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International Heat Transfer Conference 6

ISSN: 2377-424X (online)
ISSN: 2377-4371 (flashdrive)

THE BOILING MECHANISM DURING BURNOUT PHENOMENA IN SUBCOOLED TWO-PHASE WATER FLOWS

S. B. van der Molen
Netherlands Energy Research Foundation ECN, Petten (N.-H.), The Netherlands

F. W. B. M. Galjee
Netherlands Energy Research Foundation ECN, Petten (N.-H.), The Netherlands

Abstract

The development of a bubble boundary layer and vapour slugs along the surface of a heating element in low pressure two-phase water flows (1−2.5 m/sec) accompanying the boiling crisis have been investigated by means of high speed photography and high speed film techniques.
The results give rise to a model describing the occurrence of burnout in terms of the coalescence of small hemispherical bubbles growing on the surface forming whether a bubble layer or a vapour slug isolating a thin liquid layer from the subcooled bulk liquid flow. This thin liquid layer can be evaporated in few milliseconds. The passage time of a vapour slug of the order of 0.1 sec. or the residence time of 0.1 sec. of a bubble layer gives rise to dry area and wall temperature increase sufficient to exceed the Leidenfrost temperature, to prevent rewetting and to trigger burnout.

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