ID 369

Onion-to-Cone Transition of the Liquid Film from a Pressure-Swirl Atomizer by Imparting Swirl Air and its Hysteresis

 

Abstract:

In the present study, improvement of atomization quality of a pressure swirl fuel injector of a flow number of 15×10-2 mm2 by imparting swirling air motion around the onion- or tulip-shaped liquid film formed at very low injection pressures (or near the lower end of fuel flow rates) has been investigated. It was designed for the pilot fuel injection in a staged lean-premixed combustor for a small turbo-fan engine. The shape of the liquid film from the orifice was observed for water and kerosene at injection pressures equal to and lower than 100 kPa, where the liquid film was in the onion or tulip stage. Two types of air swirl units were provided coaxially with the body of the injector. Tests were conducted at swirling air pressure drops up to 12%, though the normal pressure drop is about 4% in gas turbine combustors. Threshold pressure drops for the transition between onion and cone stages and tulip and cone stages were determined as a function of injection pressure for each air swirl unit. A hysteresis was found in the transitions: once established with increasing air pressure drop, the cone stage continued for a while until a very low swirling air pressure drop was reached when decreasing swirling air pressure drop. Another interesting phenomenon was found: imposing external stimulus, such as a blow of air jets from small holes (1.5mm in diameter) against the onion bubble, resulted in an instantaneous transition. The swirling air pressure drop required for the transition was significantly reduced with external stimulus.