Dynamic Characteristics Study Of Radial Inflow Turbine Integrated In ORC System

Organic Rankine Cycle(ORC)transforms low-grade heat energy into high grade electric energy,applicable to industrial waste heat,geothermal energy,ocean thermal energy and other domains.ORC system is featured with simple structure,environmental friendliness and no noxious gas emission,and its key transduction equipment is organic working medium expander.Based on the laboratory-built platform of ORC low temperature waste heat power generation,this article has studied the performance of self-designed radial-inward-flow turbine.It has respectively studied the working performance of radial-inward-flow turbine,the vibration characteristics of rotor system;it conducted experiments and numerical researches on flow characteristics of radial-inward-flow turbine,critical speed and unbalance response characteristics of rotor system,and it has optimized the turbine structure.With R123 being the working medium,the experiments studied the rules change of radial-inward-flow turbine and its performance.Radial vibration of spindle of radial-inward-flow turbine has been measured and vibration characteristics of rotor system has been studied.Results show that with working medium flow increasing,the rotational speed goes up,and there is an optimal rotational speed,which makes isentropic efficiency of radial-inward-flow turbine be maximized.With the temperature of hot source increasing,the optimal rotational speed increases constantly.With cold source flow increasing,the pressure ratio,rotational speed and expansion power of the turbine rise constantly,and system thermal efficiency increases gradually.With rotational speed of the turbine increasing,radial vibration amplitude of spindle increases,and characteristic frequency of vibration peak value increases;the maximum rotational speed of the turbine reaches 54850r/m,and the maximum isentropic efficiency is 83.6%,and the maximum power of the expander is 3.041kW,thus reaching the design requirements.Making use of the software ANSYS Fluent,the experiment sets up the calculation model of single channel number of the internal flow of the turbine,and it has studied different established angles of guide blades,the number of static blades of guide blades and the impacts of non-spreadable paraboloid of movable blades on internal flow characteristics.According to the experimental data,by comparing calculated values of parameters,such as working medium flow,expander power and isentropic efficiency,it verified the correctness of single channel model;in accordance with numerical research results,the turbine structure has been optimized.After the optimization of guide blades,circumferential speed increases by 7.4%and velocity coefficient increases by 1.12%.After the optimization of movable blades,the efficiency of impeller increases by 1.05%and its maximum efficiency is 90.3%.After the integration and optimization of the turbine,total static efficiency rises by 1.7%.With ANSYS Workbench,the modal analysis of impellers has been made;natural frequency and vibration modes of impellers and first-eight-order rotors were obtained;the critical speed of turbine rotor system is analyzed.Results show that when the natural frequency of impellers under pretension is greater than the natural frequency of impellers without pretension,stiffening effects of centrifugal force on impellers;the first-order critical speed of rotor system is 33218 r/min,and the second-order critical speed is 65061 r/min.With the bearing stiffness increasing,the critical speed increases rapidly at first,and then it increases slowly and gradually.Selecting bearing stiffness of rotor system as 1×10~8N/m is relatively reasonable;with the elongation of spindle end increasing,the critical speed of rotor decreases constantly;with the density of impellers increasing,the critical speed decreases gradually.It is found by sensitivity analysis that the impacts of bearing stiffness on critical speed rotor are greatest,and impacts of impellers density takes the second place,while those of elongation of spindle end are minimal.Based on ANSYS APDL,combining with the analysis results of rotor modal and critical speed,this article made unbalance response analysis of rotor system of the turbine,and respectively studied the relations between response amplitude and rotor structural parameters,unbalanced positions and balance accuracy.Results show that with spindle diameter increasing,unbalance response amplitudes of nodal points in rotor system decrease;with the increase of axial span,response amplitudes of nodal points of impellers rise gradually;those of spindle end first decrease and then increase and those of bearing node go up.With the length of spindle end increasing,unbalance response amplitudes of nodal points in rotor system decrease gradually.In the same balance accuracy,when intermediate distance between impellers and two bearings increases,unbalance response amplitudes of nodal points in rotor system first decrease and then increase;when the unbalance location is fixed,with balance accuracy decreasing,unbalance response amplitudes of nodal points in rotor system all increase correspondingly.The first-order response amplitudes of nodal points in spindle end are larger than those of the second-order,and the first-order response amplitudes of impellers and bearing nodes are both less than those of the second-order.