Heat Transfer of Ferrofluid in Fluted Tubes with an Electromagnetic Field.

The present study considers the effects of the electromagnetic field and pulsating flow on nanofluids heat transfer and flow characteristics in the three-start helically fluted tube. The experiments are performed using anofluids with the Reynolds number varying from 8000 to 13000, and with an Fe3O4/water nanofluids (Ferrofluid) concentration of 0.005% by volume with and without an electromagnetic field effect. The measured results are verified with the predicted results from the proposed correlations and the published experimental results. As a result of the disturbed flow boundary layer, higher swirling Brownian motion, higher local thermal conductivity, and the mixing of turbulent intensity, the heat transfer performance also increases. For a given pulsating flow of 30 Hz, the Nusselt number enhancement increases significantly to 137.7% and 156.7% for the helical depth ratios of 0.028 and 0.093, respectively. The helically fluted tube with a helical depth ratio of 0.093 and a helical pitch ratio of 0.93 yields the highest Nusselt number improvement. The Nusselt number under the electromagnetic field effect increases by 19.0% and 14.4% for the pulsating flow of 30 Hz and 20 Hz, respectively. Finally, a comprehensive evaluation index is applied to estimate thermo-hydraulic performance. It is found that the comprehensive evaluation index increases with the increasing Reynolds number at first and then decreases as the Reynolds number increases further. [ABSTRACT FROM AUTHOR]