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.

To achieve clean and sustainable energy for power generation and displacement, the engine designers demand a high performance and powerful propulsion. The Compressor blades have the task of increasing the loss coefficient and should be considered in the design to prevent the destructive phenomena such as the flow separation. If the reverse pressure on the vane could be engineered in such a way as to prevent the flow separation and control the vortices, a higher loss coefficient would be achieved. A reliable way to achieve this goal is to use a tandem, which is obtained by placing a small secondary blade behind the main blade. In the present numerical analysis, a tandem rotor and stage designed and tested at NASA's Lewis Research Center are studied. The desired geometry is extracted from the mentioned source and a high-quality network with about 896 thousand nodes is applied to it, and then considering the SST turbulence model, it is analyzed by the CFX commercial software. The rotor and its stage are studied in 5 rounds and therefore 5 different pressure ratios, and the resulting vortices are also subject to investigation and interpretation. Finally, it can be seen that at 2105 revolutions (half of the nominal revolution), both in the rotor and in the tandem Compressor stage, the vortices are not fully restrained and occupy a very small area of the pressure diagram in terms of chords, thus demonstrating inappropriate performance. At a pressure ratio of 0. 9, we also see a lot of turbulence after the vane, which is not suitable for the operation of the Compressor. On the other hand, at 4210 rpm (nominal rpm), the vortices are well restrained and a good reduction in the pressure is observed. It also occupies a lot of space in the graph of pressure by chord. Also, at the pressure ratio of 1. 1038, we see a proper formation and control of vortices to reduce the turbulence.

Primary nozzle is one of the most important factors which has a large influence on the performance of thermo-Compressors. Most of studies carried out until now, have been performed on single-nozzle thermo-Compressors. In this paper, an actual industrial thermo-Compressor is considered and the purpose is to study the effect of increasing primary nozzle number on the performance of this thermo-Compressor. For this purpose, a triple-nozzle thermo-Compressor is simulated numerically and its performance is compared with single-nozzle thermo-Compressor. Ideal gas thermodynamic properties are considered to simulate the compressible flow within the thermo-Compressor and numerical result is validated using the experimental result. In addition, the effects of variation in mixing chamber convergence angle and position of nozzles at the radial direction in triple-nozzle thermo-Compressor are investigated. The numerical results show that at the same condition, a triple-nozzle thermo-Compressor is able to provide superior critical back pressure and entrainment ratio than single-nozzle thermo-Compressor. The proximity of nozzles (with 34% changes in radial distance) increases the critical back pressure about 8% and decreases the entrainment ratio about 5%. By increasing mixing chamber convergence angle about 66%, the value of critical back pressure decreases about 29% and the value of entrainment ratio decreases about 16% in single nozzle-thermo-Compressor and 10% in triple-nozzle thermo-Compressor. Also, by 8% reduction of mixing chamber convergence angle, the critical back pressure decreases 6% but entrainment ratio decreases about 2% in single-nozzle thermo-Compressor and increases about 3% in triple-nozzle thermo-Compressor.

The unstable flow with rotating-stall-like (RS) effects in a rotor-cascade of an axial Compressor was numerically investigated. The RS was captured with the reduction in mass flow rate and increasing of exit static pressure with respect to design operating condition of the single rotor. The oscillatory velocity traces during the stall propagation showed that the RS vortices repeat periodically, and the mass flow rate was highly affected by the blockage areas made by stall vortices. The results also showed that large scale vortices highly affects on the generation and growth of the new vortices. An unsteady two-dimensional finite-volume solver was employed for the numerical study which was developed based on Van Leer’s flux splitting algorithm in conjunction with TVD limiters and the κ-ε turbulence model was also employed. The good agreement of the computed mass flow rate with the experimental results validates the numerical study.

In this study, the main objective is to develop a one dimensional model to predict design and off design performance of an operational axial flow Compressor by considering the whole gas turbine assembly. The design and off-design performance of a single stage axial Compressor are predicted through 1D and 3D modeling. In one dimensional model the mass, momentum and energy conservation equations and ideal gas equation of state are solved in mean line at three axial stations including rotor inlet, rotor outlet and stator outlet. The total to total efficiency and pressure ratio are forecasted using the Compressor geometry, inlet stagnation temperature and stagnation pressure, the mass flow rate and the rotational speed of the rotor, and the available empirical correlation predicting the losses. By changing the mass flow rate while the rotational speed is fixed, characteristic curves of the Compressor are obtained. The 3D modeling is accomplished with CFD method to verify one dimensional code at non-running line conditions. By defining the three dimensional geometry of the Compressor and the boundary conditions coinciding with one dimensional model for the numerical solver, axial Compressor behavior is predicted for various mass flow rates in different rotational speeds. Experimental data are obtained from tests of the axial Compressor of a gas turbine engine in Sharif University gas turbine laboratory and consequently the running line is attained. As a result, the two important extremities of Compressor performance including surge and choking conditions are obtained through 1D and 3D modeling. Moreover, by comparing the results of one-dimensional and three-dimensional models with experimental results, good agreement is observed. The maximum differences of pressure ratio and isentropic efficiency of one dimensional modeling with experimental results are 2.1 and 3.4 percent, respectively.

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.

This paper describes the application of eigenvalue analysis for surge prediction in multi-stage axial flow Compressors. In this approach, the compression system is modeled using a stage-by-stage nonlinear modeling based on the conservation equations of mass, momentum and energy. By line arising the Compressor flow model at each steady state point, the small perturbation system matrices have been obtained. By analyzing the eigenvalues of the linearised model, the stability of the Compressor flow has then been investigated, and surge points for both low and high Compressor rotational speeds have been predicted. Finally, the results for a seven stage axial flow Compressor have been compared with experimental data in order to show the ability of the method.

A honeycomb panel consists of an array of open hexagonal cells which their walls are perpendicular to face sheets although other panel sandwiches don’t have these perpendicular walls. Their design is often performed based on minimum weight. This research is aimed at minimizations of weight by means of computing honeycomb core girth. Weight optimization is done by means of Naive and numerical procedures. Numerical optimization is done by the sequential quadratic programming (SQP) method. Geometric parameters and optimized weight are calculated for hexagonal and square cells. Optimized weights for these two cross-sections are compared.