Numerical simulation and experimental validation of cavitating flow in a multi-hole diesel fuel injector

Abstract

This study assesses the predictive capability of the ZGB (Zwart-Gerber-Belamri) cavitation model with the RANS (Reynolds Averaged Navier-Stokes), the realizable k-epsilon turbulence model, and compressibility of gas/liquid models for cavitation simulation in a multi-hole fuel injector at different cavitation numbers (CN) for diesel and biodiesel fuels. The prediction results were assessed quantitatively by comparison of predicted velocity profiles with those of measured LDV (Laser Doppler Velocimetry) data. Subsequently, predictions were assessed qualitatively by visual comparison of the predicted void fraction with experimental CCD (Charged Couple Device) recorded images. Both comparisons showed that the model could predict fluid behavior in such a condition with a high level of confidence. Additionally, flow field analysis of numerical results showed the formation of vortices in the injector sac volume. The analysis showed two main types of vortex structures formed. The first kind appeared connecting two adjacent holes and is known as “hole-to-hole” connecting vortices. The second type structure appeared as double “counter-rotating” vortices emerging from the needle wall and entering the injector hole facing it. The use of RANS proved to save significant computational cost and time in predicting the cavitating flow with good accuracy.

Predictions of Vortex Flow in a Diesel Multi-Hole Injector Using the RANS Modelling Approach

“…Only one-sixth of the flow domain was numerically simulated, due to symmetrical nozzle geometry, so as to reduce computational effort, as shown in Figure 3a. Unstructured grids were used-the same as those of [13], as shown in Figure 3b,c-with tetrahedral cells and five layers of prism cells on the walls in order to accurately resolve near-wall turbulence. The maximum aspect ratio and skewness of all grids were 11.2 and 0.759, respectively.…”

“…When the local pressure at the core of these vortices goes below the vapour pressure, the liquid starts to form cavitation pockets. As explained by Kumar et al [13], there are two types of such cavitating structures, which have been observed frequently in several experimental studies: The first type is hole-to-hole connecting vortex cavitation, which has been noted as an arc-shaped vortex, connecting two adjacent holes and the recess between the needle and the injector wall. The second observed type of these cavitation structures is a double counter-rotating vortical cavitation structure, originating from the needle wall and entering the opposing injector holes.…”

“…The simulation was conducted on a model injector 20 times larger than the real size, with the same geometry and setup as used by the authors of [8], who performed an extensive experimental work; a summary of the experiment is given below (for more details, see [8,13]). The 3D model geometry represents a multi-hole axisymmetric fuel injector nozzle of a Bosch conical mini-sac type six-hole (60 • apart) vertical diesel fuel injector, as shown in Figure 1.…”

“…Meanwhile, the latest CFD simulation [20] showed that the origin of in-nozzle vortex cavitation structures can be traced back to the sac volume and on-needle surface, and that the nonuniformity of the oil viscosity (due to viscous heating) gives rise to vortex formation. LDV data [8] have been recently used by Kumar et al [13] to assess CFD simulations in a multi-hole fuel injector; they used the RANS modelling approach to approximate turbulence, and the Eulerian-Eulerian cavitation modelling method to reproduce cavitation; they achieved reasonably good agreements with experimental data for mean and RMS velocity with LDV counterparts, and for cavitation structures with experimental high-speed digital images.…”

“…LDV (Laserdoppler velocimetry) experimental results have often been used to assess and validate different CFD modelling approaches. Full details are given by the authors in their previous paper [13], and a brief summary is given below. The first such attempt was by Arcoumanis et al [4], who used their inhouse CFD solver to simulate non-cavitating conditions in mini-sac injectors, and achieved reasonably good agreement with experimental measurements of mean velocity and RMS (Root mean square) velocity using a low-resolution mesh.…”