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HE-II Film Boiling Specific Features

DOI: 10.1615/ICHMT.1986.IntSympHMTinRefCryo.390
pages 567-579

E. V. Ametistov
Moscow Power Engineering Institute, Moscow, USSR

V. U. Sidyganov
Moscow Power Engineering Institute, Moscow, USSR

Аннотация

The existence of two types of film boiling - noiseless and noisy - is as is known, one of the most interesting peculiarities of superfluid helium (He-II) film boiling which basically differs it from film boiling of any other liquids.
Noiseless boiling regime is characterized by a distinctive smooth or somewhat wavy interface boundary and by a rather steady in size and time vapour film - fig 1. With this vapour does not accumulate in the film and bubble departure is not observed. Film shape and size in a general case do not depend on the density of the heat flux on the heater, qw, its immersion depth, H, and helium bath saturation temperature, T'.
He-II noisy film boiling is accompanied by high frequency noise similar to the acoustic noise which one hears when conventional subcooled liquids boil. Visual observations (with the use of microphotography with short, less than .001 s, delay time) show that with noisy film boiling, on a wire heater for instance, the vapour film represents a system of vapour bubbles which are arranged on the heater like beads strung on a thread. Unlike noiseless regime the volume of vapour bubbles constantly changes; they grow and collapse which produces characteristic noise. Between vapour bubbles there are sections of vapour film with smooth cylindrical interface similar to that in noiseless film boiling regime - fig. 2. From time to time small vapour volumes depart from vapour bubbles, the latter collapse on departing.
Heat transfer intensity in this film boiling regime is lower than with noiseless heat transfer regime, according to many researchers' data.
The feasibility of small vapour bubbles departure with rather durable time of their existence in liquid near the heater can be attributed to the existence of some layer of liquid with 'normal' thermal conductivity having the temperature close to the saturation one corresponding to the pressure in the vapour film. It is due to the presence of this layer that He-II boiling in noisy regime is so similar to the boiling of conventional subcooled liquid. Bearing this in mind, one can understand heat transfer intensity decrease when the transition from noiseless to noisy boiling regime, connected with the failure of helium superthermal conductivity near the heater.

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