Prediction of the liquid film distribution in stratified-dispersed gas–liquid flow

• A model for liquid film distribution in gas–liquid stratified dispersed flows has been derived.
• The model allows the numerical calculation of the local axial liquid film height and velocity profiles.
• Droplet deposition, gravitational drainage and wave spreading are relevant.
• The strength of each mechanism depends on the underlying flow conditions.
• The wave spreading affect is modelled as function of a modified Froude number.

A mathematical model for predicting the circumferential liquid film distribution in stratified-dispersed flow is presented. Objective of the model is to describe the typical flow conditions of wet gas transportation in long, near-horizontal pipelines. In these applications, depending on the gas velocity and pipe diameter, a large asymmetry of the liquid film distribution may arise. The model is based on the assumption that in stratified-dispersed flow, liquid droplets can only be entrained by the gas from the thick liquid layer flowing at pipe bottom. It is also assumed that the deposition of smaller droplets is related to an eddy diffusivity mechanism and regards the entire pipe circumference, while larger droplets deposit by gravitational settling on the pipe bottom. These assumptions explain the formation of a thin, non-atomizing film in the upper part of the pipe. The presence and flow structure of this film appreciably affect the pressure gradient and the liquid hold-up in the pipe and are of great importance in flow assurance studies. The model has been validated against i) the experimental observations recently published by Pitton et al. (2014), the data collected by ii) Laurinat (1982), iii) Dallman (1978), and iv) the predictions of three-dimensional CFD simulations conducted by Verdin et al. (2014). It is shown that the relevant mechanisms which are responsible for the liquid film distribution are the gravitational film drainage, droplet entrainment/deposition and wave spreading. In particular, at high gas velocities and/or small pipe diameters, the asymmetry of the liquid film diminishes owing to the wetting mechanism of wave spreading which makes the distribution of the film more uniform in the circumferential direction. As the gas velocity diminishes and/or for larger pipe diameters, wave spreading is less effective and for these flow conditions only gravitational drainage and droplet entrainment/deposition are responsible for the more asymmetric shape of the liquid film.