The organic Rankine cycle (ORC) offers great potential for waste heat recovery and use of low-temperature sources for power generation. However, the ORC thermal efficiency is limited by the relatively low-temperature level, and it is, therefore, of major importance to design ORC components with high efficiencies and minimized losses. The use of organic fluids creates new challenges for turbine design, due to dense gas behavior and the low speed of sound. The design and performance predictions for steam and gas turbines have been initially based on measurements and numerical simulations of flow through two-dimensional cascades of blades. In case of ORC turbines and related fluids, such an approach requires the use of a specially designed closed cascade wind tunnel. In this contribution the design and process engineering of a continuous running wind tunnel for organic vapors is presented. The wind tunnel can be operated with heavy weight organic working fluids within a broad range of pressure and temperature levels. For this reason, the use of classical design rules for atmospheric wind tunnels is limited. The thermodynamic cycle process in the closed wind tunnel is modeled, and simulated by means of a professional power plant analysis tool, including a database for the ORC fluid properties under consideration. The wind tunnel is designed as a pressure vessel system and this leads to significant challenges particular for the employed wide angle diffuser, settling chamber, and nozzle. Detailed computational fluid dynamics (CFD) was performed in order to optimize the important wind tunnel sections.

A circular cylinder was tested in the cross-flow of an organic vapor (Novec™ 649) and of air over the subsonic (M < 0.4) and high subsonic (0.4 < M < 0.8) speed range in a continuously running pressurized closed-loop wind tunnel test facility. Time-averaged pressure measurements gave information on surface pressure distributions, and the corresponding drag and base pressure drag coefficients were obtained. Due to the charging of the wind tunnel, different values of the compressibility factor (0.876 < Z < 0.999) could be achieved for the organic vapor flow. This enabled in combination with the results for air an assessment of the impact of non-ideal gas dynamics on the form drag of a cylinder in the considered highly subsonic flow regime. The new experimental data were compared with available literature results. Changes in surface pressure distribution at higher subsonic velocities were identified and discussed. It was found that non-ideal gas effects did not strongly affect the overall drag. The variation of drag coefficient over the Mach number range was comparable with literature data for ideal-gas compressible flow, including shock-less and intermittent shock wave, and permanent shock wave flows regimes. At Mach 0.4, the flow of Novec™ 649 was in the shock-less regime and exhibited a pronounced dependency on the Reynolds number. An increase in drag was observed at Mach 0.6 which was attributed to the commencement of vortex shedding. Non-ideal thermodynamics only affected the flow locally and a reduction of the critical pressure coefficient in the high subsonic flow regime was observed in the surface pressure distribution. However, this mechanism did not alter significantly the overall drag behavior.Graphic abstractDrag coefficientC_Dagainst Re for several Mach numbersMand comparison with available literature results obtained for air (colored symbols indicate different Mach number clusters)

This paper presents a focusing schlieren system designed for the investigation of transonic turbine tip-leakage flows. In the first part, the functional principle and the design of the system are presented. Major design considerations and necessary trade-offs are discussed. The key optical properties, e.g., depth of focus, are verified by means of a simple bench test. In the second part, results of an idealized tip-clearance model as well as linear cascade tests at engine representative Reynolds and Mach numbers are presented and discussed. The focusing schlieren system, designed for minimum depth of focus, has been found to be well suited for the investigation of three-dimensional transonic flow fields in turbomachinery applications. The schlieren images show the origin and growth of the tip-leakage vortex on the blade suction side. A complex shock system was observed in the tip region, and the tip-leakage vortex was found to interact with the suction side part of the trailing edge shock system. The results indicate that transonic vortex shedding is suppressed in the tip region at an exit Mach number around ${\mathrm{M}}_{2,is}$ = 0.8.

The rising number of applications of the organic Rankine cycle (ORC) or supercritical CO₂ (sCO₂) power systems have shaped a new branch of fluid mechanics called non-ideal compressible fluid dynamics (NICFD). This field of fluid mechanics is concerned with flows of vapors or gases, which are characterized by substantial deviations from the perfect gas model. In extreme cases, even non-classical gas dynamic phenomena could occur. Although these non-ideal compressible flows are the subject of sophisticated numerical simulation studies today, there is also a growing need for experimental data for validating purposes. In the last couple of years, new experimental test rigs designed for investigating non-ideal compressible fluid dynamics have been developed and commissioned. Classical practical measurement techniques are currently being re-developed and applied to non-ideal compressible flows. Despite its substantial relevance, information about these measurement techniques and their differences from conventional methods in the open literature is scarce. The present review article is an attempt to reduce that gap. After briefly discussing the thermodynamics and fluid dynamics of non-ideal compressible flows, the currently available test rigs and their utilized measurement techniques are reviewed. This review discusses schlieren optical investigations, pneumatic and laser-optical methods, and hot-wire anemometry for non-ideal compressible flows.