Abstract
Growing concern over global warming due to carbon pollutant emissions such as CO2 and CO has led the aviation industry to take numerous steps to reduce carbon emissions. One viable option is to use biofuels instead of conventional hydrocarbon fuels. The net carbon emission produced while using biofuel is still neutral, and therefore, the usage of biofuels has significantly increased over the past decade. However, the physical and chemical properties of biofuels are drastically different from those of hydrocarbon fuels, which can significantly affect the spray characteristics. For example, biofuels generally have a higher viscosity, which can affect the atomization and vaporization process, resulting in lower combustion efficiency and higher pollutant emissions compared to hydrocarbon fuels. Biofuels also have higher cetane numbers and lower calorific values, which can increase the ignition delay time and release lower net heat energy. Despite these parameters being crucial to spray characteristics in a reactive environment, past studies have underrated their importance. The present study aims to investigate the spray characteristics of a commercially available biofuel, particularly under reactive swirl flow conditions.
In the current study, a simplex swirl atomizer housed in a spray burner rig is used to generate the fuel spray. The rig has provision to incorporate an axial swirler in the annular air pathway such that the swirling airflow interacts with the fuel spray at the burner exit. To understand the effect of air swirl on the spray characteristics, experiments are conducted for three different conditions in the presence and absence of air co-low and air swirl. The air and the liquid mass flow are maintained the same. Different optical diagnostics are employed to characterize the spray combustion process. The primary breakup length of the liquid sheet (near the nozzle exit) and the spray cone angle are measured by the shadow imaging technique. The spray flame photographs are captured using a SLR camera. The size of the spray droplets is measured using Interferometric Laser Imaging for Droplet Sizing (ILIDS). In this technique, the spray field is illuminated by a high-power laser sheet. Each droplet is imaged as a rectangular fringe pattern that is generated on a defocused plane due to the interference of the reflected and first-order refracted light scattered from the droplet. The number of fringes formed in an interference pattern is proportional to the droplet size. The ILIDS measurements are obtained at different axial and radial measurement stations. The measured droplet and flame characteristics for different air swirl strengths provide insight into the practical spray combustion behavior of the biofuel, which has not gained much attention in the past.
