چكيده به لاتين
Studying the effect of air jet injection as a useful tool to control and reduce destructive effects of the tip leakage flow at the rotor’s blade row in an axial compressor, is the goal of this research. One of the major challenges in the design process of axial compressors is achieving the optimized performance condition while maintaining stability. As the maximum efficiency is close to the conditions where the instabilities appear and the occurrence of surge and stall phenomena, developing methods to increase the range of the stable operation is of prime importance. The method of air jet injection at the blade tip is considered as an effective method in increasing the stability range and even recovering the compressor from non-stable condition. Thus, this study is organized about two main axes including the study of the rotating stall phenomenon and controlling it through air jet injection. Experimental measurements along with numerical simulations are utilized to arrive at a comprehensive analysis of the stall formation process and the way of its elimination with the use of air injection. In the first step, the spike stall formation and propagation in the compressor are being investigated. The results from measurements through the usage of frequency analysis, results in introducing an image from the flow field and its oscillations at the beginning of the stall process and the development of the rotational stall around the compressor’s circumference and along the blades. The results show that the spike stall structure is comprised of flow separation at the blade’s leading edge and afterwards a vortex. The spike stall is not only concentrated at the blade tip region but also develops along the blade span. Also at the tip region, the tip leakage vortex is strengthened at the operating points near the stall and results in the spillage from the leading edge and backflow from the trailing edge. In the second step, the experiments and numerical analyses at different conditions for the injection-free and air injection states at the blade row tip were done. Increasing the stable operating range of the compressor is one of the most important achievements of using the air injection method to control the instabilities. Based on this, in the compressor under study, by injecting the air with the magnitude of 0.8% of the total mass flow rate, the stall margin increased by 9%. Air injection results in the improvement of the pressure rise capability of the compressor and with the decrease in the loading at the blade tip region, guarantees the rotor’s stable performance. According to the understandings on the stall inception, the stall recovery with the aid of air injection is analyzed by details and the results are introduced in the form of raw signals, frequency spectra, and velocity and pressure contours. Also the frequency analyses are done through FFT, STFT, and wavelet methods. Based on this, during the stall recovery process, first the spike vortex is attenuated and further disappeared followed by the elimination of the flow separation at the blade’s leading edge. In the studied compressor, this process occurs in a time equivalent to less than 20 rotor revolutions from the start of the air injection. Also, at the blade tip region, by the increase in flow momentum, the tip vortex is strongly attenuated and the two stall inception criteria (spillage and the backflow from the trailing edge) are eliminated from the flow passage. So, the air injection results in the stable performance of the compressor and the creation of higher pressure ratio at the near stall flow coefficients.
