Thesis title: Global and Local Characterization of Small-Scale Heat Transfer Devices
Microfluidic devices gained a significant amount of interest in recent years through specific studies on phenomenology and applications regarding heat transfer. The objectives of the present Ph.D. thesis are aimed at evaluating the global efficiency and local thermofluidic phenomenology of microfluidic devices. In the first part, the efficiency of heat transfer and the related fluid-mechanic performances inside a microfluidic device were investigated experimentally by varying the carrier fluid at different flow rates. The experimental analysis was conducted on a serpentine channel configuration by considering five different fluid mixtures, including water, glycerol, alcohol, and mixtures of these. The results displayed and emphasized the relevant role of fluid viscosity on thermal performances in the laminar and transaction regime. The corresponding Nusselt numbers were measured and related to Reynolds and Prandtl numbers. At small flow rate, i.e., small Reynolds number, the high viscosity mixtures exhibited a high positive correlation between Nusselt and Prandtl numbers, and to a lower extent also between Nusselt and Reynolds numbers. However, as Reynolds number increases, the thermal performance enhancement is limited by the action of viscous dissipation. On the other hand, mixtures with low viscosity showed a higher correlation between Nusselt and Reynolds numbers. In these conditions, the importance of local phenomena due to turbulence became one of the main factors in heat transfer enhancement. Overall, the results showed markedly different thermo-hydraulic behaviors, not only related to device efficiency but also to the local flow dynamics, particularly at curvilinear regions.
Thus, in the second part of the thesis, a different experimental set-up was designed in order to allow investigations of the carrier fluid local behaviors in terms of velocity and temperature fields. A multiplanar study was carried out on a horseshoe-shaped microfluidic cell, in order to bear a complete view of the thermo-fluidic system. The multiplanar and local thermal field experiments were carried out by a high-resolution infrared camera, mapping the thermal field on the microchannels external walls and inferring the internal thermal profiles by means of specific transfer functions in laminar and transitional regimes. Maps of thermal transients allow for deriving cooling performances which are used to identify the local thermal efficiency of the microchannel and relate it to the global efficiency. The multiplanar velocimetry measurements were conducted with a Micro-Particle Image Velocimetry (µPIV) setup at different flow rates. This analysis is set in relation to thermal results in order to obtain a comprehensive description of the effects of local phenomena on the global heat transfer performances of a microfluidic device.