Influence of Trailing Edge Height on Primary Atomization: Numerical Studies Applying the Smoothed Particle Hydrodynamics (SPH) Method

Air assisted atomizers for aircraft engine applications often include a prefilmer lip, where the atomization of the liquid fuel takes place. Recently, the trailing edge height of this prefilmer lips has been experimentally identified as a parameter influencing spray characteristics, in particular the breakup frequency and droplet size [1], [2]. Depending on the flow configuration, the trailing edge causes to establish a liquid reservoir, which is exposed to temporal fluctuating aerodynamic forces. However, if the trailing edge height is smaller than the average film thickness, the impact of the trailing edge seems to be reduced. Therefore, experimental investigations [3] using very thin lips do not reveal any effect of trailing edge thickness. The present paper addresses the question how the trailing edge thickness does affect the atomization and at which scales a further reduction of the trailing edge height no longer has an influence. The present investigation is based on a numerical study using the meshfree method Smoothed Particle Hydrodynamics (SPH) [4]. Because of its Lagrangian nature, the SPH method has some inherent advantages over commonly used grid based methods such as the Volume of Fluid (VoF) method when it comes to the modeling of multiphase flows in particular at high momentum ratios. The predictions are compared to an experimental investigation of a generic planar prefilming airblast atomizer. The flow parameters have been chosen to match those of the experiment. Prefilmer lips with various trailing edge heights have been studied. The results are compared in terms of droplet size distribution, ligament lengths and breakup frequencies. It is shown, that for trailing edge heights below 100 μm, a further height reduction does not significantly alter the breakup mechanism.