Three-dimensional unsteady numerical simulations were carried out to analyse tip clearance flow in a low-speed isolated axial compressor rotor blades row. A flow solver has been used for the current study utilizing the large eddy simulation (LES) technique. Periodic tip leakage flow and its propagation trajectories were simulated in detail. A number of pseudo pressure transducers were imposed on the pressure side of the blade for detection of unsteady surface pressures to provide a calculation of tip leakage flow frequencies. Two different sizes of tip clearance were considered for simulations and analyses. Non-dimensional frequencies of the tip leakage flow were calculated and final results were compared to those of existing numerical and experimental data. Final results demonstrated that in contrast to the Reynolds averaged Navier-Stokes (RANS) model, the LES method shows considerable dependency of frequency characteristics of the tip leakage flow to the gap size and can detect different frequency spectrums along the blade surface. All the results obtained through the current numerical approach were in close agreement with those of existing experimental data.
This paper illustrates a numerical technique undertaken for preliminary design optimization of profile losses in multi-stage axial compressors. Design process has been carried out based on one-dimensional row by row calculations along compressor mean line. The main objective of the optimization process was to find the best distribution of pressure ratios along compressor stages in order to maximize the overall efficiency, which is itself a non-linear function of governing variables. In this respect, only profile losses, identified as the most dominant ones, are focused on and minimized during the optimization process. Pressure ratios of stages have been taken as design variables. Diffusion factor and De Haller number of each blade row were considered as the main constraints during the optimization process. Numerical optimization was based on a sequential search technique, referred to as complex method. Design process in parallel to the numerical optimization is examined on a ten-stage axial compressor of known general performance data. Final results showed an increase of about 2.7 per cent in the total efficiency relative to its initial value calculated during the preliminary design process.
Aerodynamic performance of a centrifugal compressor is investigated under various casing treatment configurations of stepped tip gap type, numerically. This technique is introduced as an effective method for improvement of compressor stall margin and consequent enhancement of the compressor overall performance. In this respect, flow field within the proposed compressor was simulated by solving the Navier–Stokes equations utilizing the well-known k-ɛ turbulence model. Different geometries of the compressor stepped tip gap were examined to be able to obtain the optimum configuration. Results of velocity contours and streamlines patterns on various azimuthal and meridional planes showed that the stepped tip gap can weaken the tip leakage flow strength and reduce the blockage to the main stream near the casing. Hence, it can be concluded that using stepped tip gap can extend the stable operating range of the compressor and delay the occurrence of stall phenomenon. For small clearances, stepped tip gap was found to improve the stall margin of the proposed compressor at different rotor speeds without any negative effects on its efficiency, except to high flow conditions. But, inclusion of the stepped tip gap with large clearances tends to increase the stall margin with considerable reduction in the compressor efficiency.
Effects of air injection on a centrifugal compressor performance are studied numerically and results are presented in this paper. Flow field is simulated based on solution of the Reynolds-Averaged Navier-Stokes equations utilizing the well-known k-ε turbulence modeling. To find optimum arrangement of the air injectors, including their numbers, positions and setting angles and also their mass flow rates, 10 different cases were proposed in this investigation. Results of velocity and pressure fields and streamlines patterns on various blade-to-blade and streamwise planes provided necessary data for analyses and discussions. These results revealed that a proper air tip injection can weaken the tip leakage flow strength and alleviate the blockages to the main stream near the casing. Hence, this technique can be used to extend the stable operating range of the compressor and delay the probable stall commencement. In addition, compressor can produce higher pressure ratios in cases of proper injections in comparison to no-injection case. Optimum injection configuration improved the stall margin of the proposed compressor by nearly 15.8%. Numerical results for no-injection case were compared with those of the authors own experimental works carried out on a proper test rig, which showed close agreement.
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