J. De la Morena

Comparative study of the internal flow in diesel injection nozzles at cavitating conditions at different needle lifts with steady and transient simulations approaches

The motion of the needle during the injection process of a diesel injector has a marked influence on the internal flow, the fuel characteristics at the nozzle exit, the spray pattern and the fuel–air mixing process. The current paper is focused on the computational study of the internal flow and cavitation phenomena during the injection process, with inclusion of the opening where the needle is working at partial lifts. This study has been performed with a homogeneous equilibrium model (OpenFOAM) customized by the authors to simulate the real motion of the needle. The first part of the study covers the analysis of the whole injection process with a moving mesh using the boundary conditions provided by a one-dimensional (1D) model of the injector created in AMESim. This 1D model has offered the possibility of reproducing the movement of the needle with real lift law and real injection pressure evolution during the injection. Thus, it has been possible to compare the injection rate profiles provided by OpenFOAM against those obtained both in AMESim and experimentally. The second part compares the differences in mass flow, momentum flux, effective velocity and cavitation appearance between steady (fixed lifts) and transient (moving mesh) simulations. The aim of this comparison is to establish the differences between these two approaches. On the one hand is a more realistic approach in its use of transient simulations of the injection process and where the needle movement is taken into account. On the other hand, is the use of steady simulations at partial needle lifts. This analysis could be of interest to researchers devoted to the study of the diesel injection process since it could help to delimit the uncertainties involved in using the second approach which is more easily carried out, versus the first which is supposed to provide more realistic results.

Comparison of Different Techniques for Characterizing the Diesel Injector Internal Dimensions

The geometry of certain parts of diesel injectors is key to the injection, atomization and fuel-air mixing phenomena. Small variations on the geometrical parameters may have a strong influence on the aforementioned processes. Thus, OEMs need to assess their manufacturing tolerances, whereas researchers in the field (both experimentalists and modelers) rely on the accuracy of a certain metrology technique for their studies. In the current paper, an investigation of the capability of different experimental techniques to determine the geometry of a modern diesel fuel injector has been performed. For this purpose, three main elements of the injector have been evaluated: the control volume inlet and outlet orifices, together with the nozzle orifices. While the direct observation of the samples through an optical microscope is only possible for the simplest pieces, both Computed Tomography Scanning and the visualization of silicone molds technique have proven their ability to characterize the most complex internal shapes corresponding to the internal injector elements. Indeed, results indicate that the differences observed among these methodologies for the determination of the control volume inlet orifice diameter and the nozzle orifice dimensions are smaller than the uncertainties related to the experimental techniques, showing that they are both equally accurate. This implies that the choice of a given technique for the particular application of determining the geometry of diesel injectors can be done on the basis of availability, intrusion and costs, rather than on its accuracy.