Two-phase Flow In Vertical Channel Research Articles

Simplified two-group two-fluid model for three-dimensional two-phase flow Computational Fluid Dynamics for vertical upward flow

Recent progress in nuclear thermal-hydraulics simulations has been largely focused on coupling with other computational packages, improved closure models for subcooled boiling and for bubbly flows, and the development of higher-fidelity simulation capabilities (Kulesza et al., 2016). While high-fidelity 3D simulation is important for model validation, scientific understanding, and some design calculations, it can be prohibitively expensive for system design applications or applications involving large geometries. Thus, there is also a need for practical, simplified approaches for those applications. The two-fluid model strikes a balance between detail and computational resources, but requires the accurate specification of several key constitutive models. These include (1) interfacial forces, (2) interfacial area concentration, (3) two-phase turbulence, and (4) wall and bulk boiling and condensation. In many modern CFD packages, uncertainties in the local interfacial area concentration can have strong effects on the ability to predict the other key parameters. This paper demonstrates that the drag force in 3D CFD can be formulated in much the same way as in 1D system analysis codes and that this approach can be used to formulate a model for interfacial area concentration. The method is also applied to two-group approaches to consider the difference in transport properties for different bubble size classes. This approach may open a method to calculate the interfacial forces without the need for interfacial area transport equations. This reduces the number of differential equations and avoids the modeling challenges associated with bubble breakup and coalescence kernels and the need to specify the inlet interfacial area concentration a priori. The new method is expected to decouple the effects of interfacial area uncertainty and calibrated coefficients, and should provide reasonable local bubble diameters for both group-1 and group-2 bubbles. The approaches proposed in this study are applicable to two-phase flow simulations in rather simple geometries such as upward two-phase flow in vertical channels. In view of many applications for upward two-phase flow in vertical channels, including nuclear reactor systems, the proposed methods are considered useful.

Subcritical and supercritical bifurcations for two-phase flow in a uniformly heated channel with different inclinations

In the past few decades, the stability of two-phase flow in vertical channels has been studied quite extensively. However, such studies for horizontal channels are relatively limited. Moreover, stability analysis of inclined channels is not available in the literature on two-phase flows. Therefore, linear stability analysis using a reduced order model for two-phase flow in inclined channels is carried out. In addition, bifurcation analysis is also carried out to capture the non-linear dynamics of the system and to identify the regions in parameter space for which subcritical and supercritical bifurcations exist. The study is carried out for different inclination angles in order to characterize the effect of inclination on the stability of the system. It is observed that the stability characteristics of two-phase flow in horizontal and inclined channels are significantly different from those of vertical channels even for the same operating conditions. Furthermore, from the non-linear analysis it is seen that the dynamics of the system for inclined and horizontal channels are quite complex. At an inclination of 45°, three generalized Hopf (GH) points (these points separate subcritical region from supercritical region) have been observed in the stability map. These GH points do not exist for vertical channels. Moreover, numerical simulations of the time-dependent, non-linear ODEs are carried out for the selected points in the operating parameter space to verify the stability behavior as predicted by stability maps.

D114 縮小路をもつ気液二相流の流れの可視化

This paper described about flow visualization of a gas-liquid two-phase flow in vertical channel with contraction. We have found that the pulsation for void fraction and pressure are occurred by the contraction. However, the mechanism of pulsation is not cleared. Therefore, we visualize the gas-liquid two-phase flow at contraction and the downstream region using high-speed camera system.

A104 縮流部を有する鉛直管路内の気液二相流の流れの可視化

This paper described about flow visualization of a gas-liquid two-phase flow in vertical channel with contraction. We have found that the pulsation for void fraction and pressure are occurred by the contraction. However, the mechanism of pulsation is not cleared. Therefore, we construct Dynamic PIV/IST (Infrared Shadow Technique) system using high-speed camera and laser systems to visualize a two-phase flow. We use the system and analyze the profiles of bubble segments and flow structure of liquid phase around bubble segments.

A model for turbulent polydisperse two-phase flow in vertical channels

A polydisperse two-phase flow model is developed and used to analyze the effect of the bubble size on the radial phase distribution in vertical upward channels. The two-fluid model is evaluated considering that the bubble size distribution can be represented with groups of constant mass. The model accounts for interfacial momentum transfer terms arising from drag, lift, turbulent dispersion and wall forces for the different bubble sizes. The turbulence is modeled with the k– ε model for bubbly flow. A two-phase wall logarithmic law is developed to evaluate the boundary conditions for the k– ε and the two-fluid models. The turbulence in the buffer and laminar near-wall regions is evaluated considering the asymptotic consistency of the k– ε model approaching the solid surface. The model is able to predict the transition from the near-wall gas volume fraction peaking to the core peaking beyond a critical bubble size. The double gas volume fraction peak experimentally observed when both, small and big bubbles, are present can be also simulated. The model was numerically solved for fully developed flow by means of a finite difference method and the results were compared against the experimental data measured by others in air/water vertical ducts.

Numerical simulation of lateral phase distribution in turbulent upward bubbly two-phase flows

New constitutive models for the interfacial forces acting on bubbles were developed for accurately predicting the lateral phase distribution in turbulent bubbly two-phase flow in vertical channels. Several experimental measurements have revealed that the lateral void profile in bubbly two-phase flow varies from the void peaking near the wall to the almost flat distributions as the liquid velocity increases. However, within the authors' knowledge, the effect of liquid velocity on the void profile has not been successfully predicted by the existing models; this would indicate the strong limitation of the existing multidimensional two-phase flow models. In view of these, the validity of the present constitutive models was tested in varied conditions of the liquid velocity as well as the bubble size. Since several assumptions were required in the models mainly due to the insufficient knowledge of the bubble motion, further improvements should still be needed. Nevertheless, the predicted lateral phase distributions were found to be in reasonably good agreement with available experimental data. It is hence expected that the present constitutive models can effectively be used in the practical applications and also be the base of the more sophisticated ones.

The hydrodynamics of countercurrent two-phase flow in vertical channels

Countercurrent flow of gas and liquid in vertical tubes is investigated. Processes that occur under various conditions of flow (including the “flooding” mode) are treated. Equations are suggested that describing the hydrodynamic characteristics of countercurrent two-phase flow.

Mechanism and Effects of Predominant Parameters Regarding Limitation of Falling Water in Vertical Countercurrent Two-Phase Flow

In this study, an investigation was carried out to clarify the mechanism of countercurrent flow limitation (CCFL) or flooding, that is, limitations in the falling water mass flux in countercurrent two-phase flow in vertical channels, and to identify the effects of predominant parameters regarding CCFL, adopting the criterion that the CCFL condition be given by an envelope of momentum equation applied for the entire length of the channel with respect to any void fraction. As a result, it was found that the analytical model proposed could adequately predict all existing experimental results investigated in this study. In the channel configuration, circular, rectangular, and annular or planar channels, channel dimensions of diameter, gap size, width or circumference, and length, interfacial and wall friction, water injection mode, and inlet water subcooling were dominant parameters. Therefore, both the mechanism and the quantitative effects of CCFL have been identified.

Study of Subcooling Effect of Injected Water on Falling Water Limitation in Countercurrent Two-Phase Flow in Vertical Channels.

This study analytically investigated the subcooling effect of injected water on falling water limitation in countercurrent two-phase flow (CCFL) in vertical channels, by applying a new model of momentum balances for both liquid and gas phases to the entire length of the channel. The subcooling effect of injected water on CCFL, which is one of the dominant parameters, had been clarified neither analytically nor experimentally because the CCFL phenomena is so complicated thermodynamically. As a result of the present study, it was clarified that the analytical model proposed here could give good predictions of the existing data on the subcooling effect of experiments simulating the performance of emergency core cooling water injection during a loss-of-coolant accident in pressurized light-water reactors.

Study of injected water subcooling effect on falling water limitation in countercurrent two-phase flow in vertical channels

This study analytically investigated the subcooling effect of injected water on the falling water limitation in countercurrent two-phase flow (CCFL) in vertical channels, by applying a new model of momentum balances for both liquid and gas phases over the entire length of the channel. The subcooling effect of injected water on CCFL, which is one of the dominant parameters, had been clarified neither analytically nor experimentally because the CCFL phenomena is very complicated thermodynamically. As a result of the present study, it has been clarified that the analytical model proposed here could give good predictions of the existing data on the subcooling effect of experiments simulating the performance of emergency core-cooling water injection during a loss-of-coolant accident in pressurized light-water reactors.

Critical Heat Flux of Comparatively Low-Velocity, Upward-Saturated Two-Phase Flow in Vertical Channels.

A quantitative analysis of critical heat flux (CHF) in both vertical circular tubes and rectangular channels under about 0.1 to 14 MPa was successfully carried out by application of the completely separated two-phase flow model to the comparatively low-velocity, upward-saturated two-phase flow, based on existing model of vapor blanket over the thin liquid sublayer on the heated walls at the CHF point, which was originally proposed by Haramura and Katto [Int. J. Heat Mass Transfer, 26-3 (1983), 389-399]. Over 1000 experimental CHF data were compared with analytical results and it was clarified that the analytical results gave good predictions of the existing experimental results of circular tubes of 3.0 to 7.8 mm in diameter and 0.15 to 0.6 m in length at 0 to 300 K and rectangular channels of 1.03 to 12.7 mm in channel gap and 0.15 to 0.94 min channel length at 4 to 328 K of inlet subcooling. The differences in characteristics of CHF between circular tubes and rectangular channels in upward-saturated two-phase How were also identified.

A finite difference computational model of annular film-flow boiling and two-phase flow in vertical channels with offset strip fins

This paper describes a detailed computational model of annular two-phase flow through offset strip fin heat exchanger geometries. In this model, a modified finite difference scheme is used to compute the velocity and pressure fields throughout the vapor core of the fin matrix. The vapor phase model is iteratively matched to a second finite difference model of the liquid film flowing along the channel walls. Three-dimensional interactions between the liquid film flowing along the prime and secondary surfaces of the matrix are also incorporated into the model. By solving the Navier-Stokes equation in the vapor phase, this model provides, for the first time, detailed local information about the shear stress and static pressure variation along the liquid film. This makes it possible to study the local variation of the heat transfer coefficient and wall shear stress all along the matrix. The model is validated against experimental data for two different offset fin geometries and then used to systematically study the effects of fin and channel geometry on two-phase transport. Special properties of two-phase flow through offset fin matrices discovered using the model are also described and the effects of these properties on the accuracy of previous annular flow models is discussed.