Enhancement of heat transfer by flow pulsations

The effects of pulsating the water stream in a steam-heated heat exchanger were investigated. Two heat exchangers were used in this study, and both were horizontal, double-pipe exchangers with steam in the shell. The pulsations were generated by means of a reciprocating pump which was designed such that the frequency of the pulsations could be varied without affecting the amplitude of the displacement and vice versa.
The frequencies used ranged up to 300 cycles per minute, and five different displacement amplitudes were used at each flow rate investigated. Furthermore, the frequency and the amplitude of the pulsations could be varied so as to cause periodic reversals of the water flow. Average flow rates studied corresponded to a Reynolds number range of 1,000 to 50,000. The rate of heat transfer was found to increase with pulsations, with the highest enhancement in the heat transfer coefficient observed in the transition flow regime. This corresponded to an 8-fold increase in the water-side heat transfer coefficient using the 6 ft-long heat exchanger and to a 6-fold increase using the 3 ft-long heat exchanger.
The results were correlated using a quasi-steady state theory, according to which the time average heat transfer coefficient in pulsatile flow can be obtained from the instantaneous coefficient calculated from steady flow relationships. It was seen that this theoretical model offered a sound basis for comparison with the experimental data, and that it predicted the increase in the heat transfer coefficient that can be expected with different values of the operating variables. To this end, it was also shown that the findings of this study were consistent with the results obtained by various other investigators in the field.