Flow phase transitions

If the gas-flow rate is increased, one eventuaHy observes a phase transition for the abovementioned regimes. Coalescence of the gas bubbles becomes important and a regime with both continuous gas and Hquid phases is reestabHshed, this time as a gas-flUed core surrounded by a predominantly Hquid annular film. Under these conditions there is usuaHy some gas dispersed as bubbles in the Hquid and some Hquid dispersed as droplets in the gas. The flow is then annular. Various qualifying adjectives maybe added to further characterize this regime. Thus there are semiannular, pulsing annular, and annular mist regimes. Over a wide variety of flow rates, the annular Hquid film covers the entire pipe waH. For very low Hquid-flow rates, however, there may be insufficient Hquid to wet the entire surface, giving rise to rivulet flow. 

For systems where the bulk freezing transition is well understood, one may want to go one step further and investigate the modifications of the phase transition and the sohd phases in the event of external influence on the system. Flow does freezing happen in a confined situation where external boundaries are present What is freezing in porous media like A related question is What does the interface between sohd and liquid look like This is an intrinsic inhomogeneity that the system builds up by itself (if, as usual, the transition is first order). Let us describe some papers dealing with freezing under external influence. 

Haberstroh, R. E. and P. Griffith, The Slug-Annular Two-Phase Flow Regime Transition, Am. Soc. Mech. Engr., Paper 65-HT-52,... 

Weekman and Myers (W2) examined the fluid-flow characteristics of cocurrent downward flow of gas and liquid. The pulsing effect first noted by Larkins et al. was also observed in this work. Pressure-drop data could be correlated satisfactorily by a relation similar to those used for two-phase flow in pipes. Surface-active agents were observed to have a pronounced influence upon flow regime transition and pressure drop. 

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...

This flow regime map can be compared with other maps developed previously for air-water two-phase flow in small-diameter horizontal and vertical channels. Figure 5.21a-d shows comparisons with the results of Damianides and Westwa-ter (1988), Fukano and Kariyasaki (1993), Triplett et al. (1999a) and Zhao and Bi (2001 a), respectively. The solid lines represent the flow regime transition boundaries observed in the 1 mm diameter channels and the flow regime names in parentheses are those given by the respective authors. 

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... 

Hence, close to the critical point thermodynamic quantities at comparatively distant spatial locations become correlated. Especially in the case of liquid micro flows close to a phase transition, these considerations suggest that the correlation length and not the molecular diameter is the length scale determining the onset of deviations from macroscopic behavior. 

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. 

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-... 

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