【作者】 张磊；

【导师】 张扬军；

【作者基本信息】 清华大学 ， 动力工程及工程热物理， 2017， 博士

【摘要】 通过有机朗肯循环回收利用内燃机余热是实现汽车节能减排的重要途径。涡轮低比速设计是提高车用有机朗肯循环系统轴系可靠性、降低系统成本的有效手段。低比速涡轮效率低是车用有机朗肯循环系统研发所面临的主要难点和瓶颈,深入研究低比速涡轮内部流动机理,探讨提高低比速涡轮效率的流动控制方法,具有重要的理论意义和工程价值。论文的研究工作主要包括以下几个方面:论文仿真研究了车用有机朗肯循环低比速涡轮在高、低压比工况的流动机理。发现转子叶片前缘载荷高导致通道损失增加、轮盖刮削力强导致间隙损失增加是引起涡轮效率降低的主要原因。在高压比工况,叶片前缘载荷高使得流体在吸力面分离形成通道涡,导致转子通道有效流通面积减小,引起叶片压力面中后部流速增强,造成压力面摩擦损失和尾缘损失增加。在低压比工况,轮盖刮削力强导致从吸力面流至压力面的间隙刮削流增强,刮削流体跨通道迁移从相邻叶片间隙流出,使得近轮盖壁面刮削掺混损失增加;定义了定量评估刮削力强弱的无量纲参数刮削比,发现了导致间隙泄漏流体跨流道迁移的临界刮削比。论文深入研究了转子叶片中部后弯、前缘倾角对叶片前缘载荷的影响规律。发现后弯角对叶片前缘载荷影响较大,后弯角增加能降低转子叶片前缘载荷,减小吸力面分离损失,但使得转子通道面积减小引起压力面相对速度增加,增大压力面摩擦损失和尾缘损失,适当提升后弯角可降低吸力面分离损失且减小压力面摩擦损失和尾缘损失;前缘倾角对叶片载荷影响较小,前倾使得叶片前缘载荷略有增加而后倾使得叶片前缘载荷略有降低。仿真结果表明通过后倾后弯复合叶型设计,涡轮效率在高压比工况可提升约3%。论文深入研究了叶尖中心型线角度和轮盖半径对叶尖刮削比及间隙速度流场的影响规律。发现叶尖中心型线角度降低能减小叶片前缘至中部叶尖刮削比,削弱刮削掺混损失,但增大叶尖尾缘刮削比及刮削掺混损失;轮盖半径降低可减小叶片中部刮削比,但较小轮盖半径降低反而增加了叶片尾缘刮削比,而轮盖半径降低较多可减小叶片尾缘刮削比。仿真结果表明通过叶尖几何低刮削比设计,涡轮在低压比工况叶尖间隙损失降低约40%,效率提升5%以上,对有机朗肯循环低比速涡轮性能进行了试验研究,验证了叶尖几何低刮削比设计流动控制方法在低压比工况的有效性。更多还原

【Abstract】 Internal combustion engine waste heat recovery through organic Rankine cycle is one of the most important technologies for automobile energy conservation and emission reduction.Turbine low specific speed design is an effective way to largely improve the vehicle organic Rankine cycle system reliability and furthermore reduce the system cost.Low thermal efficiencies of low specific speed turbines are the main difficulties and challenges of automotive organic Rankine cycle system development.It is of great theoretical significance and engineering value to research the flow mechanism and find out the flow control method to improve the efficiency of low specific speed turbines.The work of this PhD thesis mainly includes the following aspects:In this paper,the flow mechanisms of low specific speed turbine under high and low pressure ratio operating conditions are researched.It is found that large rotor passage loss caused by high blade leading edge loading and large tip clearance loss caused by strong casing scrapping force mainly lead to the low efficiencies of low specific speed turbine.In the high pressure ratio operating condition,high blade leading edge loading leads to flow separation on the blade suction surface,and then the formed separation vortex reduce the effective flow passage area and further increase the relative velocity on the pressure surface from the midchord to trailing edge,and finally increase the friction loss and trailing edge loss on the pressure surface.In the low pressure ratio operating conditions,strong casing scrapping force leads to the fluid moving from suction side to pressure side through blade tip clearance,which is called scrapping flow.The scrapping flow can further move from one channel suction side to the neighboring channel pressure side,which largely increa se the mixing loss near the casing.The dimensionless parameter scrapping ratio to quantitatively assess the strength of scrapping force is defined,and the critical scrapping ratio to cause the migration of scrapping flow across the channel is found.In this paper,the influences of rotor blade midchord backward bowed angle and leading edge leaned angle on the blade leading edge loading are discussed deeply.It is found that midchord backward bowed angle has great influence on the blade leading edge loading.The increase of backward bowed angle can reduce the blade leading edge loading,decrease the separation loss on the blade suction surface,but increase the friction loss and trailing edge loss on the pressure surface because of the increase of pressure surface relative velocity caused by effective flow passage area decreasing.Appropriate midchord backward bowed angle can reduce both separation losses on the suction surface and friction and trailing edge losses on the pressure surface.The influence of leading edge leaned angle is small,and in general forward leaned angle can increase the blade leading edge loading slightly and backward leaned angle can reduce the blade leading edge loading slightly.Simulation results indicate that through the combination of backward leaned and backward bowed blade design,the turbine isentropic efficiency at high pressure ratio operating condition can be improved by nearly 3%.In this paper,the influences of rotor blade shroud camberline angle and casing radius on the tip clearance scrapping ratio and velocity flow field are discussed deeply.It is found that the decrease of shroud camberline angle can reduce the scrapping ratio and mixing loss caused by scrapping flow from leading edge to midchord of tip clearance,but increase the scrapping ratio and related mixing loss on the trailing edge of tip clearance.Reducing the casing radius can largely decrease the scrapping ratio on the midchord of tip clearance,but small casing radius decreasing increase the scrapping rat io on the trailing edge on the contrary,only large casing radius decreasing can reduce the scrapping ratio on the trailing edge.Simulation results indicate that low scrapping ratio design of tip clearance related geometry parameter can largely decrease the tip clearance loss nearly 40% and improve the turbine isentropic efficiency by no less than 5% in the low pressure ratio operating condition.And furthermore low specific speed turbine performance is tested and the effectiveness of the low scrap ratio design strategy to reduce the tip clearance loss in the low pressure ratio operating condition is verified.更多还原