Pattern transition

Steam-liquid flow. Two-phase flow maps and heat transfer prediction methods which exist for vaporization in macro-channels and are inapplicable in micro-channels. Due to the predominance of surface tension over the gravity forces, the orientation of micro-channel has a negligible influence on the flow pattern. The models of convection boiling should correlate the frequencies, length and velocities of the bubbles and the coalescence processes, which control the flow pattern transitions, with the heat flux and the mass flux. The vapor bubble size distribution must be taken into account. 

Fig. 5.19 Two-phase flow pattern transition. Reprinted from Serizawa et al. (2002) with permission...

Galbiati L, Andreini P (1992) Elow patterns transition for vertical downward two-phase flow in capUlary tubes. Inlet mixing effects. Int Comm Heat Mass Transfer 19 791-799 Garimella S, Sobhan C (2003) Transport in microchannels - a critical review. Ann Rev Heat Transfer 13 1-50... 

Taitel Y, Bamea D, Dukler AE (1980) Modeling flow pattern transitions for steady upward gas-liquid flow in vertical tubes. AlChE J 26 345-354 Triplett KA, Ghiaasiaan SM, Adbel-Khalik SI, Sadowski DL (1999a) Gas-liquid two-phase flow in microchannels. Part 1 two-phase flow patterns. Int J Multiphase Flow 25 377-394 Triplett KA, Ghiaasiaan SM, Abdel-Khalik SI, LeMouel A, McCord BN (1999b) Gas-liquid two-phase flow in microchannels. Part 11 void fraction and pressure drop. Int J Multiphase Flow 25 395 10... 

Two-phase flows are classified by the void (bubble) distributions. Basic modes of void distribution are bubbles suspended in the liquid stream liquid droplets suspended in the vapor stream and liquid and vapor existing intermittently. The typical combinations of these modes as they develop in flow channels are called flow patterns. The various flow patterns exert different effects on the hydrodynamic conditions near the heated wall thus they produce different frictional pressure drops and different modes of heat transfer and boiling crises. Significant progress has been made in determining flow-pattern transition and modeling. 

A variety of mechanisms have since been suggested to explain the physical basis for the observed transition between flow patterns (Ishii, 1975 Taitel and Dukler, 1976b Taitel et al., 1980 Barnea, 1987 Taitel and Barnea, 1990). Dukler and Taitel (1991b) summarized the various mechanisms to explain such flow pattern transitions (Table 3.1), where the letters in Table 3.1 identify the theoretical curves shown in the accompanied graphs (Fig. 3.7) for different flow directions. A word of caution was given in the reference ... 

The matter of predicting flow pattern transitions is still in a developing state. In some cases the transition mechanisms are reliably understood, other mechanisms suggested are more in the nature of speculations.. .. If the ability to predict these transition conditions is to become more reliable and more generalized, it will require considerable additional research into mechanisms and into the basic fluid mechanical factors that enter into these mechanisms. 

Pattern transition in horizontal adiabatic flow. An accurate analysis of pattern transitions on the basis of prevailing force(s) with flows in horizontal channels was performed and reported by Taitel and Dukler (1976b). In addition to the Froude and Weber numbers, other dimensionless groups used are... 

Pattern transition in vertical adiabatic flow. Upward vertical flow has been studied intensively, both because of the simplicity of the geometric condition and the relevance in applications. The map shown in Figure 3.4 is the result of rather recent and relevant studies into the interpretation of regime transition mechanisms. In this figure, the transition between bubbly flow and slug flow occurs be-... 

The effect of flow obstructions on the flow pattern transitions in horizontal two-phase flow was studied by Salcudean and Chun (1983). The practical importance of the problem is related to the use of rod spacing devices in water-cooled nuclear reactors. In general, these devices are expected to affect the flow distribu-... 

Figure 3.12 Comparison of flow pattern transition boundaries. (From Salcudean and Chun, 1983. Copyright 1983 by Elsevier Science Ltd., Kidlington, UK. Reprinted with permission.)...

Venkateswararao et al. (1982), in evaluating the flow pattern transition for two-phase flow in a vertical rod bundle, suggested the calculation of pressure gradient for annular flow by... 

The phenomenon of critical flow is well known for the case of single-phase compressible flow through nozzles or orifices. When the differential pressure over the restriction is increased beyond a certain critical value, the mass flow rate ceases to increase. At that point it has reached its maximum possible value, called the critical flow rate, and the flow is characterized by the attainment of the critical state of the fluid at the throat of the restriction. This state is readily calculable for an isen-tropic expansion from gas dynamics. Since a two-phase gas-liquid mixture is a compressible fluid, a similar phenomenon may be expected to occur for such flows. In fact, two-phase critical flows have been observed, but they are more complicated than single-phase flows because of the liquid flashing as the pressure decreases along the flow path. The phase change may cause the flow pattern transition, and departure from phase equilibrium can be anticipated when the expansion is rapid. Interest in critical two-phase flow arises from the importance of predicting dis-... 

Analysis of flow-pattern transition instability The boundary of flow pattern transitions is not sharply defined, but is usually an operational band. As discussed in Chapter 3, analytical methods for predicting the stability of flow patterns are quite limited and require further development. The same is true for analyses of nonequilibrium state instability. 

Check the static instabilities by steady-state correlations, to avoid or alleviate the primary phenomenon of a potential static instability, namely, boiling crisis, vapor burst, flow pattern transition, and the physical conditions that extend the static instability into repetitive oscillations. 

Barnea, D., 1987, A Unified Model for Predicting Flow Pattern Transitions for the Whole Range of Pipe Inclinations, Int. J. Multiphase Flow 13 1. (3)... 

Barnea, D., O. Shoham, and Y. Taitel, 1982a, Flow Pattern Transition for Downward Inclined Two-Phase Flow, Horizontal to Vertical, Chem. Eng. Sci. 37(5) 735-740. (3)... 

Dukler, A. E., and Y. Taitel, 1991 b, Flow Pattern Transitions in Gas-Liquid Systems, chap. 3 Modeling Two-Phase Annular Flow, chap. 5, and Modeling Upward Gas-Liquid Flow, chap. 7, in Two-Phase Gas-Liquid Flow A Short Course on Principles of Modelling Gas-Liquid Flow and on Modern Measuring Methods, University of Houston, Houston, TX. (3)... 

Salcudean, M., J. H. Chun, and D. C. Groeneveld, 1983a, Effect of Flow Obstructions on the Flow Pattern Transitions in Horizontal Two-Phase Flow, Int. J. Multiphase Flow, P( 1) 87. (3) Salcudean, M., D. C. Groeneveld, and L. Leung, 1983b, Effects of Flow Obstruction Geometry on Pressure Drops in Horizontal Air-Water Flow, Int. J. Multiphase Flow 9(l) 73-85. (3)... 

Taitel, Y., D. Barnea, and A. E. Dukler, 1980, Modelling Flow Pattern Transitions for Steady Upward Gas-Liquid Flow in Vertical Tubes, AIChE J. 26(3) 345. (3)... 

In the vaporization process, the formation of a continuous vapor phase causes a transition from bubble to annular flow. This flow-pattern transition is accompanied by a gradual change in the heat-transfer mechanism. Both... 

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