Flow churn

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. 

As shown in Fig. 5.2, churn flow (C) appeared at moderate gas superficial velocities and the entire range of liquid superficial velocities. How was extremely chaotic and the gas-liquid interface was rather irregular. The gas phase and liquid phase had no distinct shapes. 

As more air was added to the channel, the slug flow became unstable, the slug bubble broke down, and eventually the churn flow occurred in the channel. As shown in Fig. 5.3d, the most significant feature of flow characteristics in the churn flow is that the pressure oscillated at a relatively high amplitude, since the gas plug and liquid bridge flowed through the test section alternatively. 

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. 

Figure 3.4 Vertical upflow regime map (d = 2.5 cm), air-water at 25°C and 0.1 MPa. A, B, C, D, E. (From Taitel et al., 1980. Copyright 1980 by American Institute of Chemical Engineers, New York. Reprinted with permission.) A, D. D and D are the boundary between slug and churn flow. (From Mishima and Ishii, 1984. Copyright 1984 by Elsevier Sci. Ltd., Kidlington, UK. Reprinted with permission.)...

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... 

Fig. 4.44 Flow types in a vertical, unheated tube with upward flow, a bubble flow b plug flow c churn flow d wispy-annular flow e annular flow f spray or drop flow...

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. 

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