The internal structure of Diesel fuel injectors is known to have a significant impact on the nozzle flow and the resulting spray emerging from each hole. In this paper the three-dimensional transient flow structures inside a Diesel injector is studied under nominal (in-axis) and realistic (including off-axis lateral motion) operating conditions of the needle. Numerical simulations are performed in the commercial CFD code CONVERGE, using a two-phase flow representation based on a mixture model with Volume of Fluid (VOF) method. Moving boundaries are easily handled in the code, which uses a cut-cell Cartesian method for grid generation at run time. First, a grid sensitivity study has been performed and mesh requirements are discussed. Then the results of moving needle calculations are discussed. Realistic radial perturbations (wobbles) of the needle motion have been applied to analyze their impact on the nozzle flow characteristics. Needle radial motions are based on high-speed X-ray phase-contrast imaging collected at Argonne National Laboratory. Different types of wobbles are presented and the results are compared to the nominal in-axis motion behavior. Hole-to-hole differences are discussed and quantified. Complex flow structures are observed in the sac region of the nozzle and an explanation for the different flow structures emerging from each hole is provided. From this work it has been observed that in presence of needle wobble, at low and medium lifts hole-to-hole differences are observed. Also, some holes exhibits swirling flow which affects the near nozzle jet structure, the liquid mass distribution and the mass flow rate, resulting in potential spray modifications.

This paper focuses on the analysis of low-pressure regions inside fuel injector nozzles, where fuel vapor formation (strictly referred to as cavitation, or vaporous cavitation) and expansion of noncondensable gas (also referred to as pseudo cavitation, or gaseous cavitation) can simultaneously occur. Recently, X-ray radiography experiments of a 500 µm diameter cavitating nozzle showed that the presence of dissolved gas in the fuel can cause significant changes in the apparent distribution of projected void fraction. In this article, the effect of dissolved gas on cavitation measurements is investigated in further detail through experimentation and numerical simulations. Test conditions have been selected to have highly cavitating conditions. Tests with a standard gasoline calibration fluid and equivalent degassed fluid are compared and discussed. Numerical simulations have been conducted under the same conditions as the radiography experiments. The primary goal of the study is a quantification of the separate contributions of gas expansion as opposed to actual cavitation to the measurement of total void fraction. The multiphase flow is represented using a mixture model. Phase change is modeled via the homogeneous relaxation model. Particular attention is paid to quantifying the effective amount of noncondensable gas included in the mixture, in order to predict the response of regular and degassed fuels. The presence of dissolved gas in the multiphase flow is taken into account using a compressible fluid model with three distinct components (liquid, vapor, and gas). Issues surrounding estimation of the effective amount of noncondensable gas are discussed. Numerical simulation results match well with the experiments and indicate that when a sufficient quantity of gas is dissolved in the fuel, a void is evident in the central region of the channel that can be attributed to local expansion of noncondensed gas. Conversely, degassed fuel shows only intense cavitation at the nozzle wall, with very little contribution from noncondensed gas.