Experimental And Simulation Research On Windage Of Turbomachinery Rotor In Carbon Dioxide Working Fluid

The supercritical carbon dioxide Brayton cycle has the advantages of high system efficiency,compact system components and good economy.It has become one of the most promising energy conversion systems at present.As the carbon dioxide working medium in the circulation system leaks into the stator-rotor cavity of the turbomachinery,the friction between the working medium and the rotor surface brings amounts of windage loss to the system and reduces the efficiency of the whole machinery.In this paper,based on the problem of rotor windage of supercritical carbon dioxide Brayton cycle turbomachinery,an experimental measuring device for rotor windage is built and the characteristics of rotor windage are studied by experiment and simulation.According to the experimental results,the rotor windage increases with the increase of the speed and increases with the increase of the system pressure.The rotor windage does not change much under the influence of temperature.The semi-empirical relationship of rotor windage which is suitable for carbon dioxide working fluid under medium pressure conditions is obtained by fitting in the paper.Comparing the numerical simulation results with the experimental results,a reliable numerical simulation method for calculating the rotor windage is obtained.The Taylor vortex is successfully simulated through the numerical simulation calculation,and the basic flow state in the annular gap is analyzed.It is found that rotor speed doesn’t change the shape of the Taylor vortex in the annular gap,but it obviously changes the strength of the Taylor vortex in the annular gap.As the speed increases,the Taylor vortex becomes stronger and the rotor windage becomes larger.The system pressure and temperature conditions do not significantly change the shape and strength of the Taylor vortex in the annular gap.The rotor windage which is affected by the system pressure and temperature is mainly due to changes in the physical properties of the working fluid.The Couette Reynolds number is used to describe the flow state in the gap,and the ratio of the gap width to the rotor radius is used to characterize the geometric factor of the rotor-stator structure.Through numerical simulation of the rotor windage calculation model,it is found that the friction coefficient decreases with the increase of Couette Reynolds number,and the larger the Couette Reynolds number is,the larger the strength of Taylor vortex becomes.The friction coefficient increases with the increase of the ratio of the gap width to the rotor radius,and the larger the ratio is,the smaller the strength of the Taylor vortex becomes.From the comparison between the simulation results and the calculation results of the empirical relationships,the most suitable empirical relationship form at this stage is Cf=af(δ/r1)Reδb.