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Session: Parallel Session: Working fluids
Session starts: Friday 23 September, 14:00
Presentation starts: 15:20
Room: Senaatszaal

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Frithjof Dubberke ()
Jadran Vrabec ()

Abstract:
Heat recovery systems, such as organic Rankine cycles (ORC), play an important role for the efficient use of fossil as well as of renewable fuels. Performance improvements of ORC processes are based on the technical development of the hardware and on the selection of appropriate working fluids. The selection of suitable working fluids is made on the basis of their thermo-physical properties and safety requirements, considering the characteristics of the heat source. Thus, efficiency increases for technically well advanced ORC plants require reliable and accurate thermo-physical property data on working fluids. One group of ORC working fluids are the siloxanes, which belong to the wider class of organosilicone compounds. In particular, hexamethyldisiloxane (HMDS) appears to be an eligible candidate for becoming a widely used working fluid for high-temperature processes. However, the current lack of accurate thermo-physical data for siloxanes may lead to sub-optimally designed cycles and processes. Therefore, the measurement of such data is necessary. Thermal properties in the homogeneous fluid region, together with the speed of sound and the vapor pressure are used to generate accurate equations of state (EOS) that describe the entire range of fluid states. “Properly designed multiparameter equations of state are able to represent thermodynamic properties of a certain substance within the accuracy of the most accurate experimental data” [2]. This work aims at closing the gap with respect to the speed of sound of HMDS. The measurement mechanism of the apparatus is based on the puls-echo technique and operates up to 150 MPa in the temperature range between 250 and 600 K [1]. While emitting a high frequency modulated burst signal by a piezoelectric quartz crystal, which is positioned between two reflectors in the fluid, the speed of sound is determined by the traveled distance divided by time the signal propagates through the fluid. For designing and constructing an ORC plant, knowledge on the speed of sound is particularly relevant for the expansion in the turbo-machine. E.g., single stage radial flow turbines rely on supersonic expansion in an upstream Laval nozzle to transform the enthalpy of the superheated working fluid vapor into kinetic energy before interacting with the turbine-wheel. The mass flow rate in Laval nozzles is directly related to the speed of sound. Even small inaccuracies in the underlying data can decrease the performance of the turbo-machine significantly.