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1.
孙霖  姜勇  周爱民  陈剑 《化学工程》2011,39(10):92-95
在二流体模型基础上,采用RPI模型计算加热壁面的相间质量传输,对竖直圆管内高压流体过冷沸腾流动过程进行数值模拟.研究发现:壁面温度沿流动方向有明显拐点.根据壁温变化趋势可以判断,转折点位置即为过冷沸腾起始点;管内流体速度的增长曲线并不是简单单调递增的过程,而是速度增长由刚开始的缓慢上升到急剧上升,与空泡份额的分布曲线相...  相似文献   

2.
在低温泡状流中,鉴于液相中离散的气泡尺寸大小不一,文中将MUSIG模型应用于低温流体过冷沸腾计算中。通过模拟液氮在竖直圆管内过冷流动沸腾过程,对各尺寸组气泡的分布、轴向空泡份额的分布规律及流动不稳定性进行了分析。计算结果表明:各尺寸组气泡在壁面和中心区呈现不同的分布规律;平均空泡份额沿轴向呈非线性递增变化;由于流型的转变,管内压降将出现突变。  相似文献   

3.
为了分析低温流体在过冷沸腾中的相分布特性,对液氮在竖直圆管内的过冷沸腾过程进行了CFD数值模拟。通过修正气泡参量模型和改变过程的操作参数,系统地分析了空泡份额在加热上升管内的分布情况。结果表明,气液相间的热质传递决定着空泡份额的分布,截面平均空泡份额沿轴向非线性递增,沿径向呈U形分布。轴向截面平均空泡份额随着热流密度的增大而增大,随着入口质量流率和过冷度的增大而减小。  相似文献   

4.
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...  相似文献   

5.
邵雪锋  李祥东  汪荣顺 《化学工程》2011,39(10):82-86,95
在多尺寸组模型的基础上,从加热壁面上脱离汽泡的受力分析人手,对液氮过冷流动沸腾模型进行了修正.将新模型应用于环形通道内液氮过冷流动沸腾的数值模拟,同时为了比较,采用基于Kirichenko,Fritz汽泡脱离直径公式的多尺寸组模型对同一管道内液氮过冷流动进行了数值模拟.结果表明:结合脱离汽泡受力分析模型的多尺寸组模型可...  相似文献   

6.
摇摆条件下圆管内过冷沸腾局部空泡时空分布特性   总被引:1,自引:0,他引:1       下载免费PDF全文
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   

7.
高压高过冷度下过冷流动沸腾数值模拟   总被引:1,自引:0,他引:1       下载免费PDF全文
窦从从  毛羽  王娟  王江云 《化工学报》2010,61(3):580-586
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   

8.
采用欧拉双流体模型结合RPI沸腾模型对高压管内过冷沸腾进行三维非稳态模拟,考察了涡流发生器对管内过冷沸腾的影响,模拟了高压下光管内的过冷沸腾、层流下内置涡流发生器的换热管内的流动和湍流下内置涡流发生器的换热管内的过冷沸腾。结果表明,内置涡流发生器的换热管在层流状态下换热能力明显提升,过冷沸腾时管内换热能力有一定提升,且壁面附近的气泡由于扰流作用被大量卷入锥形片内,降低了壁面附近产生气膜的可能性,延迟了过冷沸腾起始点的位置。  相似文献   

9.
沸腾工况汽液两相流汽泡尺寸的研究对于深入理解两相流动特性以及相关模型的发展具有重要意义。文中采用双探头光学探针对受热上升管内汽液两相流汽泡尺寸的径向分布特性进行了测量与研究。此外,通过对汽泡尺寸概率密度数的分析,得出了对应工况的两相流流型,根据理论分析及实验结果,总结了汽泡尺寸径向分布与流型的关系。  相似文献   

10.
高过冷度下直管内气液两相传热特性   总被引:1,自引:1,他引:0       下载免费PDF全文
窦从从  毛羽  王娟  王江云 《化工学报》2010,61(9):2314-2319
基于双流体模型,建立了适合高压高过冷工况的过冷沸腾模型,考察了液体过冷度和壁面热通量对垂直圆管内气液两相传热特性的影响。结果表明,高过冷度下整个截面上气相分布十分不均匀,壁面附近气泡比较密集。随着入口过冷度的减小,气相体积分数增加,促进了沸腾传热的发展。随着壁面热通量的增加,表面传热系数沿流体流动方向总体上呈增大趋势,局部区域内出现下降,并且传热系数的下降趋势随入口过冷度的减小逐渐变得不明显。  相似文献   

11.
垂直圆管内液氮流动沸腾的理论模型及数值模拟   总被引:1,自引:0,他引:1  
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   

12.
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   

13.
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   

14.
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   

15.
群体平衡模型对复杂气液泡状流数值模拟   总被引:3,自引:3,他引:0       下载免费PDF全文
引言 气液泡状流广泛出现于航空航天、石油化工、核工程技术等领域,且多为复杂湍流泡状流.在工程领域中,准确预测这种复杂泡状流的含气率、气一液速度、气泡直径等参数对工业设备安全与优化分析十分重要.  相似文献   

16.
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   

17.
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   

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