Experimental Observation of Non-Ideal Nozzle Flow of Siloxane Vapor MDM

The first experimental results from the Test-Rig for Organic Vapors (TROVA) at Politecnico di Milano are reported. The facility implements an Organic Rankine Cycle (ORC) where the expansion process takes place within a straight axis convergent-divergent nozzle, which is the simplest geometry representative of an ORC turbine blade passage. In order to reduce the required input thermal power, a batch operating mode was selected for the plant. Experimental runs with air allowed to verify the throttling valve operation and the measurement techniques, which include total pressure and temperature measurements in the settling chamber, static pressure measurements along the nozzle axis. A double-passage Schlieren technique is used to visualize the flow field in the nozzle throat and divergent section and to determine the position of shock waves within the flow field. The first experimental observation of non-ideal nozzle flows are presented for the expansion of siloxane fluid MDM (C8H24O2Si3, octamethyltrisiloxane) for vapor expansion in the close proximity of the liquid-vapor saturation curve, at relatively low pressure of operation. A supersonic flow is attained within the divergent section of the nozzle, as demonstrated by the observation of an oblique shock wave at the throat section, where a 0.1 mm recessed step is located. Schlieren visualizations are limited by the occurrence of condensation along the mirror side of the nozzle. Pressure measurements are compatible with the observed flow field.