The Influence Of Stagnation Pressure On Discharge Coefficient Of Sonic Nozzle

Due to many advantages, such as good stability, low price, and so on, the sonic nozzles were widely used as standard meter to calibrate other types of gas flow meters. The discharge coefficient is the most important parameter for the sonic nozzle, which had close relationship with stagnation pressure. Based on the high pressure p VTt standard facility in National Institute of Metrology, the characteristics of the discharge coefficient with stagnation pressure was investigated in this thesis.At the end of 2014, a set of high pressure gas flow standard facilities had been constructed in National Institute of Metrology, including high pressure p VTt primary gas flow standard facility, high pressure sonic nozzle secondary gas flow standard facility and high pressure close loop working gas flow standard facility. The high pressure p VTt primary gas flow standard facility was used as the experimental system, and the expanded uncertainty of the high pressure p VTt standard facility could be 0.06%(k=2). With strict process control, the expanded uncertainty of discharge coefficient of sonic nozzle was 0.08%(k=2).Based on the capability of the high pressure p VTt standard facility, total of 18 nozzles were manufactured with throat diameter of(1.921~12.444) mm range. Within(0.1~2.5) MP stagnation pressure, the discharge coefficients of the sonic nozzles were investigated. The following three conclusions were gotten:1) At different stagnation pressure, discharge coefficient could be changed up to 2.3% for the same nozzle. At different boundary layer conditions, the relationship between discharge coefficient and the Reynolds number was different: in laminar boundary layer, the discharge coefficient significantly increased with Reynolds number increasing; in transition boundary layer, the discharge coefficient had a reduced "jump" with Reynolds number increasing; in turbulent boundary layer, the discharge coefficient slowly increased with Reynolds number increasing.2) Based on the ISO9300-n model and NMIJ-2013 model, the NIM-2016 model was presented, and the uncertainty of predicted discharge coefficient was less than 0.30%(k = 2). With 3 sonic nozzles as transfer meter, the bilateral comparison between NIM and PTB was conducted. The comparison results showed good consistence, which verified the capability of the new high pressure facility of NIM and the accuracy of the NIM- 2016 model.3) The Reynolds number of boundary layer transition showed close relationship with the throat diameter. On the base of the experimental results, the empirical equations between the Reynolds number of boundary layer transition and throat diameter was presented, which was verified by other available experimental results.