Critical nozzle is one of the flow metering devices which are being extensively used in industrial area dealing with gases. It makes use of the concept of flow choke that occurs at the nozzle throat. Under the choked flow conditions, pressure variations in the flow field downstream of the nozzle have negligible influence on the mass flow rate, and the coefficient of discharge is easily obtained only by the flow properties measured upstream of the nozzle. Recently, Kim et al. have reported the discharge coefficients of a variety of gases for a quite wide range of Reynolds number, using a computational fluid dynamics method. The flow characteristics through the critical nozzle have been well documented at both considerably low and high Reynolds number regimes.
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