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Title: The Effect Of Using Atmospheric Air In Critical Flow Nozzles.
Author: D. G. Stewart, J. T. R. Watson, A. m. Vaidya
Source: 1999 International Symposium on Fluid Flow Measurement
Year Published: 1999
Abstract: The critical flow nozzle is commonly used with atmospheric air. This poses a problem as the equations of state used to calculate the theoretical mass flowrate for air are based on dry CO2-free air. The air must be dried, and the CO2 removed before use or the effect of atmospheric air accounted for. This paper analyses the effect of water vapour and CO2 on the critical flow of air. The analysis utilises the AGA8 equation of state, which can be used for dry and moist air. Using recent data for the water vapour pressure enhancement factor, the analysis covers the temperature range 0 - 50C at pressures up to 15 MPa. The results show that the effect of the carbon dioxide present in atmospheric air is small, but that the effect of water vapour can be significant. As the maximum mole fraction of water vapour in air goes down with increasing pressure and up with increasing temperature, the effects can be most significant at low pressures and high temperatures. A previous correction factor for water vapour in air, developed for low pressure, is shown to apply well over the range. A new correction factor is presented which removes the need to calculate the mole fraction of water in air.

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