Experimental investigation of critical pressure ratio in orifices

Abstract.

To determine the value of the critical pressure ratio in orifices, critical mass flow rate of air through straight-bore orifices and knife orifices was measured. The straight-bore test orifices with varying orifice diameters of 4, 7, 10 and 12 mm were installed in a 20-mm pipe. The knife or sharp-edged test orifices with orifice diameters of 10, 15 and 18 mm were installed in a 40-mm pipe. The test orifices with diameter ratio from 0.2 to 0.6 exhibited a constancy of discharge at ratios of the downstream to upstream pressures of less than 0.17, which is considerably lower than the theoretical critical pressure ratio for an ideal nozzle. An empirical expression to calculate the value of the critical pressure ratio, which includes the relevant primary parameters and which fits the data well, is suggested for engineering design purposes.