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

Cylindrical vortex generators placed transversely over the span of a micro-channel can enhance heat transfer performance, but adding full-span vortex generators incurs a substantial pressure drop penalty. This paper examines the benefits of introducing various gaps along the length of the vortex generators, both for reducing pressure drop and improving the thermal conductance of the system. Three particular configurations are considered with varied dimensions: symmetrical gaps at each end of the vortex generator, i.e. adjacent to the channel side walls; a single central gap; and a combination of a central and end gaps. The performance is investigated numerically via 3D finite element analysis for Reynolds number in the range 300–2300 and under conditions of a uniform heat flux input relevant to microelectronics cooling. Results demonstrate that having end gaps alone substantially improves heat transfer while reducing the pressure drop. As well as generating longitudinal vortices which draw heat from the adjacent channel side walls, hot fluid passing through the gaps is swept directly upwards and inwards into the bulk flow, where it remains as it flows to the outlet. A thermal-hydraulic performance evaluation index is improved from 0.7 for full-span vortex generators to 1.0 with end gaps present. The central and central-plus-end gap geometries are less effective overall, but do offer localised improvements in heat transfer

A practical evaluation of the performance of Al2O3-water, TiO2-water and CuO-water nanofluids for convective cooling

The convective heat transfer, pressure drop and required pumping power for the turbulent flow of Al2O3-water, 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