共查询到17条相似文献,搜索用时 187 毫秒
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摇摆条件下附加惯性力的作用会对两相流动的压降及汽泡受力产生影响。考虑相变能量和质量输运,采用流体体积(VOF)多相流模型对附加惯性力条件下竖直矩形流道内过冷流动沸腾进行了数值模拟。汽液界面位置通过分段线性插值(PLIC)的方法获得。模拟结果获得了孤立汽泡周围压力、速度、温度分布以及二次流动现象,分析了汽泡聚合过程汽泡形态及内部速度矢量的演变过程,模拟结果与文献中结论吻合良好。附加惯性力作用使得流动压降比静止条件下要大,过冷流动沸腾压降由于汽相产生会在单相流动的基础上产生波动,且热通量越大,压降波动幅度越大。摇摆产生的附加惯性力相对汽泡所受的其他力而言可以忽略不计,而摇摆导致的流量波动会改变汽泡受力大小,进而影响沸腾换热。 相似文献
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过冷流动沸腾在核能、动力、化工等工业领域广泛存在,局部空泡分布特性的准确预测对构建两相流数理模型及两相流动压降和传热特性计算均具有重要意义。实验借助光学探针测量手段研究摇摆条件下圆管内过冷沸腾局部空泡分布特性,并提出多周期同相位叠加方法获得摇摆运动条件下局部瞬时空泡份额。实验结果表明:摇摆条件下圆管内局部空泡分布呈现周期性波动规律,摇摆条件下圆管轴心区及近壁区均会出现局部空泡份额峰值,且流道近壁区局部空泡份额波动幅度最大;与静止条件下不同,摇摆运动周期内过冷沸腾局部空泡份额空间分布形态随时间发生变化;摇摆周期和摇摆角度对过冷沸腾局部空泡分布特性也存在一定影响。 相似文献
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采用双流体模型对高压高过冷度下垂直圆管中水的过冷流动沸腾进行了数值模拟。通过对比不同气泡直径模型,揭示了气泡直径对于壁面传热方式的影响,确定了适合高压工况的气泡直径模型。考察了压力及壁面热通量对流动及传热特性的影响。计算结果表明,压力增加气泡脱离直径减小,单相对流传热所占比例增加,表面传热系数减小。高压高过冷度特征决定了气泡相分布极不均匀,随着热通量的增加,壁面附近容易形成气泡的密集,对过冷流动沸腾中的传热特性有重要影响。 相似文献
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分析了液氮流动沸腾过程中气液两相间动量、能量以及质量的传输规律,建立了相应的理论模型,新模型重点修正了界面面积浓度和气泡挣脱直径的计算式;采用新建立的理论模型作为封闭方程对CFX-4.3中内建的双流体模型进行了修正,并采用修正后的双流体模型模拟了液氮在垂直圆管内的流动沸腾过程.数值模拟的结果与文献中的实验数据吻合较好,证明了本文所建模型的合理性.通过数值模拟发现,两相流参数分布的不均匀性对液氮流动沸腾过程中的热质传输特性有重要影响. 相似文献
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Multiple size group (MUSIG) model combined with a threedimensional twofluid model were em ployed to predict subcooled boiling flow of liquid nitrogen in a vertical upward tube. Based on the mechanism of boiling heat transfer, some important bubble model parameters were amended to be applicable to the modeling of liquid nitrogen. The distribution of different discrete bubble classes was demonstrated numerically and the distribu tion patterns of void fraction in the wallheated tube were analyzed. It was found that the average void fraction in creases nonlinearly along the axial direction with wall heat flux and it decreases with inlet mass flow rate and sub cooled temperature. The local void fraction exhibited a Ushape distribution in the radial direction. The partition of the wall heat flux along the tube was obtained. The results showed that heat flux consumed on evaporation is the leading part of surface heat transfer at the rear region of subcooled boiling. The turning point in the pressure drop curve reflects the instability of bubbly flow. Good agreement was achieved on the local heat transfer coefficient aalnst experimental measurements, which demonstrated the accuracy of the numerical model. 相似文献
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The two‐phase flow structure of an air‐water, bubbly, upward flow in a 20 cm diameter pipe is presented with particular emphasis on the local interfacial area concentration. The radial distribution of void fraction, bubble velocity, bubble size, bubble frequency, and interfacial area concentration were measured using a local dual‐optical probe. The experimental results showed that the saddle‐type distribution of void fraction and interfacial area concentration, which are common for bubbly flow in small diameter pipes, only appeared in the present experiments under conditions of very low area‐averaged void fraction (<?> < 0.04). The values for the interfacial area concentration were higher in large diameter pipes when compared with data obtained under the same flow conditions in small pipes. The area‐averaged void fraction data were correlated using the drift‐flux model. 相似文献
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A semi-empirical gas entrainment model was proposed for the ventilated cavity in vertical pipe, based on which, a complete numerical scheme was established by coupling with the Eulerian-Eulerian two-fluid model to predict the multiscale flow field created by ventilated cavity. Model predictions were validated against experimental measurements on void fraction and bubble size distributions. Simulations were carried out to explore the effect of ventilation rate and inlet turbulence intensity on the macroscale cavity shape and the bubbly flow downstream of the ventilated cavity. As the ventilation rate increasing, a reverse trend was observed for the void fraction and bub- ble size distributions. It is concluded that the average void fraction in the pipe flow region is determined by the volumetric ratio between liquid and gas. However, the bubble size evolution is dominated by the breakage effect induced by turbulence in the vortex region. Furthermore, simulations were conducted to analyze geometric scale effect based upon Froude similitude. The results imply that the velocity distributions were properly scaled. Slight scale effect was seen for the void fraction caused by faster dispersion of bubbles in the larger size model. The com- paratively greater bubble size was predicted in the smaller model, implying significant scale effects in terms of tur- bulence and surface tension effect. It reveals that empirical correlations valid in wide range are required for the ex- trapolation from small-size laboratory models. 相似文献
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Vertical distributions of local void fraction, bubble size and gas–liquid interfacial area in air–water dispersions at 24 and 81 °C have been measured with a dual electric conductivity probe in a fully baffled dished base stirred vessel of 0.48 m diameter holding 0.145 m3 liquid. The agitator was a hollow blade dispersing turbine below two up-pumping hydrofoils. The vertical distribution of the void fraction in the hot conditions is similar to that at ambient temperature though the void fraction is significantly lower in the hot system. The vertical distributions of bubble size show maxima with large bubbles above the bottom impeller, near the top impeller and close to the free surface. With given operating conditions, the overall Sauter means bubble size in the hot systems appears to be about 21% greater than when cold. Estimates of the local interfacial area show a maximum just above the level of the top impeller. 相似文献
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Two population balance approaches based on the MUltiple-SIze-Group (MUSIG) model and one-group average bubble number density (ABND) model for handling the bubble size distribution of gas-liquid bubbly flows under isothermal conditions are assessed. Three forms of coalescence and breakage mechanisms by Wu et al. [1998. One-group interfacial area transport in vertical bubbly flow. International Journal of Heat Mass Transfer 41, 1103-1112], Hibiki and Ishii [2002. Development of one-group interfacial area transport equation in bubbly flow systems. International Journal of Heat Mass Transfer 45, 2351-2372] and Yao and Morel [2004. Volumetric interfacial area prediction in upwards bubbly two-phase flow. International Journal of Heat Mass Transfer 47, 307-328] are incorporated in the ABND model. To examine the relative merits of both approaches, local radial distributions of five primitive variables in bubbly flows: void fraction, Sauter mean bubble diameter, interfacial area concentration, and gas and liquid velocities, are compared against the experimental data of Liu and Bankoff [1993a. Structure of air-water bubbly flow in a vertical pipe—I. Liquid mean velocity and turbulence measurements. International Journal of Heat Mass Transfer 36, 1049-1060; 1993b. Structure of air-water bubbly flow in a vertical pipe—II. Void fraction, bubble velocity and bubble size distribution. International Journal of Heat Mass Transfer 36, 1061-1072] and Hibiki et al. [2001. Axial interfacial area transport of vertical bubble flows. International Journal of Heat Mass Transfer 44, 1869-1888]. In general, both of the ABND model and MUSIG model predictions yield close agreement with experimental results. To account for the range of different bubble sizes in the gas-liquid bubbly flows, the resolution required is achieved through the application of the MUSIG model. Nevertheless, computational times increase by a factor of two when compared to applying the simpler ABND model. To further exploit the models’ capabilities, investigations are carried out by extending the two population approaches beyond the bubbly flow regime of higher void fraction, particularly in the transition regime. The numerical results are found to be grossly over-predicted, which expose the inherent limitations of the models. It is known that bubbles in this regime are generally highly distorted and closely packed instead of spherical shape and allowed to move freely in bubbly flow regime. 相似文献