Study On Flow Interference And Improved Design Of Axial Turbine Coupled With Inlet And Exhaust Casings

Inlet and exhaust casings are important components matching axial turbine,especially exhaust casing is one of the most potential units for further improving the output power of the power plant.The coupled flow field between axial turbine and exhaust casing,which has a great impact on the aerodynamic performance of the turbine and the exhaust casing,is unsteady and complex.Therefore,it is of great theoretical significance and engineering application value to carry out research on unsteady flow interference characteristics of axial turbine coupled with inlet and exhaust casings,as well as three-dimensional flow control and one-dimensional design analysis on this basis.Firstly,full scale unsteady numerical simulation was carried out towards the flow field of turbocharger axial turbine coupling with inlet and exhaust casings,the interference characteristics of turbine rotor blade pressure induced by asymmetric flow field of the exhaust casing was analyzed emphatically so as the distribution of excitation force.In addition,the source of interference was investigated by analyzing the flow structure of the exhaust casing,and the influence of different operating conditions was compared.Results show that inlet casing affects the circumferential distribution of stator static pressure at the leading edge,while the asymmetric back pressure caused by the exhaust casing flow field lead to low frequency pressure fluctuations on the rotor blade surface.The complex flow separation and vortex structure in the exhaust casing is the major contributors for the above flow interference.Secondly,according to the above flow structure and superposition characteristics of high and low frequency excitation forces on blades,a modified scheme of excitation force weakening based on asymmetric vanes was proposed and verified with full scale unsteady numerical simulation on the basis of source analysis.Results show that among the present four asymmetric flow control modified schemes,asymmetric vane spacing wth ‘upper15 and lower13’ layout and asymmetric vane stagger angle with ‘upper +1degree and lower-1degree’layout effectively reduced the flow excitation force level on rotor blade on the premise of hardly affecting the turbine performance.Besides,the high-frequency amplitude of ‘upper15and lower13’ layout is reduced by 98.6% at trailing edge while the low-frequency amplitude is reduced by 52.6% at most,thus it is the optimal layout for its extra slight enhancement on turbine efficiency.Finally,a one-dimensional design model for axial turbine coupling with exhaust diffuser was established and corresponding calculation program was developed based on theoretical analysis,at last the influences of coupled design parameters on turbine and diffuser performance was investigated.The results show that the design parameters of the diffuser are area ratio,mean wall cant angle,divergence semi-angle and skin friction coefficient,which not only affect the pressure recovery performance of the diffuser,but also affect the total to static efficiency of the axial turbine.The mach number at turbine outlet,blade tip clearance leakage flow and outlet swirl flow affect the diffuser performance;therefore,sufficient consideration should be paid to the selection of turbine parameters mentioned above in the further coupling design.The flow interference characteristics,three-dimensional improved schemes,one-dimensional coupled design model and parameter selection rules of coupled axial turbine and exhaust casing studied in present paper can provide a certain reference for the improved design of coupled axial turbine and exhaust casing for the later research.