Effects of Thermodynamic Conditions and Nozzle Geometry in Gaseous Fuels Direct Injection Process for Advanced Propulsion Systems

Direct injection of gaseous fuels usually involves the presence of under-expanded jets. Understanding the physics of such process is imperative for developing Direct Injection (DI) internal combustion engines fueled, for example, by methane or hydrogen. An experimental-numerical characterization of the under-expanded jets issued from an innovative multi-hole injector, designed for application in heavy-duty engines, is carried out. The experimental characterization of the jet evolution was recorded by means of schlieren imaging technique and, then, a numerical simulation procedure was validated, allowing a comprehensive injection process analysis. A high-order and density-based solver, capable of reproducing the most relevant features of the under-expanded jets, was developed within OpenFOAM framework. Initially the effects of the upstream-to-downstream pressure ratio, namely Net Pressure Ratios (NPR), on the spray morphology were investigated. Then, the attention was focused on the effects of the nozzle geometry and consequentially on the mixture formation process. The simulations, allowing a comprehensive characterization of the turbulent and transonic flow, demonstrated how the nozzle's geometry strongly affects the quality of the air/fuel mixture and, generally, the global performances of the injection process.