Plate-fin and tube heat exchangers are extensively studied both experimentally and numerically. With a large eddy simulation technique (LES), this study performs a detailed investigation of the temporal oscillations of streamwise, spanwise and normal velocity components in the exchanger passage and their effects on heat transfer between fins and fluid. Focus is on the instability and transitional behavior of fluid flow in the exchanger. Temporal variations of instantaneous velocity found that even under very low frontal velocities, the fluid exhibits some degree of oscillations, which change from periodic, regular fluctuations to aperiodic, chaotic turbulence with increasing Reynolds numbers. Contrary to the vortex shedding in a bare tube bank, the flow in the exchanger is more like a duct flow, with steady vortexes formed behind each tube. A frequency spectrum analysis of velocity fluctuations further proves that vortex shedding is not the dominant mechanism for momentum and heat transfer. Rather, tube induced oscillations are the dominant factor.

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