| The compactness and high surface-to-volume ratios of microscale liquid flow devices make them attractive alternatives to conventional flow systems for heat transfer augmentation, chemical reactor or combustor miniaturization, aerospace technology implementations, as well as biomedical applications. While experimental evidence indicates that fluid flow in microchannels, especially in terms of wall friction and heat transfer performance, differs from macrochannel flow behavior, laboratory observations are often inconsistent and contradictory. Some researchers attributed the deviations to unknown microscale effects, which often turned out to originate from inappropriate approaches to analyze the new phenomena. Specifically, system parameters were neglected, which are not important on the macroscale but play important roles in microscale analyses.; The main objectives of the study are to identify important parameters for microscale liquid flows and nanoparticle suspensions, to find a physically sound way to analyze the new phenomena, and to provide mathematical models to simulate them.; Employing the porous medium layer (PML) idea, surface roughness effects on momentum- and heat-transfer in micro-conduits were numerically investigated and verified with experimental data. The friction factor and Nusselt number either increase or decrease depending on the PML model parameters, expressed in terms of the relative surface roughness, Darcy number, Reynolds number, and effective thermal conductivity. Variations in the viscous dissipation effect were found to increasingly affect the friction factor and Nusselt number with decreasing system size.; When nanoparticles are added to liquid flow systems, scalar transport properties can be significantly enhanced. Specifically, nanofluids, i.e., dilute suspensions of nanoparticles in liquids, are used to enhance heat transfer performance. Focusing on micro-scale heat transfer, it was found that the particle Brownian motion and the induced surrounding liquid motion are key mechanisms for the experimentally observed high increase of the effective thermal conductivity of nanofluids. A new, experimentally validated effective thermal conductivity model has been developed based on kinetic theory. The model predicts both the effective thermal conductivity and dynamic viscosity of nanofluids in terms of nanoparticle concentration, size, density and their interaction potential as well as the density, thermal capacity and dielectric constant of the base liquid. |
