Study Of Effects Of Water Ingestion On Compressor Performance And Compression Stall Mechanism

Aero-engines will inevitably face the threat of severe weather during operation due to the changes in flight altitude and meteorological conditions.The Civil Aviation Administration of China(CAAC)has clearly stipulated that the aero-engine must meet the technical indicators that the engine does not shutdown,does not appear unacceptable loss of power or thrust,and does not experience irrecoverable surge or stall or other abnormal situations during the rain ingestion period.In the process of water ingestion in an aero-engine,the component that is most negatively affected by droplets is the compressor.How to correctly and effectively evaluate the compressor performance is of great engineering significance and value.However,the compressor has evolved into a complex two-phase flow process including gas-liquid,liquid-liquid,and liquid-solid coupling interactions after water ingestion.An effective simulation and evaluation method has not been obtained until now.Therefore,the numerical simulation method of two-phase flow inside the compressor and the effect of water ingestion on the compressor performance and compression stall mechanism are deeply studied in this thesis.The Euler-Lagrangian particle tracking method is selected firstly as the two-phase flow numerical method based on the typical flow characteristics of droplets inside the compressor,the simulation requirements and the adaptability of method.Subsequently,the governing control equations for the two-way coupling of droplets/wet air are established,and the interaction between droplet and airflow,droplet and wall are considered.In addition,the precision and accuracy of the numerical method and model are verified and checked by the existing experimental data.The results confirm that the numerical prediction results can meet the calculation requirements.Based on this,a compressor performance simulation and research method suitable for the inlet water ingestion condition is obtained.Firstly,the breakup of droplets and the formation of water film in NASA Stage 35single-stage compressor at 70% speed was deeply studied in this thesis.It can be seen that the overall breaking effect is not good,and the water film on the rotor pressure surface exhibits a unique “crescent-shaped” distribution.Then,the degree of influence on the performance degradation of compressor working point under different water content(2%～8%)and droplet size(50 μm～150 μm)conditions was compared and analyzed.The results show that the reduction ranges of inlet mass flow rate,total pressure ratio and compression efficiency are0.49%～1.96%,0.23%～0.94% and 7.77%～12.85%,respectively,the increase ranges of stage entropy increase and compression specified work are 77.75%～118.80% and 7.29%～10.42%,respectively,both the increase in inlet water content and droplet size will aggravate the deterioration of compressor performance,but the effect of water content is greater.Generally,the compressor can still maintain a relative high inlet mass flow rate and total pressure ratio after water ingestion,but it needs to sacrifice the compression efficiency and increase the compression work.The change law of the compressor characteristic line is basically consistent with that of the working point.With the help of fast Fourier transform and spectrum analysis,the fluctuation amplitude of flow field and performance parameters of the compressor in the time domain and the main frequency movements in the frequency domain are investigated in this thesis.It is found for the first time that water ingestion will lead to a significant increase in the fluctuation amplitude of the flow field and performance parameters,and the main frequency of fluctuation moves from the blade passing frequency of 7220 Hz to the rotor rotating frequency of 200 Hz.This phenomenon indicates that water ingestion will change the original fluctuation characteristics of the compressor parameters,forcing the compressor to operate under a series of non-optimal working point with the characteristics of low frequency,large fluctuation and long period.The operational stability of the compressor is therefore threatened.When the inlet water content or droplet size increases,the main amplitude of the fluctuation is further increased,but the main frequency will not be changed.Furthermore,by comparing the difference in the stall process and mechanism of the compressor before and after water ingestion,it can be concluded that water ingestion does not change the compression stall mechanism,and the reason for the stall phenomenon of NASA Stage 35 at 70% speed can always be summarized as follows: the radial migration flow accumulates on the tip region,and continuous to develop through the positive feedback mechanism of flow deterioration in the passage,and finally integrates with the tip low-speed region to completely block the tip region.However,water ingestion will promote the development of the stall process at a faster rate,and even induce the compressor to enter an unstable state at the near-stall point of dry conditions.On the other hand,whether the compressor is stall after water ingestion at the small flow rate point depends on whether the negative effect of water ingestion is sufficient.Finally,the numerical simulations of water ingestion for a turbofan engine compressor under two typical operating conditions(i.e.,flight descent and take-off conditions)are carried out based on the standard atmospheric parameters,the concentration and particle distribution of rain in airworthiness regulations,the occurrence range of the rainstorm,and the flight handbook of commercial aircraft.The results show that the fan is the component most negatively affected by the droplets,and the overall change trend is the same as that of the above working point.The wet compression gain effect brought by the evaporative cooling of droplets in the low-pressure compressor begins to appear,which makes the compressor performance improve temporarily in a certain period of time,and the improvement effect is more obvious under the take-off conditions.The change law of the fluctuation amplitude and main frequency of the performance parameters in the fan and low pressure compressor is consistent with the law presented in Chapter 3 of this thesis.The main frequency shift phenomenon can be used to guide the discrimination of water ingestion signals.