Enhanced Forced Convection Heat Transfer using Small Scale Vorticity Concentrations Effected by Flow Driven, Aeroelastically Vibrating Reeds

Abstract : The 3-Yr experimental/modeling/numerical research program focused on the formation, shedding, and advection of small-scale vortical motions induced by autonomous, aeroelastic fluttering of a cantilevered thin-film reed at the centerplane of a rectangular air channel. The flow mechanisms and scaling of the interactions between the reeds and the channel flow were explored to develop the fundamental knowledge needed to overcome the limits of forced convection heat transport from air-side heat exchangers at low (laminar or transitional) Reynolds numbers. Nonlinear, inviscid vortex-sheet simulations were used to determine the flow stability to the reeds flapping motions. Simulations using coupled flow-structure-thermal models and immersed boundary solver provided insight into the reed-enhanced heat transfer. Measurements captured the nominally time-periodic interactions of the reed with the channel flow using high-resolution PIV. Concave/convex surface undulations of the reeds surface lead to formation and advection of vorticity concentrations and to alternate shedding of spanwise CW and CCW vortices that scale with the reed motion amplitude and channel width, and ultimately to motions of decreasing scales and enhanced dissipation that are reminiscent of a turbulent flow. Transitory vorticity shedding and a local increase in the turbulent kinetic energy as a result of the reeds impact on the channel's surfaces lead to strong enhancement in heat transfer (the channels thermal coefficient of performance is enhanced by 2.4-fold and 9-fold for base flow Re = 4,000 and 17,400, respectively, with corresponding decreases of 50 and 77 in the required channel flow rates). These improvements can be leveraged to enhance cooling rates or to reduce flows rates and/or the size of conventional heat sinks.