Aeroelastic aspects of Axial Compressor Stage with Self-Recirculating Casing Treatment

For successful implementation of casing treatment designs in axial compressors, apart from the stall margin improvement benefits, aeroelasticity also plays a major role. This manuscript addresses the not often discussed aeroelastic aspects of a new discrete type of passive Self-Recirculating Casing Treatment (RCT) designed for a transonic axial compressor stage. Experiments are carefully designed for synchronized measurement of the unsteady fluidic disturbances and vibrations during rotating stall for compressor with baseline solid casing and Self-RCT. The modal characteristics of the axial compressor rotor-disk assembly are studied experimentally and numerically. Experimentally it is observed that the rotating stall cells excite the blades in their fundamental mode in a compressor with baseline solid casing at the stall flow condition. In contrast, there is no excitation of the blades in the compressor with self-recirculating casing treatment at the same solid casing stall flow condition. Also, the self-recirculating casing treatment compared to the solid casing can significantly reduce the overall vibration levels of the blades that are excited at the stall flow condition. The casing treatment is able to alter the flow field near the tip region of the rotor blade, and hence influencing the forcing function of the rotating cantilever blades to have the aeroelastic benefit.

Effect of Rotor Tip Casing Treatment on the Performance and Stability of a Transonic Axial Compressor

In a transonic compressor rotor, tip leakage flow interacts with passage shock, casing boundary layer and secondary flow. This leads to increase in total pressure loss and reduction of compressor stability margin. Casing treatment is one of the passive endwall geometry modification technique to control tip leakage flow interaction. In the present investigation effect of rotor tip casing treatment is investigated on performance and stability of a NASA 37 transonic compressor stage. Existing literature reveals, that endwall casing treatment slots i.e., porous casing treatment, axial slots axially skewed slots, circumferential grooves, recirculating casing treatment etc. are able to improve compressor stability margin with penalty on stage efficiency. Turbomachinery engineers and scientists are still focusing their research work to identify an endwall casing treatment configuration with improves both compressor stall margin as well as stage efficiency. Hence in the current work, as an innovative idea, effect of casing treatment slot along rotor tip mean camber line is investigated on NASA 37 compressor stage. Casing treatment slot with rectangular cross-section was created along the rotor tip mean camber line. Four different casing treatment configurations were created by changing number of slots on rotor casing surface. In all four configurations casing treatment slot width and height remains same. Flow simulation of NASA 37 compressor stage was performed with all these four casing treatment configurations. A maximum stall margin improvement of 3% was achieved with a particular slot configuration, but without any increase in compressor stage efficiency.

Numerical Investigation on a Axial Slot Casing Treatment of a Large Circumferential Interval and Small Opening Area

Axial slot casing treatment is a common method to extend the stall margin of a compressor. Based on the mechanism of unsteady flow control, this paper redesigns axial slots with large circumferential interval and small opening area. To test the effect of this axial slot structure, unsteady numerical simulations were carried out with different slot areas and circumferential intervals. The results show that this novel axial slot casing treatment can significantly improve compressor stall margin. Meanwhile, compared with the traditional axial slot, the efficiency loss is greatly reduced. The flow field analysis shows that the new axial slot structure proposed in this paper can suppress the development of tip leakage vortex and unsteadiness in the tip region at the near stall condition through decreasing the tip loading periodically. Moreover, we find that the slot area is proportional to the improvement of stability margin. Under the same slot area, an excessive number of slots is not conducive to the improvement of the stability margin.

Redesign of casing treatment for a transonic centrifugal compressor based on a hybrid global optimization method

As a typical black-box problem, recirculating casing treatment (RCT) optimization of compressor stages is computationally intensive and time consuming, even though surrogate models are usually employed. In order to improve efficiency and robustness of the optimization, an expected-improvement (EI) based hybrid global optimization (EHGO) algorithm is developed by coupling an EI-based surrogate model with a hybrid optimization algorithm. Highly nonlinear and multiple modality mathematical tests show that the EHGO algorithm is able to create a high-fidelity surrogate model near targeted regions with less evaluated samples, and to obtain the global optimal solution simultaneously. The RCT of a compressor stage is optimized based on this algorithm. The number of CFD simulations required for obtaining an optimum solution is greatly reduced, as compared to similar studies using conventional methods. The optimization results show that the aerodynamic performance is improved over the whole speed line and the flow range is also extended. The dominant factors for the performance improvements and the enhanced stall margin are addressed by analyzing the local flow characteristics before and after optimization. It is found that those factors include: removing a larger amount of low-momentum fluid, achieving a more uniform flow of impeller passage in circumferential direction, and reducing the radial distortion of impeller inlet flow. The proposed algorithm has the potential to considerably speed up the optimization process and make the optimization much more accessible. It can be generalized to deal with other computationally intensive black-box problems, for example, turbomachinery optimization.

Numerical investigations on the effect of volute casing treatment for performance augmentation in a centrifugal fan

This article presents the effect of volute chamfering on the performance of a forward swept centrifugal fan. The numerical analysis is performed to obtain the performance parameters such as static pressure rise coefficient and total pressure coefficient for various flow coefficients. The chamfer ratio for the volute is optimized parametrically by providing a chamfer on either side of the volute. The influence of the chamfer ratio on the three dimensional flow domain was investigated numerically. The simulation is carried out using Re-Normalisation Group (RNG) k-[Formula: see text] turbulence model. The transient simulation of the fan system is done using standard sliding mesh method available in Fluent. It is found from the analysis that, configuration with chamfer ratio of 4.4 is found be the optimum configuration in terms of better performance characteristics. On an average, this optimum configuration provides improvement of about 6.3% in static pressure rise coefficient when compared to the base model. This optimized chamfer configuration also gives a higher total pressure coefficient of about 3% validating the augmentation in static pressure rise coefficient with respect to the base model. Hence, this numerical study establishes the effectiveness of optimally providing volute chamfer on the overall performance improvement of forward bladed centrifugal fan.