The evaluation of critical pressure ratios of sonic nozzles at low Reynolds numbers

A sonic nozzle is presently used as a reference flow-meter in the area of g as flow-rate measurement. The critical pressure ratio of the sonic nozzle i s an important factor in maintaining its operating condition. ISO 9300 sugg ested that the critical ratio of a sonic nozzle should be a function of are a ratio. In this study, 13 nozzles designed according to ISO 9300, with dif fuser half angles of 2 degrees -8 degrees and throat diameters of 0.28 to 4 .48 mm were tested. The testing result for the angles of 2 degrees -6 degre es are similar to that of ISO 9300. But the critical ratio for the nozzle o f 8 degrees decreases by 5.5% in comparison with ISO 9300. However, ISO 930 0 does not predict the critical pressure ratio at Reynolds numbers lower th an 10(5). To express the critical pressure ratio of sonic nozzles at low Re ynolds numbers, it is found that the critical pressure ratio should be rela ted as a function of Reynolds number rather than area ratio, as used by ISO 9300, A correlated relation of critical pressure ratios and low Reynolds n umbers for small sonic nozzles is suggested in this investigation, with an uncertainty of +/-3.2% at 95% confidence level. (C) 2001 Published by Elsev ier Science Ltd. All rights reserved.