Design and Commissioning of a Laser Doppler Velocimetry Seeding System for Non-ideal Fluid Flows

The design, the construction and the commissioning of a seeding system for Laser Doppler Velocimetry operating in non-ideal conditions, namely in the close proximity of the liquid-vapor saturation curve and critical point, is presented. The system is implemented in the Test Rig for Organic VApors (TROVA), a facility built at Politecnico di Milano with the aim of characterizing non-ideal gas flows representative of those occurring in Organic Rankine Cycle turbine passages. The tested fluid is the siloxane MDM (Octamethyltrisiloxane – C8H24O2Si3), a silicon oil of particular interest for high temperature ORC applications. Depending on the test operating conditions, the fluid under scrutiny expands in a convergent-divergent nozzle from total pressure and temperature ranging from 4 bar to 25 bar and from 253°C to 310°C respectively, therefore the seeding has to be injected in a high temperature and high pressure environment, without altering the thermo-fluid dynamic behavior of the fluid.
A suspension of the tracer particles (titanium dioxide, TiO2 or silicon dioxide, SiO2) in the working fluid is atomized into the flow, in a plenum ahead of the nozzle inlet. Since the surrounding fluid is in superheated vapor (or supercritical) conditions, the spray then evaporates leaving the solid particles free to follow the flow. The designed system consists of a tank, pressurized with nitrogen and containing the MDM/seeding suspension, of a jet mixing system, to maintain the suspension stirred, and of a drawing line ending with the atomizing nozzle. During normal operation, the tank is pressurized at a pressure higher than the plenum one and the fluid flows naturally through the atomizer.
The system has been commissioned and validated through the verification of its operation. The system is suitable for all cases where optical measurements (LDV, PIV, etc.) have to be applied in high temperature, high pressure conditions similar to those occurring in the TROVA and whenever the use of auxiliary fluids different from the working one is not feasible. The reported test proves the suitability of the system in properly seeding the flow.