Investigation of water spray characteristics in the near field of a novel swirl burst injector

Abstract

The limit of fossil fuels and more stringent emission standards have prompted the exploration of renewable biofuels and clean combustion technology. Clean and efficient combustion of liquid fuels mainly relies on spray fineness achieved by fuel injectors. Conventional injectors such as the air blast (AB) nozzles are only effective for low-viscosity fuels. Hence, viscous source oils of biofuels have to be converted into “drop-in” fuel such as biodiesel, leading to the high production cost of biofuels. In the past decade, the flow blurring (FB) injection using a two-phase flow concept has proved to generate fine sprays for even highly viscous liquids in the near field of injector exit, rather than a typical fuel jet from AB injectors. Thus, the FB injection has achieved cleaner burning of fossil fuels and enabled clean lean-premixed combustion of viscous oils including biodiesel source oils, signifying cost-effective biofuel utilization without the conversion cost. Recently, our group designed a novel twin-fluid swirl burst (SB) injector that integrates the advantages of FB and further improves secondary atomization caused by Rayleigh-Taylor instabilities. The SB design innovatively introduces swirling atomizing air (AA) to stimulate more vigorous interaction between AA and large droplets or ligaments from the primary FB atomization by bubble-bursting. The present study compares the water spray micro-characteristics of FB and SB injectors in the near field of the injector exit at the same flow conditions using high spatial resolution particle image velocimetry and shadowgraph imaging systems. Results show that, compared to the FB atomization, the novel SB injector has successfully generated a wider spray with finer droplets immediately at the exit orifice and a shorter atomization completion length, indicating the improved secondary atomization. Estimate of Weber number and Ohnesorge number consistently substantiates the improved SB atomization and suggests bag breakup dominating the secondary atomization for both injectors. Radial profiles of Sauter Mean Diameter indicate the SB injection yields uniform droplet size distribution compared to the FB injection that has relatively larger droplets mainly at the spray periphery. Regardless of the variations in the atomizer near region, both SB and FB injector generate fine sprays with similar final droplet size.