【作者】 叶天震；

【导师】 马一太；

【作者基本信息】 天津大学 ， 工程热物理， 2007， 博士

【摘要】 自然工质CO2有着广阔的应用前景。本文以研究CO2满液式蒸发器为目的,研究了CO2沸腾换热的强化及水-水热泵中采用满液式蒸发器的相关问题,并重点实验研究CO2在水平光滑铜管和机械加工表面强化管管外沸腾换热性能。在充分调研了制冷剂沸腾换热的性能的基础上,对CO2的沸腾换热性能做了深入分析和研究。研究结果表明,CO2沸腾传热符合一般物质沸腾传热的规律,与其他常规工质相比,CO2具有换热系数高、易干涸,受静液高度影响小等特点。论文在水-水热泵实验台上对CO2在水平光滑铜管和机械加工表面强化管外池沸腾换热进行了研究。实验装置解决了较高压力下的密封、换热管温度测量和可视化的技术难点,根据实验数据,拟合出一定压力和热流密度下,CO2水-水热泵中光管和实验用强化管表面平均沸腾换热系数的关联式,为满液式蒸发器换热计算提供了必要的实验数据,具有重要工程价值。当蒸发压力P=3.2MPa时,在实验热流密度(28.69~54.27kW/m2)范围内,实验用强化管的强化倍率为2.02～2.24。实验结果表明,现有商用强化管对CO2管外沸腾换热有较好的强化作用,可直接应用于CO2系统满液式蒸发器。论文对光管和内螺纹管在内压和外压作用下的强度进行了有限元分析,指出对于内螺纹管,螺距对管强度影响不明显;在相同压力下,管受内压时最大等效应力发生在内螺纹根部的管内壁处,比受外压作用更容易受到破坏,为满液式蒸发器强化传热管的选择提供了依据。论文对CO2水-水热泵中使用满液式蒸发器进行了分析,并对CO2用满液式蒸发器提出了结构上的建议。论文还对纳米气泡的现象进行了分析,提出在CO2液态物质中,可能存在微小的异相气泡。更多还原

【Abstract】 Natural refrigerant CO2 has a broad perspective of application. This thesis aims to study flooded evaporator used in CO2 refrigeration system by digging into the relevant questions of adopting flooded evaporator in water-to-water heat pump system and the enhancement of boiling heat transfer of CO2. Also this thesis emphasizes the experimental study on boiling heat transfer of CO2 on smooth and fabricated horizontal copper tubes.Based upon the thorough investigation and research of boiling heat transfer of refrigeration preparation, a deep analysis and study on boiling heat transfer of CO2 was performed. The research result shows that the boiling of CO2 is according with the rules of that of ordinary substances. And comparing to other general refrigerants, CO2 has the characteristics of high heat transfer coefficient, easy dry-out phenomenon, less influenced by the liquid height.The thesis studies the boiling heat transfer of CO2 on smooth and fabricated horizontal copper tubes on a water-to-water heat pump system test platform. The experimental device solved the technical difficulties of sealability, the measurement of temperature and visualization of heat transfer under relatively high pressure. This thesis provides the necessary experimental data for the calculation of heat transfer in flooded evaporator and has important project value. Correlations for the average boiling heat transfer coefficients of CO2 on smooth and fabricated horizontal tubes in water-to-water heat pump with certain evaporation pressure and heat flux are obtained from the experimental data.The enhancement ratios of fabricated tube are 2.02-2.24 within the range of experimental heat flux (28.69-54.27kW/m2) when the saturation pressure is 3.2MPa. The experimental results show that the existing commercial enhanced tubes have good strengthening effect for the pool boiling heat transfer of CO2 and thus can be directly applied to flooded evaporator in CO2 system.The thesis performs a finite element analysis of the intensity of smooth tube and internal-thread tube under both internal and external pressure. It points out that for the internal-thread tube, the distance between threads has no significant impact on the strength of the tube. When under the same pressure, the maximum equivalent stress occurs at the inner wall of the root of the internal-thread as the tube is under high internal pressure. It is easier to be damaged than under the impact of high external pressure. This result provides the gist of choosing enhanced heat transfer tubes for flooded evaporator.The thesis performs an analysis of adopting flooded evaporator in CO2 water-to-water heat pump system and brings forward the structural suggestions for flooded evaporator used in CO2 system.The thesis also analyzes the phenomenon of Nano bubbles and indicates that the tinny heterogeneous bubbles might exist in liquid state of CO2.更多还原