In recent years, unsteady flow-control technology has been considered as more promising for reducing secondary flow loss in turbines. In particular, because of their large sweeping range, sweeping jet actuators (SJAs) have gradually been applied to turbomachinery to reduce loss. However, most of the preliminary studies were focused mainly on computational fluid dynamics (CFD) calculations, with very few on wind-tunnel experiments to verify the flow-control effectiveness. To fill this gap, an innovative attempt to investigate experimentally the influences of SJAs on the aerodynamic performance and leakage flow is presented in a high-pressure turbine cascade. By comparing SJAs and hole-style steady jet actuators (HSJAs), the influence on the tip leakage flow and the loss characteristics with different incidences and injection frequencies are discussed in detail. The results indicate that the SJAs effectively reduce the magnitude and extension of the leakage vortex and the passage vortex, and weaken the interactions between them. When the jet flow of an SJA is 0.2% of the inlet flow, the loss at the cascade outlet decreases by up to 10.3%, which is significantly better than that with an HSJA (6.3%).
This paper describes a detailed experimental investigation into the impact of steady and pulsed blowing on endwall secondary structures and losses in a compressor cascade. Owing to their high configuration flexibility, injection holes are integrated in the cascade sidewalls to manage the secondary flows. Loss reductions of 3.2 and 5.72% relative to the uncontrolled case are achieved by steady blowing with straight and optimized inclined holes, respectively. Superior loss reduction of 7.85% is obtained by pulsed blowing through inclined holes. To identify the secondary flow structures near the endwall and suction surfaces, a self-developed oil visualization method and spatial-spectral analysis are performed. Experimental results show that two concentrated shedding vortices exist in the cascade corner region. Loss reduction is achieved as the blowing suppresses the dominant vortex. Pulsed blowing intensifies the acceleration effect on the boundary layer, resulting in better performance with the same injection velocity. The impact of the pulse frequency on loss generation is investigated, and it is found that the optimal frequency is close to the shedding frequency of the dominant vortex in the cascade corner region.
The boundary layer development on a low-pressure turbine blade surface modified by recessed dimples, U-grooves, and rectangular grooves has been investigated through the large-eddy simulations. The simulations are performed at a Reynolds number of 50,000 (based on the inlet velocity and axial chord length) and extremely low freestream turbulence conditions. The characteristic parameters of the boundary layer are used to estimate the development of the boundary layer, and spectral analysis has also been performed to identify the dominant frequency of shedding vortices. The results of simulations indicate that three surface modifications all reduce the profile losses by restraining the separation bubble size. However, the grooves and dimples show different mechanisms in inhibiting laminar separation. Grooves tend to promote the formation of spanwise vortices, which is more difficult to break into turbulence. A high-speed shedding vortex is induced by the particular 3D structure of dimple, and its shedding frequency is nearly twice the Kelvin–Helmholtz (K-H) instability frequency. The interaction between the shedding vortices and the K-H vortices promotes the breakdown process of the spanwise vortices, which leads to an earlier transition of the boundary layer at a low disturbance level. The current study reveals the different mechanisms of dimples and grooves and shows the great potential of dimples for flow control in low-pressure turbines. Besides, the flow structures inside the dimples with adverse pressure gradients are also explored.
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