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Heat Transfer in the Wake Behind a Longitudinal Vortex Generator Immersed in Drag-Reducing Channel Flows
Jens F. Eshcenbacher, Mizuho Joko, Kazuyoshi Nakabe and Kenjiro Suzuki

In order to use additives for the reductions of flow friction and pumping power, and thus to increase the energy efficiency of far-district heating/cooling transport systems, the experimental study of wall heat transfer in the wake behind a single delta-shaped winglet vortex generator (WVG) was made in drag-reducing flows. The experiments have been conducted with different mass concentrations of the water solution of a non-Newtonian surfactant, cetyl-trimethyl-ammonium-chloride (CTAC), with the same mass concentration of sodium salicylate (NaSal). The CTAC concentration of the solution was varied in three steps, 0ppm, 30ppm and 200ppm. The WVG was mounted on the heat transfer target surface of the bottom wall in a duct flow. The Reynolds number, based on the cross-sectional area-averaged flow velocity and the duct height, was set in the range from 7.3 x 103 up to 4.4 x 104 in consideration of the temperature dependence of CTAC. It was found that small amounts of CTAC/NaSal added in water reduce the heat transfer coefficients drastically, and that this reduction could be locally recovered in the wake behind the WVG. Longitudinal vortices generated behind the WVG were found to play an important role for heat transfer enhancement of the drag-reducing flows.

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