Wall heat/mass transfer in pulsatile flow

Abstract

Heat/mass transfer is considered from a solid surface to a fluid undergoing pulsatile flow, consisting of sinusoidal pulsations superimposed on a mean flow. Mass transfer was measured from small strips using the diffusion-controlled electrode method for both non-reversing and reversing flows, corresponding to small and large pulsation amplitudes, respectively. Heat/mass transfer rates from the surface consist of a mean component, with a superimposed oscillatory component that is dependent on the amplitude, α, of the fluctuating wall shear rate, and the dimensionless frequency parameter, F p . For small α, the amplitude and phase of the first harmonic of the measured transfer rates agree well with the theory of Mizush for all values of F p . The same is true for larger α (< 1), only when F p is also large; for large α and small F p , there is significant deviation from theory. In the latter case, the experimentally measured amplitudes of the second harmonic become large, and the assumptions the theory are no longer valid. For α > 1, flow reversal occurs, and the theory again does not apply. For α < 1, the shapes of the mass transfe time curve are almost sinusoidal, and space-and-time-averaged transfer rates decrease slightly compared to steady flow as α increases. For a > 1, f takes place, and as α increases to about 1.5–2, the average transfer rates increase and become equal to those in steady flow. For α > 2 the tra become greater than the steady flow values and the second harmonic of the fluctuating transfer rate is as significant as the first harmonic. The change of the transfer rate vs time curves is quite significant, with two maxima and minima appearing in a period compared to the single maximum and minimum w characteristic of small α. These changes in average transfer rates and in the shapes of the transfer rate vs time curves are substantial for small va frequency parameter, F p . As F p is increased, the changes become less pronounced, and the trend of the data indicates that for F p large enough, there is no increase in the transfer rate in pulsatile flow, and this appears to be independent of the value of α.