Validation of a CFD Model of a Hollow-Cone Spray with Gasoline Fuel Blends

Abstract

This paper presents the summary of the development of a two-phase spray model of a hollow-cone fuel injector commonly applied to spray-guided, gasoline direct injection, (SGDI) engines. The model was simulated using the Ricardo VECTIS CFD code and takes into account the physical and chemical effects of oxygenated fuel blends (flexfuels). The characteristics of the fuel sprays at typical gasoline part-load conditions, identified in a parallel study, were of particular interest. An injection duration of 0.3 ms was chosen which represented a stratified charge, unthrottled, part-load operating condition in a spray guided GDI engine with a piezoelectric fuel injector and a fuel injection pressure of 200 bar gauge. In the first instance, the spray model was validated against data recorded in a constant volume spray chamber. Secondly, the robustness of the model was tested against data measured in an optically-accessed engine. The Ricardo WAVE 1-D gas dynamics code was used to determine the gas phase boundary conditions in the engine. Initial spray input data for the model were obtained using an injection rate tube and the high-pressure and temperature (HP-HT) spray chamber. The quiescent gas pressure and temperatures in the chamber were varied in the range of between 1 and 7 bar absolute and 293 and 423 K respectively. The fuels used were pump grade, 95 RON gasoline, a blend of gasoline and ethanol (E85) and a blend of gasoline and methanol (M30) mixed by volume. In each case, the fuel injection processes (geometry and penetration rate characteristics) were visualised using Mie imaging, illuminated with a LASER sheet, as well as high-speed shadowgraphy. The camera frame rates were 10 Hz and 4 kHz respectively. The droplet size and velocity distributions in a plane coincident with the injector nozzle that bisected the axis of symmetry of the injector were simultaneously measured using Phase Doppler Anemometry (PDA). Analysis of the data sets were used to define the boundary conditions and to optimise the parameters of the spray model in a given time step. In addition, the data from the experimental HP-HT chamber and the spray model were compared with complementary data obtained in a new Ricardo single cylinder, spray-guided, optically-accessed, Hydra engine. High-speed photography, performed at frame rates in the range of 10 to 200 kHz was carried out, in the motored engine, using a glass piston bowl and a specialist cylinder liner that protruded into the pent-roof. The optimised model was implemented into a 3D CFD simulation of the optical engine, incorporating models for spray and mixture preparation. The distribution of the local air to fuel ratio, predicted by the VECTIS simulation, showed good agreement with planar laser-induced fluorescence (PLIF) measurements of the liquid and vapour fuel distributions in the optical engine. The applicability of the validated spray model, with respect to the accurate prediction of mixture preparation was discussed in the context of multiple fuel injection and flexfuel strategies.