Numerical Simulation of Interfacial Phenomenon of Air-Water Adiabatic Intermittent Flow in Helically Coiled Tubes

2016 ◽  
Author(s):  
Guangyu Zhu ◽  
Hongye Zhu
Keyword(s):  
Heat Transfer ◽  
Centrifugal Force ◽  
Two Phase Flow ◽  
Intermittent Flow ◽  
Geometrical Parameters ◽  
Phase Flow ◽  
Two Phase ◽  
Coiled Tube ◽  
Helically Coiled Tube ◽  
Coil Diameter

Helically coiled tube are widely used as the basic heat transfer elements in steam generators of the next generation reactors, such as HTR-PM (High Temperature Gas-cooled Reactor), IRIS (International Reactor Innovative and Secure) and SMART (System-integrated Modular Advanced Reactor), because of the advantages in reducing space, enhancing heat transfer, accommodating thermal stress and preventing two-phase flow instabilities. Owing to the presence of gravity and centrifugal force that being perpendicular to the main flow, two-phase flow in helically coiled tubes has different features with either vertical flow or horizontal flow. To ensure safety and reliability of the plant, it is necessary to carry out detail investigation on the two-phase flow phenomena and mechanisms in helically coiled tubes. However, less research has been carried out on this subject than on straight tubes. In this work, the upward air-water slug and plug flows in helically coiled tubes have been numerically analyzed based on the computational fluid dynamics (CFD) techniques. Three dimension models of helically coiled tubes with inner diameter of 16 mm, coil diameter of 0.1 and 0.4m, pitch of 0.08 and 0.16m are constructed, for which the structural meshes are generated by software ANSYS ICEM. The gas-liquid interface is captured by the volume of fluid (VOF) approach adopting geo-reconstruction scheme for interface interpolation, which is solved by a pressure-based transient solver in the commercial CFD software ANSYS FLUENT 14.5. Bubble chord length, slug/plug frequency, bubble velocity and void fraction under different superficial velocities have been investigated. The numerical results meet well with the pictures recorded by a high speed camera. It is revealed that in slug regime, the bubbles mainly migrate towards the top and inner wall of the tube due to the combined action of gravity and centrifugal force, leading to a highly asymmetrical internal phase distributions. Meanwhile, the secondary flow in the cross section introduced by the centrifugal force enhances the turbulence and prevents small bubbles to coalescent into enlarged bubbles. Accordingly the intermittent flow regime in helically coiled tubes is narrower than that in straight horizontal tubes. Furthermore, the influences of geometrical parameters on phase distribution characteristics are predicted. The results show that the bubble length will increase along with the increase of the coil diameter or the pitch of the helically coiled tube. And the bubble frequency will increase with the decreasing of the tube coil diameter.

Numerical Simulation of Flow Regime Transition From Slug to Wavy Flow in Helically Coiled Tubes

2016 ◽  
Author(s):  
Zhaoxu Li ◽  
Hongye Zhu
Keyword(s):  
Heat Transfer ◽  
Numerical Simulation ◽  
Centrifugal Force ◽  
Slug Flow ◽  
Two Phase Flow ◽  
Flow Regimes ◽  
Phase Flow ◽  
Pressurized Water Reactor ◽  
Two Phase ◽  
Pressurized Water

Two-phase flow in helically coiled tubes is becoming the interest of many investigators because of its importance in various applications, such as nuclear engineering, chemical engineering, refrigerating engineering and power engineering. Compared with U-type tubes used in pressurized water reactor (PWR), helically coiled tubes have advantages in size, heat transfer capacity, thermal stress toleration and two-phase stability. Accordingly the helically coiled tubes have been utilized in the steam generators of the next general reactors, such as gas-cooled reactor, fast breeder reactor and integrated pressurized water reactor. In helically coiled tubes the characteristics of momentum and heat transfer are distinct from those in straight tubes due to the presence of centrifugal force, especially for two-phase flow. Meanwhile, the transitions of flow regime, which is the crucial knowledge for the designers to determine the heat transfer rates and flow resistance, are also significantly affected by the centrifugal force. In this study, two-phase flow regimes in helically coiled tubes are investigated. Computational fluid dynamics (CFD), using fractional volume of fluid (VOF) model, is carried out to simulate wavy and slug flow regimes in helically coiled tubes. The corresponding experiment is also conducted to visualize these flow regimes at different superficial flow velocities. Numerical simulation results actually reflect the influence of centrifugal force on the two-phase flow and show a good agreement with the photographs captured from the experiment. Based on the simulations at different superficial flow velocities, the boundary between the slug and wavy flow regimes is predicted, in addition, compared with that in inclined tubes. The comparison indicates that centrifugal force could induce the appearance of wavy flows in advance and prompt the transition from slug flow to wavy flow.

A Mechanistic Model for Predicting Heat and Mass Transfer in Vertical Two-Phase Flow

Volume 3 ◽  
2004 ◽  
Author(s):  
Siamack A. Shirazi ◽  
Ebrahin Al-Adsani ◽  
John R. Shadley ◽  
Edmund F. Rybicki
Keyword(s):  
Heat Transfer ◽  
Mass Transfer ◽  
Heat And Mass Transfer ◽  
Two Phase Flow ◽  
Mechanistic Model ◽  
Intermittent Flow ◽  
Phase Flow ◽  
Empirical Correlations ◽  
Transfer Coefficients ◽  
Two Phase

The mass transfer coefficient plays an important role in predicting corrosion rates. Using similarities between heat and mass transfer mechanisms, a mechanistic model is proposed to predict heat and mass transfer coefficients for two-phase flow in vertical pipes. The mechanistic model is evaluated by using water-air heat transfer experimental data obtained from the literature. The mechanistic model is also compared with commonly used empirical correlations. In comparison with available heat transfer correlations, the mechanistic model performs very well for vertical annular flow, bubbly flow and slug or intermittent flow that were considered. The mechanistic model is based on physics of two-phase flow and thus is expected to be more general than empirical correlations.

Forced Convective Boiling Heat Transfer and Pressure Drop Characteristics of Two-Phase Flow Inside a Small Horizontal Helically Coiled Tubing Once-Through Steam Generator

2003 ◽  
Author(s):  
Liang Zhao ◽  
Liejin Guo ◽  
Bofeng Bai ◽  
Yucheng Hou ◽  
Ximin Zhang
Keyword(s):  
Heat Transfer ◽  
Experimental Data ◽  
Heat Flux ◽  
Pressure Drop ◽  
Boiling Heat Transfer ◽  
Phase Flow ◽  
Two Phase ◽  
Convective Boiling ◽  
Coiled Tube ◽  
Helically Coiled Tube

The pressure drop and boiling heat transfer of steam water two-phase flow were studied in a small horizontal helically coiled tube once-through steam generator of 9-mm inside diameter with 292-mm coil diameter and 30-mm pitch. Experiments were performed at a range of qualities up to 0.95, a system pressure range of 0.5∼3.5MPa, a mass flux range of 236∼943kg/m2s and a heat flux range of 0∼900kW/m2. Based on the experimental results, a new two-phase frictional pressure drop correlation was developed on the basis of Chisholm’s B-coefficient method. In the present experimental range, boiling heat transfer was found to be dependent not only on mass flux but also on heat flux. This result implies that both the nucleation mechanism and the convection mechanism have the same importance to forced convective boiling heat transfer in small horizontal helically coiled tube over the full range of qualities (pre-critical heat flux qualities of 0.1∼0.9) which is contrary to situations in larger helically coiled tube where the convection mechanism dominates at qualities typically > 0.1. Traditional single parameter Lockhart-Martinelli type correlations failed to satisfactorily predict present experimental data and in this paper a new flow boiling heat transfer correlation was put forward to better predict the experimental data of the present study.

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