Patterns churn flow

In high heat flux (heat transfer rate per unit area) boilers, such as power water tube (WT) boilers, the continued and more rapid convection of a steam bubble-water mixture away from the source of heat (bubbly flow), results in a gradual thinning of the water film at the heat-transfer surface. A point is eventually reached at which most of the flow is principally steam (but still contains entrained water droplets) and surface evaporation occurs. Flow patterns include intermediate flow (churn flow), annular flow, and mist flow (droplet flow). These various steam flow patterns are forms of convective boiling. 

Figure 5.3e shows the situation when the air velocity was increased to Ugs = 20 m/s. It is seen from this figure that the liquid bridges in churn flow disappeared and a liquid film formed at the side walls of the channel with a continuous gas core, in which a certain amount of liquid droplets existed. The pressure flucmations in this case became relatively weaker in comparison with the case of the churn flow. The flow pattern displayed in Fig. 5.3f indicates that as the air velocity became high enough, such as Ugs = 85 m/s, the liquid droplets entrained in the gas core disappeared and the flow became a pure annular flow. It is also observed from Fig. 5.3f that the flow fluctuation in this flow regime became weaker than that for the case shown in Fig. 5.3e, where Ugs = 20 m/s. 

In the study by Qu et al. (2004), experiments were conducted with adiabatic nitrogen-water two-phase flow in a rectangular micro-channel. The bubbly, stratified and churn flow patterns commonly encountered in macro-channels were never observed in the study. No water droplets were observed in the nitrogen bubble, nor were any nitrogen bubbles present in the water slugs. 

The flow regime maps shown in Fig. 5.16a,b indicate that typical flow patterns encountered in the conventional, large-sized vertical circular tubes, such as bubbly flow, slug flow, churn flow and annular flow, were also observed in the channels having larger hydraulic diameters ([Pg.216]

Figure 5.16c indicates that as the channel size was reduced to Jh = 0.866 mm, the dispersed bubbly flow pattern vanished from the flow regime map. Figure 5.16a-c indicates that the slug-churn flow transition line shifted to the right, as the channel size was reduced. Similar trends were also found in small circular tubes by the... 

A new approach was developed by Lee and Mudawar (2005a) to improve the accuracy of pressure drop prediction in two-phase micro-channels. Since the bubbly and churn flow patterns are rarely detected in high-flux micro-channel flow, the separated flow model was deemed more appropriate than the homogeneous. 

This well-defined flow pattern is destroyed at higher flow rates and a chaotic type of flow, generally known as churn flow, is established. Over... 

Churn flow If the velocity of a two-phase mixture in slug flow is increased, the large slugs of gas will tend to become unstable, with the possibility of breakup. The result is the destruction of the slug flow pattern, with an oscillating characteristic being established. 

FIGURE 10.37 Flow pattern of gas-liquid two-phase flow in a tubular duct, (a) Bubble flow, (b) slug flow, (c) churn flow, and (d) annular flow. 

Churn flow The flow is highly unstable with the fluid traveling up and down in an oscillatory fashion but with a net upward flow. This flow pattern is in fact an intermediate regime between the slug flow and annular flow regimes. 

The different flow patterns largely resemble those known from flow in other continuous flow conduits as pipes, tubes, capillaries, and monoliths [29-40]. Bubbly flow, slug flow (Taylor flow), annular, and churn flow are found and a few more intermediate regimes between the ones mentioned. These comprise different gas-liquid configurations such as segmented flow (bubble-train), gas core with encompassing stable thin liquid film, which wets the channel wall, and dynamic wavy liquid films. In case of high gas contents, spray is created with small droplets in... 

Fig. 5.1a-j Representative photographs of flow patterns in the 1.097 mm diameter circular test section. Test section (a), (b) bubbly (c), (d) slug (e), (f) churn (g), (h) slug-annular (i), (j) annular. Reprinted from Triplett et al. (1999a) with permission... 

M Annular to slug churn Matching of voids for slug and annular flow pattern... 

A flow-pattern map was derived for nitrogen/acetonitrile flows in the dual-channel micro reactor [274]. Bubbly, slug, churn and annular flows as well as wavy annular and wavy annular-dry flows with smaller region of stability were found (see Figure 4.35). 

Figure 4.39 Flow patterns for the nitrogen-water (a) Slug flow Ug — 0.5 m/s, U = 0.1 m/s (b) system observed for the smooth mixer in the annular flow Ug — 5.5 m/s, U = 0.07 m/s (c) microchannel used attheTU/e. The images were ring flow Ug = 20m/s, U = 0.2m/s (d) churn recorded atthe indicated superficial gas (Ug) and flow Ug = 50m/s, U — 0.5 m/s (by courtesy of superficial liquid velocities (U ). The channel has Wiley-VCH Verlag GmbH) [279]. a rectangular cross section of 100 pm X 50 pm.

Vertical Ducts. Typical flow patterns in upward vertical two-phase flow in a tube are presented in Fig. 17.48a. At low vapor qualities and low mass flow rates, the flow usually obeys the bubbly flow pattern. At higher vapor qualities and mass flow rates, slug or plug flow replaces the bubbly flow pattern. Further increase in vapor quality and/or mass flow rates leads to the appearance of the churn, annular, and wispy annular flow patterns. 

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