HEAT TRANSFER ENHANCEMENT IN RECTANGULAR CHANNELS USING AUTONOMOUS, AERO-ELASTICALLY FLUTTERING REEDS
Jha, Sourabh Kumar
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Heat Transfer Enhancement in Rectangular Channels using Autonomous, Aero-Elastically Fluttering Reeds Sourabh Kumar Jha 260 Pages Directed by Dr. Ari Glezer Forced convection heat transfer in compact, air-cooled heat exchangers with high-density, high aspect ratio fin channels is typically limited by the volume flow rate of the cooling air and consequently by the low channel Reynolds number. In addition, the heat transfer in the developing inlet flows of the fin channels is constrained by thin thermal boundary layers over the fin surfaces and by limited mixing of the heated air near the fin surfaces with the cooler flow within the center of the channel. These limitations are commonly overcome by increasing the air volume flow rate, the fin planform dimensions and density, or by deliberate, passive generation of small-scale motions (e.g., using vortex generators and dimples) resulting in significant increases in flow losses. The present dissertation builds on earlier research at Georgia Tech which revealed that heat transfer within these high aspect ratio rectangular channels can be significantly enhanced with minimal penalty in flow losses by using cantilevered thin-film aero-elastically fluttering reeds to induce the formation of small-scale vortical motions within the channels. The present investigations demonstrated that reed flutter occurs when the flow speed exceeds a critical level that depends only on and can be adjusted by the reed’s mass ratio, and that the reed’s oscillation frequency as measured by its Strouhal number depends on its mass ratio and its reduced critical speed. It was shown that the reed interactions with the channel’s inlet flow engender a hierarchy of small-scale motions of decreasing scales which result in “turbulent-like” characteristics when the channel base flow is laminar, and accelerate the onset of turbulence when the base flow is transitional at higher Reynolds numbers. It was also shown that the reed-induced small-scale motions enhance local and global heat transfer in the channel through modulation of the thermal wall boundary layers and cross-stream mixing between the wall-bounded and central flows. Even when the Reynolds number of the inlet base flow is sufficiently high so that the flow is nearly fully-turbulent near the channel’s exit, the reed significantly enhances the overall heat transfer in the channel. For a given flow power, the reed can enhance the global Nusselt numbers by up to 1.8-fold relative to the base flow and by up to 1.44-fold relative to conventional heat augmentation techniques. An important finding of the present investigations is that because the channel’s losses in the presence of the reed as measured by the friction factor scale only with the reed’s Strouhal number, the losses can be significantly lowered with minimal degradation in the enhanced heat transfer. Therefore, the channel losses in the presence of the reed can be decreased up to 3-fold by reducing the reed’s Strouhal number while maintaining a nearly-invariant Nusselt number. It was also shown that the thermal performance of the reeds in isolated channels can be used to predict the thermal performance of geometrically-similar fin array channels at low Reynolds and Strouhal numbers. Finally, based on the present findings it is estimated that for a given heat dissipation in air-cooled condensers of industrial power plants, the fan power can be reduced by up to 64% by using oscillating reeds in the condensers’ fin channels.