【作者】 管海清；

【导师】 马一太；

【作者基本信息】 天津大学 ， 工程热物理， 2005， 博士

【摘要】 环境问题的凸现,使自然工质CO2作为传统工质的替代物重新回到制冷空调领域。本文的主要研究目的就是努力提高CO2跨临界循环系统的性能,使之达到与传统工质相应的COP,促使CO2空调热泵系统早日走向实际有效应用,为传统工质的替代提供最终解决方案,实现空调热泵技术的可持续发展。本文根据对目前国内外对自然工质CO2跨临界循环系统的理论和实验研究现状的分析,以提高系统效率为目标,对CO2热泵系统的性能提升潜力进行了理论分析,并且以CO2跨临界循环性能提高最具价值的膨胀机-压缩机技术研究为核心,为CO2空调热泵技术走向实际有效应用提供理论依据和技术支持。CO2跨临界循环的特点为系统压力较高、简单循环运行效率较低。本文针对其高压特点,分析了系统的安全性及其最优压力对系统性能的影响。为提高CO2跨临界循环系统的性能,对简单蒸气压缩制冷循环进行的主要技术改进方案可分为两类:减小节流损失;减小放热过程不可逆损失,使放热过程向等温放热过程逼近。通过引入回热器、喷射器、经济器、膨胀机等辅助制冷部件来缩小实际循环与卡诺循环的差别。在热力学理论的指导下,对于各种新型循环方式的系统性能提高潜力进行评价。本文根据热力学理论,对超临界CO2降压、闪蒸过程膨胀做功与能量输出特性方面进行理论上的探讨。对气液相变中的亚稳平衡特性进行研究,分析影响相变延迟的因素。建立膨胀机膨胀过程中CO2过热液的成核长大热力学模型,计算CO2气化核心的临界半径和临界势垒。通过CO2过热液中能量涨落值与其临界势垒值的对比,分析非平衡相变发生的条件。探讨通过干扰降低膨胀过程生成气化核心的能量势垒,促进相变发生的机理与可行性。采用膨胀机对于提高CO2跨临界循环系统性能具有特殊重要的意义。为减小能量转换和储存损失,尽量提高系统效率,膨胀机所产生的机械功应该被直接利用,可使膨胀机和压缩机同轴联接。研制CO2膨胀机-压缩机是推动CO2走向有效应用的重要途径。以摆动转子式膨胀机-压缩机为开发对象,采用新型的轴向进气控制方案并根据系统容量计算出设计参数。运用摩擦学系统分析影响CO2膨胀机-压缩机润滑性能的因素。对用于CO2跨临界循环系统的四类合成润滑油POE、PAG、PAO、AN/AB的主要性能如混合性、流动性、可溶性、粘温特性、稳定性和润滑性进行分析比较。以膨胀机-压缩机的工作过程(吸气、压缩、排气;进气、膨胀、排气)为研究对象,综合考虑各种因素,建立起工作过程的数学模型并进行数值求解。求出膨胀机-压缩机中工质的主要热力参数(如压力、温度、比容),以及其它特性(如泄漏更多还原

【Abstract】 CO₂ is under serious consideration worldwide as a replacement for conventionalrefrigerants because the severe environmental problems. The objective of this dissertationis to develop a CO₂ transcritical cycle that has comparable or better performance whencompared to a cycle with conventional refrigerants. Therefore this research will try topush the CO₂ transcritical cycle into practice application that may be benefit to thesustainable development in the refrigeration and air-conditioning field.Based on the existing theoretical and experimental research on CO₂ transcriticalcycle of other talent researchers worldwide, this dissertation takes the performanceimprovement of CO₂ transcritical cycle as the main target and focuses on the developmentof rotary expander-compressor in theory analysis and experimentation.The typical characteristics of CO₂ transcritical cycle may be somewhat higheroperating pressure and lower performance in baseline system when compared with asystem using conventional refrigerants. The safety problem and the effects of theoptimism high pressure to its performance are analyzed toward its high operating pressure.In principle, a large number of modifications to the basic CO₂ transcritical cyle arepossible. Those potential improvements of components and advance cycles withadditional components such as IHX, ejector, economizer and expander etc. are analyzedtheoretically. The technology modification can be approximately divided into two classes:reducing the pressure drop power loss and reducing the irreversible loss of heat rejectionprocess.It is investigated on the characteristics of the supercritical CO₂ expansion power inthe process of decompression and flashing process theoretically adopting thermodynamics.Gas-liquid phase change hysteresis in the meta-stable state is investigated and theaffecting factors are proposed. The irreversible thermodynamic model about the bubblenucleation in the CO₂ superheated liquid in the expansion process of expander is founded.The critical radius and the critical potential barrier of CO₂ nucleation are calculated. TheCO₂ energy fluctuation of meta-stable CO₂ liquid is calculated with the statisticthermodynamic method. Therefore the theories about adopting disturbance factors such asforeign substance and ultrasonic to accelerate the nucleation and the bubble growth areproposed.Employing an expander is paramount important to improve the performance of theCO2 transcritical cycle. Integrating an expander and a compressor together will be good toutilize the recovered power directly since the transmission loss is expected to be small.The design parameters of the swing piston type rotary expander-compressor adopting thenewly designed inlet control method are calculated according to the heat pump systemcapacity. The lubrication of the prototype is analyzed with tribology. Those four kind ofsynthetic lubricants: POE, PAG, PAO and AN/AB are compared and valued on their maincharacteristics such as miscibility, fluidity, solubility, viscosity-temperature character,long-term stability and lubricity.The model on the swing piston type rotary expander-compressor is founded tosimulate the operating process e.g. suction, compression and discharge for compressorand inlet, expansion and discharge for expander. The main thermodynamic parameterssuch as pressure, temperature and density of the refrigerant in the unit and othercharacteristics such as internal leakage loss, friction loss and etc. are numerical calculated.The expander-compressor potential can be well estimated and optimization geometricparameters can be gotten through the comprehensive analysis on the simulation results.The CO2 transcritical cycle water-water heat pump test rig was founded to test theperformance of the expander-compressor unit. The experiments indicated that theexpander had obvious improvement effects on the heat pump performance, COP of thesystem can be increased about 20%⁴0% which are favorable results. However, theinternal leak rates were still too high and both an optimized design and improvedmachining accuracy are required to fabricate and operate a CO2 expander at acceptableperformance levels. The disturbance action of minor non-condensation gas NC-1 totrigger the phase change in expander is tested. The result shows the disturbance isfavorable to improve the efficiency of expander. But further research in experiment andtheory is still needed to investigate the mechanism of disturbance to expansion process.更多还原