Experimental characterization of nozzle flow expansions of siloxane MM for ORC turbines applications

This paper reports extensive experimental results characterizing the supersonic expansion of organic vapor MM (hexamethyldisiloxane, C6H18OSi2) in conditions representative of actual Organic Rankine Cycle turbine operation, in the close proximity of the liquid-vapor saturation curve. Experiments were conducted on the Test Rig for Organic VApors, at the Laboratory of Compressible fluid dynamics for Renewable Energy Applications of Politecnico di Milano. A planar nozzle featuring a design exit Mach number of 1.6 was characterized by measuring total pressure, total temperature, static pressures along the nozzle axis and by schlieren visualizations. The non-ideal influence of inlet total conditions on nozzle expansions was verified by gathering data exhibiting total compressibility factor ZT ranging from 0.39 (strongly non-ideal) to 0.98 (almost ideal conditions). Pressure ratio measured at the lowest ZT exceeded by 10%–20% the one at highest ZT along the whole nozzle. An extensive experimental dataset was analyzed to assess the influence of both total compressibility factor ZT and total fundamental derivative of gasdynamics ΓT on the non-dimensional pressure distribution along the nozzle axis. It was investigated whether one parameter only can univocally identify a non-dimensional nozzle expansion. Significant validation data for simulation codes and design tools are provided for MM and for siloxanes in general.