螺槽排气余速利用单螺杆膨胀机容积和速度膨胀的能量转换与耦合机理

The adoption of positive-displacement machines is an important direction of small-scale Organic Rankine Cycle (ORC) expanders. However, in large expansion conditions or variable working conditions, the performances of positive-displacement expander deteriorate obviously mainly due to under expansion and over expansion losses, and this limits the efficiency improvement of ORC system. The problem can be solved by a combined adoption of the first volume expansion stage generated by the changing basic volume of single screw expander and the second velocity expansion stage generated by utilizing the kinetic energy of screw groove exhaust gas. The present project will study the multiphase flow behaviors of working fluid and lubricant oil in the rotating nozzle located at the end of the screw grooves, the energy conversion mechanisms during velocity expansion, the coupling mechanisms between volume expansion and velocity expansion, the optimization theories and methods of single screw expander with two-stage expansion process. In addition, the power generated and energy conversion efficiency in volume expansion and velocity expansion will be experimentally obtained by a combined adoption of performance tests and rotating telemetry tests under different working conditions. The investigations will deeply reveal the correlation mechanisms between the energy conversion efficiency and the multiphase flow behaviors of working fluid and lubricant oil, the operational conditions of the expander, the structures of the screws and the groove nozzles, reveal the coupling mechanisms between volume expansion and velocity expansion, provide a theoretical support for optimizing the new two-stage expansion process and improving performances of single screw expander in large expansion conditions or variable working conditions.

容积式膨胀机是小规模有机朗肯循环膨胀机重要的发展方向。然而在大膨胀比工况或变工况条件下，由欠膨胀或过膨胀导致的膨胀机性能衰减严重影响了有机朗肯循环的性能。把单螺杆膨胀机封闭容积变化形成的第一级容积膨胀和螺槽内高速流出的工质动能形成的第二级速度膨胀结合起来可以解决其性能衰减问题。本项目主要研究速度膨胀过程中螺槽旋转尾喷管内工质和润滑油多相流动特性与能量转换机理；研究容积膨胀和速度膨胀的耦合机理以及二级膨胀单螺杆膨胀机优化设计理论与方法；同时把旋转遥测技术和性能试验结合起来，实际测算不同工况下容积膨胀和速度膨胀过程中各自轴功、能量转换效率等的变化规律。通过实施本项目可以深入揭示速度膨胀过程中能量转换效率和尾喷管内多相流动、膨胀机运行工况、尾喷管结构等参数之间的关联机理以及容积膨胀和速度膨胀的耦合机理，从而为优化这种新型二级膨胀行为、提高单螺杆膨胀机在大膨胀比工况和变工况条件下性能提供理论支撑。

有机朗肯循环是中低温热能热功转换领域最具前途的技术路线之一。膨胀机是有机朗肯循环实现热功转换的关键部件，采用高性能膨胀机是有机朗肯循环效率提升的关键。单螺杆膨胀机等容积式膨胀机是小规模有机朗肯循环膨胀机重要的发展方向。然而在大膨胀比工况或变工况条件下，由欠膨胀或过膨胀导致的膨胀机性能衰减严重影响了有机朗肯循环的性能提升。目前国内外文献中十分缺乏提升容积式膨胀机变工况性能的有效手段。本项目提出把单螺杆膨胀机封闭容积变化形成的第一级容积膨胀和螺槽内高速流出的工质动能形成的第二级速度膨胀结合起来以解决其性能衰减问题。项目团队构建了螺槽排气余速利用单螺杆膨胀机容积膨胀过程和速度膨胀过程耦合模型，得到了不同条件下两级膨胀过程中压力、温度等参数随螺杆转角的变化规律，揭示了速度膨胀过程中的能量转换机理，发现引入速度膨胀过程可以使膨胀机轴效率提高2~9个百分点，同时高效工况范围得以拓宽；开展了螺槽排气余速利用单螺杆膨胀机三维瞬态仿真，优化了螺槽排气尾喷管结构，使工质流出螺槽的瞬间，其流动方向与螺杆转动的线速度反方向基本一致，可以实现螺槽排气动能高效利用；以此为指导了研制了三台螺槽排气余速利用单螺杆膨胀机，并把其安装到有机朗肯循环实验台上，在变工况条件下开展了性能测试，发现三台样机适宜的膨胀比均约大于其内容积比，最高轴效率达到了65%，当膨胀比增加到最佳膨胀比以上时，轴效率降幅很小，这说明大膨胀比工况下的欠膨胀损失得以减小。利用这些成果可以合理匹配容积膨胀和速度膨胀，进而掌握螺槽排气余速利用单螺杆膨胀机优化设计理论与方法，为优化这种新型二级膨胀行为、提高单螺杆膨胀机在大膨胀比工况和变工况条件下性能提供理论支撑。本项目把基于螺槽排气余速利用的速度膨胀过程引入单螺杆膨胀机，突破和拓展了容积式膨胀机研究内容和研究框架，丰富和发展容积式膨胀机理论体系，这不仅具有重要的学术意义，还有很高的工程应用价值。