In a counter-current two-phase flow system, flooding can be defined as the onset
of flow reversal of the liquid component which results in an upward co-current flow.
Flooding in the surge line of pressurized water reactors poses a significant technical
challenge in the analysis of several postulated nuclear reactor accident scenarios.
Despite the importance of flooding in these analyses, previous work does not
identify a universally accepted rigorous physics-based model of flooding, even for
the simple case of flooding in adiabatic, vertical tubes. This can be attributed to
a lack of conclusive understanding of the physics of two-phase counter-current flow,
specifically the mechanism of flooding, and the large amount of uncertainty among
data from various flooding experiments. This deficiency in phenomenological and
experimental knowledge has led to the use of many empirical and semi-empirical
correlations for specific system conditions and geometries. The goal of this work
is the development of a model for flooding in vertical, adiabatic tubes from first
principles.
To address a source of uncertainty in the analysis of flooding, a model for the
prediction of average film thickness in annular co- and counter-current flows has been
developed by considering the conservation of momentum of the liquid and gas flows.
This model is shown to be a quantitative improvement over the most commonly used
models, those of Nusselt and Belkin, Macleod, Monrad, and Rothfus. The new model
better considers the effects of interfacial shear and tube curvature by using closure
relations known to represent forces appropriately in co- and counter-current flow. Previous work based on semi-empirical flooding models has been analyzed to
develop a new theory on the hydrodynamic mechanism which causes flooding. It is
postulated that the growth of an interfacial wave due to interfacial instability results
in a flow reversal to ensure that momentum is conserved in the counter-current flow
system by causing a partial or complete co-current flow.
A model for the stability of interfacial waves in a counter-current flow system
is proposed and has been developed herein. This model accurately represents the
geometric and flow conditions in vertical adiabatic tubes and has been shown to have
limits that are consistent with the physical basis of the system. The theory of waves
of finite amplitude was employed to provide closure to an unknown parameter in
the new model, the wave number of the wave that generates the interfacial instabil-
ity. While this model underpredicts the flooding superficial gas velocity, the result
is a conservative estimate of what conditions will generate flooding for a system.
In the context of the analysis of a nuclear reactor, specifically a pressurized water
reactor, conservatism means that the gas flow rate predicted to cause flooding for
a fixed liquid flow rate will be less than the flow rate found experimentally, mean-
ing that liquid delivery to the core would be safely underestimated. Future work
includes the improvement of the closure relation for the limiting wave number that
will cause unstable interfacial waves, as well as an assessment of the applicability of
the stability-based model to flooding in the presence of phase change and flooding
in complex geometries.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/ETD-TAMU-2009-08-7008 |
| Date | 2009 August 1900 |
| Creators | Hogan, Kevin J. |
| Contributors | Vierow, Karen |
| Source Sets | Texas A and M University |
| Language | en_US |
| Detected Language | English |
| Type | Book, Thesis, Electronic Dissertation, text |
| Format | application/pdf |
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