A comparison of errors in four methods for determining critical flow functions for sonic flow nozzles and venturis

摘要

The errors in four methods for determining the critical flow function are given for inlet pressures to 300 atmospheres, and temperatures of 400 to 700 R (222 to 389 K). Method No.1 assumes air to be an ideal gas with a constant ratio of specific heats of 1.4 and a critical flow function of 0.6847315. The results are given as correction factors to convert from ideal gas to real gas critical flow functions. Method No.2 uses the same ideal gas equation as Method No.1 with the ratio of specific heats corresponding to the inlet stagnation state to the critical flow meter. Method No.3 uses the compressibility factor corresponding to the inlet-stagnation-state for an approximate correction of Method No.2 results for real-gas effects. Method No.4, developed by Robert C. Johnson, uses iterative calculation methods to take real gas properties rigorously into account. The critical flow functions are based on air property data available in the 1950-1960 era for pressures up to 100 atmospheres. The "true values" for the error calculations were obtained by Daniel A. Sullivan using Johnson's method with more recent air property data. The maximum error decreases progressively from Method No.1 to Method No.4.