59013 - Critical and Choking Mach Numbers for Organic Vapor Flows Through Turbine Cascades 

In high subsonic flows through cascades, two inflow Mach numbers are of practical interest, namely the critical Mach number and the choking Mach number. The critical Mach number is defined as the Mach number for which sonic conditions are first reached locally in the flow field. In such case, there will be no shocks. When the Mach number is increased further, sonic conditions are achieved at the entire throat cross section of the cascade. In this case, downstream influences are not felt upstream, and the back pressure has no influence on the upstream flow field. The cascade is said to be "choked", and the upstream Mach number is called the choking Mach number. Using perfect gas assumptions, expressions for the critical and choking Mach numbers can be derived and are quoted in several textbooks and monographs. However, much less is known regarding critical and choking Mach numbers in the case of non-perfect gas flows of organic vapor flows through turbine cascades.

In this contribution, the outcome of an experimental study dealing with critical and choking Mach numbers of the flow of air and NOVEC 649, an organic vapor used also in organic Rankine cycle ORC applications, is presented. To this end, a linear turbine cascade in a closed-loop organic vapor wind tunnel test facility was investigated. The linear cascade was based on a classical transonic turbine airfoil for which perfect gas literature data were available. The individual blade profiles were manufactured by selective laser melting (SLM) and two different surface roughness levels were established. 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 both critical and choking Mach number with high accuracy. Furthermore, the effect of airfoil roughness and the impact of Pitot probe stems on the flow characteristics were checked. It was observed that the non-perfect gas dynamics locally affected the critical Mach number but it did not significantly change the choking behavior of the turbine cascade. The experimental results were compared with theoretical and computational fluid dynamics analyses and a good agreement was found. Based on the new data, a correlation was derived for predicting choking Mach numbers in organic vapors.

This work was funded by the program PROGRES.NRW-RESEARCH, Germany, under the project title “Printed ORC Blades”.