(84h) Internal Hydraulic Jump and Drop in Two Phase Gas-Liquid Flow over an Obstacle

Internal
hydraulic jump and drop in two phase gas-liquid flow over an obstacle

Two phase gas-liquid flow
in horizontal and inclined pipes is ubiquitous in industrial applications,
ranging from petroleum industry, chemical plants, nuclear reactors, heat
transfer equipment, to even geothermal energy production. In a horizontal conduit, the two phases tend to
satisfy with a smooth or wavy interface. For low flow
rates in narrow conduits, we observe that the physics of flow is often governed
by the phenomenon of hydraulic jump which marks the abrupt transition of
fast moving supercritical flow (Fr > 1 where Fr is the ratio
of flow velocity to gravity wave velocity) to slow moving subcritical flow (Fr
< 1). An understanding of the jump phenomenon is necessary to reveal the
hydrodynamics and characteristics of heat and mass transfer under such
condition.

A survey of the past
literature reveals that researchers have studied hydraulic jump mostly in open
channels. During gas-liquid flow in closed conduits, researchers have rarely
focused on hydraulic jump formation. In the present study, we have investigated
the jump characteristics during gas-liquid flow over obstacles placed in the
center of a rectangular conduit (12 mm × 50 mm) of 1560 mm length. The study
reveals the presence of multiple jumps and also the simultaneous appearance of
hydraulic jump and drop in the flow passage under certain flow conditions.

The study is primarily
experimental in nature and the jump location (relative to the obstacle) and
jump strength (defined as the ratio of flow height downstream of jump to its
upstream) have been estimated from photographic and videographic measurements.
Air and water are used as the test fluids and the experiments have been
performed with two semi-circular obstacles, having the same base length and
different crest heights. Each obstacle is placed at the middle of the conduit
and the flow rates of the two phases have ben varied from (0 m³/s to
333.33 × 10^-5 m³/s for air and 6.67 × 10^-5 m³/s
to 20 × 10^-5 m³/s for water.

During flow over the
smaller obstacle, an increase in either of the phase flow rates cause the jump
to form upstream of the obstacle for low water flow rates (Q_w <
13.34 × 10^-5 m³/s). The distance of jump position from
the obstacle and the jump strength decreases with increase in flow rate of
either phase. For higher velocities, the flow symmetrically swells over the
obstacle without exhibiting any jump but a jump may form downstream of the
obstacle. The downstream jump shifts away from obstacle and its strength
decreases with increase in flow rate of either of the phases.

Experiments with the
larger obstacle brings out several interesting features. We observe that jumps
appear both upstream and downstream of the obstacle. In addition, we
have also noted the simultaneous formation of hydraulic drop just
downstream of the obstacle at lower flow rates. In
this case, the upstream jump moves towards the obstacle and its strength
decreases while the jump downstream of the
obstacle is generally weak and shifts away from the obstacle with increase in
flow rate of either phase. When flow velocities are sufficiently high, upstream
jump disappears and symmetric swelling of flow over the obstacle occurs
followed by formation of jump downstream of the obstacle. With further increase
in any phase flow rate, the jump moves away from the obstacle and its strength
gradually decreases.

The jump phenomenon
upstream of the obstacle is prompted mainly by the resistance to flow exerted
by the obstacle itself. This results in sudden deceleration of flow. Just downstream
of the obstacle, there is an abrupt reduction of resistance and flow
accelerates rapidly leading to the formation of hydraulic drop. As the
occurrence of hydraulic drop makes the flow supercritical, there is possibility
of hydraulic jump formation once again further downstream.

(a)

(b)

Fig.
1. Photographic view of jump configuration for (a) the smaller and (b) the
bigger obstacles.

The typical jump
configurations for the small and the large obstacle are depicted in Figs 1(a)
and 1(b) respectively.

Based on the different
flow configurations thus observed, flow regime maps
are developed illustrating the influence of
relative volume flow rates on jump configuration.