28-03-2014, 11:07 AM
Elementary Aspects of Two-Phase Flow in Pipes
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INTRODUCTION
Chemical engineers frequently encounter the flow of a mixture of two fluids in pipes. Liquid-gas
or liquid-vapor mixtures are encountered in condensers and evaporators, gas-liquid reactors, and
combustion systems. Also, the transport of some solid materials in finely divided form is
accomplished by making a slurry of the solid particles in a liquid, and pumping the mixture
through a pipe. Liquid-liquid mixtures are encountered when dealing with emulsions as well as
in liquid-liquid extraction.
Two-phase flow is a difficult subject principally because of the complexity of the form in which
the two fluids exist inside the pipe, known as the flow regime. Toward the end of these notes,
we’ll see some examples of these regimes. It is difficult to construct a model from first
principles in all but the most elementary situations. Dimensional analysis is used to establish the
relevant groups to aid in designing suitable experiments. Most available empirical results are
applicable only to gas-liquid two-phase flow. A little reflection will convince you that the
orientation of the pipe makes a difference in the flow regime because of the role played by
gravity and the density difference between the two fluids.
In two-phase flow, the concept of hold-up is important. It is the relative fraction of one phase in
the pipe. This is not necessarily equal to the relative fraction of that phase in the entering fluid
mixture.
The usual question for the engineer is that of calculating the pressure drop required to achieve
specified flow rates of the gas and the liquid through a pipe of a given diameter. To make design
calculations involving two-phase flow, Perry’s Handbook is a useful resource. It summarizes
correlations that are currently used in industry. Also, an informative chapter in Holland and
Bragg (1995) is devoted to gas-liquid two-phase flow. Here, we only consider the qualitative
features of gas-liquid flow in a horizontal pipe to give you an appreciation of the complexity of
two-phase flow when compared with the flow of a single fluid phase.