The Establishment Of Critical Back Pressure Ratio Test System For Micro Sonic Nozzle And Study On The Influence Factors

The sonic nozzle is often used as a standard meter in the gas flow standard device because of its many excellent characteristics such as high accurrrate,low price and good long-term stability.It is often utilized to transfer the quantity and value to other gas flow meters.With the development of micro gas flow measurement,micro sonic nozzles have been widely used.The critical back-pressure ratio of the sonic nozzle when the Reynolds number of throat is less than 2×10~5should be no bigger than 0.25,or is subjected to make the actual test,which is suggested in ISO 9300.Therefore,it is significant to study the critical back-pressure ratio for micro sonic nozzles.The research in this thesis is consisted with the following several parts:(1)Numerical simulation technology was employed to simulate the internal flow field of the two sets of sonic nozzles through Fluent.The influence of throat diameter and inlet diameter on the critical back-pressure ratio of sonic nozzles was studied by the difference of internal flow field.(2)The overall design of a critical back-pressure ratio test facility is presented in this paper.The design of hardware mechanical structure includes:determine the size of orifice flowmeter through flowrate;the structure,size and clamping mode of the rectifier honeycomb are designed.determine the size of the experimental pipe and design the position to measure temperature and pressure;design the method to clamp sonic nozzle.Complete the overall design of the software chart,adopting the upper computer monitoring mode and data acquisition system based on LabVIEW software platform.Complete the software debugging.(3)On the basis of the completion of the test facility construction and the device leakage rate experiment,the actual test of the critical back-pressure ratio of seven micro sonic nozzles was completed under eight different upstream installation conditions.Complete the uncertainty analysis of the device,the expanded uncertainty of the device is 0.56%(k=2).According to the actual test results,the influence of upstream installation conditions and geometry on the critical back pressure ratio of sonic nozzle was studied.