tip injection

Improvement of an axial compressor performance through suppression of flow disturbances due to inlet flow blockage, utilizing air injection at the blade tip region, is the subject of the present study. Method of investigation is based on experimental measurements conducted in a low speed isolated axial compressor rotor blade row. Four different blockage screens of different blockage ratios ranged between 5 and 20 percent of the inlet area are located at the compressor entrance. Instantaneous and time-averaged static pressure are recorded at different locations of compressor casing. Frequency analyses of pressure signals, show that the flow disturbances are being created in the presence of the blockage screens. These disturbances cause to appearance of rotating stall in the flow field when the compressor operates under distorted inflow with low blockage ratios (5 and 10%). To reduce the destructive effects of the inlet distortion, air is injected at the tip region of the rotor through 12 injectors which are located evenly spaced around the compressor circumference. Air injection in small quantities, 1.5% of main flow, has considerable effects on the compressor performance under inlet distortion. The rotor performance and compressor delivery pressure is improved up to 35% than to the no injection case. By implementation of Short-Time Fourier Transform (STFT) technique effects of air injection on elimination or reduction of flow disturbances are demonstrated, too.

Inlet distortion that may be occurred for various reasons at the entrance of a gas turbine, it is caused to disturbed in compressor performance conditions and also all engine components, so it is very important to investigate its controlling methods. The aim of this paper is numerical simulation of inlet distortion in an axial compressor rotor and active control of the instabilities by the air injection at the blade tip region. Flow simulation of inlet distortion is accomplished at compressor entry with five different geometries of circumferential blockage (amounts of circumferential blockage are: 5%, 10%, 15%, 20% and 25% of the compressor inlet duct). For active control of instabilities, 12 injectors have been mounted upstream of the rotor blade row that distributed in circumferential directions symmetrically. The injection mass flow rate does not exceed 2% of the compressor main flow rate at the design point. ANSYS CFX was used for simulation and the turbulence model of k-ω SST has been used through the calculations. The results show that increasing inlet distortion cause to decrease performance and rotor efficiency. Furthermore, for this rotor modeling condition, in 5% and 10% blockage, air injection can improve the rotor performance, but for more than 10% blockage, a strong wake region is formed after the distortion screen and air injection can cause negative effects on rotor performance. Because the strong instabilities can adversely affect the injectors flow and this method instead of modifying the flow field, make it more non uniform than before.

Rotating stall alleviation in an axial compressor with deployment of air injection at its rotor blade row tip region has been experimentally investigated. Twelve air injectors had been mounted evenly spaced around the compressor casing upstream the rotor blade row. Initially, improvement of the compressor overall performance has been examined through air injection, especially at stall point condition. Instantaneous flow velocities at various radial and circumferential positions were measured simultaneously utilizing hot wire anemometry. These unsteady results, obtained from these latter measurements together with signal frequency analyses, provided to describe the stall inception process and consequent flow induced fluctuations and also alleviation process of stall during the air injection. Results show that a small amount of air injection at the rotor blade tip region can affect the total pressure rise and specifically can increase the compressor stall margin efficiently. Air injection of less than 1% of the compressor main flow rate through the injectors has caused the stall margin to be improved by 9%. Air injection at the blade row tip has caused its beneficial effects to extend throughout the blade whole span, especially while working at the near stall conditions.