| In order to improve economic suitability and environmental friendliness,modern high-performance civil aviation turbofan engines usually utilize high bypass ratio.In the high bypass ratio turbofan engines,due to aerodynamic matching needs between the high pressure and low pressure turbine,usually there is a diffuser with large radial variation,that is,inter-turbine duct.In order to reduce weight,the overall structure of engine is becoming more compact.This leads to more aggressive design of inter-turbine duct,and towards to super aggressive inter-turbine duct.However,the large curvature and high diffusion rate of super aggressive inter-turbine duct make the complex axial and radial pressure gradient exist in the duct,which is prone to occur flow separation and induce various complex vortex structures.The result either increases the flow loss or deteriorates the inlet flow quality of low pressure turbine.Therefore,the super aggressive inter-turbine duct has become a key issue in the design of civil high bypass ratio turbofan engine.In order to complete the effective design,it is necessary to carry out intense study on super aggressive inter-turbine duct.In this paper,experimental models of super aggressive inter-turbine duct were designed based on an under-studied turbofan engine.The experimental study was carried out around the key parameters(dimensionless length and mean rise angle),and further study focused on the internal flow mechanism by numerical simulation.In the experimental study,three super aggressive inter-turbine ducts with different key parameters were measured by pressure probes and static pressure holes,based on the experiment rig in the Institute of Engineering Thermophysics,Chinese Academy of Sciences.The experimental results show that:(1)the hub and the casing static pressure of super aggressive inter-turbine ducts are repeatedly subjected to the increase and decrease process,alternating show the adverse and favorable pressure gradient.Among the whole duct,adverse pressure gradient has a very large percentage.The pressure change of the hub follows the<adverse-favorable-adverse>mode,and the casing follows the<favorable-adverse-favorable>mode,or<favorable-adverse>mode.(2)The key parameters:the dimensionless length L/h and the mean rise angle Φ reflects the compactness of the ducts in opposite direction.As L/h decreases,or as Φ increases,the ducts’diffusion efficiency decreases and the flow loss increases.(3)Based on the key parameters L/h andΦ,a new parameter S-L(Slope-Length)can be used to reflect the compactness of inter-turbine duct.As the parameter S-L increases(which means the duct is more aggressive),the diffusion capacity is reduced.In order to deeply analyze the internal flow mechanism,with the CFD softwareNumeca which has been verified by experiment,numerical simulation of three super aggressive inter-turbine ducts was carried out.The study shows that the internal flow characteristics of super aggressive inter-turbine duct include the evolution of vortices pairs(born-growth-weakening-disappearance),the disappearance of upstream wakes,and the transport of low-energy fluid in boundary layer.The vortices pairs are formed near the hub side in the front part,and continuously grow up with fluid rolls up from the hub surface.Meanwhile,they moved upwardly under the radial pressure gradient;until the second bend of the hub,the vortices pairs begin to be weakened and gradually disappeared.At the same time,the upstream wake in the duct has always experienced a process of diminishing until disappearance.In addition,the strut changes the pressure difference between the end walls and the internal flow characteristics.In the case of no strut,the pressure difference between the hub and the casing is shrinking as flow goes downstream,and the upstream wake rolls up and transports the hub boundary fluid.In the case of strut,the pressure difference of the hub and the casing roughly remains constant,and the upstream wake has no impact on the hub boundary fluid any more. |
