Investigation of the effect of different throat diameters on the flow characteristics of a sonic nozzle

Abstract

Sonic nozzles are typically used as secondary standards for gas flow measurements. Most studies have focused on toroidal sonic nozzles, whereas studies on cylindrical nozzles are limited. The aim of this research is to shed light on this type of nozzle and study the parameters affecting it to improve its performance. Numerical simulations were conducted to investigate the effect of the change in geometry between the two types of sonic nozzles (toroidal and cylindrical) on the discharge coefficient for gas flow rate measurements under constant operating conditions with throat diameters ranging from 0.4064 to 3.175 mm. According to the results, the discharge coefficient increased by approximately 4.2% compared with that of the cylindrical nozzle at the same inlet stagnation pressure. The cylindrical nozzle was significantly affected by the boundary layer due to the length of the cylindrical throat section. The effect of the length of the cylindrical throat on the discharge coefficient of a cylindrical nozzle with different lengths of 0.25, 0.5, 0.75, and 1d_th was verified using four throat diameter nozzles ranging from 0.4064 to 3.175 mm to enhance the nozzle performance and compare it with that of a toroidal nozzle under the same conditions. According to the results, when the cylindrical throat length was reduced from 1 to 0.25 d_th, the discharge coefficient increased by approximately 2.8% at a constant inlet stagnation pressure of 171 kPa and the same throat diameter nozzle of 0.4064 mm. In addition, the divergence length should be considered for both toroidal and cylindrical nozzles during design, as the discharge coefficient increases slightly with a smaller divergence length.