Pressure drop and gas holdup in air–water flow in 180° return bends

Abstract

Gas–liquid flows in curved tubes are found in a number of applications, such as heat exchangers and transport pipes. The present work deals with air–water flow in 180° tube bends (curvatures of 6.1, 8.7, and 12.2) that connect two 5-m long 26-mm ID horizontal tubes. The bend lies in the vertical position and the two-phase flow can be set as upward or downward. The straight and curved segments of test section were made from borosilicate glass to enable visual access to the two-phase flow. The behavior of the static pressure upstream and downstream of the bend was measured for a wide range of flow conditions, covering the stratified, intermittent and annular flow patterns. The pressure drop and gas holdup change associated with the bend were measured for both upward and downward flows, enabling the calculation of the frictional, accelarational and gravitational components of the total pressure drop in the bend. The distribution of the phases in the bend was investigated with a high-speed camera, revealing several two-phase phenomena responsible for large variations in gas holdup between the inlet and outlet of the bend for both upward and downward flows. An assessment of the prediction methods currently available in the literature showed that correlations for gas holdup in straight tubes give inaccurate predictions of the average gas holdup in the bend for both flow orientations. Frictional pressure drop correlations for gas–liquid flows in return bends also failed to describe with reasonable accuracy the behavior of the experimental data at low gas superficial velocities for both flow orientations. The performance of the correlations improved at high mixture velocities.