Convective transport of nanoparticle-suspensions in micro-scale cooling devices

Abstract

The miniaturization of modern device induces appreciably hike in the operating temperature, motivating the research on micro-scale heat transfer to improve the thermal management in confined space. Convective heat transfer in microchannel becomes a topical subject in view of its efficiency in the thermal regulation of micro-scale devices. However, the inherently poor thermal conductivity of conventional fluid poses a primary limitation on the thermal performance and compactness of the cooling devices. Nanofluid, a novel type of engineered colloid consisting of suspended nanoparticles, exhibits a promising potential to be the high-performance heat transfer medium due to its notable enhanced thermal conductivity. The early research focused on the thermophysical properties of nanofluid whereby the study on micro-scale convective transport of nanofluid is comparatively scarce. Therefore, the understanding on the micro-scale convection of nanofluid is still in the infancy stage, urging the need of extensive research on the subject to explore the potential of nanofluid in micro-scale thermal system. This thesis presents a comprehensive analysis on the convective transport of nanoparticle-suspension in micro-scale cooling devices, aiming to investigate various pertinent effects of nanofluid convection from the point of view of the first law and the second law of thermodynamics. Starting from the basic physical laws, analytical models are developed to investigate the effects of streamwise conduction and viscous dissipation on forced convection of nanofluids in microchannels. Subsequently, the analysis is extended to thermal non-equilibrium porous microchannels to scrutinize the performance of nanofluid flow under the effects of internal heat generations and thermal asymmetries. Analysis of the first law of thermodynamics reveals that the streamwise conduction, which is significant in low-Peclet-number flow, is greatly amplified in nanofluid. On the other hand, the viscous dissipation effect intensifies with the increase of nanoparticle volume fraction and Reynolds number, leading to heat transfer performance deterioration. Nanofluid outperforms its base fluid only when the nanoparticle diameter and the Reynolds number are lower than the threshold values. Besides, the heat transfer enhancement of nanofluid is the largest when the microchannel is heated symmetrically, and occurs over a larger range of Reynolds number when the porous-medium solid-phase heat generation is significant. Analysis of the second law of thermodynamics shows that the streamwise conduction in nanofluid induces a distinctive entropy generation characteristic in low-Peclet-number flow. With an increase of Reynolds number, the streamwise conduction effect diminishes while the viscous dissipation effect increases, intensifying the bulk temperature of nanofluid along the microchannel. Consequently, streamwise entropy generation of viscous dissipative nanofluid convection becomes indispensable. Similar to the first-law analysis, nanofluid enhances the exergetic effectiveness of microchannel when the nanoparticle diameter and the Reynolds number are lower than the threshold values. For nanofluid flow in porous media, the enhancement occurs only when the internal heat generations are relatively low. The entropy generation of nanofluid can be minimized with respect to the Reynolds number and the wall heat flux ratio, providing the ideal operating condition which optimizes the second-law performance of nanofluid flow in micro-scale device.