Flow Characteristics Research On Supercritical Carbon Dioxide Brayton Cycle Axial Flow Turbine

The supercritical carbon dioxide（SCO₂）Brayton cycle has many advantages,including high efficiency,compactness and good environmental friendliness.As one of the most promising power cycles for the future,much work has been done on the application of the SCO₂ Brayton cycle on nuclear energy,waste heat recovery,solar energy and other fields.The Brayton cycle system performance depends much on the SCO₂ turbine,which is one of the key power units of the cycle.In this paper,the SCO₂ axial flow turbine is taken as the research object,and its desig n method and flow characteristics are carried out.The one-dimensional（1D）thermodynamic design and aerodynamic design of the SCO₂ axial flow turbine were conducted.Based on the geometrical parameters,the three-dimensional（3D）model of the turbine was created with ANSYS-Blade Gen.And the ANSYS-Turbo Grid was adopted to generate the mesh of the turbine.And the CFD simulation was conduct by the ANSYS-CFX.Comparing the 1D design values and the CFD simulation results,it was can be seen that the parameters deviations were within 2%.And the isentropic efficiency of the SCO₂ axial turbine was 84.42%.The numerical simulation results could truly reflect the fl ow state in the flow passage.The temperature and pressure distribution in the rotor and stator passage were reasonable.There was no obvious flow separation and vortex in the flow field.The working medium can flow smoothly through the passages.Based on the CFD simulation,the off-design performances of SCO₂ axial flow turbine were studied with different rotational speeds,inlet temperature and mass flow rate.At high inlet temperatures,with the increase of rotational speed,the turbine efficiency first increased and then stayed in a high range.When the speed deviated from the design value,the output power decreased.A 20%change of rotational speed（±2000rpm）can result in approximately a 1.469%change of mass flow rate（（?）2.710kg/s）.With the increase of inlet mass flow rate,the turbine efficiency increased slowly at first and then decreased rapidly.With the same range of mass flow rate changed,the increase of mass flow rate made the decrease of turbine efficiency more obvious.And the output power of the turbine increased with increasing the inlet mass flow rate,and a±10.812%change of inlet mass flow（±20kg/s）can result in approximately a±25.417%change of output power（±2.576MW）.Therefore,turbine performance under off-design conditions was greatly affected by operating parameters.Reasonable parameter variation range was the basis of efficient operation of the axial turbine.The density test of SCO₂ was carried out,and the influence of tip clearance on the performance of turbine under design and off-design condition was investigated.Because the density of the SCO₂ is high,the leakage of small volume will also cause large leakage loss.The tip clearance with labyrinth structure can effectively reduce the leakage flow,but its influence cannot be ignored.With the increase of tip clearance,the discharge rate increased.And the leakage flow mixed with the main flow,resulting in a large number of vortexes in the flow field,increasing flow loss.Therefore,in the design of SCO₂ axial flow turbine,the minimum tip clearance should be adopted when the machining accuracy is allowed.