoignition of underexpanded methane jet at injection pressures between 200 and 500bar and at back pressures between 40 to 125bar is investigated numerically. Particular care is taken through a two-stage workflow to simultaneously ensure accurate chemical kinetics, inclusion of real-gas effects, sufficient resolution for the complex shock structures and reliable description of turbulence-chemistry interaction with the Conditional Moment Closure combustion model. The predicted ignition delays are conceptually interpreted and quantified as an interplay of jet reactivity and of jet aerodynamics, whereas ignition locations show much lower variability. A semi-empirical correlation for autoignition delay as a function of chamber temperature, chamber pressure, injection pressure and injection temperature is ultimately constructed, which compares favourably to independent experimental measurements from literature and shows modest improvements over the simpler Arrhenius model.