The Upstream Installation And Geometrical Parameters Effect On The CBPR Of Sonic Nozzle

Due to the characteristics of high precision, no movable parts, the sonic nozzle iscommonly used as standard meter to calibrate other types of gas flowmeters. The realizationand maintaining of critical flow is the precondition for the application of sonic nozzle. So, thecritical back pressure ratio is the key parameter for the sonic nozzle, which is directly relatedto the application of the device. Therefore, ISO9300suggests that the back pressure ratio ofthe sonic nozzle as the Reynolds number of throat part less than2×105should be no biggerthan0.25, or is subjected to actual test.Based on the pVTt device of National Institute of Metrology, the system of sonic nozzlecritical back pressure ratio test was established. In accordance with the requirements of ISO9300on the sonic nozzle geometrical parameters, three groups of sonic nozzles, such as, thesame throat diameters with different diffusion length, the same throat diameter with differentdiffuser angle and different throat diameters with the same diffuser angle, total of18nozzles,were manufactured, which the Reynolds number was within (5.25×104~1.81×105). Theupstream installation conditions and geometrical parameters on the critical back pressure ratioof sonic nozzle were investigated as the Reynolds number of throat part less than2×105. Theexperimental results showed that and results showed that,1) The critical back pressure ratio for all sonic nozzles within this paper were larger than0.45;2) For the sonic nozzles with diffuser angle of4°, the critical back pressure ratio could reachthe designed value of ISO9300, which was not influenced by the upstream installationconditions, when the Reynolds number of throat part was larger than1.1×105; Otherwise,the upstream installation condition had significant effects on critical back pressure ratio.On the basis of the experimental results, the effects of upstream installation conditionsand geometrical parameter of sonic nozzle to critical back pressure ratio were analyzed:1) Effects of upstream installation conditions: when the critical flow condition is reached, therequirement of upstream installation conditions for sonic nozzle is low. The premature unchoking phenomenon will take the critical back pressure ratio of sonic nozzle decreased,which shows diversity and instability. For the sonic nozzles with the same throat diameter,this phenomenon easily happens for the sonic nozzle with larger diffuser angle. To avoidthis phenomenon, the nozzle with small diffuser angle is recommended.2) Effects of the geometrical parameters: for the sonic nozzle with the same throat diameter,the stronger flow separation and shock closer to the throat section will lead to its criticalback pressure ratio decreasing due to the shorter diffuser length or larger diffuser angle.Within the laminar flow, for the sonic nozzles with the similar geometrical parameters, theboundary layer thickness will gradually be decreased with the sonic nozzle throat diameterincreasing, which results in weak interaction between shock and boundary layer. Finally,the critical back pressure ratio is increased.