Gas-liquid flow

In Chap. 5 the available data related to flow and heat transfer of a gas-liquid mixture in single and parallel channels of different size and shape are presented. These data concern flow regimes, void fraction, pressure drop and heat transfer. The effects of different parameters on flow patterns and hydrodynamic and thermal characteristics of gas-liquid flow are discussed. 

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In addition to flow regime, hold-up and pressure drop are two other important parameters in two-phase gas-liquid flows. Hold-up is defined as the relative portion of space occupied by a phase in the pipe. It can be expressed on a time or space average basis, with the actual method chosen depending on the intended use of the hold-up value, and the measurement method employed. There are numerous correlations in the literature for hold-up, but most are based upon a pressure drop-hold-up correlation. The following expression is a widely recognized empirical relationship between hold-up and pressure drop ... 

An extensive treaunent of gas-liquid flows encountered in industry applications, along with numerous design correlations can be found in Volume 3 of the Encyclopedia of Fluid Mechanics - Gas-Liquid Flows (N. P. Cheremisinoff, editor. Gulf Publishing Co, Houston, TX, 1986). Further discussions in this volume can be found in Chapter 4 with regard to flow regimes typically encountered in bubble columns and similar devices. 

The term three-phase fluidization requires some explanation, as it can be used to describe a variety of rather different operations. The three phases are gas, liquid and particulate solids, although other variations such as two immiscible liquids and particulate solids may exist in special applications. As in the case of a fixed-bed operation, both co-current and counter- current gas-liquid flow are permissible and, for each of these, both bubble flow, in which the liquid is the continuous phase and the gas dispersed, and trickle flow, in which the gas forms a continuous phase and the liquid is more or less dispersed, takes place. A well established device for countercurrent trickle flow, in which low-density solid spheres are fluidized by an upward current of gas and irrigated by a downward flow of liquid, is variously known as the turbulent bed, mobile bed and fluidized packing contactor, or the turbulent contact absorber when it is specifically used for gas absorption and/or dust removal. Still another variation is a three-phase spouted bed contactor. 

Dukler, A. E., Gas-Liquid Flow in Pipelines, Am. Petrol. Inst. Proc., Sept. 1967. 

Hughmark, G. A., Holdup and Heat Transfer in Horizontal Slug Gas-Liquid Flow, Chem. Eng. Sci., 20, 1964, p. 1007. 

Quandt, E., Analysis of Gas-Liquid Flow Patterns, A.I.Ch.E., 6th Nat l Heat Transfer Conference, Boston, Mass., Aug. 1963. 

Hughmark, G. A. Heat Transfer in Horizontal Annular Gas-Liquid Flow, 6 National Heat Transfer Conference, AlChE-ASME, Aug. 11, (1963), Preprint No. 49-AlChE. 

This means, of course, that an energy equation is necessary for the description of gas-liquid flows, along with the usual equations of movement and continuity. Transformation of the internal energy of dissolved gas into medium movement energy is what causes the observed pressure drop at the die entrance, e.g. the apparent decline in the amount of energy required to transport the gas-containing melt. 

Glaser and Litt (G4) have proposed, in an extension of the above study, a model for gas-liquid flow through a b d of porous particles. The bed is assumed to consist of two basic structures which influence the fluid flow patterns (1) Void channels external to the packing, with which are associated dead-ended pockets that can hold stagnant pools of liquid and (2) pore channels and pockets, i.e., continuous and dead-ended pockets in the interior of the particles. On this basis, a theoretical model of liquid-phase dispersion in mixed-phase flow is developed. The model uses three bed parameters for the description of axial dispersion (1) Dispersion due to the mixing of streams from various channels of different residence times (2) dispersion from axial diffusion in the void channels and (3) dispersion from diffusion into the pores. The model is not applicable to turbulent flow nor to such low flow rates that molecular diffusion is comparable to Taylor diffusion. The latter region is unlikely to be of practical interest. The model predicts that the reciprocal Peclet number should be directly proportional to nominal liquid velocity, a prediction that has been confirmed by a few determinations of residence-time distribution for a wax desulfurization pilot reactor of 1-in. diameter packed with 10-14 mesh particles. 

Hughmark and Pressburg (H14) studied holdup and pressure drop for cocurrent gas-liquid flow, and correlated holdup with a function of gas and liquid flow rates, surface tension, densities of gas and liquid, viscosities of gas and liquid, and total mass velocity. 

Properties of Cocurrent Gas-Liquid Flow Donald S. Scott... 

Consideration will now be given to the various flow regimes which may exist and how they may be represented on a Flow Pattern Map to the calculation and prediction of hold-up of the two phases during flow and to the calculation of pressure gradients for gas-liquid flow in pipes. In addition, when gas-liquid mixtures flow at high velocities serious erosion problems can arise and it is necessary for the designer to restrict flow velocities to avoid serious damage to equipment. 

Chhabra, R. P. and Richardson, J. F. In Encyclopedia of Fluid Mechanics, Volume 3, Gas-Liquid Flow Cheremisinoff, N, P. eds (Gulf Publishing Co. 1986). Co-current horizontal and vertical upwards flow of gas and non-Newtonian liquid. 

Mandhane, J. M., Gregory, G. A. and Aziz, K. Inti. JL Multiphase Flow l (1974) 537-553, A flow pattern map for gas-liquid flow in horizontal pipes. 

It contains six chapters related to the overall characteristics of the cooling systems single-phase and gas-liquid flow, heat transfer and boiling in channels of different geometries. 

The results of experimental and theoretical investigations related to smdy of drag and heat transfer in two-phase gas-liquid flow are presented in Chap. 5. 

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