Transition between Flow Regimes

From the visual studies on the flow patterns for circular, trapezoidal and rectangular channels it may be concluded that as the tube diameter decreases, transitions between flow regimes occur at different combinations of superficial gas and liquid velocities. 

In order to predict Lhe transition point from stable streamline to stable turbulent flow, it is necessary to define a modified Reynolds number, though it is not clear that the same sharp transition in flow regime always occurs. Particular attention will be paid to flow in pipes of circular cross-section, but the methods are applicable to other geometries (annuli, between flat plates, and so on) as in the case of Newtonian fluids, and the methods described earlier for flow between plates, through an annulus or down a surface can be adapted to take account of non-Newtonian characteristics of the fluid. 

Figure 3.22 is a plot of slug-to-annular transition lines for different pressures. Thus the boundaries between flow regimes shift from adiabatic lines depending on the... 

The main interest here has been their effect on solids carryover and heat transfer (see below). Some work on transition velocities between flow regimes has been reported, however. Thus Cai et al. (1989) studied the effect of operating temperature (50 to 500°C) and pressure (0.1 to 0.8 MPa) on the transition from bubbling to turbulent fluidization of eight powders in Groups A and B of the Geldart classification fluidized in a column 150 mm in diameter and 3.8 m in height. 

Soliman proposed that the flow is always laminar for We < 20. Kim and Mudawar (2012) proposed the following criteria for boundaries between flow regimes smooth-annular to wavy-annular We = 90X, wavy-annular to transition We = 24X°, and transition to slug We = 7X 1. 

There are two flow regimes corresponding to sonic (or choked) flow for liigher pressure drops and subsonic flow for lower pressure drops. The transition between the two flow regimes occurs at tlie dimensionless critical pressure ratio, Ter,I, which is related to tlie gas lieiit capacity ratio y via... 

Pierce proposes and illustrates good agreement between the test data and the correlation for a smooth continuous curve for the Colburn factor over the entire range of Reynolds numbers for the laminar, transition, and turbulent flow regimes inside smooth tubes ... 

Fig. 5.15 Comparison between the experimental flow patterns obtained by Triplett et al. (1999a) and the experimental flow regime transition lines of Damianides and Westwater (1988) representing their 1 mm diameter cireular test section. Reprinted from Triplett et al. (1999a) with permission...

Figure 5.6 Flow pattern map for a gas/liquid flow regime in micro channels. Annular flow wavy annular flow (WA) wavy annular-dry flow (WAD) slug flow bubbly flow annular-dry flow (AD). Transition lines for nitrogen/acetonitrile flows in a triangular channel (224 pm) (solid line). Transition lines for air/water flows in triangular channels (1.097 mm) (dashed lines). Region 2 presents flow conditions in the dual-channel reactor ( ), with the acetonitrile/nitrogen system between the limits of channeling (I) and partially dried walls (III). Flow conditions in rectangular channels for a 32-channel reactor (150 pm) (T) and singlechannel reactor (500 pm) (A) [13].

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-... 

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