Numerical Simulation of Spray Formation and Liquid Atomization

Abstract

1. IntroductionThe quality of atomization of liquid fuel is the most essential factor of the reducing emissions in Diesel engines.The atomization process depends among other things on the velocity of the fuel exiting the injection nozzle andon the extent of cavitation within the nozzle. These nozzles are very small and the fuel flow is very fast, makingexperimental observation very difficult. An additional problem introduced by the cavitation is the difficulty inidentifying and matching the correct scaling parameters for fuel injector nozzles. On the numerical side, there arenumerous other difficulties introduced by cavitation. The values of density in cavitating flows can change severalorders of magnitude between single computational cells. The difficulty of resolving this practically discontinuouschange in density can cause severe numerical oscillationsin the numerical density field. Despite these complications,there have been some successes in the study of cavitation nozzles flow. Experiments with transparent nozzles [1]clearly demonstrate that even at supply pressure of more than 200 bar cavitation may disappear for short timesprovided the pressure increases sufficiently rapidly. Conversely, cavitation extends more than expected when thesupply pressure decreases fast enough. The atomization of the liquid jet is heavily affected by disappearance andre-occurance of cavitation.2. Purpose and MethodIn the present paper a numerical model is presented which simulates the unsteady atomization process of liquidfuel in Diesel engine. The connections between transient nozzle flows and spray formation are considered. For thatpurpose the both models of unsteady nozzle flow including the transient behaviour of cavitation and two-phaseatomisation process are employed. For the calculation of transient nozzle flow the model of compressible liquiddescribed in [2] is employed. The two-phase nozzle flow (liquid and cavitating bubbles) is replaced mathematicallyby a single phase flow characterized by an artificial barotropic equation of state, where the density varies sharplybetween the density of vapour, when the pressure decreases to vapour pressure, and the density of liquid, when thepressure is slightly above the vapour pressure.