Effect of electric charge and temperature on the near-field atomization of diesel and biodiesel

Abstract

High speed backlight imaging combined with particle tracking velocimetry is used to analyse the near-field characteristics of a series of typical charge injection atomizer sprays. Diesel and biodiesels are studied as a function of applied voltage and temperature, in order to provide new information on the influence of fuel preheating on charged sprays. Detailed quantitative characterisation of ligament and droplet statistics, fragment orientation and local dimensionless scalings using “effective surface tension” are presented. These shed new light on the fragmentation mechanisms that drive primary atomization in charge injection systems. It is observed that pre-heating can reduce the droplet size of electrostatically atomized dielectric fluids. However, the advantage of pre-heating is limited by a higher ionic mobility at higher temperatures which increases the leakage current. Bimodality in the droplet size distribution is noted which confirms previous work, and this study extends the literature by analysing distributions of ligament size. Statistics of orientation and velocity of arbitrarily shaped ligament-like fragments are presented to quantify the oscillatory nature of the primary atomization zone. Analysis of the local non-dimensional Weber number using an effective surface tension demonstrates that at a critical applied voltage, atomizing arbitrarily shaped fragments approach a local critical Weber number value (We1/2 ∼0.8) which is comparable to previous work done in non-charged laminar and air assisted sprays.