Spray characterization of an air-assist pressure-swirl atomizer injecting high-viscosity Jatropha oils

The effect of the kinematic viscosity on liquid sprays injected by an air-assist pressure-swirl atomizer has been investigated in a series of experiments employing pulse-laser backlight imaging and laser diffraction droplet size distribution measurements. Sprays of crude Jatropha oil, Jatropha methyl ester, diesel fuel and propylene glycol brine were examined, and the liquid viscosity was controlled by changing the liquid temperature. It was found that atomization of the liquids was improved by introducing the assist air. Instantaneous images of the sprays taken using pulse-laser backlight illumination show that the 'spray cone' inside the nozzle cap evolves into the following five phases as the Reynolds number increases (i.e., liquid viscosity decreases): (1) a twisted liquid jet, (2) a folded liquid film, (3) a hollow spray cone with a smooth spray cone surface, (4) an unstable spray cone with periodic fluctuation, (5) a spray cone with many wrinkles on the cone surface. It was found that the droplet size in terms of the Sauter mean diameter (SMD) is small when the spray width is large. The SMD did not monotonically increase with the liquid viscosity. A local maximum of the SMD in the SMD vs. liquid viscosity curve was observed in the unstable transitional region where the transition from laminar flow to turbulent flow occurred. A local minimum of the SMD was observed at a higher liquid viscosity, where the 'spray cone' inside the nozzle cap changed from a twisted liquid jet to a folded liquid film. The change in the SMD as a function of liquid viscosity has a strong correlation with that in the flame radiation intensity as a function of liquid viscosity observed in a combustion test employing the same fuel atomizer (Hashimoto et al. submitted for publication). This indicates that the flame radiation intensity can be decreased by improving atomization characteristics. (C) 2013 Elsevier Ltd. All rights reserved.