Schlieren visualizations of non-ideal compressible fluid flows

Abstract

The peculiarities of schlieren visualizations of non-ideal compressible fluid flows are analyzed with the support of experimental evidence and numerical calculation. Schlieren visualizations are performed on a vapor flow of Siloxane MDM (Octamethyltrisiloxane, C8H24O2Si3) expanding to supersonic speeds, up to a Mach number of 2, within a converging-diverging nozzle. The nozzle constitutes the test section of the Test Rig for Organic Vapors (TROVA), an experimental facility built at the Laboratory of Compressible fluid dynamics for Renewable Energy Applications (CREA Lab) of Politecnico di Milano (Italy) to investigate non-ideal expansions of complex organic vapors close to the saturation curve and the critical point, representative of blade channel flows in Organic Rankine Cycle (ORC) turbines. The schlieren system used in this work employs a double-pass configuration featuring a mercury lamp, a bi-convex lens, a cubic beam splitter, a second focusing lens and a high-speed CMOS camera. Visualizations of MDM vapor flows showed measuringrange issues: portions of the image expected to appear bright due to negative density gradients (expansion), were dark instead. This was attributed to the strong density gradients occurring in these non-ideal flows, causing refractions intense enough to deflect light onto some system aperture stop. Ray tracing calculation indeed showed that rays crossing regions of the test area with stronger gradients were intercepted by the knife frame before reaching the camera sensor. Interestingly, these measuring range issues were found to decrease as the non-ideality of the flow decreased. Moreover, when the same nozzle geometry was used for analogous testing with air, these measuring-range issues were absent or noticeably reduced. This paper analyses both experimental and numerical evidence to investigate the two aforementioned observations and provide an explanation in terms of refractive index gradient, compressibility and pressure ratio gradient trends during an expansion.