| The aero engine is known as "the jewel in the crown of modern industry".As the core component of an aero engine,the compressor directly determines the engine development level.When the aircraft is in flight,the compressor blades under frequent air intake conditions(the airflow intake angle of attack is constantly changing),complex flow phenomena(such as flow separation,vortex shedding,etc.)will appear on the blade surface,which will affect the engine.The performance,stability,efficiency and noise caused a series of negative effects.As the compressor develops in the direction of high load and high boost ratio,when the compressor is often operated at a high angle of attack or critical angle of attack,although the compressor has the highest boost ratio at this time,the blade flow loss is also very large It is often accompanied by strong vortex shedding.Once the shedding vortex completely blocks the flow channel,it will cause the blade to stall and cause catastrophic consequences.Moreover,the vortex shedding at the critical angle of attack is also the main noise source of the compressor.Therefore,the research on aerodynamic performance and aerodynamic noise of compressor blades under critical angle of attack is of great significance to the aerodynamic design of compressor blades.As the bionic flow control technology with practical research value,the sawtooth structure of the owl wing has achieved some research results and practical application in reducing noise,such as saw-tooth trailing edge wind turbine blades and sawtooth tail nozzles.The noise reduction of the saw-tooth structure is mainly achieved by controlling the flow field,such as changing the position of the flow separation point,changing the vortex system structure,etc.Such changes also have a significant impact on the flow loss and stability.Therefore,in this paper,the aerodynamic performance,flow loss,and aerodynamic noise characteristics of the serrated trailing edge blade at the critical angle of attack are studied,in order to obtain a compressor pressure ratio that does not affect the critical angle of attack,and can improve the stability of the compressor(Stall margin),the optimal serrated trailing edge blade configuration to reduce flow loss and vortex shedding noise,provides a reference for the aerodynamic and acoustic design of the compressor blade.The main research work of the thesis is as follows:(1)The airfoil unit is the basic element for the design of impeller machinery(fans,compressors,etc.),and is also an important medium for studying the basic theory of aerodynamics.Therefore,the geometric modeling is based on the blade cascade of the aero engine compressor NACA651210.Based on the grid,a reference blade is established.Considering the engineering application and numerical grid division,the reference blades were processed with zigzag structure in an inscribed way to obtain six kinds of serrated airfoil blade models with different relative tooth heights.Unsteady constant value simulation adopts the large eddy simulation method(LES),which can accurately capture the small-scale turbulent flow field near the sawtooth fine structure and obtain detailed information on the separation of air flow.(2)Through the large eddy simulation of the Smagorinsky sub-lattice stress model,the aerodynamic noise and aerodynamic characteristics of the reference cascade under different angles of attack were studied.It was found that with the increase of the angle of attack,the airflow of the compressor cascade turned Bending angle,pressure rise coefficient and flow loss gradually increase,when reaching the critical angle of attack(? = 18 °),the compressor pressure ratio is the highest,but the flow loss is also large;when the angle of attack exceeds the critical angle of attack,the cascade suction Severe flow separation occurs on the surface,the blade stalls,the aerodynamic performance drops sharply,and the loss increases rapidly;at a small angle of attack(? = 5 °),the cascade vortex is mainly dominated by low energy and high frequency small vortices.The noise at the trailing edge appears discrete,and the peak of the sound pressure level appears at high frequencies;when the angle of attack reaches 10 °,the trailing edge vortex appears a Karman-like vortex street with a lower frequency,and the noise at the trailing edge is discrete It is more obvious,and the vortex shedding frequency is basically the same as the peak frequency of the sound pressure level of the noise;at large angles of attack(? = 15 °,18 °),not only the trailing edge shedding vortex but also the suction surface shedding vortex,due to the different scale vortex The evolution of the led to noise at the trailing edge Broadband characteristics appear in the high frequency band.(3)Through the study of the cascade angle of attack characteristics,we know that at the critical angle of attack,the cascade has the largest flow loss and the strongest pressure expansion capability,and it is also accompanied by severe aerodynamic noise caused by shedding vortices.Therefore,in order to improve the stability of the airfoil blade,without affecting the pressure expansion capability of the blade,effective measures are taken to reduce the flow loss and aerodynamic noise at the critical angle of attack.Therefore,the aerodynamic characteristics of six kinds of serrated trailing edge blades with different tooth height ratios at critical angle of attack were studied.Studies have shown that at a critical angle of attack,the sawtooth trailing edge structure can advance the separation point of the boundary layer,reduce the momentum thickness of the separation zone and the loss of the flow separation zone,reduce the thickness of the blade surface layer,thereby suppressing the separation and delay of the boundary layer of the suction surface The blade stalls.At the same time,the zigzag structure can also break the large vortex along the zigzag direction into small vortices,promote the small vortices to evolve into turbulent pulsations,avoid the shedding vortex to block the flow channel,and make the blade flow wake more uniform,thereby reducing flow loss Helps reduce vortex shedding noise.Considering the stability,flow loss,boosting capacity and aerodynamic noise,the sawtooth blades with a tooth height ratio of about 1.2 have the best performance.Compared with the benchmark blade,without affecting the boost ratio,the best serrated blade can improve the stability of 11.7% and reduce the flow loss of 28.1%;predicted by the trailing edge vorticity,it can be reduced by at least 12% Aerodynamic noise. |
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