The Effect of Different Gases on Critical Flow Nozzles

Published

May 20, 2026

Author(s)

John Wright, Christopher Crowley, Aaron Johnson, Shinichi Nakao

Abstract

Critical nozzles and critical flow venturis are widely used gas flow meters because of their excellent calibration stability and well-developed flow theory. It is often desirable to calibrate them using one gas (dry air or nitrogen) and extrapolate that calibration to other gas species. The primary effects of gas species on I tried to explain the possibility of different the flow through critical nozzles are accounted for by the molar mass (M) and the ideal gas critical flow function (C_I^*). Prior researchers derived how secondary gas species corrections enter through real gas effects, the inviscid core flow, the velocity and thermal boundary layers, and vibrational relaxation phenomena. This paper compares theoretical predictions of nozzle discharge coefficients to experimental measurements made using gases with specific heat ratios between 1.09 and 1.67, including some with vibrational relaxation effects (CO2, SF6, and N2O) as large as 2.3 %. We introduce an approximate method for calculating the vibrational relaxation correction using an effective critical flow function C_(I,eff)^* that does not require a computational fluid dynamics simulation. For the cases examined, the agreement between experimental discharge coefficients and calculated values is better than 0.7 %.

Conference Dates

May 17-20, 2026

Conference Location

Nara, JP

Conference Title

FLOMEKO 2026

Pub Type

Conferences

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Keywords

critical nozzles, critical flow venturi, gas flow, vibrational relaxation

Citation

Wright, J. , Crowley, C. , Johnson, A. and Nakao, S. (2026), The Effect of Different Gases on Critical Flow Nozzles, FLOMEKO 2026, Nara, JP, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=961539 (Accessed June 9, 2026)

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