Numerical Study on the Influence of Convergent-Divergent Nozzle Structures on the In-Nozzle Flow and Jet Breakup Based on the OpenFOAM

The non-conventional diesel nozzles have attracted more and more attention for their ability to promote jet breakup. In the present study, the internal nozzle flow and jet breakup relying on the convergent-divergent nozzle are investigated by combining the cavitation model and LES model with Multi-Fluid-Quasi-VOF model based on the OpenFOAM code. This is a novel method for which the interphase forces caused by the relative velocity of gas and liquid can be taken into account while sharpening the gas-liquid interface, which is able to accurately present the evolution processes of cavitation and jet breakup. Primarily, the numerical model was verified by the mass flow rate, spray momentum flux, discharge coefficient and effective jet velocity of the prototype Spray D nozzle from the literature. Then the inlet and outlet diameters of spray D nozzle were kept as constant, while the diameters at 1/4, 1/2 and 3/4 of axis distance from the inlet of the spray D nozzle were changed to 160 μm respectively to obtain three different convergent-divergent level nozzles. The transient internal nozzle flow and jet breakup of the three different convergent-divergent level nozzles were presented and their characteristics were analyzed. The results demonstrate that cavitation is formed downstream the throat of the convergent-divergent nozzles and the formation of supercavitation is accelerated by the high convergence-divergence level. In addition to the strong disturbance caused by cavitation, the divergence in the final part of the nozzle results in a larger radial jet velocity, which brings about the consequence that the liquid column breakup angle is greatly increased by the higher convergence-divergence level.