二氧化碳微通道蒸发器换热特性研究

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摘要

随着生活水平的提高，人类的环保意识逐渐增强，温室气体制冷剂面临着被

淘汰的危险，而二氧化碳以其高效及安全这两大特点被公认为是最有前景的替代

制冷剂之一。微通道以其高效、节能等优势被广泛关注，具有广阔的应用前景。

本文将二氧化碳与微通道技术相结合，展开对二氧化碳在微通道蒸发器内沸腾换

热与流动特性的研究，为二氧化碳微通道蒸发器设计提供理论基础，对提高二氧

化碳微通道蒸发器换热特性具有重要的学术意义和实用价值。

本文首先分析已有的二氧化碳在微通道中沸腾换热与流动特性的关联式，全

面考虑各关联式的使用范围及特点，结合本实验台实际情况，对比分析得出适用

于本实验台的二氧化碳微通道蒸发器沸腾及流动关联式。当二氧化碳处于沸腾两

相区，应采用 Cheng 所开发的基于流态的二氧化碳沸腾换热关联式；而在过热段，

当Re≥2300 时，对流换热系数应采用 Gnielinski 关联式，当 Re＜2300 时，对流换

热系数应采用 Sieder-Tate 关联式。分析几种不同形式的摩擦压降模型，得出当

G≥43 kg·m-2·s-1 时，应采用 Jassim 和Mewell 概率型压降模型，当 G＜43 kg·m-2·s-1，

采用 Friedel 分相压降模型更为合理。在此基础上，基于有限元分析法，采用

MATLAB 与REFPROP 相结合技术，综合考虑空气侧干湿工况以及二氧化碳侧两

相段及过热段，建立二氧化碳微通道蒸发器二维分布参数模型。研究各参数对二

氧化碳在微通道中沸腾换热及流动特性的影响，根据模拟结果可知二氧化碳在沸

腾两相区干涸点附近，其对流换热系数达到最大值，干涸现象发生后，对流换热

系数迅速降低，且二氧化碳在过热段压降非常小，远小于两相段压降。搭建二氧

化碳微通道蒸发器沸腾换热特性研究实验台，得出各微元段壁面及空气出口温度

分布情况，并计算出各微元段二氧化碳对流换热系数。对比分析模拟与实验结果，

各参数误差在可接受范围之内，验证所建立数学模型的合理性。采用热红外成像

技术，发现二氧化碳两相流在微通道蒸发器入口处存在较为严重的分液不均现象，

导致其在微通道中出现部分换热恶化的状况，为此提出相应的改进措施。

最后以熵产数这一无量纲来表征二氧化碳在微通道沸腾换热过程中的不可逆

损失，充分考虑微通道蒸发器空气侧干湿工况，对二氧化碳沸腾两相区与过热段

分别建立熵产模型，计算各微元段熵产数，得出各微元段不可逆损失分布情况。

结果表明，二氧化碳在微通道蒸发器中不可逆损失主要由温差传热引起的；过热

段不可逆损失远小于两相段；在干涸点附近微元段其不可逆损失达到最大值。

关键词：二氧化碳微通道蒸发器换热特性有限元熵产数

ABSTRCT

With the improvement of people’s living standards, the awareness of

environmental protection is gradually enhanced. The greenhouse gas refrigerants are in

danger of being eliminated. While carbon dioxide has been regarded as the potential

alternative refrigerant due to its high efficiency and environmental safety. As the

microchannel has the advantages of high efficiency and saving energy, it has been

widely concerned and has broad application prospects. In this paper, the carbon

dioxide and microchannel technology were combined. The heat transfer and flow

characteristics of carbon dioxide boiling in microchannel evaporator were studied. It

provides a theoretical basis for carbon dioxide microchannel evaporator design and has

important academic significance and practical value.

Firstly, the existing carbon dioxide boiling heat transfer and flow correlations in

microchannel were firstly analyzed in this paper. The use scope and characteristics of

the correlation were fully considered. According to the actual situation of the

experiment bench ，the carbon dioxide microchannel evaporator boiling and flow

correlation were obtained through comparison and analysis. When the carbon dioxide

is in a boiling two-phase region in microchannel, Cheng correlations should be used,

because the carbon dioxide flow boiling heat transfer correlation is based on the flow

state. While in superheat region, as the Reynolds number Re≥2300, the Gnielinski

correlation should be applied to solve the convective heat transfer coefficient. However,

when he Reynolds number Re＜2300, the Sieder-Tate correlation should be used to

solve it. Several different forms of frictional pressure drop model were analyzed. If G

≥43 kg •m-2 •s-1, the Jassim and Mewell correlation which is a probabilistic drop

model should be used to work out the frictional pressure drop value in boiling

two-phase region. If G＜43 kg •m-2 •s-1, the Friedel correlation which is a phase

separation type drop mode was suggested to solve the frictional pressure drop value in

boiling two-phase region. On this basis, based on finite element analysis, a steady state

distributed parameter model for microchannel evaporator was established by using

MATLAB combined with REFPROP technology and considering the dry and wet

conditions on air side as well as the overheat condition on carbon dioxide side. The

impacts of various parameters on carbon dioxide boiling heat transfer were studied.

According to the simulation result, the carbon dioxide convective heat transfer

coefficient reaches a maximum near the dryness point in two-phase region. After the

dryness, the convective heat transfer coefficient decreased rapidly. The pressure drop

value in superheat region is smaller than in boiling two-phase region. Then, the

experiment bench of carbon dioxide boiling heat transfer in microchannel evaporator

was build. According to the experiment result, the carbon dioxide and the air outlet

temperature of each segment were gotten. Meanwhile, the convective heat transfer of

carbon dioxide of each segment in microchannel evaporator was calculated.

Comparisons of analytical and experimental data were made to verify and validate the

model. The phenomenon of liquid separation inequality at the inlet of carbon dioxide

evaporator was found by the infrared imaging technology. Because of this, the heat

deterioration phenomenon occurred in microchannel. Several improvement measures

were been put forward to alleviate this problem.

Finally, the irreversible loss of heat transfer process of carbon dioxide in

microchannel was represented by a dimensionless number —entropy generation

number NS. The situation of wet and dry conditions on air side were fully considered.

At the same time, the entropy generation model in boiling two-phase and superheat

region were built separately. The distribution of each infinitesimal segment irreversible

loss was analyzed. The results from the mathematic model showed that system entropy

was mostly caused by temperature difference of heat transfer between carbon dioxide

and air sides. The overheating segment irreversible loss is far less than the two-phase

segment. The irreversible loss of the infinitesimal segment near dry-point reaches the

maximum value.

Key words: carbon dioxide, microchannel evaporator, heat transfer

characteristics, finite element method, entropy generation number

中文摘要

ABSTRCT

第一章绪论..................................................................................................................... 1

1.1 课题研究背景........................................................................................................ 1

1.1.1 制冷剂发展.................................................................................................... 1

1.1.2 跨临界二氧化碳空调与热泵系统............................................................... 2

1.1.3 二氧化碳与微通道相结合技术................................................................... 3

1.2 国内外研究现状.................................................................................................... 4

1.3 本文研究内容及意义............................................................................................ 6

第二章二氧化碳热微通道蒸发器理论研究................................................................. 8

2.1 二氧化碳蒸发器的发展....................................................................................... 8

2.2 二氧化碳在微通道中两相沸腾换热原理............................................................ 9

2.2.1 二氧化碳物性介绍........................................................................................ 9

2.2.2 二氧化碳在微通道中微尺度效应及两相流动特点................................... 9

2.3 二氧化碳在微通道中沸腾换热机理................................................................. 11

2.4 二氧化碳在微通道中沸腾换热主要影响因素................................................. 11

2.5 二氧化碳在微通道中沸腾换热干涸现象分析................................................. 13

2.6 二氧化碳在微通道中换热关联式..................................................................... 13

2.7 二氧化碳微通道蒸发器压降模型..................................................................... 18

2.8 本章小结............................................................................................................. 22

第三章二氧化碳热微通道蒸发器及实验设计........................................................... 23

3.1 微通道蒸发器结构参数...................................................................................... 23

3.2 实验概述.............................................................................................................. 25

3.2.1 二氧化碳微通道换热特性研究实验台...................................................... 25

3.2.2 焓差实验室.................................................................................................. 26

3.2.3 测试及采集系统.......................................................................................... 26

3.3 实验方案.............................................................................................................. 32

3.3.1 实验目的...................................................................................................... 32

3.3.2 实验内容...................................................................................................... 33

3.3.3 实验步骤...................................................................................................... 33

3.3.4 注意事项...................................................................................................... 34

3.4 实验数据处理...................................................................................................... 35

3.5 本章小结.............................................................................................................. 36

第四章二氧化碳微通道蒸发器传热模拟................................................................... 37

4.1 微通道蒸发器模型.............................................................................................. 37

4.1.1 微通道蒸发器物理模型............................................................................. 37

4.1.2 微通道蒸发器数学模型............................................................................. 38

4.2 微通道蒸发器模型计算过程............................................................................. 40

4.3 数值模拟结果...................................................................................................... 45

4.4 本章小结.............................................................................................................. 53

第五章实验和模拟数据处理及分析........................................................................... 55

5.1 二氧化碳微通道蒸发器模拟与实验对比......................................................... 55

5.1.1 二氧化碳微通道蒸发器传热模拟与实验对比......................................... 56

5.1.2 二氧化碳微通道蒸发器流动模拟与实验对比......................................... 59

5.2 二氧化碳微通道蒸发器性能评价指标.............................................................. 61

5.2.1 评价指标...................................................................................................... 61

5.2.2 二氧化碳微通道蒸发器熵产分析.............................................................. 62

5.3 微通道蒸发器的不足和改进方向...................................................................... 66

5.4 本章小结.............................................................................................................. 66

第六章结论与展望....................................................................................................... 68

6.1 结论...................................................................................................................... 68

6.2 本论文创新点...................................................................................................... 69

6.3 展望...................................................................................................................... 69

符号表............................................................................................................................. 70

参考文献......................................................................................................................... 71

在读期间公开发表的论文和承担科研项目及取得成果............................................. 76

致谢................................................................................................................................. 77

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摘要：

摘要随着生活水平的提高，人类的环保意识逐渐增强，温室气体制冷剂面临着被淘汰的危险，而二氧化碳以其高效及安全这两大特点被公认为是最有前景的替代制冷剂之一。微通道以其高效、节能等优势被广泛关注，具有广阔的应用前景。本文将二氧化碳与微通道技术相结合，展开对二氧化碳在微通道蒸发器内沸腾换热与流动特性的研究，为二氧化碳微通道蒸发器设计提供理论基础，对提高二氧化碳微通道蒸发器换热特性具有重要的学术意义和实用价值。本文首先分析已有的二氧化碳在微通道中沸腾换热与流动特性的关联式，全面考虑各关联式的使用范围及特点，结合本实验台实际情况，对比分析得出适用于本实验台的二氧化碳微通道蒸发器沸腾及流动关联式。当二氧化碳...

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