Numerical simulation of the onset of slug initiation in laminar horizontal channel flow P. Valluri, P.D.M. Spelt * , C.J. Lawrence, G.F. Hewitt Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK Received 30 September 2006; received in revised form 28 June 2007 Abstract Results are presented for the initiation of slug-type structures from stratified 2D, two-layer pressure-driven channel flow. Good agreement is obtained with an Orr–Sommerfeld-type stability analysis for the growth rate and wave speed of very small disturbances. The numerical results elucidate the non-linear evolution of the interface shape once small dis- turbances have grown substantially. It is shown that relatively short waves (which are the most unstable according to linear theory) saturate when the length of the periodic domain is equally short. In longer domains, coalescence of short waves of small-amplitude is shown to lead to large-amplitude long waves, which subsequently exhibit a tendency towards slug for- mation. The non-uniform distribution of the interfacial shear stress is shown to be a significant mechanism for wave growth in the non-linear regime. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Slug flow; Two-phase flow; Stability 1. Introduction The transition from stratified to slug flow is an important consideration in the design and operation of pro- cess equipment, especially in the transport of mixtures of gas with associated liquids (oil, condensate and/or water). Slug flow is associated with oscillations in pressure and flow rate and large surges of liquid, which can cause significant problems if the receiving facilities are not adequately sized. The pace of research is acceler- ating with the reliance on very long (hundreds of km) pipelines for multiphase transport from offshore fields in increasingly harsh and deepwater environments. Much research has naturally been conducted to determine the conditions under which the transition from stratified to slug flow occurs. The results are usually presented in the form of flow regime maps (e.g., Hewitt, 1982). The use of such flow maps is restricted by the fact that many parameters govern the transition. A dominant requirement for slugging is a minimum liquid level in the pipe- line, which manifests itself in commonly used flow maps as a minimum superficial liquid velocity. Other 0301-9322/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmultiphaseflow.2007.09.001 * Corresponding author. Tel.: +44 020 75941601; fax: +44 020 75945700. E-mail address: p.spelt@imperial.ac.uk (P.D.M. Spelt). Available online at www.sciencedirect.com International Journal of Multiphase Flow 34 (2008) 206–225 www.elsevier.com/locate/ijmulflow