(312h) Internal Hydraulic Jump Induced Slugging and Flooding in Two Phase Gas-Liquid Flow

Two phase gas-liquid flow in horizontal conduits is ubiquitous in engineering applications, ranging from chemical plants, petroleum industry, heat transfer equipment, nuclear reactors, to even geothermal energy production. In a horizontal conduit, the two phases exhibit stratified flow at low velocity. Under such condition, the flow physics is often governed by the phenomenon of internal hydraulic jump which marks an abrupt deceleration of fast moving supercritical flow (Froude number Fr > 1) to slow moving subcritical flow (Fr < 1). The flow deceleration during jump is associated with an abrupt increase in gas-liquid interface height which is favorable for transition of stable stratified flow to slugging in cocurrent flow and flooding in counter current flow. This calls for a detailed study of internal hydraulic jump for a holistic understanding of flow transition from stratified to slugging and flooding.

A comprehensive literature survey reveals that although jumps have been studied since the last century, the majority of these studies address planar jumps in single phase flow through an open channel. Researchers have rarely investigated internal hydraulic jumps during two-phase flow in closed conduits. Further, almost nothing is reported on the influence of side wall confinement on internal hydraulic jumps. With these considerations, the present study aims to bridge the lacunae in literature and enhance the understanding of internal hydraulic jump during gas-liquid flow through narrow rectangular conduits. Three different flow configurations namely (i) cocurrent air-water flow, (ii) counter-current air-water flow and (iii) cocurrent air-water flow across a T junction are considered for the present investigation.

The present study is primarily experimental in nature. Air and water are used as the working fluids. The flow rates of the two phases in each of the cases have been varied from 6.67 × 10^-5 m³/s to 20 × 10^-5 m³/s for water and 0 m³/s to 333.33 × 10^-5 m³/s for air. For cases (i) and (ii), the experiments have been performed in a rectangular conduit of 50 mm height, 12 mm width and 1720 mm length. In case (iii), the T junction with the side arm vertically upward is mounted at the middle of the horizontal conduit. The cross-section of the horizontal and vertical side arms of the T junction are identical to that of the narrow rectangular conduit.

In order to predict the influence of air on jump characteristics under different flow conditions, we estimate the jump strength (defined as the ratio of downstream and upstream liquid height) and its location (relative to the liquid inlet) from videographic and photographic measurements. We also classify the jump types such as oscillatory, weak and undular jumps based on the nomenclature used in the study of jumps in open channel flow. The routes of slugging and flooding are explained from the physics of hydraulic jump.

During cocurrent air-water flow, an increase in either of the phase flow rates causes the jump to shift downstream. With increase in flow rate of either phase, the distance of jump from the entry section increases and the jump strength decreases. Consequently, jump types change from oscillatory to weak to undular forms. At very high air flow rate (beyond a critical value), the jump strength becomes close to unity (i.e. no jump formation) and the air-water interface becomes unstable with waves of large amplitude. The interface eventually touches the ceiling, blocking the air flow passage, and slugging occurs.

For counter current flow, only an increase in liquid flow rate shifts the jump downstream and decreases the jump strength, as expected. At a fixed liquid flow rate, jump location and strength remains almost invariant for a range of air flow rate. Beyond a certain air flow rate, the jump starts shifting towards the liquid entry and the post-jump interface becomes highly wavy. This continues till a critical air flow rate at which the highly unstable interface touches the conduit roof, resulting in flow reversal due to blockage in air flow passage. This results in transition from stratified flow to flooding. Figure 1 depicts the successive events of hydraulic jump induced flooding. The superficial velocities of liquid and gas at onset of flooding are compared with the well-known Wallis correlation for counter-current flow limitation. Our observations also confirm the predictions of Gargallo et al. (2005, Nuclear Engineering and Design, Vol. 235, pp. 785–804) that for high liquid inlet Froude number, flooding occurs only if the liquid flow becomes locally subcritical, i.e. if a hydraulic jump occurs in the conduit.

An abrupt change in flow cross-section in the form of T junction facilitates flow of a considerable amount of air through the vertical side branch. Such an abrupt change in cross-section causes rapid reduction in air velocity in the horizontal conduit after the T junction. As a result, the effect of air flow on the jump reduces. The distance of jump from entry is lower and its strength is higher as compared to that for identical phase flow rates in a straight horizontal conduit without side branch. For transformation of jumps from oscillatory to weak to undular type, higher air flow rates are required in comparison to conduit without T. The critical air flow rate for slugging is also higher in T junction.

Based on experimental results, phase diagrams for each of the flow conditions are developed to illustrate the influence of phase flow rates on different types of jumps and the transition from stratified to slugging and flooding.

Figure 1. Successive events of flooding. Water flow rate = 8.5 LPM and air flow rate =48 LPM.