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Subcooled flow boiling can be observed when the bulk liquid temperature is
below its saturation value. But within the pool of liquid, if there is a hot surface, there
is a chance of bubble formation, which is the Onset of Nucleate Boiling
(ONB). According to the classical theory (Collier and Thome, 1994; and Tong and
Tang, 1997), the bubbles formed at the hot wall will condense as they move out of
the developing saturation boundary layer, affecting the heat transfer between
the wall and the fluid. At low heat fluxes or high level of subcooling, only
few nucleation sites are active and a portion of the heat is transferred by
single-phase convection between patches of bubbles. This stage is termed as partial
nucleate boiling. As the heat flux is increased, more nucleation sites are activated
leading to fully developed nucleate boiling which corresponds to fully active
nucleation, followed by saturated nucleate boiling. Prediction of the void fraction
profile, flow pattern and velocity distribution is essential for design and safety analysis
of such systems.
Extensive work has been done for predicting the void fraction in
subcooled convective flow boiling. But most of the studies are based on empirical
correlations due to the complexity involved in the process. Zuber et al. (1966), reported an expression for the axial void fraction considering the relative velocity
between two-phases. Using the expression proposed by Zuber and Findlay (1965) and
Levy (1967), a correlation was formulated for the vapor volumetric fraction and
was tested with the experimental data in literature. Kroeger and Zuber (1968),
developed an empirical correlation for the axial void fraction in a pipe depending
on temperature, flow and local relative velocity. There are also a number
of correlations available in literature. The major drawback of these correlations
is that they are valid only for specific conditions in which they are tested. Hu
and Pan (1995), developed a mechanistic model derived from a one-dimensional
two-phase model. However, the model was limited to only the axial direction and
no information can be obtained in the radial direction. Zeitoun and Shoukri
(1997), also reported a one-dimensional two-phase model that accounts for
interfacial mass and energy transport between two phases. However, this model also
predicts the void fraction only in the axial direction and turbulence effects were
not considered. A comprehensive review of subcooled boiling heat transfer
correlations was presented by Kandlikar (1998). Kandlikar also re-examined his
correlation for saturated flow boiling and proposed a methodology with correlations to
predict heat transfer in each region. An advanced two-phase model to subcooled
boiling flow in a pipe has been developed by Lai and Farouk (1993). The model was
quite useful for its prediction of the axial and radial void fraction profile,
temperature distribution and velocity profile in the pipe. |
