Yong Moon Choi

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

Abstract A sonic nozzle is presently used as a reference flow-meter in the area of gas flow-rate measurement. The critical pressure ratio of the sonic nozzle is an important factor in maintaining its operating condition. ISO 9300 suggested that the critical ratio of a sonic nozzle should be a function of area ratio. In this study, 13 nozzles designed according to ISO 9300, with diffuser half angles of 2°–8° and throat diameters of 0.28 to 4.48 mm were tested. The testing result for the angles of 2°–6° are similar to that of ISO 9300. But the critical ratio for the nozzle of 8° decreases by 5.5% in comparison with ISO 9300. However, ISO 9300 does not predict the critical pressure ratio at Reynolds numbers lower than 10 5 . To express the critical pressure ratio of sonic nozzles at low Reynolds numbers, it is found that the critical pressure ratio should be related 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 numbers for small sonic nozzles is suggested in this investigation, with an uncertainty of ±3.2% at 95% confidence level.

Effects of cavitation and plate thickness on small diameter ratio orifice meters

Abstract High differential pressure drop is frequently required in the process line of electric power plants. However, cavitation produced by a high pressure drop could damage the pipe and pump blades. In these experiments, the effects of cavitation and plate thickness for small-bore diameter (d) to pipe diameter (D) ratio ( β = d/D) orifice plates were evaluated in a 100 mm diameter test section of a water flow calibration facility. Three size orifice meters were tested with β = 0.10, 0.15, and 0.33 and no bevel on the throat. The cavitation number at inception was measured to be 1.0 to 1.2 for all experiments. Although cavitation occurred in all experiments, the discharge coefficient was affected by cavitation only for the smallest bore ( β = 0.10) and thickest plate tested, 7.0 mm or t/d = 0.70 where t is the plate thickness. Thickness seemed to influence the discharge coefficient only for the β = 0.10 plate. But the trends in the results for thickness were not conclusive and could be attributed to edge sharpness.

Thermal effects in small sonic nozzles

Abstract A study has been made of the effect of temperature on the coefficient used to characterise small sonic nozzles. Due to adiabatic cooling of the gas stream to −30 °C in the throat, the body of the nozzle is also cooled. The temperature drop of several bodies during operation was measured. Using a gas flow standard that can operate in continuous mode, measurements were made of the change of the nozzle coefficient with temperature. The real gas correction factor, the discharge coefficient and the area of the throat are investigated to explain the observed change in the nozzle coefficient. Changes in these quantities are inadequate to explain the results. The maximum percentage changes likely to be experienced are estimated for a range of nozzle throat diameters.

Volumetric positive-displacement gas flow standard

Abstract A new volumetric standard system has been developed capable of calibrating gas flow meters continuously. It is made of double concentric spheres. The inner sphere defines a basic volume that is divided into two parts by a flexible diaphragm. A water draw method used to measure this volume gave 25.343 l with 1 ml uncertainty. A changing pressure signal causes a change of flow direction every cycle. A careful treatment of this transient signal provides corrections to the basic formula for the mass flow rate. An uncertainty analysis of the new standard system shows that the most important factor is air temperature. Multiple nozzles were calibrated singly and in parallel by the new system to check the upper limit of the flow rate which is about 2 m 3 /h with 0.15% expanded uncertainty.

Calibration System for Three-Cup Anemometers

초록:3- , ,. 3- ,, . 3-. 3-. 3-, . 53- , 3- .Abstract: Three-cup anemometers are popular devices for measuring wind speeds in automated weather stations,environmental monitoring systems, and wind turbines. Cup anemometers usually suffer from lack of long-term stabilityowing to the wear of the bearing systems that support the rotational parts. The bearing systems are susceptible to externalpollutants, vibrations, and gusts. Therefore, these anemometers have to be calibrated regularly to maintain the desiredcharacteristics for measuring wind speed. In the present study, a new in-situ calibration system to help reduce cost andsave time by calibrating the cup anemometers at the installation site is proposed. A portable in-situ calibrator wasfabricated. After the characteristics of this calibrator were verified, it was used to calibrate cup anemometers. Some of thecalibration results were compared with the data obtained by wind tunnel testing.