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.
Description:
Papers presented at the 13th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Portoroz, Slovenia on 17-19 July 2017 .
