Abstract

A fundamental study on the characteristics of biodiesel spray is performed and compared with diesel spray at the same condition. In this vein, the liquid jet in cross flow problem is applied to compare the spray penetration depth, droplet size distribution, spatial flux distribution and breakup study of biodiesel, diesel and their blends. Both experimental and numerical analyses have been performed to shed more light on the physics of atomization of liquid jets in cross flow.
In the experimental part, shadowgraph technique and image processing have been used in order to capture the penetration of the spray. In addition, droplet size measurement and spatial flux distribution are found by using Phase Doppler Particle Analysis (PDPA). The experimental study shows less penetration depth for biodiesel in comparison with diesel. On the other hand, the droplets’ mass flux distribution with biodiesel is less in the vicinity of the windward side of the spray. In the numerical part of this work, the near field of the injection is simulated using Volume of Fluid (VOF) coupled with the Large Eddy Simulation (LES) turbulence model. As a result a considerable dissimilarity has been found between the breakup regime of biodiesel and diesel. Namely, at Weber numbers of above 40, where the breakup regime of most liquids including diesel occurs in atomization mode, the breakup regime of biodiesel is bag breakup. The main cause of this behavior can be attributed to the remarkably higher viscosity of biodiesel compared with many conventional fuels.
The geometry of the orifice can play an important role in controlling the atomization parameters. In this vein, elliptical jets with various aspect ratios between 1 (circular) and 3.85 is performed for several Weber numbers, ranging from 15 to 330. The elliptical jet is first characterized in free air in order to find its capillary behavior in Rayleigh instability regime. The axis-switching phenomenon and breakup length of the jets are the important parameters characterized in this research. Second, the elliptical jets in crossflow are simulated to find differences from the circular orifices in terms of penetration depth, surface waves and breakup length. The simulations of elliptical jets in crossflow were performed with relative gas–liquid Weber numbers of 11, 18 and 30, which cover the bag and multimode primary breakup regime in crossflow. The results show remarkable changes in liquid shapes before disintegration for different aspect ratios.