Gas-Liquid Two-phase Flow

The flow of gas-liquid mixtures in pipes and other items of process equipment is common and extremely important. In some cases the quality, that is the mass fraction of gas in the two-phase flow, will vary very little over a large distance. An example of this is the flow in many gas-oil pipelines. In other cases, boiling or condensation occurs and the quality may change very significantly although the total mass flow rate remains constant. 

It is important to appreciate that different flow regimes occur at different gas and liquid flow rates and differences also occur for different materials. In order to have any confidence when calculating pressure losses in two-phase flow it is necessary to be able to predict the flow regime and then to use an appropriate pressure drop calculation procedure. 

Design so that the two-phase flow is in the correct regime. Ap for two-phase flow is tricky to predict because the value depends on the flow regime, whether the individual gas and liquid flows are laminar or turbulent, on the pressure (because this affects the gas density), the mass fraction of the flow that is gas, x, and upon whether the flow is horizontal, vertical, cocurrent or countercurrent. In general the approach is that the actual pressure loss per unit length of pipe = (Ap/I)2(j, = (f)c(Ap/I)(- = ( )l(Ap/L)l where the last two terms are, respectively, corrections to the pressure drop assuming only the gas or only the liquid are flowing in the pipe. Usually, when the mass fraction of gas, x 0.75 we tend to use the former and vice versa. 

The correction factors, or 4) are usually 1 and can be 1000. The correlations (and approximate factors given here) apply usually for horizontal flows in the slug and bubble regime with the factors being too high for the stratified, wavy and spray flow regimes and too low for the annular flow regime. 

Example values for as a function of mass fraction gas and absolute pressure are given in Table 2.1. 

from Table 2.1, if the mass fraction of gas flowing in the line is about 0.2 and, if we estimated the pressure drop based on only the liquid flowrate to be 0.05 kPa, then for two-phase flow at 0.7 MPa abs we might expect the actual pressure drop to be perhaps 20 times this value or 1 kPa. If, from Fig. 2.2, we estimate that the flow regime is annular flow, then the actual pressure drop may be even larger. 

Avoid slug flow. Avoid the use of inclined lines, use either vertical or horizontal. Two-phase flow over a packing is given in Section 6. 17, trickle bed reactors. 

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Part 1. Presentation of the model. Int J Heat Mass Transfer 47 3375-3385 Tiselj I, Hetsroni G, Mavko B, Mosyak A, Pogrebnyak E, Segal Z (2004) Effect of axial conduction on the heat transfer in micro-channels Int J Heat Mass Transfer 47 2551-2565 Triplett KA, Ghiaasiaan SM, Abdel-Khalik SI, Sadowski DL (1999) Gas-liquid two-phase flow in microchannels. Part I. Two-phase flow patterns. Int J Multiphase Flow 25 377-394 Tsai J-H, Lin L (2002) Transient thermal bubble formation on polysihcon micro-resisters. J Heat Transfer 124 375-382... 

For adiabatic, steady-state, and developed gas-liquid two-phase flow in a smooth pipe, assuming immiscible and incompressible phases, the essential variables are pu, pG, Pl, Pg, cr, dh, g, 9, Uls, and Uas, where subscripts L and G represent liquid and gas (or vapor), respectively, p is the density, p is the viscosity, cr is the surface tension, dh is the channel hydraulic diameter, 9 is the channel angle of inclination with respect to the gravity force, or the contact angle, g is the acceleration due to gravity, and Uls and Ugs are the liquid and gas superficial velocities, respectively. The independent dimensionless parameters can be chosen as Ap/pu (where Ap = Pl-Pg), and... 

Comparison of Gas-Liquid Two-Phase Flow Characteristics Between Conventional Size Channels and Micro-Channels... 

Me et al. (2006) addressed the differences in gas-liquid two-phase flow characteristics that occur in conventional size channels and micro-channels by examining the two-phase flow pattern, interfacial wave, void fraction and friction pressure drop data obtained in circular and rectangular channels with a hydraulic diameter ranging from 50 pm to 6.0 mm. 

Akbar MK, Plummer DA, Ghiaasiaan SM (2003) On gas-liquid two-phase flow regimes in microchannels. Int J Multiphase Flow 29 855-865... 

Fukano T, Kariyasaki A (1993) Characteristics of gas-liquid two-phase flow in a capillary. Nucl Eng Des 141 59-68... 

Ide H, Kawahara A, Kawaji M (2006) Comparison of gas-liquid two-phase flow characteristics between mini-channels and micro-channels. In Proceedings of 13th International Heat Transfer Conference, Sydney Convention and Exibition Centre, Sydney, Australia, 13-18 August 2006, MPH-51... 

Ishii (1977) One-dimensional drift-flux model and constitutive equations for relative motion between phases in various two-phase regimes. AML Report ANL-77-47 Ide H, Matsumura H, Tanaka Y, Fukano T (1997) Flow patterns and frictional pressure drop in gas-liquid two-phase flow in vertical capUlary channels with rectangular cross section, Trans JSME Ser B 63 452-160... 

Zhao TS, Bi QC (2001b) Pressure drop characteristics of gas-liquid two-phase flow in vertical miniature triangular channels. Int J Heat Mass Transfer 44 2523-2534 Zimmerman R, Gurevich M, Mosyak A, Rozenblit R, Hetsroni G (2006) Heat transfer to air-water annular flow in a horizontal pipe. Int J Multiphase Flow 32 1-19... 

Triplett KA, Chiaasiaah SM, Abdel-Khahk SI, Sadowski JL (1999a) Gas-liquid two-phase flow in micro-channels. Part 1 two-phase flow patterns. Int J Multiphase How 25 377-394 Yuan H, Prosperetti A (1999) The pumping effect of growing and codapsing bubbles in a tube. Micromech Microeng 9 402-413... 

Eulerian two-fluid model coupled with dispersed itequations was applied to predict gas-liquid two-phase flow in cyclohexane oxidation airlift loop reactor. Simulation results have presented typical hydrodynamic characteristics, distribution of liquid velocity and gas hold-up in the riser and downcomer were presented. The draft-tube geometry not only affects the magnitude of liquid superficial velocity and gas hold-up, but also the detailed liquid velocity and gas hold-up distribution in the reactor, the final construction of the reactor lies on the industrial technical requirement. The investigation indicates that CFD of airlift reactors can be used to model, design and scale up airlift loop reactors efficiently. 

The first attempt to analyze the flow pattern of an adiabatic gas-liquid two-phase flow in terms of the dominant physical forces acting on the system was made by Quandt (1965). The criteria for prediction of major flow patterns were developed... 

In addition to knowledge of the flow pattern in gas-liquid two-phase flow, it is important to establish quantitatively the relative amount of each phase, or the void... 

Measurement of the velocity of a large particle. The investigation of the turbulence characteristics in the liquid phase of a bubbly flow has generated detailed studies on the use of thermal anemometry and optical anemometry in gas-liquid two-phase flows. These techniques have been proved to be accurate and reliable for the measurement of the instantaneous liquid velocity in bubble flow. However, the velocity of the gas bubbles—or, more precisely, the speed of displacement of the gas-liquid interfaces—is still an active research area. Three techniques that have been proposed to achieve such measurement were reviewed by Delhaye (1986), as discussed in the following paragraphs. 

Knoll, K. E., 1991, Investigation of an Electromagnetic Flowmeter for Gas-Liquid Two-Phase Flow Measurement, ANS Trans. TANS AO 64 720. (3)... 

The prediction of the flow regime in gas-liquid two-phase flow is rather uncertain partly because the transitions between the flow regimes are... 

Figure 7.6 illustrates a gas-liquid two-phase flow through an inclined pipe. For clarity the diagram is drawn for stratified flow but the equations to be derived are not limited to that flow regime. A momentum equation can be written for each phase but it will be sufficient for the present purposes to treat the whole flow. In this case the interfacial shear force SFs makes no direct contribution but it would have to be considered in writing the momentum equation for either of the phases individually. The net force acting in the positive x-direction is... 

For a gas-liquid two-phase flow there are four possible reference flows ... 

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