Influence of ambient air pressure on effervescent atomization

Shikui Chen, Purdue University

Abstract

All previous studies of effervescent atomization have concentrated on measurements of the Sauter mean diameter (SMD) of the spray under conditions where the air-liquid mixture is discharged into air at normal atmospheric pressure. A new atomization test facility has been constructed for the interrogation of sprays at elevated ambient pressures. Measurements of mean drop size in sprays injected into air at pressures up to 1 MPa (10 atm) were conducted by locating an effervescent atomizer in a high pressure tank fitted with observation windows. A Malvern particle analyzer was used to measure mean drop sizes over wide ranges of ambient air pressure, atomizing gas/liquid mass ratio, liquid injection pressure, liquid viscosity, surface tension and atomizer discharge orifice diameter. It was found that continuous increase in ambient air pressure above the normal atmospheric value causes the mean drop size to first increase up to a maximum value and then decline. An explanation for this characteristic was derived in terms of the various contributing factors to the overall atomization process. It was also observed that mean drop sizes and drop size distributions are virtually independent of nozzle discharge orifice diameter at all levels of ambient air pressure. The flow instability and flow regimes inside the mixing tube and at the exit orifice of the atomizer were also exmaiend in this study. A correlation for exit orifice discharge coefficient was found to represent the experimental results. A relationship between the atomizer pressure ratio and upstream conditions was also derived. This relationship allows the determination of flow conditions at the exit of the final orifice. The effects of variations in atomizer geometry and operating conditions on spray cone angle were also investigated. It was found that the sprays produced by a single-orifice effervescent atomizer have relatively narrow cone angles, ranging from 15 to 23 degrees. It was also noted that an increase in ambient air pressure above the normal atmospheric value results in a reduction in spray cone angle although the change is small. Equations for predicting the mean drop size of a spray were sought based on the conservation equations of mass and energy for a gas-liquid two-phase flow. These equations were found to predict reasonably well the Sauter mean diameters of sprays produced by effervescent atomization. (Abstract shortened by UMI.)

Degree

Ph.D.

Advisors

Lefebvre, Purdue University.

Subject Area

Mechanical engineering

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