Flow Mechanism And Control Of Organic Rankine Cycle Low Specific Speed Turbine For Vehicle Applications

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.