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Cavitation and turbulence inside a diesel injector play a critical role in primary spray breakup and development processes. The study of cavitation in realistic injectors is challenging, both theoretically and experimentally, since the associated two-phase flow field is turbulent and highly complex, characterized by large pressure gradients and small orifice geometries. We report herein a computational investigation of the internal nozzle flow and cavitation characteristics in a diesel injector. A mixture based model in FLUENT V6.2 software is employed for simulations. In addition, a new criterion for cavitation inception based on the total stress is implemented, and its effectiveness in predicting cavitation is evaluated. Results indicate that under realistic diesel engine conditions, cavitation patterns inside the orifice are influenced by the new cavitation criterion. Simulations are validated using the available two-phase nozzle flow data and the rate of injection measurements at various injection pressures (800-1600 bar) from the present study. The computational model is then used to characterize the effects of important injector parameters on the internal nozzle flow and cavitation behavior, as well as on flow properties at the nozzle exit. The parameters include injection pressure, needle lift position, and fuel type. The propensity of cavitation for different on-fleet diesel fuels is compared with that for n-dodecane, a diesel fuel surrogate. Results indicate that the cavitation characteristics of n-dodecane are significantly different from those of the other three fuels investigated. The effect of needle movement on cavitation is investigated by performing simulations at different needle lift positions. Cavitation patterns are seen to shift dramatically as the needle lift position is changed during an injection event. The region of significant cavitation shifts from top of the orifice to bottom of the orifice as the needle position is changed from fully open (0.275 mm) to nearly closed (0.1 mm), and this behavior can be attributed to the effect of needle position on flow patterns upstream of the orifice. The results demonstrate the capability of the cavitation model to predict cavitating nozzle flows in realistic diesel injectors and provide boundary conditions, in terms of vapor fraction, velocity, and turbulence parameters at the nozzle exit, which can be coupled with the primary breakup simulation.

A skeletal mechanism with 118 species and 837 reactions was developed from a detailed LLNL mechanism that consisted of 3329 species and 10806 reactions for a tricomponent surrogate mixture, consisting of methyl decanoate, methy-9-decenoate, and n-heptane, which is suitable for combustion modeling of biodiesel derived from various feedstocks. The method of directed relation graph (DRG) for skeletal mechanism reduction was improved for mechanisms with large numbers of isomers. The improved DRG together with isomer lumping and DRG-aided sensitivity analysis (DRGASA) were subsequently applied to obtain a minimal skeletal mechanism from the detailed mechanism for the given error tolerance. The reduction was performed within a parameter range of pressure from 1 to 100 atm, equivalence ratio from 0.5 to 2, and temperature higher than 1000 K in autoignition and perfect stirred reactors (PSR). Although reduced in size almost by a factor of 30, the skeletal mechanism features high accuracy for high-temperature applications both in predicting the global system parameters, such as ignition delay and extinction time, and detailed profiles of species concentrations. Furthermore, numerical simulations of jet stirred reactors were compared with experimental measurements for rapeseed oil methyl esters. The temperature and species profiles in one-dimensional atmospheric counterflow diffusion flames were well predicted as well compared with experimental data in the literature.

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