Proper orthogonal decomposition analysis of heat transfer in oscillatory flow tubes with helical coil inserts

Resumen

Helical coil inserts are a means for enhancing heat transfer in tubes of heat exchangers. The swirlflow induced by the inserts gains importance at moderate Reynolds numbers in the laminar regimeand promote early transition to turbulence for 300 < Re < 800, depending on the pitch and thicknessof the helical coil. When an oscillatory flow is superimposed to the net flow through the tube, theflow mechanisms change, and a cyclic dispersion of vortices appears periodically upstream anddownstream of the coil as the flow oscillates.In this paper, a numerical simulation of the unsteady flow and heat transfer in a circular tube with ahelical coil insert with non-dimensional pitch p/d=1.5 and non-dimensional thickness e/d=0.24 isaccomplished. Large Eddy Simulation (LES) is employed for retrieving the flow and temperaturefields during 100 oscillating cycles, and phase-averaged results are employed for describing the mainflow features as a function of the oscillating angle. A proper orthogonal decomposition (POD)technique is applied to the instantaneous flow patterns in order to derive the fundamental modes ofthe flow and their contribution to the time-averaged and time-resolved heat transfer characteristics ofthe flow, allowing to describe the physics underlying the compound heat transfer enhancementinduced by oscillatory flow in tube inserts.