The effect of geometric parameters on the flow of compressed air through small orifices

Author

Rebecca Ann Bachschmidt

Date of Award

8-1992

Degree Type

Thesis

Degree Name

Master of Science

Major

Engineering Science

Major Professor

Richard J. Jendrucko

Committee Members

Osama Soliman, Jack Wasserman

Abstract

An investigation was undertaken to determine the effect of cross-sectional geometry on the discharge of compressed air through thin orifices and to establish its relevance in the prediction of air lost in typical leakages found in industrial compressed air systems. Tests were conducted with four custom-fabricated circular and equilateral triangular orifices ranging size from 1.39(10)^-3 to 0.21(10)^-3 and 1.04(10)^-3 to 0.18(10)^-3 in², respectively, with thicknesses approximately 75% of the orifice hydraulic diameter. A compressed air orifice flow metering system was constructed to provide a constant-pressure source of dry, clean air at 100 psig and to meter the volumetric flow of air discharged through the orifices to the atmosphere. Flow approach velocity at the orifice upstream face was negligible; Reynolds numbers based on orifice diameter and maximum adiabatic compressible fluid velocity exceeded 100,000 for all experimental runs.

Coefficients of discharge calculated for the air mass flow rate through the circular orifices were about 5% higher than previously documented results for knife-edged circular orifices. However, throat thicknesses of the orifices used in investigation were about 10% greater, indicating a higher range of discharge coefficient possibly due to a decreased degree of efflux vena contracta.

Generally, the results indicated mass flow rates which were about 30% higher through equilateral triangular orifices than through circular orifices of the same cross-sectional area. Comparison of mass flow rates obtained with triangular and circular orifices of equal hydraulic diameter again revealed discharge rates through the triangular orifices which were about 30% greater than those through the circular orifices. Although the hydraulic diameter concept closely approximates flows for a variety of duct geometries, it does not appropriately describe flows through thin orifices. Similar discharge flow rates were noted for triangular and circular orifices of equal perimeter. While the larger perimeter ratio of triangle to circle may in part explain an increased mass flow rate, the influence of orifice perimeter on the downstream flow pattern is believed to be significant. Therefore, it is concluded that orifice cross-sectional geometry is of primary importance in determining flow discharge rates.

Recommended Citation

Bachschmidt, Rebecca Ann, "The effect of geometric parameters on the flow of compressed air through small orifices. " Master's Thesis, University of Tennessee, 1992.
https://trace.tennessee.edu/utk_gradthes/12165