Non-ideal flows of organic vapors : an experimental investigation for applications in organic Rankine cycle turbines

The present thesis work discusses the execution and results of an experimental investigation carried out on the Test Rig for Organic Vapors (TROVA), located at the Laboratory of Compressible fluid dynamics for Renewable Energy Applications (CREA Lab) of Politecnico di Milano, with the aim of characterizing non-ideal flows of organic vapors representative of those occurring in Organic Rankine Cycle (ORC) turbines. Fluids usually employed in ORCs feature high complexity and molecular weight, and turbine expansion occurs in the dense gas region (near the saturation curve and the critical point). As a result, turbine flows are highly supersonic and show non-neglible real gas effects, requiring accurate design tools for their modelling. The TROVA is a blow-down wind tunnel for organic vapors, currently employing Siloxane MDM (Octamethyltrisiloxane, C8H24O2Si3), a silicon oil of particular interest for high temperature ORC applications, expanding in a converging-diverging nozzle test section. Independent temperature and pressure measurements, complemented by schlieren visualizations, are performed with the aim of investigating real gas effects on parameters of interest in turbomachinery and providing one of the first ever assessments of a non-ideal compressible fluid dynamics CFD solver against experimental measurements. In this work, consistency of measurements performed during different experimental runs is verified. The validity of the numerical tools employed is proven by the very good agreement between experimental data and CFD calculation. Trends in pressure ratio, speed of sound and Mach number are observed to significantly deviate from the polytropic ideal gas behaviour: the extent of this discrepancy depends on total conditions and increases as the flow non-ideality increases. The schlieren technique is exploited in an unconventional quantitative way to provide an independent measure of the Mach number, which is found to agree with experimental and numerical results, confirming their validity. Finally, the measuring range issues associated with schlieren visualizations of non-ideal flows are investigated by means of ray tracing calculation and CFD results, showing the importance of total conditions and real gas effects in the implementation of this technique.