The discharge coefficients of critical Venturi nozzles were measured for $CO2$ and $SF6$ on the Reynolds number range from $3×103$ to $2×105.$ The results showed that the measured discharge coefficients for both gases were about 2 percent larger than the theoretical estimation based on the assumption of isentropic flow of a perfect gas and this large deviation could not be reduced even by introducing real gas effects. The experimental results also showed that the large deviation for $CO2$ could be explained through the assumption of a nonequilibrium flow at the throat. On the other hand, the reason of the deviation observed for $SF6$ has not been clear yet, but one possible explanation would be the inadequate estimation of the boundary layer at the throat because the theory is based on the laminar boundary layer of a perfect gas. [S0098-2202(00)02004-6]

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