a b s t r a c tAs a part of characterizing the bubble interaction mechanisms and flow regime transition processes in horizontal gas-liquid two-phase flow, a flow visualization study is performed in an air-water test facility constructed from 3.81 cm inner diameter clear acrylic round pipes. The test section is approximately 250 diameters in length to allow for development of the flow. Flow visualizations are performed using a high-speed video camera at 80 and 245 diameters downstream of the inlet to observe the development of the flow structures. A total of 27 flow conditions including bubbly, plug, slug, stratified, wavy, and annular flows are characterized in the present study.In highly turbulent bubbly flow conditions it is found that the distribution becomes more uniform with increasing development length through a turbulence penetration process that counters the effect of buoyancy. It is also found that plug bubbles form below a layer of small bubbles rather than at the upper pipe wall where the bubbles are most packed. In fact, it is consistently found that turbulence-based bubble interactions do not occur in the most densely packed regions as the eddies there are not large enough to effect the bubbles. Rather, bubble packing-induced coalescence occurs in these regions and contributes to the formation of plug bubbles. The newly formed plug bubbles move faster than, and ultimately pass, the smaller bubbles above due to the effect of the wall. These small bubbles are subsequently overtaken by the following plug bubble and coalesce with the nose region through a process denoted as drag-induced coalescence.Ó 2015 Elsevier Ltd. All rights reserved.
IntroductionIn two-phase flows, the transfers of mass, momentum, and energy between the phases strongly depend on the configuration of the interfacial structure. Thus, in order to provide closure to the two-fluid model (Ishii and Hibiki, 2006), an accurate description of the interfacial structure is required. For the current nuclear reactor system analysis codes, a flow regime based approach is employed to predict two-phase flow conditions (TRACE Theory Manual, 2007;RELAP5-3D Code Manual, 2009). Such an approach is taken because the closure relations for various parameters (drag coefficients, heat transfer coefficients, etc.) have conventionally been developed based on the flow regime. Therefore, to determine which closure relations should be utilized during a calculation, flow regime maps and static transition criteria are first employed to identify the flow regime. Then, the appropriate closure relations are selected. Many flow regime maps have been developed over the years, including the more common maps for horizontal two-phase flow of Mandhane et al. (1974) and Taitel and Dukler (1976). However, the use of steady-state flow regime based relations imposes significant deficiencies in dynamically modeling the interfacial structures. Mortensen (1995) and Kelly (1997) identified shortcomings related to the flow regime based approach, which can be summarized as:(1) Si...
