Thermal Effects on Critical Flow Venturis

Published

April 14, 2015

Author(s)

John D. Wright, Aaron N. Johnson, Michael R. Moldover, Woong Kang, Liang Zhang

Abstract

Critical flow venturis (CFVs) are widely used as working and transfer standards for gas flow measurement because of their long-term calibration stability (5 (e.g., a 2 mm throat diameter flowing air at 1 MPa), CFVs exhibit sensitivity to the environmental temperature of approximately 200 parts in 10⁶ / K, primarily due to the difficulty of measuring the temperature of the gas entering the CFV (temperature sampling errors) and thermal boundary layer effects. During the CCM.FF-K6 2002 key comparison, the temperature sensitivity of the CFV transfer standard accounted for as much as 40 % of the transfer standard uncertainty. Heat transfer from the CFV body to the thermal boundary layer produces an annular region of lower density gas near the CFV wall and a lower CFV discharge coefficient. We minimized temperature sampling errors by using a better gas temperature measurement design, including non-metallic approach pipe materials and a temperature sensor with low stem conduction error. We measured and accounted for thermal boundary layer effects using the correction C_T = 1 + K Re^-1⁄2 ({Δ}T / T₀) where {Δ}T is the difference between the CFV inner wall temperature and the inlet gas temperature. The value of K is approximately -7 for CFVs made of stainless steel and copper with diameters of 0.56 mm, 1.1 mm, and 3.2 mm for the particular CFV configuration we used.

Proceedings Title

International Symposium for Fluid Flow Measurement

Conference Dates

April 14-17, 2015

Conference Location

Arlington, VA

Pub Type

Conferences

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Keywords

critical, flow, venturi, boundary layer, discharge coefficient

Citation

Wright, J. , Johnson, A. , Moldover, M. , Kang, W. and Zhang, L. (2015), Thermal Effects on Critical Flow Venturis, International Symposium for Fluid Flow Measurement, Arlington, VA, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=917814 (Accessed October 14, 2025)