On the thermal and hydrodynamic characteristics of liquid-liquid Taylor flows

Two phase liquid-liquid flows offer significant heat and mass transfer enhancements over single phase flows and, as a result, have found use in numerous emerging technologies employing microfluidics. Such technologies include lab-on-chip devices for chemical and biological diagnostics, and biosensors. Liquid-liquid flows have also shown potential for use in high-heat flux removal systems. Although these flows are found in numerous applications, little is known about the complex fluid mechanics that govern them. Consequently, there is a need for a greater knowledge base to serve as a foundation for future system design and characterisation. This thesis presents a fundamental investigation of the hydrodynamic and thermal characteristics of liquid-liquid slug or Taylor flows confined to minichannel geometries. There were three principal aspects to this thesis, which encompassed the measurement of film thickness, pressure drop and heat transfer in liquid-liquid Taylor flows. Experiments were carried out using a number of different carrier fluids – while maintaining water as the dispersed phase throughout. Dimensionless slug length, Capillary and Reynolds numbers were varied over several orders of magnitude. High speed imaging was used in conjunction with microscopy to measure the mean slug velocity and liquid film thickness. Images of the dispersed slugs revealed that the thickness of the liquid film was not constant along the length of the slug. However, above a threshold dispersed slug length, a region of constant film thickness existed. The thickness of the film was found to be heavily dependent on the Capillary number. Analysis of the experimental data revealed that it fell into two distinct flow regimes: a visco-capillary regime and a visco-inertial regime. A modified Taylor’s Law is proposed for flows in the visco-capillary regime, while a novel correlation – based on the Capillary and Weber numbers – is put forward for flows in the visco-inertial regime. The pressure drop induced by the liquid-liquid flow regimes was measured using a differential pressure transducer, and the results were compared to the most referenced correlations in the literature. Comparisons highlighted a lack of robustness in the liquid-liquid pressure drop correlations. Interpretation of the data using liquid-gas Taylor flow correlations unearthed a threshold viscosity ratio, above which liquid-gas correlations may be used to model the flow. Below this threshold, a modification to an existing correlation is proposed, where the interfacial pressure drop is normalised by the volumetric channel fraction occupied by the carrier phase. A heat transfer facility was designed and commissioned to subject the flow to a constant wall heat flux boundary condition. Local temperature measurements were acquired using a high resolution infrared thermography system. Slug length and film thickness were found to have a significant effect on the local heat transfer rates, with enhancements up to 600% over conventional Poiseuille flow noted. Nusselt number oscillations were observed in the lower Capillary number flows. However, these oscillations damped out as the Capillary number, and hence film thickness, increased. Based on the characteristics identified, a novel correlation is proposed to model the flow in the thermal entrance and fully developed regions. The findings of this thesis are of fundamental and practical relevance for the design of systems and devices incorporating liquid-liquid Taylor flow regimes.