Inclination Effects on Wave Characteristics in Annular Gas–Liquid Flows A. Al-Sarkhi Dept. of Mechanical Engineering, King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia C. Sarica and K. Magrini Dept. of Petroleum Engineering, The University of Tulsa, Tulsa, OK 74104 DOI 10.1002/aic.12653 Published online May 9, 2011 in Wiley Online Library (wileyonlinelibrary.com). Measurements of wave characteristics have been conducted in a 0.0762 m internal diameter (ID) pipe at inclinations of 0  , 10  , 20  , 45  , 60  , 75  , and 90  from horizon- tal. Wave celerity and frequency are very strongly dependent on modified Lockhart– Martinelli parameter, X*, and the inclination angle. Wave amplitude increases with increasing liquid film thickness at the bottom of the pipe. Wave amplitude depends on liquid film thickness for any pipe diameter, surface tension, and viscosity. Strouhal number (dimensionless wave frequency) decreases with increasing X*. Effect of pipe diameter, surface tension, and liquid viscosity on the liquid film Reynolds number, Re LF , was studied. Re LF variation with X* is not sensitive to the surface tension and less sensitive to the pipe diameter. However, Re LF is very sensitive to the viscosity of the flowing liquid. Correlations for wave celerity, amplitude, frequency, and liquid film Reynolds number are proposed. V V C 2011 American Institute of Chemical Engineers AIChE J, 58: 1018–1029, 2012 Keywords: annular flow, wave characteristics, pipe inclination, fluid mechanics, multiphase flow Introduction An annular flow pattern can exist at high gas velocities in a pipe. In annular flow, liquid flows in a film along the pipe wall and as droplets entrained in the gas core. There is a liq- uid mass transfer between the film and the gas core, whereby droplets deposit at the film and are formed by atomization at the film–gas core interface. Because of gravity, the liquid film at the wall is distributed asymmetrically for all configu- rations except vertical. For air–water systems, the flow in the film is characterized by the intermittent appearance of distur- bances, which are a group of large amplitude waves that could be considered as patches of turbulence. The irregular waves are the source of drops that enter the gas phase. The annular flow pattern is quite different for small diam- eter pipes [less than 3 cm internal diameter (ID)] and large diameter pipes (larger than 5 cm ID) in that the disturbance waves for small diameter pipes are wrapped around the whole circumference. The pattern resembles what is observed in vertical pipes. 1 Geraci et al. 2 explained how the majority of drops in an- nular flow are created from the wall film by the action of flowing of the gas phase over it. However, drops are not cre- ated from the entire liquid film interface, but particularly they arise from disturbance waves. Azzopardi and Whalley 3 concluded that waves are the source of drops. This conclusion was proved through the experiment in which injecting small quantities of liquid into a film flow whose flow rate was just below the flow rate for the formation of waves, they were able to create waves on demand. These experiments, which used an axial viewing Correspondence concerning this article should be addressed to A. Al-Sarkhi at alsarkhi@kfupm.edu.sa. V V C 2011 American Institute of Chemical Engineers 1018 AIChE Journal April 2012 Vol. 58, No. 4