Heat Transfer Performance of Flag Vortex Generators in Rectangular Channels

Abstract

This experimental work investigated the flow and heat transfer characteristics of rectangular channels equipped with a flapping flag as a vortex generator. Specifically, it aimed to determine the influences of channel geometry, flag material properties and flow conditions on the flapping dynamics of the flag, turbulence characteristics inside the channel and the resulting heat transfer behaviour of the system. The inclusion of the flag inside the channel alters the overall nature of the flow and consequently the heat transfer. Vortices shed from the flag interact with each other and the channel walls, thus increasing the turbulence levels (as high as 20%) in the near wake of the flag. The enhanced turbulence levels lead to Nusselt number enhancement by as high as 1.34 to 1.62 times bare channel levels. As the turbulence decays in the streamwise direction, so does the thermal enhancement. The turbulence and heat transfer enhancement are accompanied by an increase in pressure drop due to the periodic blockage imposed by the flag during flapping. The friction factor, which is governed by the flag oscillation mode and frequency, was found to be as high as 1.39 to 3.56 times bare channel levels. The thermal enhancement benefit of the flag vortex generator varies with streamwise distance: a thermal enhancement factor as high as 1.14 was observed near the flag's free end. The blockage may be minimised through the use of a thicker flag which assumes a simpler oscillation mode that reduces the contact between the flag and the channel walls. The size of the channel (aspect ratios of 0.2 to 1.0) relative to the flag has profound effects on flag dynamics, turbulence levels and heat transfer. Wider channels allow wider flapping amplitudes and more complex oscillation modes, which lead to the creation of stronger vortices that decay slower inside the channel. Due to stronger vortices and delayed turbulence decay, thermal enhancement in wider channels are higher and sustained for longer distances. The present study revealed the salient mechanisms which affect the performance of flags as vortex generators for turbulence and heat transfer enhancement. The results of this study contribute to the better understanding of this complex thermal-fluid-structure problem and may serve as guides for the use flag vortex generators in various applications.