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.     

水平管降膜蒸发器管外液体流动研究及膜厚的模拟计算

针对应用于空调和制冷系统的水平管降膜式蒸发器,建立了FLUENT数值模拟计算的物理模型。以制冷剂R134a为研究对象,对不同流量、不同布液器开孔孔径、不同管束结构下管外制冷剂液体的流动情况进行了模拟计算;并实现了绕管周方向不同角度液膜厚度的读取。     

The effect of inlet subcooling on two‐phase flow instabilities in a horizontal pipe system with augmented surfaces

This research has been conducted to investigate the effect of inlet subcooling on two‐phase flow instabilities in a horizontal pipe system with augmented surfaces. Five different inlet temperatures are used to study the effect of inlet subcooling for five different heat transfer surface configurations. All experiments are carried out at constant heat input, system pressure and exit restriction. The effect of inlet subcooling on the steady‐state characteristics and two‐phase flow instabilities are studied for each configuration. The bound aries for the appearance of pressure‐drop‐type and density‐wave‐type instabilities are found and the effect of the inlet subcooling on these oscillations is studied for each configuration. Copyright © 2002 John Wiley & Sons, Ltd.     

Improving two-phase mass transportation under Non-Darcy flow effect in orientated-type flow channels of proton exchange membrane fuel cells

Orientated-type flow channels of proton exchange membrane fuel cells cause non-Darcy effect occurring in flow regions. Therefore, the species transportation is affected by inertial effect. However, how the inertial force affects convection and diffusion of different species has not been discussed before. Thus, a modified two-dimensional, non-isothermal, two-phase and steady state model considering non-Darcy effect is employed in this study, and reactants and products transportations through diffusion and convection under inertial effects are quantitatively analyzed for the first time. Simulation results reveal that the convective transportation of reactants increases more under the influence of inertial force; water vapor transportation through convection increases the water content in porous regions. At the same time, liquid water expels more rapidly from gas diffusion layers under baffle regions, and enlarging baffle volumes increases the regions where the liquid water is rapidly removed under the inertial effect.     

Mathematical model for coal conversion in supercritical water: reacting multiphase flow with conjugate heat transfer

Providing heat for supercritical water gasification (SCWG) of coal by coupling subsequent products oxidation in integrated supercritical water reactor (ISWR) provides an effective method for directional control of temperature field and avoids excessive hot spots caused by uniform heating. An exploratory numerical model incorporating particle-fluid flow dynamics, multispecies transport and thermal coupling between endothermic coal gasification and exothermic product oxidation was established to simulate the reacting multiphase flow process of coal conversion in a novel lab-scale ISWR. An eleven-lump kinetic model was proposed for the prediction of chemical reactions. And the thermal coupling relationship was described by conjugate heat transfer boundary conditions (BC). Detailed physical and chemical field distribution in ISWR were analyzed and influence factors were discussed. The results showed that oxidation of gas products as inner heat source could promote the gasification reaction with only slight or even little maximum temperature increase of the pressure-bearing wall. Coal feeding rate and oxygen supply method significantly affected the field distribution. The multi-injection compressed-air supply method provided a more uniform temperature field but would reduce heat transfer temperature difference. The carbon gasification efficiency (CGE) in the gasification zone could easily reach up to 97% under mild conditions (less than 650 °C).     

Heat transfer of air-water dispersed flow in a vertical pipe

Heat transfer of air-water dispersed flow in a vertical heating pipe and its enhancement have been studied. The axial and circumferential wall temperature distributions were measured using various mist ratios and wall heat fluxes. The measured wall temperature increased sharply at a particular streamwise location, with a notable variation in the circumferential profile. This sharp increase was conceivably caused by a breakdown of the water film rather than by its dryout. A separate unheated experiment was carried out to estimate the droplet deposition velocity and the water-film flow rate. A numerical analysis, taking into account heat and mass transfer from the water film to the bulk flow, was performed in order to estimate the mean wall temperature. Good agreement was obtained with the experimental results in the area where the entire inner surface of the pipe was covered with the water film. In this area, the rate of heat transfer was approximately seven times larger than that for single phase air flow. This enhancement was shown to be due mainly to evaporation of the water film. The mechanism of heat transfer enhancement is discussed in detail using the numerical analysis results. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(4): 255–270, 1998     

Effect of pressure on entrainment flow rate in vertical upwards gas—liquid annular two-phase flow. Part I: Experimental results for system pressures from 0.3 MPa to 20 MPa

The purpose of this study is to investigate the pressure effects on the entrainment flow rates in vertical gas—liquid annular two-phase flow. The cross-sectional entrainment flow rates were measured using an isokinetic probe method. It was found that the behavior of cross-sectional entrainment flow rate profiles is divided into low- and high-pressure regions. Also, the entrainment flow rates amount to 90 percent of the total liquid flow rate under high-pressure conditions. In this study, system pressure in the closed-loop system was changed substantially from 0.3 MPa to 20 MPa at a constant fluid temperature in vertical upward flow. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res, 25(5): 267–280, 1996     

Effect of channel length on critical heat flux under conditions of high subcooling and high velocity in short heated channels

The characteristics of critical heat flux (CHF) in existing experiments under high subcooling and high velocity in short heated channels have, for the first time, been systematically and quantitatively investigated to provide a CHF correlation that can properly predict the effect of channel length, especially when the channel length-to-channel diameter ratio L/D is less than about 20. The major test conditions of existing CHF experiments investigated in this study were channel diameter 1 to 4 mm, L/D 1 to 25, 0.1 to 1.2 MPa pressure, 34 to 117°C inlet water subcooling and 500 to 40 700 kg/(m² · s) mass flux in circular channels, and 3 to 20 mm gap size, 6 to 40 L/D_e, 0.1 to 3.1 MPa pressure, 4 to 166°C inlet water subcooling, and 940 to 27,000 kg/(m² · s) mass flux in rectangular channels. The effect of L/D on CHF was evaluated referring to the analytical solution of CHF, which was previously derived by the author for the channel flow at high subcooling and high velocity. As a result, the effect of L/D was quantitatively clarified as an effect of magnitude in heat transfer of single-phase forced-convection flow, giving a larger CHF with a smaller L/D in the case of L/D less than about 20. The proposed correlation predicts CHF to within a ±35 percent error margin. ©1998 Scripta Technica, Heat Trans Jpn Res, 27(7): 509–521, 1998     

Investigation of two‐phase flow mixing between two subchannels

The cross flow between subchannels in a BWR fuel assembly has been typically analyzed using three types of mixing models, namely, pressure difference, turbulent mixing, and void drift which are expressed by time‐averaged flow parameters. However, in our previous paper, we expressed the above cross flow phenomenon simply by a fluctuating pressure model and confirmed its validity experimentally. In this present study, we examine the relationship between the fluctuating pressure difference and the cross flow rate more precisely by using a short mixing zone with no steady pressure difference. Results show that the experimental cross flow data agree well with the calculations using this model. Furthermore, we tried to express the fluctuating pressure difference by using a sinusoidal wave as a new cross flow model. This model is shown to have no dependence on frequency. We verify that the cross flow can be analyzed using only the pressure difference amplitude. © 2000 Scripta Technica, Heat Trans Asian Res, 29(5): 412–426, 2000     

Three-dimensional coupling numerical simulation of two-phase flow and electrochemical phenomena in alkaline water electrolysis

Alkaline water electrolysis has the advantage of scalability for industrial-scale mass production of hydrogen; however, it is operated under a lower current density than other methods of water electrolysis because a high overpotential resulting from ion transport limitations will occur at high current density. Bubble dynamics can both prevent ion transport by its existence and accelerate it by bubble-induced flow. In this study, we conduct three-dimensional coupling numerical simulations of two-phase flow and electrochemical phenomena to elucidate the mechanisms by which microscale bubble dynamics influence ion transport and the cell overpotential. We find that the flow induced by rising microbubbles enhances ion transport to the anode and suppresses the cell overpotential. Moreover, bubble atomization further suppresses the overpotential because smaller bubbles approach the anode more closely than larger ones and accelerate ion transport to the anode surface.     

Forchheimer's inertial effect on liquid water removal in proton exchange membrane fuel cells with baffled flow channels

In this study, a two-dimensional, two-phase, non-isothermal and steady-state modified model of proton exchange membrane fuel cells is developed. The Forchheimer's effect (Non-Darcy effect) is coupled in the model, and its impact on liquid water removing process in flow channels with baffles having different shapes is discussed. Simulation results show that the liquid water is able to be removed more at the regions around baffles. At the same time, the baffle shapes reform the liquid water distribution. When using the baffles having larger dimensions (e.g. using rectangular baffles or trapezoidal baffles), the flow spaces around baffles decrease more and the liquid water is removed more because of the increase in local flow velocity. As a result, the concentration polarization is weakened and cell performance is improved more. Moreover, a streamline baffled flow channel that is designed for the purpose of both increasing the cell performance and avoiding excessive increase in pressure drops is discussed. Simulation results show that this flow channel design can both avoid too much increase in pressure drop and facilitate the liquid water removing out from the fuel cell.     

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]     

模拟贯穿裂纹内高速气液两相流动特性研究

压力管道和容器发生贯穿泄漏会引发严重的事故,合理估算贯穿泄漏量具有重要的工程意义.以矩形狭缝通道模拟贯穿裂纹,开展了高压氩气-水贯穿模拟裂纹的高速流动可视化试验研究,狭缝长度为20 mm,间隙宽度为80～180 μm.狭缝进口压力大于5 MPa,液体的表观速度为0.05～58.62 m/s,气体表观速度为1.71～34...     

Visualization on flow patterns during condensation of R410A in a vertical rectangular channel

The visualization experiments on HFC R410A condensation in a vertical rectangular channel （14.34mm hydraulic diameter, 160mm length） were investigated. The flow patterns and heat transfer coefficients of condensation in the inlet region were presented in this paper. Better heat transfer performance can be obtained in the inlet region, and flow regime transition in other regions of the channel was also observed. Condensation experiments were carried out at different mass fluxes （ from 1.6 kg/h to 5.2 kg/h） and at saturation temperature 28~ C. It was found that the flow patterns were mainly dominated by gravity at low mass fluxes. The effects of interfacial shear stress on condensate fluctuation are significant for the film condensation at higher mass flux in vertical flow, and con- sequently, the condensation heat transfer coefficient increases with the mass flux in the experimental conditions, The drop formation and growth process of condensation were also observed at considerably low refrigerant vapor flow rate.     

竖直矩形窄通道内流动沸腾局部换热特性实验研究

为了明确竖直矩形窄通道内各阶段流动沸腾的换热特性,优化换热器性能,以去离子水为工质,对尺寸为720 mm×250 mm×3.5 mm的单面电加热竖直矩形窄通道内的流动沸腾换热进行实验研究,分析了质流密度、进口温度、热流密度对流动沸腾局部换热特性的影响。并在已有流动沸腾传热关联式的基础上,对实验数据进行非线性回归分析,得到适用于实验工况下的新流动沸腾传热关联式。结果表明：质流密度增大对流动沸腾段换热特性有强化作用,对核态沸腾段换热特性有削弱作用;热流密度对核态沸腾影响剧烈,但对流动沸腾的影响不明显;入口温度越高,流体会越早进入过冷沸腾阶段,但对局部传热系数的影响不明显;新流动沸腾传热关联式与实验值的平均相对误差为23.87%,其中74.19%的预测值在±25%内,83.87%的预测值在±50%以内,能很好地预测本实验工况下矩形窄通道内流动沸腾的局部传热系数。     

Study on Characteristics of Steady Flow Condensation Heat Transfer in a Fube Under Zero—Gravitation

ＳｔｕｄｙｏｎＣｈａｒａｃｔｅｒｉｓｔｉｃｓｏｆＳｔｅａｄｙＦｌｏｗＣｏｎｄｅｎｓａｔｉｏｎＨｅａｔＴｒａｎｓｆｅｒｉｎａＴｕｂｅｕｎｄｅｒＺｅｒｏ－ＧｒａｖｉｔａｔｉｏｎＱｎＷｅｉ（ＨａｒｂｉｎＩｎｓｔｉｔｕｔｅｏｆＴｅｃｈｎｏｌｏｇｙ，Ｈａｒｂｉ...     

The flow characteristics of an upward gas‐liquid two‐phase flow in a vertical tube with a wire coil: Part 2. Effect on void fraction and liquid film thickness

An experimental study has been carried out to clarify the characteristics of the void fraction and the liquid film thickness of the air‐water two‐phase flow in vertical tubes of 25‐mm inside diameter with wire coils of varying wire diameter and pitch. The flow pattern in the experiment on the average void fraction and the local void fraction distribution in cross section was a bubble flow, and the liquid film thickness was in the region of semiannular and annular flows. It is clarified from these experiments that the average void fraction in tubes with wire coils is lower than that in a smooth tube and decreases with the wire diameter owing to the centrifugal force of the swirl flow which concentrates bubbles at the center of the tube, that the local liquid film thickness becomes more uniform with a decrease in the pitch of the wire coil, and that the liquid film becomes thicker after the passage through the wire coil with an increase in the wire diameter. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(8): 652–664, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10067     

The flow characteristics of an upward gas‐liquid two‐phase flow in a vertical tube with a wire coil: Part 1. Experimental results of flow pattern,void fraction,and pressure drop

Experiments were carried out to investigate the flow pattern, average void fraction, and pressure drop of an upward air‐water two‐phase flow in vertical tubes of 25‐mm inside diameter with wire coils of varying wire diameter, pitch, and number of coils in cross section. Five kinds of flow patterns—bubble, slug, churn, semiannular, and annular flow—were defined based on the observation of flow behavior in the experiments. At higher water flowrates, the bubble‐to‐slug transition occurred at lower air flowrates in tubes with wire coils than in smooth tubes. The average void fraction was found by using the drift flux model. Further, the experimental results of the friction pressure drop were compared with the Lockhart‐Martinelli correlation. As a result, a correlation with the constant C in Chisholm's equation was obtained as a function of the wire coil pitch‐to‐diameter ratio. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(8): 639–651, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10066     

Study of novel flow channels influence on the performance of direct methanol fuel cell

The existing flow channels like parallel and gird channels have been modified for better fuel distribution in order to boost the performance of direct methanol fuel cell. The main objective of the work is to achieve minimized pressure drop in the flow channel, uniform distribution of methanol, reduced water accumulation, and better oxygen supply. A 3D mathematical model with serpentine channel is simulated for the cell temperature of 80 °C, 0.5 M methanol concentration. The study resulted in 40 mW/cm² of power density and 190 mA/cm² of current density at the operating voltage of 0.25 V. Further, the numerical study is carried out for modified flow channels to discuss their merits and demerits on anode and cathode side. The anode serpentine channel is unmatched by the modified zigzag and pin channels by ensuring the better methanol distribution under the ribs and increased the fuel consumption. But the cathode serpentine channel is lacking in water management. The modified channels at anode offered reduced pressure drop, still uniform reactant distribution is found impossible. The modified channels at cathode outperform the serpentine channel by reducing the effect of water accumulation, and uniform oxygen supply. So the serpentine channel is retained for methanol supply, and modified channel is chosen for cathode reactant supply. In comparison to cell with only serpentine channel, the serpentine anode channel combined with cathode zigzag and pin channel enhanced power density by 17.8% and 10.2% respectively. The results revealed that the zigzag and pin channel are very effective in mitigating water accumulation and ensuring better oxygen supply at the cathode.     

Three‐dimensional simulation of water droplet movement in PEM fuel cell flow channels with hydrophilic surfaces

Water management is one of the critical issues in proton exchange membrane fuel cells, and proper water management requires effective removal of liquid water generated in the cathode catalyst layer, typically in the form of droplets through cathode gas stream in the cathode flow channel. It has been reported that a hydrophilic channel sidewall with a hydrophobic membrane electrode assembly (MEA) surface would have less chance for water accumulation on the MEA surface. Therefore, a comprehensive study on the effect of surface wettability properties on water droplet movement in flow channels has been conducted numerically. In this study, the water droplet movements in a straight flow channel with a wide range of hydrophilic surface properties and effects of inlet air velocities are analyzed by using three‐dimensional computational fluid dynamics method coupled with the volume‐of‐fluid (VOF) method for liquid–gas interface tracking. The results show that the water droplet movement is greatly affected by the channel surface wettability and air flow conditions. With low contact angle, droplet motion is slow due to more liquid–wall contact area. With high air flow velocities, increasing the contact angle of the channel surface results in faster liquid water removal due to lesser liquid–wall contact area. Copyright © 2010 John Wiley & Sons, Ltd.