Characterization of Hollow Cone Gas Jets in the Context of Direct Gas Injection in Internal Combustion Engines

Direct injection (DI) compressed natural gas (CNG) engines are emerging as a promising technology for highly efficient and low-emission engines. However, the design of DI systems for compressible gas is challenging due to supersonic flows and the occurrence of shocks. An outwardly opening poppet-type valve design is widely used for DI-CNG. The formation of a hollow cone gas jet resulting from this configuration, its subsequent collapse, and mixing is challenging to characterize using experimental methods. Therefore, numerical simulations can be helpful to understand the process and later to develop models for engine simulations. In this article, the results of high-fidelity large-eddy simulation (LES) of a stand-alone injector are discussed to understand the evolution of the hollow cone gas jet better. The hollow cone gas jet is characterized in terms of several parameters such as axial penetration length, maximum jet width, area of jet, volume of jet, and mixing in terms of the mass-weighted probability density of the injected gas within the jet volume. Different grid resolutions have been used to study the effect on the gas jet behavior as well as mixing. The power-law scaling of the temporal evolution of the axial penetration length, maximal width, and area of jet is compared with the previously published literature for a similar injector. The applicability of different turbulence models commonly used in computationally cheaper Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations is investigated. Both LES and URANS simulations overpredict the axial penetration length because of the initial nonlinear behavior of the jet evolution. The transient needle opening has been found to impact initial stages of the gas jet formation and is responsible for the linear jet evolution observed in experiments. Moreover, the initial condition has a strong influence on later jet evolution in case of fixed needle simulations.