SUMMARY

Air-coupled heat exchangers are widely employed in a variety of engineering applications, including power generation, electronics cooling, air-conditioning, refrigeration, and the automotive and chemical process industries. Air has poor transport properties, resulting in low air-side heat transfer coefficients and large heat exchanger surface areas. The air side typically presents that governing thermal resistance; therefore, there is a significant need to develop strategies to enhance air-side heat transfer while minimizing parasitic power requirements. This study investigates the enhancement of air-side heat transfer using auto-fluttering reeds between the air-side fins. A review of the literature on the enhancement of air-coupled crossflow heat exchangers under different flow conditions is conducted. Aeroelastically fluttering thin reeds installed inside the fin channels of crossflow air-coupled heat exchangers passively oscillate as the air flows through the heat exchanger, generating vortical structures that disrupt the thermal boundary layer and improve mixing. Heat transfer enhancement and pressure drop penalties due to the use of such auto-fluttering reeds are investigated in representative heat exchanger geometries. Experiments are conducted in a temperature- and humidity- controlled wind tunnel facility to measure the air-side Nusselt number and friction factor with and without reed-enhancement over a wide range of channel Reynolds numbers under uniform and maldistributed air flow conditions. Heat transfer is enhanced by a factor of up to 1.6 with a maximum friction factor penalty of 3.2 under uniform flow conditions. Air flow maldistribution marginally increased bare channel heat transfer but did not significantly affect reed-enhanced heat transfer compared to uniform flow conditions. The effect of reed thickness is studied to gain insights on the effect of reed flutter dynamics on the Nusselt number and friction factor. Based on these experimental results, models are developed to predict heat transfer and pressure drop for reed-enhanced heat exchangers under different air flow conditions. Fluttering reed heat transfer and pressure drop characteristics are compared with those of other common passive enhancement techniques at the component- and system- level. Insights from these experiments and analyses will guide the future development of more compact air-coupled heat exchangers using fluttering reeds.