Numerical simulation of the internal flow field for the straight pipeline after a sonic nozzle and its influence on the discharge coefficient
By establishing the model, a nozzle outlet and its internal flow field for the straight pipeline after the sonic nozzle is simulated by the CFD software. The simulation results are compared with theoretical data and experimental tests respectively. According to the simulation results, pressure changes strongly from the nozzle outlet, and gradually rises to the back pressure after a distance of 3D (D is pipe diameter) from the nozzle outlet. With the 5D length of the straight pipeline after a sonic nozzle, the nozzle throat will keep critical flow when the back pressure ratio is less than or equal to 0.849, and the straight pipeline after a sonic nozzle has little influence on the discharge coefficient. But when the back pressure ratio is less than 0.507, the pressure in the pipeline appears fluctuations.
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.
Numerical Characterization Of The Discharge Coefficient In Critical Nozzles
The discharge coefficient in critical nozzles flowing N 2, Ar, CO2, and Hz in the Reynolds number range (2 000 to 22 000) is studied computationally to assess the capability of Computational Fluid Dynamics (CFD) in assisting experimental calibration. A parametric study is conducted to investigate the effects of the wall thermal boundary condition and gas species on the calibration process. Comparisons between numerical and experimental data are found to be in good agreement. However, similar to existing analytical models, the computational model has difficulty characterizing the discharge coefficient for CO2. Based on these results, it is concluded that with continued research CFD may have the potential to aid in the calibration of critical nozzles by providing detailed flow field characteristics for operating conditions and nozzle geometries not conducive to experiment.