Three-dimensional conjugate heat transfer under laminar flow conditions within a microchannel is analysed numerically to explore the impact of a new design of vortex generator positioned at intervals along the base of the channel. The vortex generators are cylindrical with quarter-circle and half-circle cross sections, with variants spanning the whole width of the channel or parts of the channel. Micro-channels with Reynolds number ranging from 100 to 2300 are subjected to a uniform heat flux relevant to microelectronics cooling. To ensure the accuracy of the results, validations against previous microchannel studies were conducted and found to be in good agreement, before the new vortex generators with radii up to 400 µm were analysed. Using a thermal-hydraulic performance parameter expressed in a new way, the VGs described here are shown to offer significant potential in combatting the challenges of heat transfer in the technological drive toward lower weight/smaller volume electrical and electronic devices.

Benefits of spanwise gaps in cylindrical vortex generators for conjugate heat transfer enhancement in micro-channels.

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The convective heat transfer, pressure drop and required pumping power for the turbulent flow of Al2O3water, TiO2-water and CuO-water nanofluids in a heated, horizontal tube with a constant heat flux are investigated experimentally. Results show that presenting nanofluid performance by the popular approach of plotting Nusselt number versus Reynolds number is misleading and can create the impression that nanofluids enhance heat transfer efficiency. This approach is shown to be problematic since both Nusselt number and Reynolds number are functions of nanofluid concentration. When results are presented in terms of actual heat transfer coefficient or tube temperature versus flow rate or pressure drop, adding nanoparticles to the water is shown to degrade heat transfer for all the nanofluids and under all conditions considered. Replacing water with nanofluid at the same flow rate reduces the convective heat transfer rate by reducing the operating Reynolds number of the system. Achieving a target temperature under a given heat load is shown to require significantly higher flow rates and pumping power when using nanofluids compared to water, and hence none of the nanofluids are found to offer any practical benefits. Declarations of interest: noneNomenclature A Inside surface area of the test section tube (m 2 ) Cp Specific heat (J/kg.K) Pr Prandtl number V Average fluid velocity (m/s) Greek letters  Volume fraction of nanofluids Ratio between the nanolayer thickness surrounding the nanoparticle and the nanoparticle radius Kinematic viscosity (m 2 /s) Density (kg/m 3 ) µ Dynamic viscosity (Pa s)