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...
The current work seeks to develop an additional database in air-water horizontal bubbly flow through a 3.81 cm inner diameter test section with a total development length of approximately 250 diameters. The experimental conditions are chosen to cover a wide range of the bubbly flow regime based upon flow visualization using a high-speed video camera. A database of local time-averaged void fraction, bubble velocity, interfacial area concentration, and bubble Sauter mean diameter are acquired throughout the entire pipe cross-section using a foursensor conductivity probe for nine flow conditions. To investigate the evolution of the flow, measurements are made at axial locations of 44, 116, and 244 diameters downstream of the inlet.Using this database, the effects of gas flow rate, liquid flow rate, and development length on the local and area-averaged two-phase flow parameters are presented. From the local profiles, it is found that highly turbulent liquid flow conditions cause the void fraction profile to become more uniform with increasing development length through a turbulent mixing process which counters the effect of buoyancy. Bubble interactions are also observed to play a significant role in the evolution of the flow such that the area-averaged void fraction and interfacial area concentration can have different axial trends.
The objective of this study is to advance the local multi-sensor conductivity probe measurement technique through systematic investigation into several practical aspects of a conductivity probe measurement system. Firstly, signal "ghosting" among probe sensors is found to cause artificially high bubble velocity measurements and low interfacial area concentration (a i) measurements that depend on sampling frequency and sensor impedance. A revised electrical circuit is suggested to eliminate this artificial variability. Secondly, the sensitivity of the probe measurements to sampling frequency is investigated in 13 two-phase flow conditions with superficial liquid and gas velocities ranging from 1.00-5.00m/s and 0.17-2.0m/s, respectively. With increasing gas flow rate, higher sampling frequencies, greater than 100kHz in some cases, are required to adequately capture the bubble number frequency and a i measurements. This trend is due to the increase in gas velocity and the transition to the slug flow regime. Thirdly, the sensitivity of the probe measurements to the measurement duration as well as the sample number is investigated for the same flow conditions. Measurements of both group-I (spherical/distorted) and group-II (cap/slug/churn-turbulent) bubbles are found to be relatively insensitive to both the measurement duration and the number of bubbles, as long as the measurements are made for a duration long enough to capture a collection of samples characteristic to a given two-phase flow system (or a statistical ensemble). Fourthly, investigation into the orientation of a double-sensor probe in the pipe indicates that the sensors should be oriented parallel to the pipe wall to ensure symmetric bubble velocity measurements. Lastly, Monte Carlo simulations are performed to study the effects of the axial (s) and lateral (d) probe sensor separation distances. In addition to
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