Results are presented of a theoretical and experimental study dealing with critical and choking Mach numbers of organic vapor flows through turbine cascades. A correlation was derived for predicting choking Mach numbers for organic vapor flows using an asymptotic series expansion valid for isentropic exponents close to unity. The theoretical prediction was tested employing a linear turbine cascade and a circular cylinder in a closed-loop organic vapor wind tunnel. The cascade was based on a classical transonic turbine airfoil for which perfect gas literature data were available. The cascade was manufactured by Selective Laser Melting (SLM), and a comparable low surface roughness level was established by subsequent surface finishing. Because the return of the closed-loop wind tunnel was equipped with an independent mass flow sensor and the test facility enabled stable long-term operation behavior, it was possible to obtain the choking Mach number with high accuracy. It was observed that non-perfect gas dynamics affect the critical Mach number locally, but the observed choking behavior of the turbine cascade was in good agreement with the asymptotic result for the considered dilute gas flow regime.

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