【作者】 陈奇成；

【导师】 徐进良；

【作者基本信息】 华北电力大学 ， 可再生能源与清洁能源， 2014， 博士

【摘要】 中低温能源占世界能源总储量一半以上。合理、有效利用中低温能源对人类持续发展具有重要意义。有机朗肯循环作为一种高效的低温热功转换技术,为中低温能源的利用提供了有效途径。目前有机朗肯循环技术仍处于发展阶段,理论分析方法与实际应用均不成熟,具体表现为热力学分析模式单一,换热设备体积大、成本高,严重制约有机朗肯循环技术的发展与推广。本论文针对以上具体科学问题,从系统与部件两个层面,分别开展研究工作。首先基于系统层面研究,提出了有机朗肯循环热力学新方法以及求解方案。新方法的提出改变了传统有机朗肯循环热力学分析方式,不仅将膨胀机输出功与系统热效率完全统一,并且把热源与窄点温差作为重要参数同时引入循环系统中,在充分考虑热源温度与窄点温差双重影响下,通过对膨胀机输出功、窄点温差以及烟气进出口温度等因素的研究,深度剖析有机朗肯循环系统整体循环性能。研究结果表明,在给定热源与窄点温差的约束条件下,系统的热效率、膨胀机的进口压力和有机工质的质量流率随着膨胀机的进口温度的增加而减少。最佳运行工况出现在膨胀机进口处的蒸汽处于饱和状态或轻微过热状态。窄点温差的增加,使得膨胀机进口压力下降,导致了系统的热效率降低。目前将ORC热力学分析新方法作为指导思想,探索并筛选出甲苯、苯和环己烷三种有机工质,不仅适用于不同中温热源有机朗肯循环,并且具有更广泛的运行范围。在有效回收余热的同时,将膨胀机输出功最大化。蒸发器与冷凝器作为有机朗肯循环主要换热设备直接影响系统循环效率,其主要科学问题体现为小温差换热需求导致换热面积大,成本高。在最佳运行工况下,冷凝器(?)效率最低,因此提高冷凝器换热性能,降低其不可逆损失,对提高整个系统性能具有重要作用。新型相分离流动结构的构想与实施,解决了这一科学难题。本论文针对相分离流型调控过程开展数值研究,研究结果不仅从多角度揭示了垂直相分离冷凝管冷凝强化换热机理主要表现为薄液膜传热模式与自发性振荡流动循环结构,同时还建立了不同重要参数之间的定量关系。对于垂直相分离冷凝管内气泡流动过程中出现的泄漏现象给予了理论性分析,通过拟合曲线获得气泡泄漏判别式。本论文进一步将相分离冷凝管拓展到了小重力和微重力环境中,总结了重力对相分离流型调控过程的影响,在不同重力条件下,流型调控后依然呈现“气在壁面,液在中心”的全新分布模式。重力越小,调控后液膜厚度减小幅度越大,冷凝换热量越大,特别是在微重力情况下,环隙区域内完全被气体占据,膜厚度减薄到了1/3,冷凝换热量提高57.4倍。在本论文研究过程中,发展了多种数值方法。其中包括跨尺度网格系统的建立,动态参考坐标系的应用,局部动态网格加密技术。在获取精确计算结果的同时,更加节省了计算时间,解决了多尺度计算的难题。本论文的研究成果,极大的拓展了对有机朗肯循环及流型协同理论的认识,为有机朗肯循环系统设计和运行,以及相分离冷凝管的应用等提供科学指导。更多还原

【Abstract】 Low grade thermal energy (heat) such as waste heat, geothermal, and heat from low to moderate temperature solar collectors, accounts for more than one half of the total heat generated worldwide. It is significantly to utilize the energy from low grade thermal energy for human sustainable development. Organic Rankine Cycle (ORC) is applied efficiently to recover the low grade thermal energy as a kind of the thermal power conversion technology. At present, however, the development and application of the ORC are restricted due to the unitary thermodynamic analysis mode and the requirement of the large size for heat transfer equipment. In this paper, the research on ORC is carried on from both system and component to improve the ORC performance.A new design method for Organic Rankine Cycles coupling heat source and solution strategy are proposed firstly based on the ORC system analysis. The new design method is not only building the connection between expansion power and thermal efficiency, but also considering influence of the heat source and pinch temperature difference on the ORC performance. The results indicate that With constraint of the given heat source and pinch temperature difference, the system thermal efficiency, expander inlet pressure and mass flow rate of the organic fluid are decreased with increases in the expander inlet temperatures. The optimal condition appears at the saturated or slightly-superheated vapor state at the expander inlet. The increase in the pinch temperature differences yields the decreased expander inlet pressure to reduce the system thermal efficiency. Furthermore, the three organic working fluid, toluene, benzene and cyclohexane, are selected for medium temperature ORC system due to the higher thermal efficiency and larger operation range applying the new design method.As the main heat transfer equipment, the evaporator and condenser are influence on the ORC system performance strongly. It is point that the exergetic efficiency of the condenser is lowest in the ORC system via analyzing based on the new design method. Besides that, the low temperature difference heat transfer process in the evaporator and condenser leads to the requirement of the larger size and higher manufacture cost. Phase separation condensation tube is applied to solve this scientific problem. In this paper, the numerical simulation is carried on to quantify the relationship among the different parameters. The simulation results reveal that the mechanism of the phase condensation tube enhance heat transfer is contributed by the extra liquid film and three-levels of liquid circulation. The tube behaves the upward mixture flow in the annular region and downward liquid flow in the core region. Void fractions are exact zero in the core region and larger in the annular region, indicating the gas phase flowing in the annular region and inside of the mesh cylinder is liquid. Liquid film thicknesses are significantly decreased by the modulated flow. The three-levels of liquid circulation promote the liquid mixing over the whole tube length and within the radial direction. These circulations were performed through mesh pores. Besides that, the critical criterion is proposed to prevent the bubble leakage deteriorating heat transfer. The research will further developing into the small gravity and microgravity environment. The phase distribution still keeps "gas near the tube wall and liquid in the tube core". The liquid film thickness and condensation heat transfer quantity decrease significantly as the gravity decreasing. Especially for the microgravity, the annular region of the tube is occupied by gas totally.During the numerical simulation process, some kinds of the numerical technology, including the multiscale grid system, dynamic mesh adaption and the frame of reference coordinate, are developed and employed to reduce the computing period.The conclusions of this paper will promote the ORC system and modulating flow pattern theory research, and provide support for the ORC system and phase separation condensation tube design and application.更多还原