Variation of Leidenfrost temperature in liquid nitrogen pipe chilldown process and its application in chilldown performance analysis

Abstract

• A reliable model on Leidenfrost temperature of chilldown is suggested. • Re does not affect Leidenfrost temperature in low-conductivity coating tube chilldown process. • A throughout nucleate boiling-dominated chilldown could be caused by micro-fin tube. • Micro-fin pipe can reduce chilldown time and rapidly takes away thermal energy from metal wall. For a typical cryogenic pipe chilldown event, film boiling plays a dominate role in the view of temperature decrease range and time cost proportion. In the chilldown process, Leidenfrost temperature ( T L ) denotes the termination of film boiling and could be utilized to represent the chilldown progress to a certain extent. Therefore, T L is significant for the cryogenic chilldown but the associated researches are still insufficient. In the present study, an experimental system, adopting liquid nitrogen (LN 2 ) as the working fluid, is built to perform cryogenic chilldown tests under different conditions, and the variation of T L is concerned to assist the chilldown performance analysis. Current predictive models on T L are evaluated and the influences of existing chilldown acceleration approaches are discussed in virtue of T L variation. The results demonstrate that the chilldown test data sourced from different literature appear remarkable discrepancy with one another, and the model proposed by Darr and Hartwig could be suggested to estimate T L . Moreover, significant rise in T L could be observed when a chilldown acceleration approach is adopted, which demonstrates that T L variation could be used to assess the acceleration approach. If a low-conductivity coating layer pipe is adopted, increasing Re does not affect T L significantly. Comparatively, when using a micro-fin pipe in the chilldown event, Re influence on T L is still apparent. Even for the micro-fin case with Δ=50mm, the differences of different boiling regimes are hard to be recognized, and a throughout high heat flux could dominate the wall temperature decrease process. Therefore, balancing consideration of micro-fin structure and inlet Re should be involved in the design of a micro-fin pipe. In addition, although both of the low-conductivity coating layer technique and the micro-fin tube could realize the purpose of chilldown acceleration, the mechanisms and the characteristics are very different. The micro-fin pipe has the advantages of reducing chilldown time as well as rapidly taking away the thermal energy from the metal wall region. The low-conductivity coating layer appears the advantages of facilitating the liquid-wall heat transfer occurrence and reducing the flow resistance, but the existence of the coating layer simultaneously restrains the thermal removal from the metal wall region.