Baffled tubes with superimposed oscillatory flow: Experimental study of the fluid mixing and heat transfer at low net Reynolds numbers

Abstract

Experimental results of flow pattern and heat transfer in circular-orifice baffled tubes under pure oscillatory flow and compound flow conditions are presented. The hydrogen bubble visualization technique is employed for describing the unsteady flow structure, and particle image velocimetry is used in order to measure the velocity field during eight different phases of the oscillation cycle. The existence of a central jet and the cyclic dispersion of vortices upstream and downstream of the baffles is analysed. The loss of the flow axisymmetry for Re_osc>130 is clearly identified. Heat transfer measurements under uniform heat flux (UHF) conditions are obtained in a thermal-hydraulic rig, allowing for the description of the influence of net and oscillatory Reynolds numbers on the Nusselt number, using propylene-glycol as working fluid (Pr=150). The impact of chaotic mixing, for Re_osc>150, results in a uniform local heat transfer distribution along the reactor cell, as well as in thermal uniformity in the transverse plane of the tube.