Heat transfer enhancement in micro-scale geometries

• Main Author: Abed, Waleed Mohammed
• Published: University of Liverpool 2016
• Subjects: 621.402
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.721949

Micro-geometries or 'microfluidics' are commonly utilised in a widespread variety of applications such as, bioengineering devices, microelectronic devices, electronics cooling, chemical micro-reactors and mini or micro-heat exchangers. In the microscale systems (with 'small' dimensions typically less than 1 millimeter), however, fluid mixing has been understood as one of the most fundamental and difficult-to-achieve issues because the flow of Newtonian fluids becomes increasingly controlled by viscous forces rather than inertia (as molecular diffusion is dominant at these small scales). As a consequence, the enhancement of convective heat transfer is problematic under these conditions (steady and laminar flow regime). In this thesis, two different regimes of instabilities, namely 'purely-inertial' and 'purely-elastic', have been adopted to enhance the convective heat transfer in the micro-scale geometries. Purely-inertial instability refers here to the secondary flow that arise in curved channels, also known as Dean flows, due to the centrifugal forces and also in crossed channels (cross-slot), symmetry-breaking bifurcations, which results in an axially-oriented spiral vortex along the outlet channels. While, purely-elastic instability is created in the flow of non-Newtonian viscoelastic fluids through curved channels due to the non-linear interaction between elastic stresses generated within the flowing viscoelastic solutions and the streamline curvature or through cross-slot device as a consequence of the planar extensional flow field (strong elongational flow) at the stagnation point. Fluid flow and convective heat transfer characteristics have been investigated experimentally and supporting numerical calculations for Newtonian flow within two different micro-geometries: a square cross-section serpentine microchannel and a square cross-section crossslot micro-device. A group of Newtonian fluids, aqueous glycerine solutions and aqueous sucrose solutions, was utilised to carry out the experiments for purely-inertial flows whilst high-viscosity polymeric viscoelastic fluids, shear-thinning and approximately constant-viscosity Boger solutions, were used for the experiments to investigate purely-elastic instabilities.