Numerical Research On The Flow Field In Axial Compressor Under Unstable Working Conditions By Using The Harmonic Balance Method

Gas turbine is widely used in aerospace,marine engineering,power generation and other fields due to the high power density and good maneuverability characteristic.As one of the most important core components of gas turbine,the stability and working performance of the compressor plays a crucial role during the operation of gas turbine.As we all know,the design technology of compressors depends on the understanding of the internal flow of the compressor.Further research on the internal flow characteristic of the compressor under various working conditions is necessary and is meaningful to improve the compressor technology.In this paper,the numerical method is applied to investigate the unsteady flow field in the adjacent blade rows,and also the working characteristic of the stall and surge process of the axial compressor.The main research contents are summarizd as follows:1.The NASA Stage35 transonic axial compressor is used as the research object,and the internal flow field and the working characteristic were numerical simulated under the design speed.By comparing with the published experimental data,the numerical method used in this paper is firstly verified.Then,an efficient method which is used to predicate the compressor working performance based on harmonic balance method is proposed,the accuracy and the calculation speed is verified by comparing with the traditional dual-time step method.Results show that the proposed method based on the harmonic balance method is feasible to study the periodical unsteady flow field in turbomachinary.Compared to the traditional dual-time step method,the calculation efficiency is significantly improved wihle ensuring the computation accuracy.The calculaton efficiency is at least 10 times faster than the traditional method;2.The unsteady flow field between adjacent blade rows is numerically simulated by using the harmonic balance method,and then,the in-house code SPARC is used to study the unsteady flow field in the clearance between adjacent blade rows in a full annulus multistage axial compressor.Results show that the airflow in the clearance of adjacent blade rows show a nonaxisymmetric distribution due to the relative position between adjacent blades row is different.Although the non-axisymmetric distribution is negligible in front stages,but the slight nonaxisymmetric distribution will continue accumulate with the development of the airflow,and finally form a distinct non-uniform flow field in latter stages.3.The dynamic model of the airflow at the outlet is developed by using the UDF technique,a predication method for the stall process of compressor is proposed.And the generation mechanism and development process of the stall of NASA Stage35 is numerically studied in annulus domain.Results show that,as the mass flow at the compressor inlet and outlet drops,stall starts occurs,and the compressor tends to be completely stalled in next few revolutions.The stall characteristic was initially occured at the top region near the suction side of the blade,both the size and the position of the stall cell are uncertain.During the development of stall,some small cells tends to be further merged.As the compressor completely stalled,the disturbance signal caused by stall in the blade channel is rotating around 47% of the compressor speed in the circumferential direction.4.The Harmonic Balance method was extended by using a hybrid BDF/Harmonic balance method,a surge predication model is estabilished in combination with a variable condition at the outlet,and the surge characteristic is numerical simulated by using the in-house code SPARC.Results show that,when surge occurs,the airflow starts to oscillate in the compressor,the average working pressure ratio of the compressor decreases sharply,and at least 11 revolutions are required to complete a surge cycle.As the system volume and the average working flow rate changes,the surge frequency is varying in the range of 14.32-26.04 Hz.Furthermore,by comparing the surge frequency obtained through the numerical method and Helmholtz frequency,the frequency obtained by using the two different methods are at the same level and the tendency is almost the same,but there is still a significant difference between them.