Design And Flow Control Study Of A Small Scale High Expansion Ratio Organic Vapour Turbo Expander For Automotive Application

The waste heat of internal combustion engine takes up two thirds of all fuel energy,therefore waste heat recovery(WHR)technology has largest potential for energy conservation and emission reduction among all automotive technologies.Among all the WHR technologies,the system based on Organic Rankine Cycle(ORC)is outstanding and in the research front.Moreover,numerous researches focus on the expander as it is the most significant component within ORC system.And turbo expander has advantages such as compact size,high efficiency and low noise emission,hence it is more suitable for internal combustion engine WHR system compared with volumetric expander.While turbo expander usually works with high expansion ratio in order to increase the WHR system efficiency.However,the manufacture of turbo expander with automotive WHR system is problematic as it has low power output,high rotation speed and high expansion ration,and this has already become the bottle neck of ORC technology.This thesis has preliminarily designed an organic radial turbo expander which is used in a heavy duty waste heat recovery system.The radial turbo expander uses R245 fa as working fluid,the output power is about 30 kW and expansion ratio is 8.Low specific speed design characteristic is employed and the specific speed has successfully been reduced to around 50% of normal turbine.All the studies on turbine internal flow loss mechanism are conducted with help of three-dimensional computational fluid dynamic simulation method.It is obtained that shock waves and trailing edge losses in the supersonic nozzle are the main sources that damage turbine efficiency.In addition to the flow separation in the rotor leading edge parts.The influence of nozzle and rotor blade geometry on the turbine flow and efficiency is going to be investigated through computational fluid dynamic method(CFD).Major research objects include nozzle blade inlet and outlet angle,blade shape,blade height and blade inclination angle,besides,rotor geometry objects include blade leading edge bend angle and trailing edge thickness.A novel parabolic curve is put forward to reduce shock wave losses and,on the other hand,bending rotor leading edge to control flow separation.Last but not least,optimizations with the turbine vane and rotor blades are accordingly carried out.As a consequence,optimized turbo expander has an obviously lower nozzle total pressure loss coefficient and higher total-to-static efficiency,a rise with 3.3% on efficiency can be observed.Shock waves and reflected waves are suppressed and a more uniform stream at vane outlet can be obtained.