Investigation of in-nozzle flow characteristics of fuel injectors of IC engines

Almost all automotive fuel injection systems are experiencing some form of cavitation within their nozzle under different operating conditions. In-nozzle cavitation initiates in various forms and directly influences the emerging spray. Experimental studies have shown that cavitation in diesel injectors leads to smaller droplet formation, especially by the on-going trend towards higher injection pressures, which enhances fuel evaporation but also creates undesirable consequences due to transient nature of cavitation such as spray instabilities, erosion on internal surfaces, and hydraulic flip. Thus, the understanding of the internal flow of automotive fuel injectors is critical for injector design. On the other hand, biodiesel has emerged as one of the potential alternative fuel which can also be carbon neutral because it uptakes CO2 during cultivation of its feedstock and can be used in existing diesel engines with little or no modifications. Therefore, the present study is focused on assessing and outlining cost-effective methods to analyse internal flow in fuel injectors for diesel and biodiesel fuel applications. In the present study, RANS-based (Reynolds-averaged Navier–Stokes) CFD (Computational fluid dynamics) approach has been chosen to simulate quasi-steady flows in the steady state test rigs of fuel injectors of IC engines. The RANS approach is selected over computationally expensive SAS (Scale Adaptive Simulations), DES (Detached Eddy Simulations) and LES (Large Eddy Simulations) because it was considered that these quasi-steady simulations could be performed within hours and with less computing resources using RANS rather using SAS, DES and LES which may require orders more time and computing resources. Cavitation models and RANSbased turbulence models have been evaluated for single-hole and multi-hole injectors operating on steady state test rigs. Furthermore, influences of liquid and vapour compressibility were also investigated. Influences of biodiesel properties such as higher viscosity and density on cavitation were also assessed. In the first part of the study, single-phase simulations have been carried out in the mini-sac type multi-hole (6) injector. Several two-equation turbulence and near wall models were assessed, amongst most appropriate for the application were identified. Predicted mean velocity and RMS velocity were compared with measurements and showed good agreements. Flow field analysis showed predictions of different types of vortices in the injector. Two main types of vortex structures were predicted: ‘Hole-to-hole’ connecting vortex and double ‘counterrotating’ vortices emerging from the needle wall and entering the injector hole facing it. The latter create a complex 3D flow inside the injector hole when it interacts with the recirculation region at the entrance of the injector hole. Cavitation simulations inside a single-hole injector were next performed. Simulations were assessed by comparing predicted vapour volume fraction with measurements. Influences of liquid and vapour compressibility were also checked. The compressibility of vapour was modelled using ideal gas law and liquid compressibility was modelled using the Tait equation. Vapour compressibility resulted in an increase of vapour volume fraction at the low-pressure region and predictions were also in better agreements with experimental data. The liquid compressibility made no impact on the simulation results. The local sonic speed in the liquid-vapour mixture was computed using Wallis model which predicted a very low local sonic speed in the liquid-vapour mixture. Therefore, the local flow in liquid-vapour mixture became supersonic. A normal shock wave was predicted just downstream of the cavitation bubble cloud as local flow velocity was reduced from supersonic to subsonic. Finally, the cavitation simulations were performed in the enlarged mini-sac type multi-hole injector. Established turbulence, cavitation and compressibility models from above studies were used. Reasonable quantitative agreements with experimental data were obtained for the mean axial velocity and RMS velocity. Reasonable qualitative agreements were also achieved when predicted cavitation results were compared with high-speed digital images. Henceforth a parametric study to assess the influence of biodiesel fuel properties such as an increase in viscosity and density on the cavitation was performed. Viscosity and density of both phases in the fluid were parametrically increased by 20%. Results showed that cavitation was suppressed when the viscosity was increased because it increased the flow resistance, thus reduced the velocity. This caused a reduction in the size of recirculation region at the entrance of the injector hole and hence a smaller saturation pressure region was predicted. Cavitation was further suppressed when density was increased causing the reduction in the velocity at the same mass flow rate, which further reduced the recirculation region, therefore, reduced the saturation pressure region and consequently cavitation.