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 were mounted evenly spaced around the compressor casing upstream the rotor blade row. Initially, improvement of the compressor overall performance was 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. 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 stall alleviation during the air injection process. Results show that a small amount of air injection at the rotor blade tip region can affect the rise in total pressure and more specifically, increase the compressor stall margin efficiently. Air injection of less than 1% of the compressor main flow rate through the injectors has improved the stall margin by 9%. Air injection at the blade row tip enables this beneficial effects to extend throughout the blade whole span, especially while working at the near stall conditions.

Rotating-stall (RS) through a cascade of an axial compressor has been numerically investigated by employing an unsteady two-dimensional finite-volume density based computer code. To validate the computer code, three test cases were prepared, and the good agreement resulting from comparison of the results has given adequate issuance of the code. The cascade problem was studied through different geometric approaches, which were classified in two main categories. The first was the semi-stage (single rotor-cascade), and the second was the complete stage. For each geometric approach, the number of blades was varied from a minimum number to a maximum value (variable cascade length). The RS modal characteristics and its development for all geometric cases were observed. The RS was incepted with a 40% reduction in flow coefficient and a 0.4% increase in the load coefficient from their normal operating values. In all cases, the captured modal characteristic of RS varies with the variation of the cascade length to a maximum value, and this triggers the same modes of RS. The variation of the modal characteristics of the RS, in the two main geometric approaches seems to be similar, but at different levels.

The current paper reports on a new theory developed by modifying the basic Moore-Greitzer model, which can predict the performance of a compression system during the instabilities in more details. The general assumptions such as the compression system layout, the lags in the entrance and exit ducts, the compressor axisymmetric characteristic and the small disturbances are similar to those of Moore-Gereitzer model. However, a second order hysteresis is used in the current work for the pressure rise of the rotor and stator rows. As a result, some new parameters are added to the governing equations, such as the stall cell acceleration ( ), second derivative of the mean axial flow coefficient ( ), second derivative of the disturbance amplitude ( ) and slope of the compressor characteristic curve. This gives the modified model new capabilities, like investigating the transient speed of the stall cell or the effect of the throttling rate on the instabilities, which are discussed in details in the current paper.

In this paper, a new model is developed for Rotating stall in low speed axial compressors and fans. The theory is developed from Moore’s theory. The modified model makes it possible to predict the transient behavior of the stall cells, which is not possible with Moore’s theory. The general assumptions such as the layout of the compression system, the lags in the entrance and exit ducts, and the small disturbances are assumed to be similar to those of Moore’s theory. However, a second order hysteresis is used in the current work for the pressure rise of the rotor and stator rows. Comparing the experimental results with the theory shows that the modified model can predict the transient behavior of the stall cells fairly accurately. Furthermore, the current model makes it possible to study the effects of different parameters such as the stagger angle, number of stages, and number of stall cells. It has been suggested in the current study that the number of stall cells should reduce to one in a fully developed Rotating stall pattern.

The Rotating-stall (RS) through a rotor-cascade of an axial compressor were numerically investigated with an unsteady two-dimensional finite-volume density based computer code. To validate the computer code, a test case was prepared and the good agreement of the compared results has given the adequate assurance of the code. The RS was incepted with a 40% reduction in flow coefficient and a 0.4% increase in the load coefficient from their normal operating values. The velocity traces from numerical probes, for all chosen number of blades had a periodic behavior, during RS effect. It was also concluded that the captured modal characteristics of RS may vary with varying the number of blades and this variation may cause various modes of RS. Moreover, in each mode, decreasing the number of blades may cause phase lag times in stall progress. In studied cases, approximately two modes and a transient mode were captured which had different rotational speeds and modal characteristics. Finally, a minimum required number of the blades for the RS inception studies (in spite of large numbers in previous works) was proposed, to intensify the probability of using shorter cascades in stability margin calculations of a compressor

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.

Aims The use of intrauterine devices (IUDs) has been one of the most effective, the safest and cost-effective methods for preventing pregnancy, for a long time. However, some complications have been reported for IUDs. The complications may be reduced by using a specific technique for placement of IUD. The objective of this study was to introduce the Rotating withdrawal technique for the first time, which was investigated to reduce the complications of IUD insertion. Materials & Methods This historical cohort study was conducted on 1199 females who referred to Sarem Medical Center for IUD insertion from 1984 to 1995. After primary examinations, IUD was inserted by Rotating withdrawal technique. The subjects were followed up for 2 years. Findings Displacement was observed just in 20 cases (1. 67%). All of them were seen in the first month after placement. For these cases, IUD insertion was done again. After the second replacement, displacement was observed in 4 cases (0. 035%) after one month. Therefore, it was recommended to them to use another preventive method. There was no pregnancy or displacement in the other cases during 2 years. Conclusion The Rotating withdrawal technique can minimize the pregnancy rate when IUD is used. In addition to the type and quality of IUD, insertion method of IUD can increase its effectiveness.

In the present work, the air-injection as an active flow stabilizing techniques were numerically examined during the aerodynamic behavior and the characteristics of an axial compressor. First, in design condition, the characteristic curve was numerically captured for a specified test compressor. The computed results showed good agreements with those obtained from the experiments. They validate the finite-volume solver which was developed based on Van Leer’s flux splitting algorithm in conjunction with TVD limiters and the k-e turbulence model. At the second step, to examine the performance of the air injection technique unstable condition, the operating points were set for the unstable condition upon the operating map, and the expected unstable flow patterns were captured. At the final step, the numerical simulation was completed with the air injection, and surprisingly, the unstable flow patterns became highly re-stabilized. Consequently, a significant recovery of the performance was augmented. The study could successfully demonstrate the capability of the numerical studies for simulating the active flow controls specifically the inlet air-injection technique using a 2-D finite volume approach.