Numerical simulation of vertical intermittent flows

Abstract

The present research extends the application of the “slug capturing” technique, already extensively validated for the prediction of horizontal slug flow, to the case of vertical pipes. In this technique, the one-dimensional two fluid model is solved numerically in order to simulate mechanistically the growth and development of the structures which characterise the vertical intermittent flow, such as slugs and waves. In this work the technique is first successfully applied to the prediction of vertical slug flow: it is shown here that, when the appropriate correlation for the interfacial friction forces is used, the slug capturing technique is able to simulate correctly – even in vertical configurations - the complete evolution of the slugs and to predict their key characteristics with a notable accuracy when compared against experimental data. This represents certainly the most important achievement of this work. The one-dimensional two-fluid model is well known to be ill-posed and hence, as the mesh is refined, artificial instabilities may grow so as to render the simulation unreliable. For this reason, a Von Neumann analysis of the discretized form of the model is presented. It is shown that the discretization introduces a cut-off limit for short wavelengths, below which all the perturbations are damped. It is suggested here that this effect, for practical sizes of the mesh, is sufficient to stabilize the system. Furthermore, this work presents preliminary results of the application of the technique to churn flow. It is shown that, although the results may be considered promising, the model still needs development.