In this study, in order to select an appropriate method to control the heat transfer and flow of U-shaped stationary/rotating microchannel, thermal and flow characteristics of microchannel have been investigated and compared under the influence of rotating states, single and combined magnetic fields and various parameters such as Reynolds number (50≤Re≤1000), Hartmann number (0≤Ha≤80), Nanoparticles volume fraction (0.01≤∅≤0.04), rotational speed (0≤ω≤30orad/s), slip and no-slip boundary conditions. In addition, in order to investigate the arrangement effect of applying a single magnetic field on the thermal performance and pressure drop of the microchannel, four similar permanent magnets, in 16 different arrangements, have been used to apply Lorentz force to the flowing nanofluid in the microchannel. The Ansys-Fluent software platform was employed to solve the governing equations. The working fluid used in the present study is water-alumina nanofluid with different volume fractions. The single-phase and two-phase methods have been used to simulate the flow and heat transfer of nanofluid. To validate the proposed methods, the corresponding results of the homogeneous single-phase model and the discrete phase model under the influence of changing various parameters are reviewed and compared. The maximum difference between the values of heat transfer coefficient obtained from the homogeneous single-phase model and the discrete phase model is 5.9%. Therefore, considering the ability of the homogeneous single-phase model to predict the nanofluid heat transfer characteristics in an acceptable range, computational cost, and the number and range of parameters of the problem under study lead the homogeneous single-phase model to be selected for further research. Also, by using the thermal performance coefficient, the effect of increasing Hartmann and Reynolds numbers on changes in total Nusselt number and total pressure drop were investigated and compared simultaneously. It is also observed that the relation between thermal performance coefficient individually is not a suitable criterion for eva‎luating the performance of stationary microchannels under the influence of single and combined magnetic fields. Therefore, the ratio of thermal power absorbed from the microchannel surfaces to the pump power required for pumping fluid and changes in total Nusselt number compared to the absence of field has been used to compare and eva‎luate the thermal performance of stationary microchannels under single and composite magnetic fields. The results of the present study show that controlling the fluid flow caused by applying magnetic field and rotation around the coordinate axis can play a significant role in thermal performance of microchannel. For example, in the range of studied parameters, the total Nusselt number of stationary microchannels under the influence of Spiral flow resulting from the combination of single magnetic fields Bx and By (BxBy) compared to the stationary state and the absence of the field, increases by 137.7%. However, with the counter-clockwise and clockwise rotation of the microchannel under the influence of the BxBy combined magnetic field around the Z coordinate axis, the total Nusselt number can increase by a maximum of 161.4% or decrease by 62.4% compared to the stationary state and the absence of the field. In addition, by arranging the magnetic field and affecting a part of the microchannel while increasing the thermal performance of the microchannel, the increase in total pressure drop due to the application of the magnetic field can be significantly reduced.

احمد سوهانكار اصفهاني

محمد رضا سليم پور

محمد رضا توكلي، محسن دوازده امامي