CFD and ERT investigations on two-phase flow regimes in vertical and horizontal tubes

In the present study, different two-phase flow regimes in horizontal and vertical tubes have been studied experimentally and theoretically. A 3-D Computational Fluid Dynamics (CFD) modeling was carried out in order to model gas–liquid two-phase flow using volume of fluid (VOF) model. An Electrical Resistance Tomography (ERT) system was used to visualize these flow regimes, which were produced by change in the gas to liquid flow rate ratio. The reconstructed images from the ERT measurement and corresponding captured photographs for different flow regimes have been compared with the CFD predictions and a good qualitative agreement was observed between them.     

Two-phase slug flow across small diameter tubes with the presence of vertical return bend

This study examines the two-phase slug flow across small diameter tubes with the presence of vertical U-type return bends. The translational velocity of the air slug across return bend usually peaks at an angle of π/2-3π/4. The increase of translational velocity is especially pronounced when flow enters at the lower tube with a smaller curvature ratio. An approximately twofold increase of the slug velocity is observed at a curvature ratio of 3. Dimensionless correlations for flowing upwards and downwards with a mean deviation of 18.7% and 24.5% are proposed that can describe the variation of translational velocity within the return bend.     

An empirical correlation for two-phase frictional performance in small diameter tubes

Experimental two-phase pressure drop data in small diameter tubes (D<10 mm) have been collected and updated from the literature which contain eight refrigerant and three air-water datasets. Comparisons between the data and the predictions indicate that the Chisholm correlation fails to predict the data. The Friedel correlation and Souza and Pimenta's correlation give fair predictions for the refrigerant data, but fail to predict the air-water data due to the surface tension effect. The homogeneous model shows a better predictive ability (a mean deviation of 34.7%) than the other empirical correlations. In this regard, an empirical correlation based on the homogeneous model was developed. By introducing the Bond number and Weber number to the modified correlation, the new correlation gives a mean deviation of 19.1% based on 1484 data points.     

Influence of Tube‘s Diameter on Boling Heat Transfer Performance in Small Diameter Tubes

This paper reports the experiments of evaporation study in 6 mm inner copper diameter tubes using HFC-134a, HCFC-22 and CFC-12 as working fluid. The results show that the evaporation heat transfer coefficient increases with the decreasing of inner diameter of tubes. A new concept of non-dimensional tube diameter U is proposed in this paper for correction of the influence of the tube diameter on the evaporation heat transfer coefficient. And further, a convenient empirical correction method is presented.     

Flow regime transition criteria for upward two-phase flow in vertical tubes

Traditional two-phase flow-regime criteria based on the gas and liquid superficial velocities may not be suitable to the analyses of rapid transient or entrance flows by the two-fluid model. Under these conditions, it is postulated that direct geometrical parameters such as the void fraction are conceptually simpler and therefore more reliable parameters to be used in flow-regime criteria than the traditional parameters. From this point of view, new flow-regime criteria for upward gas-liquid flow in vertical tubes have been developed considering the mechanisms of flow-regime transitions. These new criteria can be compared to existing criteria and experimental data under steady-state and fully developed flow conditions by using relative velocity correlations. The criteria showed reasonable agreements with the existing data for atmospheric air-water flows. Further comparisons with data for steam—water in round tubes and a rectangular channel at relatively high system pressures have been made. The results confirmed that the present flow-regime transition criteria could be applied over wide ranges of parameters as well as to boiling flow.     

Analytical study of two-phase flow pressure-drop oscillations in a vertical heated tube

This paper focuses on an analytical investigation of two-phase flow pressure-drop oscillations in a vertical heated tube with the two-fluid model employed in the computer code MINI-TRAC. Pressure-drop oscillation is an important phenomenon appearing in a two-phase flow channel with compressible volume existing in the upstream region. In a previous investigation, pressure-drop oscillations with superimposed density-wave oscillations were observed. In this work, numerical calculations have been carried out to predict the characteristics of pressure-drop oscillations and the range of instabilities. We also offer an application of the two-fluid model to the analysis of pressure-drop oscillations at low pressure in a heated channel with a small inner diameter. Good agreement between the simulations and experiments was obtained. © 1999 Scripta Technica, Heat Trans Jpn Res, 27(8): 597–608, 1998     

Flow pattern transition for vertical downward two-phase flow in capillary tubes. Inlet mixing effects

Experimental results of flow pattern for vertical downward two-phase flow in capillary tubes are reported and flow pattern regime maps are presented. In addition theoretically based transition criteria for the flow pattern are presented. The experimental results and theory seem to match each other fairly well. When results given in this paper are compared with empirical ones presented in literature stratified flow has not been reported in this study. Additional experiments were undertaken and inlet mixing effects have been found to be extremely important for the existence of fully developed flow.     

Two-phase flow pattern in small diameter tubes with the presence of horizontal return bend

This study provides a qualitatively visual observation of the two-phase flow patterns for air-water mixtures inside 6.9, 4.95, and 3 mm smooth diameter tubes with the presence of horizontal return bend. The influence of the return bend on the two-phase flow patterns are investigated. For D=6.9 mm and at a mass flux of 50 kg m⁻² s⁻¹ having a quality less than 0.1, no influence on the flow patterns is seen at a larger curvature ratio of 7.1. However, were the curvature ratio reduced to 3, the flow pattern in the recovery region is temporarily turned from stratified flow into annular flow. The temporary flow pattern transition phenomenon from stratified flow to annular flow is not so pronounced with the decrease of tube diameter. It is likely that this phenomenon is related to the influence of surface tension and the reduction of developing length of the swirl flow. Based on the present flow visualization, three flow pattern maps are proposed to describe the effect of return bend on the transition of two-phase flow pattern.     

Onset of flooding in a small diameter tube

The main concern of this work is to obtain and investigate the experimental data of the onset of flooding (OSF) for air and water in a small diameter tube. Flooding experiments are carried out in a tube having an inner diameter of 8.75 mm and the length of 1 m at inclination angles of 0, 30, 45, 60° from the vertical. The curves of the OSF are built and shown as a function of the dimensionless superficial velocity. The effects of the inclination angles of the small diameter tubes on the OSF are discussed. The present results are compared with existing limited data reported in literature.     

竖直管内降膜流动气液两相运动数值模拟

对较高流速下强迫竖直降膜进行了模拟.建立了描述竖直管内强迫降膜的物理模型和数学模型,采用RNG k-ε模型描述管内气体和液体的复杂湍流流动过程,采用VOF方法对气液相界面进行追踪.通过观察流动过程中的液膜前端速度矢量变化、整个流场中的压力分布变化过程,对降膜前端的"托举"现象进行了分析,对竖直管内液膜形成过程中的两相复杂运动进行了分析研究,指出了避免"托举"现象出现的条件.     

Two-phase wavy-annular flow in small tubes

Base liquid film thickness distribution, wave behavior, and pressure loss measurements have been obtained for 237 horizontal two-phase (air–water) flow conditions in 8.8 and 15.1 mm I.D. tubes in the wavy and annular regimes. The behaviors measured indicate the presence of a transitional wavy-annular regime at flow rates traditionally labeled as annular in these small diameter tubes. Data for a 26.3 mm I.D. tube do not show these same trends.     

主蒸汽管径对余热锅炉设计的影响

本文以某公司F级燃机余热锅炉为例，选三种主蒸汽管径计算管道压降与温降，探求对余热锅炉高压过热器和蒸发器换热面的影响，认为高压主蒸汽管道选型应与余热锅炉设计同步优化确定，并认为管内蒸汽流速在30～40m／s之间比较合适。     

Boiling heat transfer characteristics of R-113 in a vertical small diameter tube under natural circulation condition

Boiling heat transfer characteristics of freon R-113 are experimentally investigated in a vertical small diameter tube, D=1.45 mm and L=100 mm at a wide pressure range of 19-269 kPa under natural circulation condition. Except the entrance region of the test section, the flow regime is annular in view of the measured vapor flux. The pool boiling correlations of Stephan and Abdelsalam and McNelly equally well predict the experimental data within an error of ±20%. No enhancement of heat transfer coefficient is obtained although D/B is less than 1.5, which differs from the finding of Klimenko.     

Correlation for boiling heat transfer of R-134a in horizontal tubes including effect of tube diameter

A Chen-type correlation for flow boiling heat transfer of R-134a in horizontal tubes was modified taking into account the effect of tube diameter. The effect of tube diameter on flow boiling heat transfer coefficient was characterized by the Weber number in gas phase. Results showed that this correlation could be applied to a wide range of tube diameters (0.5–11-mm-ID). In addition, the dryout point and the heat transfer characteristics after the dryout point were also investigated based on the annular flow model. The proposed experimental expressions to predict both the dryout quality and the post-dryout heat transfer coefficient could also be applied to a wide range of tube diameter (0.5–11-mm-ID).     

Pressure drop studies on two-phase flow in a uniformly heated vertical tube at pressures up to the critical point

Measurements of two-phase flow pressure drop have been made during a phase-change heat transfer process with refrigerant (R-134a) as a working fluid for a wide range of pressures right up to the critical pressure. The experiments were conducted in a uniformly heated vertical tube of 12.7 mm internal diameter and 3 m length over a heat flux range of 35–80 kW/m², mass flux range of 1200–2000 kg/m² s, exit quality range of 0.19–0.81 and for reduced pressures ranging from 0.24 to 1 with a fixed inlet subcooling of 3 °C. The measurements were compared with the predictions from the homogeneous flow model, a separated flow model using correlations drawn from the literature for void fraction and frictional pressure drop, and finally, using a flow pattern-based predictive method accounting specifically for bubbly, slug and annular flow regimes. It was found that the best results were obtained with the flow pattern-based approach with a mean deviation of ±20% over the entire pressure range.     

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. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res. 25(1): 25–38, 1996     

Study of two-phase flow regime identification in horizontal tube bundles under vertical upward cross-flow condition using wavelet transform

A wavelet-transform based approach for flow regime identification in horizontal tube bundles under vertical upward cross-flow condition was presented. Tests on two-phase flow pattern of R134a were conducted under low mass velocity and flow boiling conditions over ranges of mass flux 4–25 kg/m²·s, vapor quality 0.02–0.90. Time series of differential pressure fluctuations were measured and analyzed with discrete wavelet transform. Different time-scale characteristics in bubbly flow, churn flow and annular flow were analyzed. The wavelet energy distributions over scales were found to be appropriate for flow regime identification. Based on the wavelet energy distribution over characteristic scales, a criterion of flow regime identification was proposed. The comparison with experiment results show that it is feasible to use the discrete wavelet transform as the tool of flow regime identification in horizontal tube bundles under vertical upward cross-flow condition. __________ Translated from Journal of Shanghai Jiaotong University, 2007, 41(3): 337–341, 346 [译自: 上海交通大学学报]     

Study of two-phase flow regime identification in horizontal tube bundles under vertical upward cross-flow condition using wavelet transform

A wavelet-transform based approach for flow regime identification in horizontal tube bundles under vertical upward cross-flow condition was presented. Tests on two-phase flow pattern of R134a were conducted under low mass velocity and flow boiling conditions over ranges of mass flux 4–25 kg/m²s, vapor quality 0.02–0.90. Time series of differential pressure fluctuations were measured and analyzed with discrete wavelet transform. Different time-scale characteristics in bubbly flow, churn flow and annular flow were analyzed. The wavelet energy distributions over scales were found to be appropriate for flow regime identification. Based on the wavelet energy distribution over characteristic scales, a criterion of flow regime identification was proposed. The comparison with experiment results show that it is feasible to use the discrete wavelet transform as the tool of flow regime identification in horizontal tube bundles under vertical upward cross-flow condition.     

Improved semi-empirical method for determination of heat transfer coefficient in flow boiling in conventional and small diameter tubes

In the paper presented have been considerations regarding a model of boiling heat transfer in conventional and small diameter channels. Modified has been a model postulated earlier by Mikielewicz [J. Mikielewicz, Semi-empirical method of determining the heat transfer coefficient for subcooled saturated boiling in a channel, Int. J. Heat Transfer 17 (1973) 1129–1134] to incorporate the surface tension effects into the two-phase flow multiplier as well as the empirical correction influencing the nucleate boiling heat transfer term. The changes to the model under scrutiny were both of qualitative and quantitative character as the entirely new data bank of experimental data points has been collected to devise a new form of a semi-empirical model applicable both to conventional and small diameter channels. The fluids examined were: R141b, R134a, R113, R12 and R11 and the total number of considered experimental points, corresponding to small diameters, amounted to over 260. The range of considered tube diameters varied from 1 mm to 3 mm. The use of the model is very simple on account of its analytical form, which is its fundamental advantage. It is of general character, and its coefficients do not depend on the type of fluid. Comparisons with experimental data show good agreement.