Effect of Fuel Injection Processes on the Structure of Diesel Sprays 970799
A diesel spray model has been developed and validated against experimental data obtained for different injection and surrounding gas conditions to allow investigation of the relative importance of the different physical processes occurring during the spray development. The model is based on the Eulerian-Lagrangian approximation and the Navier-Stokes equations, simulating the gas motion, are numerically solved on a collocated non-uniform curvilinear non-orthogonal grid, while the spray equation is solved numerically using a Lagrangian particle tracking method. The injection conditions are determined by another recently developed model calculating the flow in the fuel injection system, the sac volume and injection holes area which accounts for the details of the injection velocity, the fuel injection rate per injection hole and occurrence of hole cavitation. Thus, differences between the sprays from inclined multihole injectors can be simulated and analysed.
Several spray sub-models have been implemented into the code, and their influence on the predicted droplet characteristics was evaluated. These models account for liquid core atomization, droplet secondary breakup, droplet collisions, droplet impingement, droplet turbulent dispersion and droplet evaporation. Aerodynamic-induced atomization, jet turbulence-induced atomization and cavitation-induced atomization introduced by a new sub-model are the three mechanisms considered to be responsible for the liquid core primary break-up; the vibrational, bag, chaotic, stripping and catastrophic break-up regimes are taken into account using different droplet secondary break-up models reported in the literature. A new sub-model based on the maximum entropy formalism is used for the calculation of the droplet size distribution. Coalescence, bouncing and separation are the mechanisms accounting for droplet-droplet interaction and a model assuming isotropic turbulence is used for the calculation of the gas turbulence effects on droplet diffusion. Evaporation has been considered assuming both uniform droplet temperature and its variation within the volume of the droplet. Effects attributed to droplet deformation, as calculated by the break-up model, frequently neglected in other existing spray models, are found to play an important role on the fuel evaporation rate; heat and mass transfer correlations for the flow around spheroids were used for these calculations. In the case of spray impingement on a wall, the rebounding, splashing and sticking regimes were considered, representing different physical assumptions adopted for the simulation of droplet-wall interactions; these models account for the effects of wall roughness, presence of wall film and wall temperature on the properties of the secondary droplets ejected from the wall.
Experimental test cases used for model validation include free atmospheric sprays injected into quiescent or cross flowing air, impinging sprays and diesel sprays in engine cylinders. Extensive comparison between computational and experimental results has been performed for the temporal and spatial variation of the droplet size and droplet velocities, the spray tip penetration and the global spray structure. The results revealed that the dominant parameters influencing diesel spray development and its mixing with the surrounding air are hole cavitation, injection velocity and droplet deformation.