An Investigation of the Stratified-Slug Flow Transition in Horizontal Gas-Liquid Flow in Large-Diameter Pipes
Authors: Song S. , Hill A.D. , Morriss S.L. , Podio A.L. 
Published: October 2008

ABSTRAC T
The transition from stratified to slug flow in gas-liquid horizontal flow was studied in 6.35 cm and 18.4 cm ID pipes for a wide range of air and water flow rates by video-recording the flow field and by ultrasonic measurement of the location of the air-water interface. The ultrasonic imaging techniques provided an accurate means of monitoring the liquid depth and wave characteristics in stratified flows. The experimental results were checked with theoretical models of flow regime transitions, which showed that the Taitel-Dukler model1 of the stratified to intermittent flow transition gave a reasonable prediction, particularly in the small pipe. In the large pipe, stratified flow occurred for all conditions tested.

INTRODUCTION
We have been investigating the application of ultrasonic velocimetry to the measurement of volumetric flow rates in gas-liquid flow from deep, subsea wells. In this application, it would be advantageous to locate the measurement system in the subsea pipeline near the wellhead. Our preliminary results show that the efficacy of ultrasonic velocimetry in two-phase flow depends on the flow regime, with stratified flow being a preferred measurement environment compared with intermittent flow. Thus, we are interested in gas-liquid flow regime behavior in large pipes near the pipe entrance.
This paper presents the results of a laboratory investigation of the conditions leading to the transition from stratified to slug flow in gas-liquid, horizontal flow. The results were tested with the theoretical model of Taitel and Dukler and the empirical flow regime map of Mandhane et al.2 For the 6.35 cm ID pipe, reasonable predictions of the stratified to intermittent flow transition were obtained with both of these methods. For the 18.4 cm ID pipe, both methods predicted intermittent flow to occur for some of the conditions tested, while stratified flow was observed in all cases.
Previous work has demonstrated the applicability of ultrasonic imaging and velocimetry to the metering of upwards two-phase flow in vertical or inclined pipe.3 In this study we extended these techniques to horizontal gas-liquid two phase flow. It was found that the ultrasonic imaging techniques provide a means of accurately measuring the distribution of the phases in horizontal gas/liquid flow. A clear picture of the water-air interface and many of the wave characteristics, such as wave amplitude and wave frequency, can be easily obtained from the ultrasound signals. By measuring the wave velocity from the video-recordings of the flows, the wave length can also be obtained.

EXPERIMENTAL APPARATUS

Flow Loop
Experiments were performed using a flow loop consisting of a pipeline of 6.35 cm ID and a pipeline of 18.4 cm ID (Fig. 1). Both pipes are 12.8 m long and are clear, so flow can be observed visually. Water and air are conducted to the flow loop separately through two hoses. By changing the connections between the hoses and the pipelines, either or both of the fluids can be conducted into one pipe and be exhausted from the other. In this way, flows with different flowing directions can be studied.