Topics: Fluid dynamics, Viscosity, Chemical engineering Pages: 5 (714 words) Published: April 19, 2015




FACULTY MENTOR:- Dr.Pravin kodgire ,HOD Chemical engg Dept,PDPU

Siddhartha jain
B.Tech chemical engineering

DEF:- Interacting flow of two or more phases, where the interface between the phases is influenced by their motion.examples being gas-liquid flows in evaporators and condensers, gas-liquid-solid flows in chemical reactors, solid-gas flows in pneumatic conveying, etc

Two general topologies of multiphase flow can be usefully identified at the outset, namely disperse flows and separated flows.

Disperse flows : those consisting of finite particles, drops or bubbles (the disperse phase) distributed in a connected volume of the continuous phase.
Separated flows consist of two or more continuous streams of different fluids separated by interfaces.

Multiphase modeling

There are three ways in which such models are explored:
(1) experimentally, through laboratory-sized models equipped with appropriate Instrumentation. (2) Theoretically, using mathematical equations and models for the flow. (3) computationally, using the power and size of modern computers to address the complexity of the flow.

From a practical engineering point of view one of the major design difficulties in dealing with multiphase flow is that the mass, momentum, and energy transfer rates and processes can be quite sensitive to the geometric distribution or topology of the components within the flow. For example, the geometry may strongly effect the interfacial area available for mass, momentum or energy exchange between the phases. Moreover, the flow within each phase or component will clearly depend on that geometric distribution. Therefore a detail study of flow patterns is required.

Types of flow pattern(vertiacal flow)

Of the four type of Two-Phase Flow (Gas-Liquid, Gas-Solid, Liquid-Liquid and Liquid-Solid), gas-liquid flows are the most complex, since they combine the characteristics of a deformable interface and the compressibility of one of the phases. For given flows of the two phases in a given channel, the gas-liquid interfacial distribution can take any of an infinite number of possible forms. However, these forms can be classified into types of interfacial distribution, commonly called flow regimes or flow patterns. Detailed discussions of these patterns are given by Hewitt (1982), Whalley (1987) and Dukler and Taitel (1986). The regimes encountered in vertical flows are illustrated in Figure 1. They include Bubble Flow, where the liquid is continuous, and there is a dispersion of bubbles within the liquid; Slug or Plug Flow where the bubbles have coalesced to make larger bubbles which approach the diameter of the tube; Churn Flow where the slug flow bubbles have broken down to give oscillating churn regime; Annular Flow where the liquid flows on the wall of the tube as a film (with some liquid entrained in the core) and the gas flows in the centre; and Wispy Annular Flow where, as the liquid flow rate is increased, the concentration of drops in the gas core increases, leading to the formation of large lumps or streaks (wisps) of liquid.

Homogeneous Equilibrium Model
The simplest approach to the prediction of two-phase flows is to assume that the phases are thoroughly mixed and can be treated as a single-phase flow. This homogeneous model will obviously work best when the phases are strongly interdispersed (i.e. at high velocities). For the homogeneous model, the pressure gradient (dp/dz) is given by: (1)

where τ0 is the wall shear stress, P the tube periphery, S the tube cross sectional area,  the mass flux, z the axial distance, g the acceleration due to gravity, α the angle of inclination of the channel to the horizontal and ρH the homogeneous density given by: (2)

where ρG and ρL are the gas and liquid...
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