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云计算-气液降膜流动的计算传递学研究.pdf
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云计算-气液降膜流动的计算传递学研究.pdf
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ABSTRACT
The thesis is composed by two parts. Theoretical simulation and experiments on
the falling film heat and mass transfer process are discussed in chapter 2-4, which
includes the evaporation, condensation and the effects of plate structure on liquid film
flow. In chapter 5-7, A novel liquid distribution model is established and multi-scale
theory is applied to predict the liquid distribution in packed column.
The gas-liquid two phases flow behavior and heat and mass transfer
characteristics have always been the focus of attention in falling-film evaporators and
desalting plates. Therefore, a two-dimensional two-phase flow CFD model using the
volume of fluid (VOF) method is presented to investigate the transport phenomena.
Two important momentum source terms, surface tension and interface stress shear,are
considered. Moreover, the change of water vapor concentration and heat transfer
capacity around the interface have also been described by adding the heat and mass
transfer source term in the model. Based on the simulated results, the influence of two
phase temperatures, flow rates, water vapor concentration and wall flux on heat
transfer coefficients, latent and sensible heat have been discussed. The simulation
results show that the heat transfer coefficients near the inlet decrease rapidly, and the
latent heat is the major mode of heat transfer in the interface. Moreover, the heat
released from condensation process further restricts the increase of sensible heat.
The heat and mass transfer process is significantly influenced by liquid film flow
behavior. For this, a novel heat transfer plate structure is proposed, and the heat
transfer coefficients on three different plates have been investigated, the structures of
which are flat, coarse and oriented baffled plate, respectively. The experimental
results indicate better liquid distribution and increased remixed and turbulence degree
on oriented baffled plate, which improve the heat transfer performance.
In section 2, multi-scale method which consists of microcosmic CFD simulation
and macrocosmic calculation is applied to predict the liquid distribution in structured
packing column. The minimum representative units reappearing in structured packing
is proposed and liquid film flow on the units is also been simulated to obtain the
model parameters. Based on the parameters and established liquid distribution model,
the fluid mechanics behavior in packed column is obtained. Furthermore, a refined
model is developed to take the condition of liquid film flowing on structured packing
with holes into consideration, as well as the influence of counter-current flow of gas
phase. This multi-scale method doesn’t need complex computation and any
experimental data to regress model parameters. Therefore, it provides a new approach
for the design and optimization of the large-scale packing column, and has broad
future in industrial applications.
Key words:Computational Fluid Dyanmics (CFD); Evaporation; Condensation; Heat
transfer plate structure; Structured packing; Multi-scale method; Liquid distribution
model.
I
目 录
第一章 文献综述.............................................................................................................. 1
1.1 降膜流动传热过程................................................................................................ 1
1.1.1 伴有蒸发现象的降膜流动过程.................................................................... 1
1.1.2 伴有冷凝现象的降膜流动过程.................................................................... 2
1.2 界面力对降膜流动的影响 .................................................................................... 4
1.2.1 降膜流动过程中的气液剪应力.................................................................... 4
1.2.2 降膜流动过程中的 Marangoni剪应力 .......................................................... 5
1.3 规整填料塔中的流体力学行为研究 ....................................................................... 7
1.3.1 实验研究 ................................................................................................... 7
1.3.2 理论模型研究 ............................................................................................ 8
1.3.3 CFD 流型研究 ...........................................................................................11
1.3.4 多尺度模型.............................................................................................. 13
1.4 本文主要工作及意义 ......................................................................................... 15
第二章 伴有蒸发相变的降膜流动过程 CFD 研究 ............................................................. 17
2.1 物理模型 ........................................................................................................... 17
2.2 数学模型 ........................................................................................................... 18
2.2.1 VOF 模型 ................................................................................................ 18
2.2.2 源项的确定.............................................................................................. 19
2.2.3 边界及初始条件....................................................................................... 22
2.2.4 网格划分 ................................................................................................. 23
2.2.5 数值求算方法 .......................................................................................... 23
2.2.6 公式定义 ................................................................................................. 24
2.3 结果与讨论 ....................................................................................................... 24
2.3.1 模型可靠性验证....................................................................................... 24
2.3.2 降膜流动传质分析 ................................................................................... 25
2.3.3 气相进口相对湿度的影响......................................................................... 25
2.3.4 壁面热通量的影响 ................................................................................... 28
2.3.5 液相进口温度的影响................................................................................ 30
2.3.6 液相流量和并流气速的影响 ..................................................................... 31
2.3.7 气流方向和大小的影响 ............................................................................ 33
2.3.8 气液界面处潜热与显热分析 ..................................................................... 35
2.4 小结 .................................................................................................................. 37
第三章 伴有冷凝相变的降膜流动过程 CFD 研究 ............................................................. 39
3.1 物理模型............................................................................................................ 39
3.2 数学模型 ........................................................................................................... 40
3.2.1 控制方程 .................................................................................................. 40
3.2.2 源相的确定.............................................................................................. 40
3.2.3 边界及初始条件....................................................................................... 40
3.2.4 网格划分 ................................................................................................. 41
3.2.5 数值求算方法 .......................................................................................... 42
II
3.3 结果与讨论 ....................................................................................................... 43
3.3.1 液膜波动性的影响 ................................................................................... 43
3.3.2 液膜流量的影响....................................................................................... 44
3.3.3 液膜温度的影响....................................................................................... 46
3.3.4 气速的影响.............................................................................................. 48
3.3.5 水蒸气浓度的影响 ................................................................................... 49
3.3.6 气相温度的影响....................................................................................... 50
3.3.7 高浓度水蒸气的影响................................................................................ 52
3.4 小结 .................................................................................................................. 54
第四章 降膜流动结构变化对传热性能的影响.................................................................. 55
4.1 板式换热器传热性能试验 .................................................................................. 55
4.1.1 实验装置及流程....................................................................................... 55
4.1.2 实验操作步骤与条件................................................................................ 58
4.2 实验结果分析与讨论 ......................................................................................... 59
4.2.1 不同换热板结构下的液相流动形态........................................................... 59
4.2.2 换热板的传热系数 ................................................................................... 62
4.2.3 无量纲分析.............................................................................................. 65
4.3 数值模拟 ........................................................................................................... 67
4.3.1 物理模型 ................................................................................................. 67
4.3.2 网格划分及数值求算方法......................................................................... 67
4.3.3 结果与讨论.............................................................................................. 68
4.4 小结 .................................................................................................................. 70
第五章 基于降膜流动的普通规整填料多尺度计算........................................................... 71
5.1 多尺度模型 ....................................................................................................... 71
5.1.1 代表性单元.............................................................................................. 71
5.1.2 小尺度数学模型....................................................................................... 73
5.1.3 宏观尺度模型——单元网络模型 .............................................................. 75
5.2 多尺度模型计算方法 ......................................................................................... 81
5.3 结果与讨论 ....................................................................................................... 81
5.3.1 模型参数 ................................................................................................. 81
5.3.2 填料片内的液相分布................................................................................ 86
5.3.3 填料片局部持液量分布 ............................................................................ 88
5.3.4 填料片高度的影响 ................................................................................... 91
5.3.5 整盘填料内的液相分布 ............................................................................ 92
5.4 小结 .................................................................................................................. 94
第六章 基于降膜流动的开孔规整填料多尺度计算........................................................... 95
6.1 代表性单元及数值模拟方法 ............................................................................... 95
6.2 液相分布模型 .................................................................................................... 96
6.2.1 模型假设 ................................................................................................. 96
6.2.2 模型简介 ................................................................................................. 96
6.2.3 模型计算过程 .......................................................................................... 98
6.3 结果与讨论 ......................................................................................................108
6.3.1 液膜流动形态 .........................................................................................108
6.3.2 有效润湿面积 ......................................................................................... 110
III
6.3.3 模型参数 ................................................................................................ 111
6.3.4 填料片上的液相分布............................................................................... 114
6.4 小结 ................................................................................................................. 117
第七章 气相对开孔规整填料降膜流动过程的影响.......................................................... 119
7.1 气相对流型分布的影响...................................................................................... 119
7.2 气相对模型参数的影响 .....................................................................................122
7.2.1 液膜流动方向 .........................................................................................122
7.2.2 模型参数 ................................................................................................122
7.3 小结 .................................................................................................................125
第八章 结论与展望 .......................................................................................................126
8.1 结论 .................................................................................................................126
8.2 展望 .................................................................................................................127
符号说明.......................................................................................................................128
参考文献.......................................................................................................................131
发表论文和参加科研情况说明 .......................................................................................143
附录..............................................................................................................................144
致谢..............................................................................................................................146
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