Air-assist pressure-swirl atomization

The performance of an air-assist pressure-swirl atomizer and, more importan t, its limitations are described. The design of the present atomizer is bas ed on a pressure-swirl nozzle, but differs from conventional single-phase p ressure-swirl designs in that the liquidfiltn in the exit orifice is stabil ized by axially injecting air through the upstream plane of the swirl chamb er: The present study includes drop size data obtained using a Malvern 2600 HSD particle size analyzer: Atomizer performance was assessed using three diff erent nozzle configurations and four different liquids. The drop size data indicate that an increase in liquid supply pressure, liquid mass flow, rate , or aatomizing air-to-liquid ratio by mass (ALR) leads to a decrease in Sa uter mean diameter (SMD). It also shows that spray quality (i.e., mean drop size) is independent of swirl chamber geometry at constant liquid supply p ressure and ALR for low viscosity liquids, Atomizer exit orifice diameter h as little effect on SMD when operating at constant liquid supply pressure f ar these same low-viscosity liquids. However, the effects of liquid mass fl ow rate and exit orifice diameter are coupled with an increase in exit orif ice diameter, lending to mt increase ill SMD when liquid mass flow rate is constant The influence of both swirl chamber and exit orifice diameter is e nhanced when liquid viscosity climbs to 0.010 kg/m-s. In these cases, SMD i ncreases,vith a decrease in swirl chamber diameter regardless of whether li quid mass flow rate or supply pressure is kept constant. SMD continues to d ecrease with a decrease in exit orifice diameter for both the constant liqu id mass flow rate and constant liquid supply pressure cases. Finally, the d ata indicate that mean drop size increases with an increase in either liqui d viscosity or surface tension. A first principles model was developed to explain the observed SMD scaling with operating conditions, nozzle configuration, and liquid physical proper ties. It combines the classical inviscid analysis for the flow inside a pre ssure-swirl atomizer with a correlation for the air-to-liquid velocity slip ratio, a geometric model for ligament formation, and a linear fluid mechan ical instability analysis to describe ligament breakup. Model predictions r eflect the observed SMD scaling with variations in liquid supply pressure, liquid mass flow rate, ALR, swirl chamber diameter, exit orifice chamber di ameter, and surface tension. Accuracy is best for intermediate and high liq uid delivery rates, Viscosity scaling is not captured accurately, That lack is ascribed to the inviscid internal flow model employed here, which canno t account for rite increase in liquid film thickness with an increase in li quid viscosity.