Experimental Investigation of a Non-Ideal Expansion Flow of Siloxane Vapor MDM

Abstract

Nozzle flows of siloxane fluid MDM (C8H24O2Si3, octamethyltrisiloxane) expanding to supersonic speeds in non-ideal conditions are observed experimentally for the first time in the Test-Rig for Organic Vapors (TROVA) at Politencnico di Milano. The TROVA is a blow-down facility for investigating non-ideal compressible flows of organic vapors typical of Organic Rankine Cycle (ORC) applications. Total conditions in the settling chamber are subcritical and superheated, with total presssure 3.15 bar and total temperature 246 °C. In these conditions, the thermodynamic model predicts a value of the compressibility factor and of the fundamental derivative of gasdynamics equal to Z = 0.884 < 1 and Γ = 0.885 < 1, respectively. The total pressure and temperature are monitored during the test runs, together with the static pressure at selected stations along the nozzle axis. A double-passage Schlieren is used to visualize the density gradients within the nozzle. The nozzle is a convergent-divergent planar nozzle with a throat area of 16.8 mm (height) × 18.7 mm (width). Differently from dilute gas conditions, for a constant total-to-ambient expansion ratio, the static-to-total pressure ratio measured at the geometrical throat is observed to slightly depend on the reservoir conditions.