Some industrial processes are associated with the flow of aqueous foams inside horizontal channels. Examples are found in the oil, food and cosmetic industries. This type of flow presents an important pressure loss, originated from the shear stress exerted by the channel walls. Foam flow is one of the most complex fluids. In a macroscopic point of view, the physical-chemical interaction between the bubbles can be related to some non-Newtonian models (Bingham law, power law, etc.) or an apparent viscosity. These last can represent the internal deformations of fluid elements when shear stress is applied. An experimental facility able to create this type of flow is not so easy to design. Many parameters must be taken into consideration. So, Computational Fluid Dynamics (CFD) constitutes an ideal technique for analyzing this kind of problem. The aim of this study is to validate the use of Computational Fluid Dynamics in order to correctly predict the pressure losses and the velocity fields of a foam flowing through a straight channel and singularities (fence and half-sudden expansion). Simulations for a realistic scenario: two-phase flow, change in the surface tension, bubble size, were undertaken. Obtained results showed that simulations are not able to accurately reproduce for such a complex fluid, the important aspects of this study, such as the pressure losses and the velocity fields. Therefore, an approximation to a Bingham fluid was made. For a foam flow quality of 70% and a velocity of 2 cm/s, the numerical results are justified by experimental evidence. Experiments have been done and predictions for the flow behavior are extrapolated. Results show that the software is able to recreate the behavior of foam flow through a straight channel and singularities. However, this approach is extremely sensitive to the choice of several parameters, like the apparent viscosity, the yield stress, the viscosity consistence, etc.

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Aqueous foam flow over horizontal channels present significant pressure losses originated by the wall shear stress. Understanding this phenomenon is of paramount importance for the oil, food and cosmetic industries. In this study, we validate the use of the innovative polarographic method, used to measure the wall shear stress. It measures an oxy-reduction reaction controlled by the convection and diffusion phenomenon. The most reliable way of obtaining the wall shear stress is through the pressure losses. They allow obtaining the pressure gradient along the length of the channel, which can be related to an averaged wall shear stress. These measurement techniques were applied over a horizontal foam flow enclosed in a square duct of section 21 × 21 mm2; with a velocity of 2, 4 and 6 cm/s; and a void fraction of 70%. Results validate the use of the polarographic method to obtain the wall shear stress produced by foam flow inside a channel.

In this study, the two main transport characterization problems of the foam flow are studied: foam flow stability, through the evolution of the velocity at the core of the foam, and rheology, with the study of the wall shear stress over the lateral walls, for different void fractions. The same velocity profile (block flow, mean velocity 1.75 cm/s) is imposed to the foam flow, at the inlet of the channel, for several void fractions (air/water relation) going from 55 to 85 %. Later on these ones are passed through a singularity (fence) to study the different behaviours induced by the particular properties of each case. The velocity fields, the lateral liquid film thickness and the lateral wall shear stress fields are obtained and compared with one another to comprehend and remark the difference in such a complex flow. The results show that as we move closer to very dry foams the shear at the foam core increases and its velocity becomes higher. However, the wall shear stress at the lateral wall does not present big deviations from one void fraction to the other.