โ Scribed by T.E. Degaleesan; G.S. Laddha
No coin nor oath required. For personal study only.
This length corresponds to a certain time taken by the fluid to adjust its velocity profile, and if in pulsed flow the period of the pulsations is much greater than the adjustment time, then the quasi-steady state assumption is valid. The condition for validity can be written:
In the present work a was in the range 6.7-10, so the quasisteady assumntion is hardly valid. It is likely (a) that turbulent conditions persisted in the-water even when the instantaneous Reynolds number was less than 2000 and (b) that some of the eddies generated at velocity peaks persisted during the slower parts of the cycle. Effect (b) may account for the unexpected positive effect of pulsation at low values of Y.
From the practical point of view, it is concluded that pulsations will improve heat transfer in the turbulent flow regime, particularly if the flow can be made to reverse in direction for part of the cycle, The maximum improvement ratio obtained was R = 1.41, on the basis of the overall heat transfer coefficient. The corresponding improvement in the water-side coefficient may be estimated from Eq. ( 3) for Y = I and n = 0.8 to give R = 3.25. In future work it would be desirable to measure the tube wall temperature and thereby directly obtain a value for the water-side heat transfer coefficient.
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