There are a number of performance indices for a turbomachine on the basis of which its strength is evaluated. In the case of axial compressors, pressure ratio, efficiency and stall margin are few such indices which are of major concern in the design phase as well as in the evaluation of performance of the machine. In the process of improving the blade design, 3D blade stacking, where the aerofoil sections constituting the blade are moved in relation to the flow. Tilting the blade sections to the flow direction (blade sweep) would increase the operating range of an axial compressor due to modifications in the pressure and velocity fields on the suction surface. On the other hand, blade tip gap, though finite, has great influence on the performance of a turbomachine. The present work investigates the combined effect of these two factors on various flow characteristics in a low speed axial flow compressor. The objective of the present paper is thereby confined to study the collective effects of sweep and tip clearance without attempting to suggest an outright new design. In the present numerical work, the performance of Tip Chordline Sweeping (TCS) and Axial Sweeping (AXS) of low speed axial compressor rotor blades are studied. For this, 15 computational domains were modeled for five rotor sweep configurations and three different clearance levels for each rotor. Through the results, 20°AXS rotor is found to be distinctive among all the rotors with highest pressure rise, higher operating range and less tip clearance loss characteristics. TCS rotors produced improved total pressure rise at the low flow coefficients when the tip gap is increased. Hence there is a chance that an “optimum” tip gap exists for the TCS rotors in terms of total pressure coefficient and operating range, while AXS rotors are at their best with the minimum possible clearance.
This article presents the study of Tip Chordline Sweeping (TCS) and Axial Sweeping (AXS) of low-speed axial compressor rotor blades against the performance of baseline unswept rotor (UNS) for different tip clearance levels. The first part of the paper discusses the changes in design parameters when the blades are swept, while the second part throws light on the effect of sweep on tip leakage flow-related phenomena. 15 domains are studied with 5 sweep configurations (0 • , 20• TCS, 30• TCS, 20• AXS, and 30• AXS) and for 3 tip clearances (0.0%, 0.7%, and 2.7% of the blade chord). A commercial CFD package is employed for the flow simulations and analysis. Results are well validated with experimental data. Forward sweep reduced the flow incidences. This is true all over the span with axial sweeping while little higher incidences below the mid span are observed with tip chordline sweeping. Sweeping is observed to lessen the flow turning. AXS rotors demonstrated more efficient energy transfer among the rotors. Tip chordline sweep deflected the flow towards the hub while effective positive dihedral induced with axial sweeping resulted in outward deflection of flow streamlines. These deflections are more at lower mass flow rates.
Loading of a swept aerofoil blade is typically reduced as its lifting line is inclined (at the sweep angle) with the radial direction. Various estimations are available in the literature about the correction to be made on the loading of a wing when the same is swept. However, it is difficult to obtain an accurate and a more general estimation of correction for a range of sweep angles and for a range of mass flowrates. This article presents two aspects of the flow field in forward swept compressor blade passages: (a) Loading of swept rotors from the present computational study (on a baseline unswept rotor and forward swept rotors of 20 • , 30 • , and 40 • sweep angles; for different tip clearance and stagger configurations) is compared with a number of load correction estimations reported in the literature. Loading of the swept rotors obtained through numerical simulations is compared with the existing estimations obtained from the loading of unswept rotor. A correction factor blend is evaluated, which is found to provide good estimations of loading characteristics for various sweep configurations. (b) Spanwise and streamwise resolved loss coefficient distributions are studied, which showed that the high endwall losses observed in the unswept rotor were significantly reduced with blade sweeping. However, tip clearance is found to result in increased overall loss in the swept rotors.
Tilting the blade sections to the flow direction (blade sweep) would increase the operating range of an axial compressor due to modifications in the pressure and velocity fields on the suction surface. On the other hand, blade tip gap, though finite, has great influence on the performance of a turbomachine. The present paper investigates the combined effect of these two factors on various flow characteristics in a low speed axial flow compressor. For this present study, nine computational domains were modeled; three rotor sweep configurations (0°, 20° and 30°) and for three different clearance levels for each rotor. Commercial CFD solver ANSYS CFX 11.0 is used for the simulations. Results indicated that tip chordline sweep is found to improve the stall margin of the compressor by modifying the suction surface boundary layer migration phenomenon. Diffusion Factor (DF) contours showed the severity of stalling with unswept rotor. For the swept rotors, the zones of high probable stall are less severe and they become less in size with increasing sweep. Increment in the tip gap is found to gradually affect the performance of unswept rotor, while the effect is very high for the two swept rotors for the earlier increments. As a minimum clearance is unavoidable, swept rotors suffer relatively higher deviation from the idealistic behavior than the unswept rotor due to tip clearance.
Blade tip gap, though finite, has great influence on the performance of a turbomachine. Tilting the blade sections to the flow direction (blade sweep) would increase the operating range of an axial compressor by modifying the pressure and velocity fields on the suction surface. The present paper investigates the combined effect of these two factors on various flow characteristics in a low speed axial flow compressor. For the present study, six computational domains were modeled: two rotor sweep configurations (0° and 20°) and three different clearance levels for each rotor. Commercial CFD package ANSYS ® CFX 11.0 is used for the simulations. Results indicated that the effect of tip clearance is more predominant in swept rotor than unswept rotor in terms of change in total pressure rise and efficiency. Traced streamlines indicated that blade sweep guides the suction surface boundary layer fluid towards the trailing edge, preventing it from getting accumulated near the tip. Studies near the tip region showed the pattern of velocity streamlines through the tip gap. Tip clearance vortices are found and their trajectory and effect on the flow are studied. Total pressure rise contours at the rotor exit showed that radial migration of low energy boundary layer fluid towards the tip sections is well prevented with sweep.
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