Researches On Cooling Characteristics Of Supercritical CO₂ High-Temperature Turbine Rotating Shaft Annulus

In supercritical carbon dioxide(SCO2)Brayton cycle power generation system,SCO2 high-temperature turbine is an indispensable component.Because the dry gas seal of SCO2 high-temperature turbine has poor heat resistance and is difficult to work in the high temperature environment of the turbine,it is necessary to take active cooling measures for the turbine main shaft to reduce the environmental temperature of the dry gas seal and ensure the operational stability of the seal.The active cooling design of the SCO2 high-temperature turbine introduces a low-temperature SCO2 fluid into the gap between the impeller and the seal for rotating convective heat exchange,achieving the purpose of cooling the main shaft and protecting the dry gas seal.In view of the dry gas seal cooling problem of SCO2 high-temperature turbine,this investigation carried out a numerical study of the coaxial rotating flow and heat transfer of SCO2 in the gap between the main shaft and the casing(called Taylor-Couette-Poiseuille flow,TCP flow for short),and completed the design of a SCO2 rotating cooling experimental system.First,the physical model of numerical simulation was established,and the turbulence model,meshing method,boundary conditions of numerical simulation were introduced.Then,the grid independence and turbulence model were verified.Due to the lack of reliable SCO2 TCP flow experimental data,the numerical simulation method was verified by heat transfer experimental data of air TCP flow,and heat transfer experimental annulus SCO2 flow.By comparing with the existing experiment data,it was shown that the numerical simulation method used in this thesis had sufficient precision to analyze the convective heat transfer of SCO2 TCP flow quantitatively.Secondly,in order to explore the local flow and heat transfer characteristics of the SCO2 TCP flow,a typical case was selected for local flow field and heat transfer analysis.The results showed that Taylor vortices in the TCP flow were secondary flow caused by the coupling effect of the rotating centrifugal force and the pressure gradient of the inner and outer walls.Taylor vortex was continuously formed and moved with the axial flow to the annulus exit until it is completely dissipated.It appears that Taylor vortex plays a certain effect on enhancing the overall heat transfer of the TCP flow.The labyrinth structure can restrain the formation of the Taylor vortex and strengthen the overall heat transfer effect of TCP flow.Thirdly,to further explore the flow and heat transfer law of SCO2 TCP flow,the four key factors of Taylor number,axial Reynolds number,aspect ratio,and radius ratio have been investigated respectively.It is found that the reduction of Taylor number,the increase of axial Reynolds number,the decrease of axial ratio,and the increase of radial ratio can significantly restrain transition and development of Taylor vortex,improve the flow stability.The increase of the Taylor number increases the circumferential flow velocity of the TCP flow and strengthens the Taylor vortex,which is the best enhancement on the overall heat transfer.The increase of the axial Reynolds number and the radial ratio increases the axial flow velocity and the Taylor vortex correspondingly decreases,which also fairly enhances heat transfer.The increase of axial ratio makes the axial distance of the heat transfer section increased,and weakenes the overall heat transfer performance.In order to predict the stability of the TCP flow and the starting position of the Taylor vortex transition,the stability distribution of each working condition of the TCP flow and the relationship between the Taylor vortex transition point and the four key factors are summarized.To predict the overall heat transfer performance of SCO2 TCP flow,the influence of temperature field and geometric structure are introduced,and a new heat transfer correlation is presented.Finally,in order to verify the reliability of the SCO2 turbine shaft cooling design scheme and the reliability of the numerical simulation research of SCO2 TCP flow,the overall schematic design of the SCO2 rotational cooling principle experimental system has been carried out.This scheme solved the test problem under the coupled conditions of high temperature,high pressure,and high speed.In the experimental system,a noncontact electromagnetic induction heating system was designed to achieve the purpose of rapid heating of the high-speed rotating surface and stable temperature control.In order to simplify the high-pressure sealing of the experimental system,a magnetic coupling was used for a fully enclosed design to ensure the system operating reliability and safety under high-pressure condition.A shaft surface temperature measuremen system formed by the combination of a micro thermocouple and a high-speed slip ring was designed to achieve accurate measurement of the wall temperature on a high-speed rotating under high pressure.Through the numerical simulation of SCO2 TCP flow,the local flow and heat transfer characteristics,and the influence of various factors on the flow and heat transfer are studied.The vortex transition position calculation formula and heat transfer correlation are presented,and a new rotating cooling experimental system is designed,which has laid a theoretical foundation for carrying out relevant experimental research.