A review of solid–fluid selection options for optical-based measurements in single-phase liquid, two-phase liquid–liquid and multiphase solid–liquid flows

Abstract

Experimental techniques based on optical measurement principles have experienced significant growth in recent decades. They are able to provide detailed information with high-spatiotemporal resolution on important scalar (e.g., temperature, concentration, and phase) and vector (e.g., velocity) fields in single-phase or multiphase flows, as well as interfacial characteristics in the latter, which has been instrumental to step-changes in our fundamental understanding of these flows, and the development and validation of advanced models with ever-improving predictive accuracy and reliability. Relevant techniques rely upon well-established optical methods such as direct photography, laser-induced fluorescence, laser Doppler velocimetry/phase Doppler anemometry, particle image/tracking velocimetry, and variants thereof. The accuracy of the resulting data depends on numerous factors including, importantly, the refractive indices of the solids and liquids used. The best results are obtained when the observational materials have closely matched refractive indices, including test-section walls, liquid phases, and any suspended particles. This paper reviews solid–liquid and solid–liquid–liquid refractive-index-matched systems employed in different fields, e.g., multiphase flows, turbomachinery, bio-fluid flows, with an emphasis on liquid–liquid systems. The refractive indices of various aqueous and organic phases found in the literature span the range 1.330–1.620 and 1.251–1.637, respectively, allowing the identification of appropriate combinations to match selected transparent or translucent plastics/polymers, glasses, or custom materials in single-phase liquid or multiphase liquid–liquid flow systems. In addition, the refractive indices of fluids can be further tuned with the use of additives, which also allows for the matching of important flow similarity parameters such as density and viscosity

Optical measurements in evolving dispersed pipe flows

“…Measurements in all three dimensions are also possible with volumetric illumination and more than one camera (Maas et al 1993;Kreizer and Liberzon 2011). A recent review by Wright et al (2017) demonstrates previous works conducted in matched refractive index systems.…”

Particle Seeded Grains to Identify Highly Irregular Solid Boundaries and Simplify PIV Measurements

“…We have also demonstrated that hydrogel may be fractured into grains for the study of packed bed flows. Other RIM liquid-solid pairs have the solid material with the potential for seeding as reported in Budwig (1994), Wiederseiner et al (2011), andWright et al (2017). A likely candidate for solid seeding, in addition to the presently tested hydrogel, would be silicone rubber, which is paired with aqueous solution of sodium chloride and glycerol (Shuib et al, 2011).…”

“…These limitations become even more restrictive when PIV is coupled with RIM because both fluid and solid objects may become indistinguishable. In the RIM method, the refractive index of two transparent materials are matched such that light is not refracted or reflected at the interface between the materials making the solid disappear in the fluid (Budwig, 1994;Wiederseiner et al, 2011;Wright et al, 2017).…”

Dynamics of liquid–liquid flows in horizontal pipes using simultaneous two–line planar laser–induced fluorescence and particle velocimetry

“…Ideally, a fully RI-matched system is desired to avoid such optical distortions ( Wright et al, 2017 ). However, this is a challenging task to accomplish for liquids and pipe materials with properties comparable to those encountered in actual practical applications.…”

“…The average uncertainty in the elevation difference at the horizontal configuration was estimated at ± 2.1 mm over the total pipe length. For the purpose of reducing optical distortions, the optical measurement location was located in the quartz pipe section, L / D = 209 from the inlet, since quartz matches the RI of the oil phase ( Wright et al, 2017 ).…”