Heat transfer enhancement using rectangular vortex promoters in confined quasi-two-dimensional magnetohydrodynamic flows

Heat transfer efficiency from a duct side-wall in which an electrically conducting fluid flows under the influence of a transverse magnetic field is investigated. A quasi-two-dimensional magnetohydrodynamic model is employed to model the flow using high-resolution numerical simulation. The gap height and angle of attack of a rectangular cylinder, with aspect ratio alpha = 1/2 and blockage ratio beta = 1/4, are independently varied to establish relationships between obstacle configuration and heat transfer efficiency. The heat transfer efficiency is measured through an efficiency index given by the ratio of heat transfer enhancement to pressure drop penalty in comparison to an empty duct case. At gap height ratios 1.15 <= G/L-c < 2 for an upright cylinder above a heated lower wall, thermal enhancement and efficiency can be improved; with a peak thermal efficiency of eta = 1.6 occurring at G/L-c = 1.5. Additional increases in thermal efficiency for an obstacle at the duct centre-line (G/L-c = 2) can be achieved through inclining the cylinder at gamma = -7.5 degrees, gamma = -37.5 degrees and 0 degrees < gamma <= 22.5 degrees. However, these configurations offered no improvement over simply offsetting an upright cylinder at a gap height ratio of G/Lc = 1.5. For a cylinder offset at G/L-c = 1.5, varying the incidence angle through -37.5 degrees < gamma <= 22.5 degrees, -7.5 degrees <= gamma < 0 degrees and 0 degrees < gamma <= 15 degrees can lead to additional thermal efficiency benefits; with a global peak efficiency of eta = 1.7 occurring at gamma = -37.5 degrees. The streamwise distribution of the local time-averaged Nusselt number and the effect of Hartmann dampening for 100 <= Ha <= 2000 on heat transfer and flow dynamics are also investigated. A net power balance analysis reveals that in fusion applications the heat transfer enhancement dominates over the pumping power cost to produce net benefits for even modest heat transfer enhancement. (C) 2015 Elsevier Ltd. All rights reserved.