Search

An in-house large eddy simulation (LES) code based on three-dimensional compressible N-S equations is used to research the impact of incoming wakes on unsteady evolution characteristic in a low-pressure turbine (LPT) cascade. The Mach number is 0.4 and Reynolds number is 0.6 × 105 (based on the axial chord and outlet velocity). The reduced frequency of incoming wakes is Fred = 0 (without wakes), 0.37 and 0.74. A detailed analysis of Reynolds stresses and turbulent kinetic energy inside the boundary layer has been carried out. Particular consideration is devoted to the transport process of incoming wakes and the intermittent property of the unsteady boundary layer. With the increase of reduced frequency, the inhibiting effect of wakes on boundary layer separation gradually enhances. The separation at the rear part of the suction side is weakened and the separation point moves downstream. However, incoming wakes lead to an increase in dissipation and aerodynamic losses in the main flow area. Excessive reduced frequency (Fred = 0.74) causes the main flow area to become one of the main source areas of loss. An optimal reduced frequency exists to minimize the aerodynamic loss of the linear cascade. [ABSTRACT FROM AUTHOR]

In this paper, the control mechanism of the honeycomb tip structure on the tip leakage flow of a turbine cascade is studied experimentally and numerically, and the sensitivity of tip leakage flow characteristics to different clearance heights from 0.5% to 2% based on the blade span are mainly discussed. A flat tip is considered as a comparative case. The results show that a part of the leakage flow enters the tip honeycomb cavity, forming small-scale vortices and mixes with the upper leakage fluid, which increases the flow resistance within the clearance. In the range of clearance height variation investigated, honeycomb tip structure can effectively reduce the leakage flow, and reduce the size and strength of the leakage vortex, so that the loss of the cascade is reduced. At a large tip clearance height, the unstable split of the vortex cores causes the vortex in the honeycomb cavities near pressure side to grow in size, so that the vortex extends further into the upper gap, where the turbulent blocking effect of the vortices on the leakage flow is increased. However, due to the vortex movement and the mixing between honeycomb vortices and the upper clearance flow, there is no obvious advantage in reducing the total loss of the cascade compared to the small tip clearance height. [ABSTRACT FROM AUTHOR]

This paper presents flow separation control conducted on a highly loaded compressor stator cascade using endwall synthetic jets. Numerical methods are employed and mechanisms of endwall synthetic jets in improving the cascade performance are discussed in detail. The influence of several actuation parameters is also investigated. Results show that endwall synthetic jets are able to improve the flows in the blade passage significantly, a maximum loss reduction of 21.63% and a pressure rise increment of 5.60% are obtained at design condition. Apart from energizing the low momentum fluid inside endwall boundary layer by streamwise momentum addition, endwall synthetic jets could induce a streamwise jet vortex and impede the transverse movement of endwall boundary layer through upwash and downwash. Hence, at the expense of slightly degraded near-wall flows, the formation and further evolution of passage vortex would be delayed and flows in the midspan region would be improved notably. The effectiveness of endwall synthetic jets relies on the proper selection of actuation position and jet angle. Flow control turns out to be the most efficient when the actuator is positioned at just upstream of corner separation region with a relatively small jet angle, and a large enough injected momentum is also necessary. Additionally, the adaptability of the actuation at off-design conditions is validated in the present study. [ABSTRACT FROM AUTHOR]

In this paper, numerical simulation of the whole structure of a half flush-mounted S-shaped air-intake and the rear fan stage was conducted for investigating the impact of air-intake exit distortion on the fan-stage performance. Considering substantial boundary layer ingesting, a scheme of blowing control imposed near the first bend of the inlet where the flow separation occurs was also carried out. The results show that the air intake internal separation is eliminated, and the distorted air region at its exit decreases significantly after blowing, simultaneously, the aerodynamic performance of the rear fan stage improves dramatically. The choked mass flow increases about 1.45%, and the maximum isentropic efficiency as well as the corresponding total pressure ratio at this point increases about 1.83% and 1%, respectively. In addition, the distorted fluid always covers several blade passages no matter with blowing control or not when it goes through the fan stage. Both relative total pressure and velocity of air in these distorted blade passages are lower than that in other uniform passages and have slightly increased with blowing. Under effect of the inlet flow angle increase caused by the non-uniform air admission and the adverse pressure gradient in the rear part of the flow passage, serious flow separation occurs at the stator blade suction side but mainly exists at 50% span and below and weakens after blowing control. [ABSTRACT FROM AUTHOR]