| Research Abstract |
An experimental study was conducted on pressure drop of water-air foam flowing through circular/rectangular channels to clarify the flow resistance of the foam. Foams with wide rage of expansion ratio and channels with various sizes were used in this study.
The results showed that apparent viscosity of the foam is very high, i.e., tens or hundreds times higher than that of the foaming liquid. These high values, were resulted from the flow characteristics of the foam that velocity change exists only in the vicinity of the pipe wall and most part of the foam do not participate in the shearing motion.
It was also shown that the thickness of a thin liquid layr existing on the wall of a pipe, which lubricates the bulk foam flow, has predominant effects on apparent viscosity of the foam. The experimental results of pressure drop for rectangular channels of narrow height suggested that the foam flow can be represented by a plug flow lubricated by the liquid slip-layr on the wall of the channel.
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The thickness of the slip layr was estimated to be the same order of inter-lamella thickness of the bubbles in the foam.
The existing model of foam flow, i.e., C-N model, which assumes foam to be a continuous fluid, can predict the pressure drop of foam flows through channels if the appropriate values of flow parameters are employed. However, it is difficult to determine these values for particular foam flows. Its applicability is limited to channels of relatively large size.
For the foam flow in a channel of narrow height, values of the apparent viscosity has a tendency to decrease for foam of intermediate expansion ratio, i.e., from 6 to 10.This is due to the increase of the slip layr thickness by deformation of the moving bubbles and due to movement of bubble-liquid interface at the top of the slip layr. It is also shown that the apparent viscosity for a narrow channel is well correlated with the thickness of the slip layr by a theoretical model of plug flow.
This study shows that, in addition to the properties of the foam, a kinetic quantity such as the velocity of the foam must be taken into account to predict the apparent viscosity of the foam flowing in a channel. Less
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