The variation of the void fraction for vertical upward
and downward orientation at constant liquid superficial
velocity and with increasing gas superficial velocity is presented
in Figs. 8 and 9, respectively. For vertical upward flow low void
fraction values are associated with the bubbly (0 < a 6 0.4) and
slug flow (0.2 < a 6 0.7) regimes and are observed to increase rapidly
with the small increase in the superficial gas velocity. The void
fraction continues to increase gradually with the gas superficial
velocity as the flow pattern transits from froth to annular flow regime.
This observed trend in the void fraction is consistent with the
observations of Oshinowo and Charles [5] and Yijun and Rezkallah
[6].
In the case of downward two phase flow it is observed that for
the bubbly and slug flow regimes, the void fraction increases rapidly
even with the slight change in the gas superficial velocity.
Whereas for the falling film and annular flow regimes where the
gas flows through the core surrounded by the liquid film, the void
fraction virtually remains constant with increasing gas flow superficial
velocity. This trend of the void fraction for varying liquid and
gas superficial velocities is consistent with the documentation of
Yijun and Rezkallah [6]. It is observed that for both upward and
downward two phase flow this change in the trend in the variation
of the void fraction with change in the flow patterns occurs for void
fraction around, a = 0.7.
Thus the rapid variation of the void fraction with the gas superficial
velocities explains the need for a correlation that is sensitive
to the change in gas superficial velocity and predicts the void fraction
accurately in this region of the steep slope typically for
0 < a 6 0.7. In addition to the flow patterns, the two phase parameters
such as gas volumetric flow fraction and the slip ratio were
also observed to be influenced by the interaction of the buoyancy,
inertia and gravity forces for both orientations. Here we present
the variation of the void fraction against the gas volumetric flow
fraction and slip ratio and attempt to link this phenomenon with
the physical mechanism governing vertical upward and downward
two phase flow.