高压流体过冷流动沸腾的数值模拟

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...     

加热上升管内过冷流动沸腾数值模拟

采用计算流体动力学（CFD）程序CFX4.4对加热上升管内过冷流动沸腾工况下气水两相流动局部两相流参数（空泡份额和汽泡尺寸）进行了数值模拟。对数值差分方法、相关模型（界面力和气泡诱导的紊流）和汽泡尺寸进行了敏感性分析。空泡份额分布计算结果与实验结果比较表明;在低空泡份额工况下;两者符合较好;在高空泡份额工况下两者存在一定偏差;并且气相速度和汽泡尺寸的计算结果不理想。计算结果与实验结果之间的差异说明程序模型对于加热上升管内过冷流动沸腾模拟并不完善;建立更为合理的汽泡尺寸模型;考虑汽泡的合并和撕裂是必要的。     

液氮过冷沸腾空泡份额分布规律的研究

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。     

高压高过冷度下过冷流动沸腾数值模拟

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。     

基于MUSIG模型的竖直圆管内液氮过冷沸腾数值模拟研究(英文)

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.     

竖直窄流道内过冷流动沸腾的单汽泡生长模型

The process of bubble growth on heating wall in subcooled boiling includes the micro-layer evaporation on heating wall and the bubble top coagulation when the bubbles grow to a certain size and emerge into the subcooled mainstream fluid. Based on this consideration, a model for the single bubble growth of subcooled flow boiling in vertical narrow rectangular channel was proposed.Compared with experimental results, the error of the simulation results using the proposed model is less than ±25%. The simulation results indicated that as the wall superheat increases, the bubble growth gets faster, with the subcooled degree of mainstream increases, the bubble growth in later stage would be slowed, with the contact angle increases, the contact radius of the bubble bottom and the wall tension would be strengthened, resulting in faster bubble growth to make the bubble to be flat and more easily exposed to the mainstream. The velocity of mainstream has no significant effects on bubble growth rate.     

垂直圆管内液氮流动沸腾的理论模型及数值模拟

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.     

竖直圆管内液氮过冷流动沸腾数值模拟研究

在现有数学模型的基础上,从沸腾换热的机理入手,对过冷流动沸腾模型中的气泡参量模型进行了修正,分析确定了壁面热流密度的拆分方法,构建出适应于液氮的过冷沸腾计算模型。将新模型应用于CFX4.4中,对液氮在三维竖直圆管内的过冷流动沸腾进行了数值模拟。研究发现,在过冷流动沸腾形成之后,蒸发热流成为壁面换热的主要部分,沸腾换热占据换热的主导地位。数值模拟结果与已有的实验数据吻合较好,证明了新建模型的正确性。     

液氮过冷沸腾壁面换热特性的数值研究

应用计算流体力学(CFD)研究手段,对液氮过冷沸腾过程进行了模拟。系统分析了过冷沸腾壁面换热过程中存在的各传热模式,得到3部分热流沿管长的分布规律。计算结果表明,在过冷沸腾后期,蒸发热流成为壁面换热的主要模式。同时,就壁面热流密度的变化对3部分热流的影响进行了分析。随着壁面热流的增加,过冷沸腾出口处淬火热流和对流热流将减小,只有蒸发热流增大,且所占总热流份额绝对占优,壁面热流的增加促进了沸腾换热。     

内置涡流发生器的管内过冷沸腾与强化换热的模拟

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。     

Forced convection subcooled boiling of binary mixtures

A mode based on an additive mechanism of heat transfer is proposed for forced convection subcooled boiling of binary mixtures. The contributing modes of heat transfer are: (i) the heat transferred as latent heat by the rising bubbles, (ii) the heat transferred as the heat contained in the superheated thermal layer that is removed from the surface in the wake of the rising bubbles and (iii) the single phase forced convection heat transfer from the heating surface not influenced by the bubbles. Experimental data from the literature on binary systems show good agreement with the model, validating the postulated mechanism.     

Numerical study of condensing bubble in subcooled boiling flow using volume of fluid model

In this numerical study, the behavior of condensing bubble was investigated using the volume of fluid (VOF) model in the FLUENT code. In order to simulate the condensing bubble with the FLUENT code, the bubble condensation was modeled using the user-defined function (UDF). For the validation of the UDF of bubble condensation, the results of CFD simulation were compared with the results of a bubble condensation experiment performed in Seoul National University (SNU). Simulation results showed good agreements with the experimental data. Moreover, the fundamental behavior of the condensing bubble was investigated in various conditions. The effects of condensation on bubble behavior were analyzed by comparing the behavior of condensing bubbles with that of adiabatic bubbles. It was found that the behavior of the condensing bubble was different from that of the adiabatic bubble in many respects including the bubble shape, velocity, rise distance and moving trajectory.     

竖直环形通道内液氮流动沸腾的数值模拟

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...     

干涸后膜态沸腾传热计算模型

提出了膜态沸腾传热的物理模型,建立相应的数学模型,对干涸后膜态沸腾传热特性进行分析.通过矩形通道试验数据确定关系式系数,得到了适用于矩形通道的干涸后膜态沸腾传热计算模型,并对计算偏差进行了统计计算.文中提出的理论计算模型与现有不同工况范围内取得的试验数据和已有计算模型进行了比较分析.结果表明:模型计算值与现有试验数据符...     

低温甲醇脱碳工艺热力学模型和过程模拟研究

为了进行含高压气体和强极性物质的体系的低温甲醇脱碳工艺的模拟研究,建立相应的热力学模型.采用Soave-Redlich-Kwong(SRK)立方型状态方程,结合Huron-Vidal混合规则和非随机双流体Non-Random-Two-Liquid(NRTL)活度系数模型建立的热力学模型,从气体溶解度和气液平衡数据拟合获...     

管束异型排列制冷蒸发器沸腾传热性能数值模拟

制冷蒸发器是制冷系统中的重要设备,其沸腾传热性能对整个系统的能源利用效率有着显著影响。文中以管束异型排列的制冷蒸发器为研究对象,通过FLUENT软件模拟计算,分析探讨了管束排列方式对蒸发器壳程流场及沸腾传热性能的影响。计算结果表明,在相同的流体进料量下,异型管束排列蒸发器的压力变化趋势与正方形排列的相似。在综合性能上,具有合理气流通道的异型管束排列蒸发器明显优于管束正三角形排列和正方形排列的蒸发器。