In this paper results from experimental investigations on the behaviour and interaction of elongated air bubbles in water in a 254 mm diameter vertical pipe are presented. For the experiments three identical observation/measurement stations, each of them comprising a transparent Perspex window, a pair of electrical resistivity tomography (ERT) sensor, a differential pressure (DP) transducer and a pressure transducer, were implemented along the riser. During the tests the water velocity in the riser ranged from 0-1.4 m/s (corresponding to a Reynolds number range of 0-350,000). Air bubbles were released from the riser base from a pressurised air supply by opening/closing a valve manually. A high speed high definition video camera was used to image the elongated bubbles in the riser. It was found that a Taylor bubble could form in stagnant or low water flow rates, however its shape was increasingly deformed as the water flow rate was increased. In stagnant water, the Froude number of the Taylor bubbles was in a good agreement with its counterpart in small diameter risers. Although the bubble was deformed significantly in a high water flow rate, a good linear correlation was found between the bubble rise velocity and liquid superficial velocity. The development length for the rise velocity of elongated bubbles was examined, and it appeared to be approximately 20 times of the riser diameter. The interaction between two consecutive elongated bubbles was also examined. It was found that the interaction between two bubbles was substantially small when they were more than 20 times of the pipe diameter apart.
Large diameter risers, of which the inner diameter is larger than 100 mm, are frequently encountered in offshore pipelines. Hence the knowledge of multiphase flow behaviours in large diameter riser systems is critical in terms of both economic and technological importance.