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

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 (TL) denotes the termination of film boiling and could be utilized to represent the chilldown progress to a certain extent. Therefore, TL is significant for the cryogenic chilldown but the associated researches are still insufficient. In the present study, an experimental system, adopting liquid nitrogen (LN2) as the working fluid, is built to perform cryogenic chilldown tests under different conditions, and the variation of TL is concerned to assist the chilldown performance analysis. Current predictive models on TL are evaluated and the influences of existing chilldown acceleration approaches are discussed in virtue of TL 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 TL. Moreover, significant rise in TL could be observed when a chilldown acceleration approach is adopted, which demonstrates that TL variation could be used to assess the acceleration approach. If a low-conductivity coating layer pipe is adopted, increasing Re does not affect TL significantly. Comparatively, when using a micro-fin pipe in the chilldown event, Re influence on TL is still apparent. Even for the micro-fin case with & UDelta;=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 re-strains the thermal removal from the metal wall region.

Automated summary

In a cryogenic chilldown process, film boiling is crucial and Leidenfrost temperature (TL) represents the termination of film boiling. A study evaluating predictive models on TL and discussing the impact of chilldown acceleration approaches on TL variation revealed discrepancies in test data and the potential of TL as an indicator for acceleration assessment. Different pipe materials and structures, such as low-conductivity coating and micro-fin pipes, have unique advantages in chilldown acceleration but contribute differently to thermal energy removal.