| Turbine is also the most important component in the power system of trans-media vehicle,which needs to take into account both air and underwater operating modes.It is one of the key technologies to be considered in the power system design of trans-media vehicle.Therefore,it is necessary to carry out a detailed design of the turbine,the main working component of the trans-media vehicle engine,and study its internal unsteady flow mechanism.First of all,this paper proposes a design method for trans-media engine turbines using the same set of turbine rotors in two modes of air and water,aiming at the situation that the turbine speed,thermal properties of working medium,flow rate and flow boundary conditions are quite different in two modes of air and water.The design process mainly focuses on the design points of airborne mode turbines and takes into account the design points of underwater mode turbines.Turbine efficiency,rotor stress requirements and local air intake rate under the two modes in air and underwater are taken as the decision conditions for cyclic iteration until the design requirements are met.Unsteady computational fluid dynamics method was used to analyze turbine performance under two modes of air and water.Flow characteristics,flow loss sources and rotor blade load distribution were studied under different sequential positions.By adjusting turbine speed and expansion ratio,turbine flow characteristics under two modes were studied.Secondly,in view of the strong three-dimensional,unsteady and non-uniform flow characteristics at the oblique outlet of the turbine nozzle of the trans-media engine,the very large expansion ratio turbine was optimized by adjusting the geometric configuration and arrangement of the nozzle.The maximum height of the nozzle ellipse miter outlet is lower than the inlet height of the rotor blade,and there is a rotation margin around the center of the nozzle ellipse outlet.The turbine model under different pitch angles of the nozzle was calculated to explore the turbine flow characteristics under different pitch angles of the nozzle.The boundary layer on the long side of the elliptic nozzle outlet was separated seriously,so the two adjacent nozzle outlets were fused.The flow of the short side outlet of the downstream nozzle was used to rectify the wake on the outlet side of the upstream nozzle,and the turbine model under different nozzle overlap degrees was calculated to explore the turbine flow characteristics under different nozzle overlap degrees.The nozzle model with rectangular section is established,and the flow characteristics of the nozzle turbine model with different sections are compared and analyzed.There is an axial clearance between the nozzle outlet and the rotor blade,and the high-speed fluid ejects from the nozzle generates flow dissipation in the circumferential and radial directions in the clearance.Turbine models with different axial clearances are established and calculated,and turbine flow characteristics under the influence of different axial clearances are explored.Finally,in view of the windage effect in the non-intake region of the turbine of the transmedia engine,a geometric model of the turbine exhaust housing is established to explore the interference characteristics of the coupled exhaust housing on the unsteady flow of the super large expansion ratio turbine.Unsteady calculation was carried out for turbine models with different exhaust baffle coverage,and the uniformity of pressure distribution at inlet and outlet of rotor blade runner was analyzed under time-uniform results.Moreover,the influence of windage effect on overall turbine performance caused by pressure fluctuation at inlet and outlet of turbine runner in non-intake area under unsteady conditions was explored.The calculation model of axial clearance between different exhaust baffle and rotor blade outlet was established to analyze the influence degree of axial movement of gas in rotor blade passage in non-intake area caused by circumferential movement of gas in clearance.The design method of super expansion ratio turbines for trans-media engines and the study of unsteady flow characteristics in turbines proposed in this paper can provide certain references for the design of trans-media power system and local intake turbines with super expansion ratio. |
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