Fluid Mechanics and Transport Phenomena zyxw Initiation of Slugs in Horizontal Gas-Liquid Flows z Z. Fan, F. Lusseyran, and T. J. Hanratty zyxw Dept. of Chemical Engineering, University of Illinois, Urbana, IL 61801 zyxw Experiments were conducted with air-water flow in a horizontal 0.095-m pipeline at atmospheric pressure to examine the mechanism by which slugsform in a stratified flow. A specially designed entrance box was used to avoid disturbances. In these experiments, at superficial gas velocities less than 3 m/s, the slugs are found to evolve from waves, with a length zyxwvu of about 0.085 m, that are generated by a Jeffreys mechanism. These waves grow in height and eventually double in wavelength by a nonlinear resonance mechanism. Depending on the height of the liquid, the growth can lead to a breaking wave or to a wave that fills the whole pipe cross section. At superficial gas velocities equal to or greater than 4 m / s capillary-gravity waves with a wide range of lengths are generated by a linear Kelvin-Helmholtz mechanism. These rapidly evolve into long waves outside the range of linear instability. zyx If the liquid height is large enough, these waves canform slugs through a nonlinear Kelvin- Helmholtz instability that is aided by wave coalescence. Introduction When gas and liquid flow in a pipeline, a slug pattern can be observed, for which aerated blocks of liquid bridge the whole cross section and move intermittently downstream. The manner by which they are formed is of considerable importance in predicting the conditions for initiating the slug pattern and for predicting the frequency of slugging. Most theoretical work has focused on a Kelvin-Helmholtz mechanism whereby the destabilizing effects of inertia and of gas-phase pressure variations 180 degrees out of phase with the waveheight are larger than the stabilizing effects of gravity and surface tension. Wallis and Dobbins (1973), Lin and Han- ratty (1986), and Wu et al. (1987) implemented this notion by examining the stability of a flowing liquid layer to long wave- length disturbances. According to these analyses, slugs evolve directly from infinitesimal waves, which grow to touch the top of the pipe. Kordyban and Ranov (1970), Taitel and Dukler (1976), and Mishima and Ishii (1980) examined the stability of finite amplitude waves. These analyses are not complete in that the origin of the wave is not indicated and information, such as the wavelength and the amplitude of the unstable wave, has to be specified. Bontozoglou and Hanratty (1990) suggested that slugs could evolve in air/water systems from small-amplitude capillary- gravity waves generated by a Jeffreys sheltering mechanism 2. Fan is currently at Fractionation Research, Stillwa:er, OK. F. Lusseyran is visiting scientist from LEMTA-ENSEM, Nancy, France. (an imbalance between energy fed to the wave by gas-phase pressure variations in phase with the wave slope and viscous dissipation). When these waves grow to a certain amplitude, a bifurcation exists whereby they have the possibility of dou- bling in wavelength. Bontozoglou and Hanratty argued that this bifurcation is the first step in the evolution of these waves to slugs and that the bifurcation is triggered by a subcritical Kelvin-Helmholtz instability. No experimental evidence is available to support this mechanism. Lin and Hanratty (1987b) reported on visual observations of the initiation of slugs in horizontal 0.051-m and 0.095-m pipelines. The mixer for the air and water at the inlet was a simple tee in which the liquid moved along the run. At gas superficial velocities (defined as the gas volumetric flow divided by the cross section of the empty pipe) below 3 m/s, the first slugs were observed to originate far downstream from the entry. The formation occurred so rapidly that the details could not be documented. With increasing liquid flow rate, the lo- cation of the transition was observed to move closer to the inlet. At large enough liquid flow rates, several slugs existed in the pipe at a given time and the slugs originated in the mixing section where the process could not be observed. At superficial gas velocities above 3 m/s, slugs were reported to form as the result of the coalescence of Kelvin-Helmholtz waves. This study uses both photographic and instrumental methods to obtain information about how slugs evolve. Results are given for air/water flow at atmospheric pressure in a 0.095-m hor- AIChE Journal November 1993 Vol. 39, No. 11 1741