• Transactions of the Korean Society of Mechanical Engineers B
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• v.24 no.7
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• pp.924-929
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• 2000

This paper addresses a new technique of measuring the mean flow velocity over the cross sectional area of the pipe using sound field reconstruction. When fluid flows in the pipe and two plane waves propagate oppositely through the medium, the flow velocity causes the change of wave number of the plane waves. The wave number of the positive going plane wave decreases and that of negative going one increases in comparison to static medium in the pipe. Theoretical backgrounds of this method are introduced in detail and the measurement of mean flow velocity using the sound field reconstruction is not affected by velocity profile upstream of microphones.

• Journal of the Korean Society of Marine Environment & Safety
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• v.15 no.1
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• pp.63-69
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• 2009

Gas volumetric fractions and pressure loss are very important parameters in understanding and predicting gas-liquid two-phase flows. They are also essential to design large heat exchanging system in many industries, boiler and refrigerating systems mounted at ships. This paper therefore presents a theoretical method of predicting the pressure loss and gas volumetric fractions in gas-liquid two-phase flows for the whole range of pipe inclinations. The theoretical analysis is based on the two-fluid stratified flow model. It also provides the results of the comparisons between this theoretical analysis results and previous experimental results.

• Nuclear Engineering and Technology
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• v.44 no.8
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• pp.871-888
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• 2012

This paper aims at presenting the state of the art, the recent progress, and the perspective for the future, in the modelling of two-phase flow in the horizontal legs of a PWR. All phenomena relevant for safety analysis are listed first. The selection of the modelling approach for system codes is then discussed, including the number of fluids or fields, the space and time resolution, and the use of flow regime maps. The classical two-fluid six-equation one-pressure model as it is implemented in the CATHARE code is then presented and its properties are described. It is shown that the axial effects of gravity forces may be correctly taken into account even in the case of change of the cross section area or of the pipe orientation. It is also shown that it can predict both fluvial and torrential flow with a possible hydraulic jump. Since phase stratification plays a dominant role, the Kelvin-Helmholtz instability and the stability of bubbly flow regime are discussed. A transition criterion based on a stability analysis of shallow water waves may be used to predict the Kelvin-Helmholtz instability. Recent experimental data obtained in the METERO test facility are analysed to model the transition from a bubbly to stratified flow regime. Finally, perspectives for further improvement of the modelling are drawn including dynamic modelling of turbulence and interfacial area and multi-field models.

• Journal of Advanced Marine Engineering and Technology
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• v.8 no.1
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• pp.85-99
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• 1984

The stability of the two-phase flow in a heated channel is of great importance in the design and operation of the boilers and light water nuclear reactors, because it can cause flow oscillations and lead to a violation of thermal limits with resultant overheating of the channels and cladding. This paper presents a systematic evaluation to the variation effects of the basic four (4) dimensionless parameters in a homogeneous equilibrium model. The flow stability is examined on the ground of static characteristic curves. The complicated transfer function of flow dynamics which gives consideration to the transport lag of density wave is derived, and the transient flow stability is analysed by applying the Nyquist stability criterion in control engineering. The analysis results summed up as follows 1. The coolant flow becomes stable in large friction number and specific flow, while it is unstabale in small friction number and flow. 2. Large phase-change number and Froude number destabilize the two-phase flow, but small numbers stabilize it. The effect to variation of phase-change number is more dominant compared with Froude number. 3. The dynamic analysis is required to hold the sufficient safety of heated channels since only static results does not keep it. The special attention could be payed in the design and operation of heat engines, because the unstaable region exists within the stable boundary at small and middle phase-change number and Froude number.

• Transactions of the Korean Society of Mechanical Engineers
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• v.5 no.3
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• pp.159-169
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• 1981

The mixing length theory is extended to close the momentum queations for two-phase turbulent flow at a first-order closure level. It is assumed that the mass fraction of the particles is of the order of unity, that the particle size is so small that the particles are fully suspended is the primary fluid, and that the relaxation time scale of the particles is of the same order as the time scale of the energy containing eddies so that the suspended particles are responsive to the fluctuating turbulent field. The bulk motion of the particles is treated as a secondary fluid with its own coefficient of momentum transport. The proposed closure is uniformly destributed acress the pipe section. Predicted velocity profiles and the friction factors are in good agreement with avaiable experimental data.

• Nuclear Engineering and Technology
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• v.56 no.7
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• pp.2881-2892
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• 2024

Annular flow refers to a special type of two-phase flow pattern in which liquid flows as a thin film at the periphery of a pipe, tube, or conduit, and gas with relatively high velocity flows at the center of the flow section. This gas also includes dispersed liquid droplets. The liquid film flow rate continuously changes inside the tube due to two processes-entrainment and deposition. To determine the liquid holdup, pressure drop, the onset of dryout, and heat transfer characteristics in annular flow, it is important to have proper knowledge of flow characteristics. Especially a better understanding of entrainment fraction is important for the heat transfer and safe operation of two-phase flow systems operating in an annular two-phase flow regime. Therefore, the objective of this work is to develop a computational model for the simulation of the annular two-phase flow regime and assess the various existing models for the entrainment rate. In this work, Computational Fluid Dynamics (CFD) in ANSYS FLUENT has been applied to determine annular flow characteristics such as liquid film thickness, film velocity, entrainment rate, deposition rate, and entrainment fraction for various gas-liquid flow conditions in a vertical upward tube. The gas core with droplets was simulated using the Discrete Phase Model (DPM) which is based on the Eulerian-Lagrangian approach. The Eulerian Wall Film (EWF) model was utilized to simulate liquid film on the tube wall. Three different models of Entrainment rate were implemented and assessed through user-defined functions (UDF) in ANSYS. Finally, entrainment for fully developed flow was determined and compared with the experimental data available in the literature. From the simulations, it was obtained that the Bertodano correlation performed best in predicting entrainment fraction and the results were within the ±30 % limit when compared to experimental data.

• The KSFM Journal of Fluid Machinery
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• v.5 no.1 s.14
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• pp.7-12
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• 2002

As the first step to investigate the fundamental mechanism of a dispersed two-phase flow, we studied the detailed interactions between bubble or particle motion and flow around it. Experiments were carried out with a rising bubble or particle in stagnant water in a vertical pipe. Particles with different densities, and/or different shapes were used for comparison with a bubble. We adopted 3D-PTV (Three-Dimensional Particle Tracking Velocimetry) for measuring the bubble or particle motions, and PIV (Particle Image Velocimetry) for measuring the water flow simultaneously (Hybrid PIV). The experimental results showed that the oblate spheroidal solid particle rose along the longer axis direction at the point that the inclination of the longer axis reached the maximum, and the inclination direction changed after moving. The bubble moved to the direction that the spheroid's projected width grew up to the largest, and the minor axis of the oblate spheroidal body of the bubble was parallel to the moving direction. The trajectory of the center of the particle/bubble which was measured with 3D-PTV, was marked on the section (x-y) of the pipe. It exhibited the pattern of the particle/bubble motion.

• Nuclear Engineering and Technology
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• v.44 no.1
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• pp.71-78
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• 2012

In many semi-empirical analyses of flow boiling heat transfer, an annular flow is often assumed as a model flow and the local liquid film thickness is a key parameter in the analysis. This work considers a simple electrical conductance technique to estimate the local liquid film thickness in two-phase annular flows. In this approach, many electrodes are mounted flush with the inner wall of the pipe. Voltage differences between two neighboring electrodes for concentric annular flows with various liquid film thicknesses are obtained before the main experiments and logged in a look-up table. For an actual application in the annual flow, voltage differences of neighboring electrodes are measured and then corresponding local film thicknesses are determined by the interpolation of the look-up table. Even though the proposed technique is quite simple and straightforward, the numerical and static phantom experiments support its usefulness.

• Proceedings of the Korean Nuclear Society Conference
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• 2002.10a
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• pp.154.1-154
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• 2002

• Transactions of the Korean Society of Mechanical Engineers
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• v.14 no.1
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• pp.214-224
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• 1990

A "two-fluid" model using thermal eddy diffusivity concept and Lumley's drag reduction theory, is proposed to analyze heat transfer of the turbulent dilute gas-particle flow in a vertical pipe with constant wall heat flux. The thermal eddy diffusivity is derived to be a function of the ratio of the heat capacity-density products .rho. over bar $C_{p}$ of the gaseous phase and the particulate phase and also of the ratio of thermal relaxation time scale to that of turbulence. The Lumley's theory dictates the variation of the viscous sublayer thickness depending on the particle loading ratio Z and the relative particle size $d_{p}$/D. At low loading ratio, the size of viscous sublayer thickness is important for suspension heat transfer, while at higher loading, the effect of the ratio .rho. $_{p}$ over bar $C_{p}$$_{p}$/ .rho. $_{f}$ over bar $C_{p}$$_{f}$ is dominant. The major cause of decrease in the suspension Nusselt number at lower loading ratio is found to be due to the increase of the viscous sublayer thickness caused by the suppression of turbulence near the wall by the presence of solid particles. Predicted Nusselt numbers using the present model are in satisfactory agreements with available experimental data both in pipe entrance and the fully developed regions.