Influence of diffuser angle on discharge coefficient of sonic nozzles for flow-rate measurements

A series of numerical simulations were conducted to investigate the influence of diffuser angle on discharge coefficient of sonic nozzles for gas flow-rate measurements satisfied with the regulation of ISO-9300 and operated at atmospheric condition with throat diameter from 0.15 to 5 mm and 2.0×103≤Red≤6.6×1042.0×103≤Red≤6.6×104. When throat diameters are larger than 1 mm, the discharge coefficients are almost the same for nozzles with different diffuser angles, while they increase with increasing diffuser angle for smaller nozzles. A concept or effective critical flow was introduced to explore the effect of diffuser angle on discharge coefficient. Using a new parameter, effective throat diameter or deviation of effective throat diameter from the minimum effective diameter δδ, a very good description is derived to discuss the effect of diffuser angle on the discharge coefficient.