Design of a Closed-Loop Optical-Access Supersonic Test Facility for Organic Vapours

Despite significant research activities into organic Rankine cycles for the conversion of low-temperature heat into power, there remain uncertainties with regards to non-ideal gas effects and their role in turbine performance. Moreover, existing performance models and numerical solvers have yet to be validated for turbines operating with organic fluids. This paper documents the design of a closed-loop supersonic test facility intended for experimental characterisation of the flow of organic fluids under typical operating conditions experienced within an ORC turbine. The test section forms part of a wider test facility, developed for the study of ORC expanders, which includes a screw compressor, the supersonic test section, a heat exchanger and an expander test section. The working fluid is R1233zd, and the test facility is sized to deliver test conditions up to 20 bar and 125 °C with a mass-flow rate of 1 kg/s. After an overview of the test facility, the detailed design of the upstream diffuser, settling chamber, contraction zone and converging-diverging nozzle to deliver a flow with a Mach number of 2 to the test section is discussed. The performance of the test section is confirmed by CFD simulations. Finally, the intended flow visualisation using particle-image velocimetry is discussed. This includes the identification of a suitable seeding method considering both liquid and solid tracer particles. The assessment is completed considering constraints such as the operating conditions, the required particle size to accurately trace the fluid flow, maintenance issues, and compatibility with the working fluid. In particular, the possibility of using the compressor lubricating oil as the seeding particle is evaluated.