The Aerodynamic Integrated Design Of Airframe And Podded Propulsion System For Blended Wing Body Transport

Blended wing body(BWB)has become an ideal configuration for the next generation of wide-body airliners to meet the requirements of "Green Aviation" due to its integrative benefits.The conventional fuel high bypass ratio turbofan engine propulsion system is the best choice for the application of BWB in near term.The integration of BWB and propulsion system brings both fuel saving and noise reduction benefits,as well as special technical challenges,which is one of the critical technologies of the BWB transport.The aft-upper centerbody mounting position of the podded engines is an option with low risk and high feasibility,which can provide better noise shielding and more space for the installation of the high bypass ratio turbofan engine.However,it also leads to greater interference between the airframe and the engine as their close distance than that of the conventional under-wing mounted engines.Therefore,the propulsion airframe integration has become a research focus and a critical technology in BWB design.Aiming at the development of the next generation of wide-body airliners in China,the paper work is done based on the conceptual design of a 300 seat class airliner designed by our research team.The design technologies of airframe-nacelle interference mechanism,powered nacelle design,propulsion airframe integration and engine installation effect for BWB transport with podded engines are investigated in this paper as technical reserve for the development of the next generation of wide-body airliners.The main research work and conclusions are shown as follows:(1)Investigation on the interference mechanism of BWB transport with podded engines.For the problem of the flow interaction between the airframe and the nacelle,by investigating flow characteristics of typical high and low speed conditions,it is found that the upper surface of the BWB centerbody has good performance,which can provide uniform and stable intake for the podded engines.However,the strong shock wave and flow separation are easily generated due to the flow interference between the airframe and the nacelle at high speed.The physical mechanism of the flow interference is as follows: firstly,the large supersonic region and shock wave on the nacelle external surface interferes with airframe surface flow seriously,which induces shock wave and flow separation;secondly,a convergent-divergent channel is formed between the airframe and the nacelle,resulting in the "throat" effect,which produces shock wave and flow separation.The philosophy of propulsion airframe integration is proposed based on the flow mechanism.The powered nacelle design should adapt to the flow characteristics of the BWB transport with podded engines and avoid large supersonic region of the external surface at high speed condition.At the same time,the flow channel between the airframe and the nacelle should be designed to improve the area distribution.Therefore,the “throat” effect is alleviated,and the shock wave and flow separation are eliminated.(2)Investigation on the podded nacelle design.The isolated nacelle is designed based on the philosophy of propulsion airframe integration proposed in this paper and the flow characteristics of the BWB transport with podded engines.The design principles for podded nacelles are proposed with the aim of reducing supersonic region near the external leading-edge and control the rest region to maintain subsonic.The optimization design method for powered nacelles is established on the basis of the parameterization method and the Kriging surrogate model.The initial axisymmetric nacelle is designed based on the design principles of the podded nacelle.By investigating the influence of the geometric parameters on the internal and external flow of the nacelle,the key parameters of the nacelle design and the optimization design principles are determined.Then,the single point and multi-point optimization design are carried out,and the axisymmetric nacelle design options meeting the requirements of podded configuration are obtained.Based on the systematic analysis of the impacts and the mechanisms of inlet distortion for the axisymmetric nacelle under crosswind,the crosswind design requirements are given by changing the curvature of the inlet lip to decelerate the inlet flow and improve its anti-distortion performance,and then the optimization design and its aerodynamic performance of the non-axisymmetric nacelle are obtained.The final design is selected after evaluating the performance of each axisymmetric and non-axisymmetric design options comprehensively.The single point optimization design of the axisymmetric nacelle is chosen as the nacelle design in the following investigation.The selected nacelle has small supersonic region and low flow speed at external leading-edge,and the rest region keeps subsonic,together with preferred intake quality at various low and high speed conditions.(3)Investigation on the application of the nacelle model in aerodynamic integrated design.The applicability of two nacelle models in propulsion airframe integration is investigated.By comparing the engine simulation effect,internal and external flow characteristics and the aerodynamic performance of the isolated flow through nacelle and the powered nacelle,it is found that the intake massflow is the major factor affecting the internal and external nacelle flow.By comparing the flow characteristics of two nacelle models mounted on BWB configuration,it is found that that the intake and exhaust effect on the airframe surface flow at high and low speed conditions is distinct.At high speed condition,the reasonable interference could be identified when simulating the engine by the flow through nacelle,however,the result will be quiet different at low speed condition.Therefore,in order to simplify the design and improve efficiency,the flow through nacelle could be adopted in integrated design to analyze the influence of propulsion system on airframe at high speed condition,and then investigate the power effect on the integrated design at low and high speed condition.(4)Propulsion airframe integration.Based on the philosophy of propulsion airframe integration proposed in this paper and the result of nacelle design and its application,the propulsion airframe integration is carried out by adopting the flow through nacelle model.Firstly,the selection principles of the engine position parameters are proposed by investigating their interference effects.The flow interference should be reduced and the constraints of structural weight,noise shielding and longitudinal control should be taken into account simultaneously.The optimal engine position can improve the area distribution in the flow channel,however,it still cannot eliminate the convergent-divergent effect fundamentally.Secondly,an integration method to channel the flow between the airframe upper surface and the nacelle is constructed.The selection principles of design parameters of the airframe upper surface is proposed by investigating their influences and mechanisms on the airframe surface flow.The negative pressure peaks of the centerbody under the nacelle should not exceed the critical value,and the local pressure “bump” should be avoided at transition sections.After redesigning the airframe upper surface,the shock wave and flow separation between the airframe and the nacelle are eliminated due to the improved area distribution and the decelerated flow in the channel,and the cruise lift-to-drag ratio(L/D)is increased significantly by 11%.Thirdly,the influence of pylon cross section shape on the airframe/pylon/nacelle interference is analyzed systematically.The pylon design method for podded engines are given,which is that the maximum thickness of the pylon and the maximum diameter of the nacelle should not be in the same position,and the pylon length should by increased properly to prevent the rear pylon from contraction immediately.The pylon designed by the above method could reduce the flow velocity in the forepart and the adverse pressure gradient in the rear part,make the surface flow stable,eliminate the flow separation,and avoid the flow interference on the nacelle and the airframe.Finally,the high-speed aerodynamic performance of each design in the evolution history of aerodynamic integrated design is analyzed.The results show that the airframe/pylon/nacelle interference is almost eliminated through aerodynamic integrated design,and the transonic aerodynamic efficiency of the final integrated design is efficient.The cruise L/D of the integrated design is close to that of the clean wing after removing the component drag of nacelle and pylon.(5)Investigation on podded engine installation effect.The engine intake and exhaust effect on aerodynamic performance of the integrated design is investigated by CFD method at both high and low speed conditions,which indicates engine intake and exhaust effect and their mechanisms.The intake and exhaust mainly affect drag characteristics at high speed,while affect lift and pitching moment at low speed.The intake massflow has great influence on the longitudinal aerodynamic performance of the BWB transport,and the jet make the airframe surface pressure increased.The installation effect on engine trust and intake performance is analyzed.The results show that the total drag of installed nacelle increases and the effective thrust loss is about 2.37% at cruise.At low speed condition with high angle of attack,the quality of engine intake flow is not affected by the separation flow,however,the pressure of inner and bypass jet is decreased due to the airframe interference,which leads to the reduction of the installed net thrust and the effective thrust loss by about 2.41%.At crosswind takeoff condition,the intake performance of installed nacelles on each side is lower than that of the isolated nacelle,but still meeting the engine operation requirements.(6)Wind tunnel test validation.The aerodynamic performance and flow characteristics of the integrated design are validated by test in large-scale production wind tunnel.The CFD results are in good agreement with wind tunnel test in aerodynamic performance and the flow characteristics.The numerical method shows high accuracy for the simulation of the integrated design,which could satisfy the design requirement.At high speed condition,the integrated design reduces the flow interference between the airframe and the nacelle.The surface flow keep attached at cruise,and the design has satisfied aerodynamic efficiency.At low speed condition,the power effect of the engine is simulated by the ejector powered engine simulator.The influence of the intake and exhaust on the longitudinal aerodynamic performance is in good agreement with the CFD results.The intake and exhaust mainly affect the lift and pitching moment at low speed,while affect the lateral-directional performance unapparently.