An experimental study on liquid nitrogen pipe chilldown and heat transfer with pulse flows

In order to provide the optimized chilldown strategy for a cryogenic transport line, a study on pulse flow chilldown was conducted. A comparison between continuous flow and pulse flow patterns on heat transfer characteristics was presented to reveal the effect of flow oscillation. Liquid nitrogen was used as the working fluid and upward flows with Reynolds numbers ranging from 2500 to 7000 in a round tube were examined. To gain a better understanding of the effect of flow pulsation on the chilldown process, different square wave pulse flow strategies were examined. Analysis of the ability of different flow patterns to chill down transport lines indicated that there is no single best strategy for all boiling regimes and Reynolds numbers. In terms of the efficiency, the continuous flow is optimal for most boiling regimes at low Reynolds numbers. On the other hand, pulse flow is ideal in the film boiling regime at higher Reynolds numbers. In terms of the total efficiency, continuous flows with high Reynolds numbers are generally more efficient at transferring heat than other patterns or those of lower Reynolds numbers. If a short chilldown duration is a priority, the continuous flow consistently offers the quickest chilldown times. A general guideline for chilldown strategy is suggested as a conclusion.