جستجوی مقالات مرتبط با کلیدواژه "aerodynamic instability" در نشریات گروه "مکانیک"

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.

In this article, aerodynamic stability of J79 compressor was analytically investigated. For this purpose, conservation equations of mass, momentum and energy were applied with several assumptions. By accomplishment of equations related matrix as well as investigating its eigenvalues, the effect of parameters on aerodynamic stability were assessed. Due to the complexity of the equations used in the matrix, the equations codes in MATLAB and the desired matrix have been provided. By applying the geometry and flow characteristics of J79 engine on the matrix and extract the relevant diagrams, the effect of some parameters on the aerodynamic stability of the engine is evaluated. According to the results, by reducing the mass flow rate through the compressor, the system tends to be working in an unstable condition. By increasing the ratio of upstream to downstream compressor duct length, the aerodynamic stability of the system will increase. Increasing the plenum volume will yield to a considerable reduction in stability. But by increasing the volume of compressor, the system stability will increase. Enhancing the combustion chamber temperature and also increasing the compressor flow area have favorable effects on system stability. The results of this study compared with previous outcome. Significant achievements were observed between both aforementioned results Relatively good alignment between the results has also been observed.

Due to aerodynamic instabilities, their desired operation takes place in a small and limited range, so knowledge of the instabilities seems to be inevitable. In this article, aerodynamic instability of compressors are investigated analytically. For this purpose, the equations of mass, momentum and energy have been used. After a series of assumptions, the formation of a matrix related to equations and its eigenvalues are calculated, the effect of effective parameters on aerodynamic stability is evaluated. These relationships are coded in MATLAB software and by applying geometric characteristics and flow characteristics on it; the effect of important parameters on the aerodynamic stability of the engine is investigated. According to the results, it can be seen that this phenomenon occurs when the compressor is unable to produce sufficient head to overcome its lower resistance. Or, more simply, the output pressure produced by the compressor is lower than the downstream pressure. In this case, a cyclic motion will be generated in the gas stream, and the compressor's inability to overcome the disturbances that may occur around its operating point, which can cause instability in it .In the following, the effects of decreasing the mass flow through the compressor, increasing the length of the downstream duct length to the length of the lower duct, increasing the plenum volume, and increasing the cross-sectional flow of the compressor in the system stability were investigated. The results are compared with the results of previous studies and have a good correlation between the results.