The enhancement of heat transfer in fluids is an important factor in the design of many practical engineering devices. One particular technique for increasing heat transport is to impose oscillatory flow on a duct connecting two reservoirs. In the current study, axial heat transfer enhancements by incompressible laminar oscillatory flow between cold and hot reservoirs connected by a bundle of tubes are examined experimentally and numerically. The dimensionless frequency parameter or Womersley number Wo was varied from 0.1 to 100 at different fluid tidal displacements. The oscillatory thermal conductivity was found to scale quadratically on the tidal displacement and pressure-gradient amplitude, and on the square root of the frequency of oscillation, and could exceed its molecular counterpart by up to five orders of magnitude.Correlations were determined for the oscillatory thermal conductivity enhancement as a function of Wo and tidal displacement. The correlations showed that the axial heat transfer rate scaled in proportion to Wo1.62 for Wo > 3 and behaved exponentially for low Wo with different scalings depending on pressure gradient amplitude. The study suggested a classification for the oscillatory heat transfer which partitioned the flow into four different regions varying from low tidal displacement to bulk convective exchange. It was shown that for unsteady flow the unsteady axial conduction is usually small for Wo > 3, but becomes significant below Wo = 3. This criterion is in contradiction to results of previous studies for Wo < 3, which underestimated effective conductivity because unsteady differential axial conduction was neglected. Finally, enhancement by a further factor of ten was observed in the axial heat transfer rate in tubes when walls were conductive and of sufficient thickness.The effect of viscous heat dissipation in raising the field temperature of incompressible oscillatory air flow was also studied. A threshold was established for when the viscous heat dissipation term in the thermal energy equation changes or does not change significantly the temperature field for the case of oscillatory air flow in a tube connecting two reservoirs. According to this threshold, the effect of dissipative bulk heating can be described by a correlation in terms of Womersley number Wo and axial tidal displacement Z of the oscillatory flow. These results were determined using numerical simulations of oscillatory air flow (Pr = 0.7) for different adiabatic non-conductive tube-reservoirs' systems configurations over a wide range of oscillatory frequencies and tidal displacements. The effect of viscous heat dissipation in oscillatory air flow can be ignored only below a specific limit of unsteadiness depending on Womersley number and axial tidal displacement. Otherwise, it becomes more significant with increasing Wo and Z.
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