化工学报 ›› 2022, Vol. 73 ›› Issue (7): 2912-2923.doi: 10.11949/0438-1157.20220076

• 流体力学与传递现象 • 上一篇    下一篇

跨临界CO2快速膨胀过程中非平衡冷凝和闪蒸机理的数值研究

李亚飞1,2(),邓建强1,2(),何阳1,2   

  1. 1.西安交通大学化学工程与技术学院，陕西 西安 710049
    2.陕西省能源化工过程强化重点实验室，陕西 西安 710049
  • 收稿日期:2022-01-14 修回日期:2022-05-19 出版日期:2022-07-05 发布日期:2022-08-01
  • 通讯作者: 邓建强 E-mail:lyfxjtu@stu.xjtu.edu.cn;dengjq@mail.xjtu.edu.cn
  • 作者简介:李亚飞（1994—），男，博士研究生，lyfxjtu@stu.xjtu.edu.cn
  • 基金资助:
    国家重点研发计划项目(2021YFF0306802);国家自然科学基金项目(51676148)

Numerical study on non-equilibrium condensation and flashing mechanisms in rapid expansion process of transcritical CO2

Yafei LI1,2(),Jianqiang DENG1,2(),Yang HE1,2   

  1. 1.School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an 710049, Shaanxi, China
    2.Shaanxi Key Laboratory of Energy Chemical Process Intensification, Xi’an 710049, Shaanxi, China
  • Received:2022-01-14 Revised:2022-05-19 Published:2022-07-05 Online:2022-08-01
  • Contact: Jianqiang DENG E-mail:lyfxjtu@stu.xjtu.edu.cn;dengjq@mail.xjtu.edu.cn

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摘要：

跨临界CO2在高速膨胀时，压力和温度剧烈下降，会发生非平衡相变。其中在天然气超声速分离设备和超临界CO2离心压缩机中CO2会发生非平衡冷凝相变；在引射膨胀制冷系统中，跨临界CO2在引射器主动喷嘴中发生非平衡闪蒸相变。为解决跨临界CO2在膨胀过程中物性变化剧烈，非平衡相变模拟困难的问题，构建了新型非平衡相变CFD模型，以研究跨临界CO2在超声速缩放喷嘴中的非平衡冷凝和非平衡闪蒸的相变过程和膨胀机理，模型耦合了温度驱动的蒸发-冷凝相变机制和压力驱动的空化-冷凝相变机制，并用文献中的试验结果验证了模型的准确性。研究结果表明，在冷凝相变过程中，由压力驱动的冷凝传质具有主要影响，压力驱动的冷凝传质主要存在于喷嘴喉部与内流区域，温度驱动的冷凝传质主要存在于喷嘴渐扩段壁面。冷凝传质速率随着进口压力的增加和进口温度的降低而增加，从而使冷凝的非平衡程度和喷嘴内的干度降低，喷嘴渐扩段内达到声速的位置也相应延后。在闪蒸相变过程中，由温度驱动的蒸发传质占据主导，蒸发相变主要发生在喷嘴喉部附近，空化相变主要发生在喷嘴渐扩段，两相CO2在喷嘴的渐扩段达到声速。随着喷嘴进口压力的增加和进口温度的降低，闪蒸的非平衡程度增加，使喷嘴内的干度减小。本研究有助于厘清跨临界CO2快速膨胀中的非平衡闪蒸和冷凝相变机理，并为跨临界CO2膨胀设备的分析和优化设计提供参考。

关键词: 二氧化碳, 闪蒸, 冷凝, 超声速喷嘴, 气液两相流, 计算流体力学

Abstract:

When transcritical CO2 expands at a high speed, the pressure and temperature drop sharply, and a non-equilibrium phase change occurs. The non-equilibrium condensation phase change of CO2 would occur in natural gas supersonic separation equipment and supercritical CO2 centrifugal compressor. Besides, the non-equilibrium flashing phase change of transcritical CO2 occurs in the ejector primary nozzle in the ejector expansion refrigeration system. In order to solve the problem that the physical properties of transcritical CO2 change sharply during the expansion process and it is difficult to simulate the non-equilibrium phase change, a new non-equilibrium phase change CFD model was constructed to study the transcritical CO2 non-equilibrium condensation and flashing phase change process and expansion mechanisms in the supersonic converging-diverging nozzle. The model coupled the temperature-driven evaporation-condensation phase change mechanism and the pressure-driven cavitation-condensation phase change mechanism, and the accuracy of the model was verified by the experimental results in literature. The results showed that the pressure-driven condensation mass transfer had a major influence on condensation phase change. The pressure-driven condensation mass transfer mainly existed in the nozzle throat and internal flow zone, and the temperature driven condensation mass transfer mainly existed on the wall of the nozzle diverging section. The condensation mass transfer rate increased with the increase of inlet pressure and the decrease of inlet temperature, so that the non-equilibrium degree of condensation and the quality in the nozzle were reduced, and the position of speed of sound was delayed accordingly in the nozzle diverging section. In addition, the temperature-driven evaporation mass transfer dominated the flashing phase change, the evaporation phase change mainly occurred near the nozzle throat, the cavitation phase change mainly occurred in the nozzle diverging section, and the two-phase CO2 reached speed of sound in the nozzle diverging section. With the increase of inlet pressure and the decrease of inlet temperature, the non-equilibrium degree of flashing increased and the quality in the nozzle decreased. This study was helpful to clarify the non-equilibrium flashing and condensation phase change mechanism in the rapid expansion of transcritical CO2, and it provided a method for the analysis and optimization design of transcritical CO2 expansion equipment.

Key words: carbon dioxide, flashing, condensation, supersonic nozzle, gas-liquid flow, CFD

中图分类号: 

  • TB 131

图1

Berana等[32]试验喷嘴的几何参数"

表1

喷嘴进出口条件"

进口压力pin/MPa进口温度Tin/K出口压力pout/MPa
9.5323.554.06

图2

含喷嘴的CO2引射器几何结构"

表2

CO2引射器的几何参数"

几何参数数值几何参数数值
γ129.59oLmix38.00 mm
γ271.47oWmix2.82 mm
γ388.19oLd130.50 mm
γnc23.73oLd224.00 mm
Wn8.00 mmWd10.00 mm
Ws8.00 mmγd13.43o
Ls26.55 mmHn0.78 mm
NXP8.40 mmHmix1.78 mm
NDA2.00o

图3

CO2喷嘴结构模型"

表3

CO2引射器的进出口操作条件"

算例编号

主动流压力

pp/MPa

主动流温度

Tp/K

引射流压力

ps/MPa

引射流温度

Ts/K

引射器出口压力

peo/MPa

A19.50306.593.73297.173.80
A29.00304.922.99299.533.10
A38.49303.173.04299.473.13

图4

网格无关性分析"

图5

模拟的喷嘴轴向压力和试验结果的比较"

图6

不同进口压力的温度驱动冷凝传质速率"

图7

不同进口压力的压力驱动冷凝传质速率"

图8

不同进口压力的喷嘴轴线干度分布"

图9

不同进口压力的喷嘴轴线压力分布"

图10

不同进口压力的Mach数云图"

图11

不同进口温度的温度驱动冷凝传质速率"

图12

不同进口温度的压力驱动冷凝传质速率"

图13

不同进口温度下的喷嘴轴线压力分布"

图14

不同进口温度下的喷嘴轴线干度分布"

图15

不同进口温度下的Mach数"

图16

网格无关性分析"

表4

CFD模拟得到的质量流量和试验结果的比较"

算例编号主动流流量试验值mp,exp/(g/s)引射流流量试验值ms,exp/(g/s)主动流流量模拟值mp,num/(g/s)引射流流量模拟值ms,num/(g/s)主动流流量 误差/%引射流流量 误差/%
A119.2510.7017.0510.96-11.432.43
A215.556.6016.036.923.094.85
A314.676.2115.066.752.668.70

图17

不同进口压力的喷嘴轴线蒸发传质速率"

图18

不同进口压力下喷嘴内的蒸发传质速率"

图19

不同进口压力下的喷嘴内的空化传质速率"

图20

不同进口压力下的喷嘴轴向干度"

图21

不同进口压力下的喷嘴轴向Mach数"

图22

不同进口温度的喷嘴轴向蒸发相变传质速率"

图23

不同进口温度的喷嘴轴向空化相变传质速率"

图24

不同进口温度的喷嘴轴线Mach数"

图25

不同进口温度的喷嘴轴线干度分布"

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