A comprehensive analysis is performed to investigate the effect of the oscillating flow of gap fluid on the shuttle heat transfer in reciprocating expanders. For a sinusoidal motion of displacer over cylinder having an axial temperature gradient, a new exact expression for shuttle heat transfer is derived from the analytical solution of the velocity and the temperature distributions for the gap fluid and the two walls. Through a rigorous analysis, the most significant parameter in the shuttle phenomena is proven to be the ratio of the inertial force to the viscous force in the oscillating fluid. For the ratio values smaller than unity, the predicted shuttle heat transfer from the present expression is in good agreement with the previously published results. For large ratios, however, a notable discrepancy exists between the results of the present analysis and the previous investigations. The reason for this discrepancy is that the wall-to-wall heat flux is not in phase with the temperature difference because of the fluid motion that was not included in the previous investigations.
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