Physical interpretation of geysering phenomena and periodic boiling instability at low flows Page: 1 of 11

                                
CO/N F- /( 0306 --//
BNL-62703

PHYSICAL INTERPRETATION OF GEYSERING PHENOMENA AND
PERIODIC BOILING INSTABILITY AT LOW FLOWS

*

Romney B. Duffey
Department of Advanced Technology
Brookhaven National Laboratory
Upton, New York
Upendra S. Rohatgi
Department of Advanced Technology
Brookhaven National Laboratory
Upton, New York

ABSTRACT
Over 30 years ago, Griffith showed that unstable and periodic
iniialboilrng occurredin stagnant liquids in heatedpipes coupled
to a cooler or condensing plemn volume. This was called
"geyering", and is a similar phenomenon to the rapid nucleation
and voiding observed in tubesfiled with superheated liquid It is
also called "bumping" when non-iunformly heated water or a
chemical suddenly boils in laboratoryglassware. In engineering,
de stability and predictability has importance to the onset of bulk
boiling in a natural and forced circulation loops The latest
available data show the observed stability and periodicity of the
onset of boiling flow when there is a plenum, multiple heated
channels, and a sustained subcooling in a circulating loop.
We examine the available data, both old and new, and develop a
new theory to illustrate the simple physics causing the observed
periodicity of the flow. We examine the validity of the theory by
comparison to all the geysering data, and develop a useful and
simple correlation. We illustrate the equivalence of the onset of
geyering to the onset of static instability in subcooled boiling. We
alo derive de stability boundaryfor geysering utilizing turbulent
transport analysis to determine the effects ofpressure and other
key parameters. This new result explains the greater stability
region observed at higherpressures
The paper builds on the 30 years of quite independent thermal
hydraulic work that is stillfresh and useful today. We discuss the
physical interpretadon of geysering onset with a consistent theory'
and show where refinements would be useful to the data
correlations.

INTRODUCTION
We are interested in predicting both the onset and development of
unstable bulk boiling in a heated stagnant or slowly-flowing liquid.
Since heating induces buoyancy forces which give natural
circulation and convection, there is a small initial flow. In a tube or
pipe, after an interval of subcooled nucleate boiling, the onset of
bulk boxing is often characterized by periodic bubble and large void
region growth. The liquid is then alternately expeled from and mo-
enters the heated section. The vapor will exit the end of the tube or
pipe, and its expansion is determined by the balance between
eva poaon and condensation, and any inertial forces. The onset of
the flow excursion and the initiation of the expulsion is a classic flow
instability, and has been extensively studied for subcooled flow in
parallel channels in both forced and natural circulation [see the
papers referenced in Duffey and Hughes, 1991].
The periodicity and instability of the flow depends on having
condensation of the vapor [Khartabil and Dirnuick, 1995].
Evaporation is sustained by the power input Cif any), the decrease in
hydrostatic pressure as voids form, and continued vaporization of
any superheated liquid on the walls or bulk flow. Condensation
occurs at the vaporliquid interfae, due to the local subcooling, or
vapor venting. If the pipe or tube is open ended, or the loop is in
natural or forced circulation, then liquid is replenished also by inlet
flow. Other parallel or connected channels may also supply liquid.
Inada et al [1995] give a useful summary of the literature for
natural circulation reactors, with an emphasis on experimentally
determining the stability boundary. We distinguish three cases from
the literature of potentially unstable boiling onset and 'geyseing"
in an initially fiquid-flled tube
1)      nucleation of initially superheated liquid [Playle,1973].
where the rapid non-equilibrium vaporization is from the

*Sponsored by the U.S. Department of Energy

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