[en] It is suggested that the one-time spatial correlation function of density fluctuations can be used for two-phase flow identification. The correlation function of density fluctuations is assumed locally isotropic for most flow regimes with a correlation distance which is small compared to the system dimensions. A new procedure, based on the cross-correlation of radiation attenuation signals, is proposed by which the local spatial correlation function can be measured at any point of a propagating two-phase medium. The method is also suitable for determining local vapour velocity and local average void fraction. (author)

[en] It is very important issues to monitor the phase distribution, void fraction and water level for effective design and analysis of system in various engineering fields such as chemical and nuclear industry. For these reasons various measurement techniques have been proposed, for example, radiation, micro wave, ultrasonic, and electrical impedance method. However, most techniques have limitations in terms of economic point of view and also these are difficult to apply to the case of the rapid change of phase distributions. The probe level meter which has been widely used for water level measurement tends to disturb the flow fields, in case of ultrasonic level meter water level measurement can be hard when numerous bubbles forms in the interface of two phases. This study considers electrical impedance method which does not disturb the flow field and is simple to design. In this work, a ring-type sensor was designed to measure the water level in the stratified regime. Also, measured water levels converted from the sensor signal were compared with those captured by high-speed camera

[en] The basic concept of two-group average bubble number density equations along with three-fluid model has been demonstrated for vertical gas-liquid flow. Specifically, the current study focused on: (i)classification of bubble interaction between spherical bubbles (Group-1) and cap bubbles (Group- 2), (ii) preliminary consideration of source and sink terms in the averaged bubble number density equations via the model of Hibiki and Ishii [1] and (iii) assessment by means of experimental data sets at bubbly-to-cap flow transition. Reasonable agreement was achieved between measured and predicted distributions of void fraction, interfacial area concentration (IAC) and volume equivalent bubble diameter. (author)

[en] This report describes a void-quality relationship for horizontal stratified two-phase flow. The present predictions compare reasonably well with the experiments and are an improvement over the Armand and Martinelli correlations

[en] Instability of flow around a body moving in a fluid can induce lift force acting on the body. One example of this phenomena is a bubble rising. Lift acting on a bubble effects on a void fraction distribution of a bubbly flow, which can be related to two phase flow in a nuclear reactor, bubble column reactor, and a flow around a ship. This leads researchers to use experimental or numerical methods. Study of Tomiyama et al.(2002) is the most well known experimental results on a bubble rising in a linear shear flow. They used water-glycerin mixture as a liquid and rotating belt to make linear shear flow, and measured lift coefficients. VOF simulations are conducted for investigation of lift acting on single bubble rising in a high Re linear shear flow. In spite of small amount of data and numerical error about spurious current near bubble interface, some insights can be obtained. First, a turbulence model generates large difference on C_L of large Re bubble. Second, for lift acting on a large Re bubble in a linear shear flow, Re is better scale than E_o_H and C_L is proportional to inverse of Re. Despite of some quantitative difference between results of experiment(Yang et al.(2013), Li et al.(2016)) and present numerical study, all results shows -7< C_L<0 for large Re bubbles

[en] In this study, a three-field pilot code is developed for a one-dimensional channel flow. Unlike the study in the reference, the collocated grid system is used instead of the staggered one. Using staggered grid typically results in stable and robust solutions, as shown in the reference. For general unstructured meshes, however, the staggered methods tend to become rather complex. To avoid this difficulty, an alternative approach is to use collocated pressure and velocity nodes. However, the collocated grid leads to indeterminate oscillation in the pressure field due to decoupling of odd node values from even node values. To provide linkage between adjacent pressure nodes, Rhie and Chow interpolation scheme imitating the staggered grid solution is used. Then, comparative simulations using the staggered grid and the collocated grid codes have been performed for several test cases

[en] The characteristics of two-phase flow in a vertical pipe are investigated to provide information for understanding the excitation mechanisms of flow-induced vibration. An analytical model for two-phase flow in a pipe was developed by Sim et al. (2005), based on a power law for the distributions of flow parameters across the pipe diameter, such as gas velocity, liquid velocity and void fraction. An experimental study was undertaken to verify the model. The unsteady momentum flux impinging on a 'turning tee' (or a 'circular plate') has been measured at the exit of the pipe, using a force sensor. From the measured data, especially for slug flow, the predominant frequency and the RMS value of the unsteady momentum flux have been evaluated. It is found that the analytical method, given by Sim et al. for slug flow, can be used to predict the momentum flux

[en] The main objective of the present study is to design a bubble collecting section for use in ventilated supercavitation experiments. The large amounts of air ventilated around a cavitator split into small bubbles that follow the water’s passage through the water tunnel. The presence of these bubbles in the test section of the water tunnel prevents effective observation of supercavitation. To enable the clear observation of cavitation shape, a bubble collecting section with large volume is necessary upstream of the test section to collect bubbles. The buoyancy of bubbles provides a simple means for their collection. However, the bubbly flows in such systems have rather high velocities and a non-uniform velocity distribution, which degrades the buoyancy effect. In the present study, a bubble collecting section with three porous plates that produce a uniformly low velocity distribution is designed and analyzed with numerical methods. The effectiveness of this approach is assessed experimentally with the 1/10 miniature model. The reduction of the void fraction downstream of the bubble collecting section was also assessed in the test section. The bubble collecting section in the full-scale water tunnel was also eventually found to be well designed through flow speed measurements and bubble visualization in the test section.

[en] Interfacial area concentration is one of important parameters in the constitutive relations relating to the interfacial transfer terms in the two-fluid model. In this paper, the measuring method for the interfacial area concentration by using a five-sensor conductivity probe is briefed. To generate IAC data, an air/water test loop is constructed, which has the 0.08m diameter of test section. The IAC transport phenomena are examined by measuring the IAC atvertical three positions: L/D=12.2, 42.2, 100.7. The test range is 0.47∼2.87m/s of superficial liquid velocity and 0.11∼1.58m/s of superficial gas velocity, which corresponds to the bubbly and slug flow in the flow regime map. In this study, the bubbles are categorized into two groups based on their size, and IAC and void fraction data of each group are generated respectively. Since the IAC data are classified into two groups, the data could be directly used to model the IAC by two group interfacial area transport equations

[en] INSCSP-R7 Standard Problem based on ENTEK BM Test Facility is investigated by RELAP5 code for prediction of averaged cross-section void fraction in vertical boiling channel of 7 m height. This standard problem also gives a challenge in application of CFD code such as ANSYS CFX to predict void fraction along the channel mentioned above due to: (a) only ten measured averaged cross-section void fraction given along the channel of 7 meters and (b) CFD simulation of boiling flow is mainly appropriate with sub cooled boiling. This study presents prediction of averaged cross-section void fraction along the channel of INSCSP-R7 Standard Problem using ANSYS CFX with calibration of parameter in boiling model based on experiment measured results. (author)