Investigations On The Flow Characteristic And Forced Response Of Turbocharger In Pulsating And Distortion Inlet Conditions

Turbocharged internal combustion engine is an important technical approach for the vehicle engine to achieve the high performance and small displacement.Moreover,it is also an essential means to satisfy the increasingly stringent emission standard.However,the vehicle exhaust turbocharger has the characteristics of small size and high speed,and the dynamic response of the turbocharger rotor is outstanding.Additionally,the inlet condition of the turbine is fluctuating accompanied with the engine exhaust pulsation,and the inlet of the compressor is distorted flow field caused by the bent inlet duct.In the effects of pulsation and inlet distortion,the internal unsteady flows of turbocharger impellers are enhanced significantly,and the forced response of turbocharger rotor has some novel characteristics.In this paper,aiming at the pulse and distortion inlet conditions of the turbocharger,the internal flow patterns,blade loading fluctuations and forced response characteristics of turbocharger rotor was studied in numerical and experimental methods.The dynamic response mechanism of turbocharger rotor and its influencing factors were investigated in the coupling effects of pulse and distortion inlet conditions.The main research contents and results are as follows:The flow pattern,blade loading distribution and forced response characteristic of variable radial turbine in pulse inlet conditions were studied.The pressure pulse waveform of the sine wave was designed as the inlet condition of the turbine through the reasonable simplification of the engine exhaust pulse wave.The blade loading fluctuation characteristics were determined by conducting the unsteady flow calculation of the turbine in pulse inlet conditions,which is in the coupling effects of the low frequency disturbance caused by the pulse flow and the high frequency disturbance of the guide nozzle vane.The forced vibration characteristics of turbine impeller in pulse inlet conditions were studied using the finite element method.The study shows that the inlet flow angle of the turbine blade is varied with the change of pulse inlet conditions,this causes the unsteady flow in the turbine passage to be intensified.There is an asynchronous phenomenon between the turbine efficiency and the inlet pulse wave in the pulse inlet condition.In a pulse period,the tip clearance leakage flow of the turbine blade is approximately linear with the pulse inlet pressure.The blade loading of turbine not only reflects the low frequency fluctuation of the inlet pulse wave,but also reflects the high frequency disturbance characteristic of the interference between turbine rotor and stator.The inlet pressure pulse is transmitted through the impeller passage and acting on the blade surface of turbine,which makes the loading distributions on the blade surface has positively correlated with the inlet pulse waveform.The forced response of the turbine in pulse inlet condition is not only embodied in the low frequency fluctuation of the pulse inlet,but also accompanied with the characteristics of the high frequency excitation.In a pulse cycle,the minimum vibration response of turbine corresponding to inlet pulse pressure wave trough,and the vibration response process of turbine follows the inlet pulse waveform.Near the peak of the inlet pulse wave,the vibration response of the turbine reaches its peak,and the periodic oscillation occurs at the same time.The effects of different bent duct inlet conditions on the aerodynamic excitation and harmonic response of compressor impeller were investigated.The 0°(U-type),90°,270° three typical compressor inlet bending ducts were designed.The influences of vortex and total pressure distortions on the pressure distribution patterns in compressor internal were studied in different bending inlet ducts using experimental and numerical methods.And the association relationship of different inlet bent duct conditions and the harmonic response of compressor impeller was determined.The results show that the bending inlet ducts lead to vortex and total pressure distortions at compressor inlet,causing the circumferential non-uniform pressure distribution of compressor inlet.The bending ducts results in the pressure drop at compressor inlet,the sequencing of pressure drop degree is 270° bending inlet duct model,0° bending duct model and 90° bending duct model.Moreover,the bending inlet duct causes the static pressure drop at corresponding position of compressor wheel shroud and its upstream and downstream wall,the pressure decrease degree has the same regularity of the bending inlet ducts acting on the static pressure distributions at compressor inlet.Compressor inlet bending ducts vary the pressure harmonic components of blade surface,and the effects of inlet bending ducts on the harmonic components of static pressure are different.Among them,the 0° bending duct model causes the maximum first-order harmonic amplitude of blade surface static pressure drop.Compared with the straight inlet condition,90° and 270° bending duct models reduce the forced response amplitude of compressor blade,and the 0° bending duct model significantly enhances the resonant response of compressor blade.The forced response and its influencing factors of the turbocharger rotor in the coupling effects of pulse and distorted inlet conditions were investigated.The turbocharger rotor dynamic model consists of the turbine,shaft and compressor was established by simplifying the support condition of floating-ring bearing.The effects of bending duct inlet conditions of compressor on the forced response of the turbocharger rotor was revealed.The study shows that the bearing stiffness affects the natural frequency,mode and critical speed of the turbocharger rotor.The rotor vibration modes vary with the change of support stiffness.Compared with the bearing supporting condition,the rotor supported by the equivalent spring will not appear rotor conical and translational mode.Low order modes of the rotor embody the bending vibration of the shaft,and gradually transit to the impeller mode from shaft mode.High order modes of the rotor reflect the local vibration mode of the compressor and turbine.Compressor inlet bending ducts have different effects on the forced response of the turbocharger rotor.In the range of low excitation frequency,the vortex and total pressure distortion of compressor inlet caused by the bent ducts have less influence on the low order vibration of turbocharger rotor especially for the bending vibration of shaft.At high excitation frequency range near the 1st order natural frequency of compressor blade,the compressor inlet distortions affect the forced response of turbocharger rotor significantly.