An experimental study of ice formation and convection during cryogenic pipe freezing

Abstract

Cryogenic pipe freezing is introduced as a pipeline maintenance procedure, and some recent applications of the technique are reported. Recent research into pipe freezing at the University of Southampton is reviewed, along with other published experimental research into related fields. The development of suitable experimental apparatus for studying natural convection and ice formation during pipe freezing in a vertical pipe is outlined, and the selection of a suitable flow visualisation technique is described. Results from an experimental programme are examined and discussed, and conclusions about ice formation and natural convection are drawn. Flow visualisation confirms an earlier hypothesis by showing that the downward flowing boundary layer flow over the forming ice, and the upward return flow through the core, interfere once the ice plug neck is small enough, and this interference and mixing effectively isolates the upper part of the plug from the lower part. The isolation provided by this interference is shown to be more marked at higher liquid temperatures, where the effect causes greatly increased ice formation rates just above the ice plug neck. Flow visualisation also reveals a substantial mixing region just below the freezing section of the pipe. The size of this region is controlled by the fluid temperature: at higher temperatures it extends well up into the freezing section and some way down the pipe. Natural convection is shown to influence ice formation even at very low liquid temperatures, but these effects are local, and confined to the top section of the ice plug. The shape of an ice plug formed at lower liquid temperatures is mainly controlled by the cryogen jacket filling characteristics. The design of the cryogen jacket is shown to have considerable effect on ice formation. A method of calculating local heat transfer coefficients at the ice/water interface is demonstrated, using ice interface position data and temperature measurement at fixed points at short time intervals. Results are shown to be consistent with observations of ice growth, but inconsistent with observations of convection flow. It is concluded that the values of the heat transfer coefficient are so low at ambient fluid temperatures, that ice formation is controlled almost entirely by the local bulk water temperature. Local bulk temperature is in turn controlled by the convection patterns induced by cooling. Possible future work is outlined, including further experimental work that would be possible with the existing apparatus. Further work making use of the techniques developed for this investigation is also suggested.

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