Two-phase flow models

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A number of models have been proposed to calculate pressure drop and heat transfer in two-phase flow (Chapter 4). These models provide a simple and accurate representation of the flow regimes. The models that have been extensively used by most researchers in two-phase are the homogeneous model and the separated flow model. The latter is a simple version of multi-fluid model (Section 4.5); it allows two phases to have different properties and one-dimensional velocities, while the conservation equations are written for the combined flow. The major difference between these two models is the way phase velocity is addressed in each. The homogeneous model lumps both phases together to provide a homogeneous flow, and the behavior of the homogeneous flow is then determined. In the separated flow model, on the other hand, the flow of each phase is determined independently and the effects of the two phases are then summed.

The homogeneous flow model provides an easier approach to determining flow properties and behaviors, but it underestimates the pressure drop, particularly in a moderate pressure range. Furthermore, the homogeneous model is less accurate when velocity and flow conditions for both phases are more dispersed. A separated flow model, on the other hand, is somewhat more complex but tends to produce more accurate results. The one-dimensional models for homogeneous flow and separated flow are presented in Sections 11.3.1 and 11.3.2, respectively.

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