Heat transfer of nanofluid through helical minichannels with secondary branches

Heat transfer, fluid flow characteristics and entropy generation of water-Al2O3 nanofluid for cylindrical heat sinks with helical minichannels that have secondary branches are investigated experimentally. The minichannels helix angles are 45, 60 and 90 degrees; the volume fraction of nanoparticles variation is 0.05% – 0.1%; the Reynolds number range is between 113 to 478. The effects of the helix angle of minichannels, Reynolds number and nanoparticle volume fraction are studied. The heat transfer enhances with increasing Reynolds number, decreasing helix angle and increasing volume fraction of nanoparticles. However, with increasing Reynolds number, increasing helix angle and decreasing nanoparticles volume fraction, the friction factor decreases. The experimental results show that the secondary branches decreased the friction factor and Nusselt number. The maximum enhancement of Nusselt number in helix angle 60 and 45 for pure water are 31.1% and 51.3% greater in comparing with the straight minichannel, respectively; moreover, maximum enhancement of the Nusselt number of nanofluid compared with the pure water was about 14.3% for 0.1% vol. The thermal entropy generation rate decreases and, the frictional entropy generation rate increases with increasing the nanoparticles volume fraction and decreasing the helix angle. Furthermore, two new correlations are obtained for the Nusselt number and friction factor.