Experimental And Numerical Investigation Of Compressor Tip Clearance Flow Control Using Blade Tip Winglet

It is well known that tip clearance flow is detrimental to the pressure rise capability, stability and efficiency of the compressor. To improve the performance and reliability of gas turbine, it is important for the researchers to understand how to control the tip clearance flow and corporate the interaction of the vortices. Recently, blade tip winglet controlling gap flow technique has also been concerned in the field of turbomachinery. In order to enhance the reliability, validity and universality of tip winglet technique application, it is necessary to investigate the action effect of winglet and its mechanism.In this paper, a detailed experimental and numerical study was conducted to investigate the influences of blade tip winglet on tip clearance flow control in compressor cascade and rotors, and reveal the mechanism of the winglets. Results for a variation of the tip clearance size, incidence and winglet geometry are presented.First of all, detailed experimental investigations were carried out to uncover the interaction of the tip clearance flow with three-dimensional separation in the corner region of a compressor cascade and the influences of three different winglet geometries, suction side winglet, pressure side winglet and double-side winglet. The results show that the three-dimensional separation on the blade suction surface is largely removed by the clearance flow for a proper tip clearance size and the aerodynamic performance of the cascade is improved. For the suction side winglet case, the tip leakage vortex is shift toward the core of the passage and pushes the passage vortex near the tip toward the pressure side of the adjacent blade, the strength of the interaction between tip leakage, passage vortex and concentrated shed vortex is reduced. For the pressure side winglet case, the tangential momentum of tip leakage flow is reduced; the pressure side winglet manages to displace the tip leakage vortex to the left, or toward the tip/outer casing corner of the suction side. For the double side winglet case with a high ratio of blade thickness to tip clearance height, the strength of tip leakage flow is reduced clearly due to more friction resistance between the blade tip and the endwall, and the flow undergoes a less underturning near the easing.Extensive traverse measurements were made of the three dimensional flows in a low speed linear cascade with tip winglet of different widths for various tip clearance si- zes and for various cascade inlet flow angles. It is found that the winglets increase aerodynamic loss of the cascade for small tip gap height. For middle and large tip clearance sizes, the suction side winglets reduce loss in the tip leakage vortex region and passage vortex region. The pressure side winglet with the largest width can reduce the aerodynamic loss of the wake in50%-80%span region noticeably. At large incidence conditions, the winglets can only reduce the strength of the rear part tip clearance flow.A numerical study was conducted to explore the effects of three different blended tip winglet geometries on tip clearance flow field of compressor cascade. The current results show that a significant tip leakage velocity and strength of tip leakage vortex reduction is possible by using blended winglet. The blade loading near the tip and the tip leakage vortex trajectories are changed by the tip winglets, thus the interaction between passage vortex and tip leakage vortex is also changed. The effect of endwall movement on the3D flow field in a compressor cascade with different tip clearances was studied; the simulation results show that when relative movement of endwall exists, the scrapped tip leakage vortex appearing in the cascade passage will move toward the pressure surface of adjacent blade; both the upper passage vortex and tip separation vortex are to be inhibited; the load near the tip region will rise which may result in tip leakage mass flow rate increased. Two types of blended tip winglet were investigated to control tip clearance flow in an isolated axial compressor rotor. The suction side blended tip winglet shifts the tip vortex trajectory toward the pressure side of adjacent blade, which results in the corner separation on the suction surface becoming seriously. For the pressure side winglet case, the presence of a tip leakage vortex near the suction surface induces spanwise flow towards the blade tip and helps to wash away the corner separation zone on the blade surface, and this type of winglet is found to increase the compressor rotor stall margin with a little loss in efficiency.3D numerical simulation was conducted for a single transonic compressor Stage37. It is found that the low energy zones due to breakdown of the tip leakage vortex, radial vortex and corner separation of the stator are the factors leading to flow instability at design speed. Based on an improved understanding of compressor rotor tip leakage flow development from CFD simulations, two kinds of blended tip winglet have been designed for a transonic compressor rotor and numerical simulation of three dimensional flows in the rotor were carried out to verify the designed winglet. For the case with suction-side winglet, the breakdown of the tip leakage vortex occurs due to the interaction of shock/tip leakage vortex appeared early than datum rotor, thus leading to more low energy fluid accumulated near the pressure side of the rotor tip passage. For the pressure-side winglet case, the passage choke caused by fluid of low energy and tip leakage flow can be improved, thus the compressor rotor surge point can be effectively delayed.In order to carry out the experimental study of the stage aerodynamic performances of the original and redesigned transonic compressors in the next step, the structure of the compressor test rig was introduced firstly, and the single transonic compressor stage with tip winglet was design. The traditional average flow field measurements and instantaneous velocity measurements obtained with3D-PIV will make it a powerful technique to understand the flow field occurring within compressor with winglet.