Research Summaries

Back Understanding and Mitigating Vortex-Dominated, Tip-Leakage and End-Wall Losses in a Transonic Splittered Rotor Stage: Research Area 1.1, Including 1.1

Fiscal Year 2012
Division Graduate School of Engineering & Applied Science
Department Mechanical & Aerospace Engineering
Investigator(s) Hobson, Garth V.
Sponsor Army Research Office (Army)
Summary The requirement for higher power-to-weight ratios in modern jet engines leads to a reduced number of stages at increased loading per stage or blade row. The use of a tandem cascade, instead of a single blade row, will allow higher diffusion factors be blade row. Tandem, or splittered, blade rows enable a large flow deflection and a correspondingly high-pressure rise within a short distance. Tandem cascades have been applied to compressors stators, particularly as the final blade row of a high-pressure axial compressor or as an exit guide vane for the axial part of a mixed flow compressor: In the 1970's Wennerstrom and Hearsey undertook the task of designing, building and testing a supersonic axial flow stage with a pressure ratio of 3.0 and an isentropic efficiency of 0.82. Upon testing, the stage fell dramatically short of design. This was blamed largely on poor flow control within the rotor passage. Recognizing the splitters had long been used to improve performance of centrifugal compressors, the decision was made to use a splitter, in the hopes that better flow control could be achieved without incurring additional losses. Due to time constraints, many decisions regarding the splitter were based upon engineering judgment with little analysis. The result of adding the splitter was that the rotor performance improved, but the overall stage performance was still short of the goal, as the pressure ratio was 2.76 and the efficiency was only 0.68. However the stage was much less sensitive to incidence variations at off-design conditions, indicating that the splitter did indeed improve the flow control within the rotor.
Inviscid and viscous, three-dimensional calculations were performed by Tzuoo, et. al. in 1990 on Wennerstrom's rotor. They determined that additional shocks existed between the splitter vanes and main airfoils and they also noted that by moving the splitter closer to the main airfoil suction side the likelihood of passage chocking was reduced. Most recently, McGlumphy, et. al. performed a numerical study on a rear stage of a core compressor that incorporated splitter vanes on the rotor. The shock-free fully turbulent flow analysis showed that the tandem rotor outperformed its single blade counterpart by attaining a higher, pressure ratio and efficiency as well as numerical stall margin. In spite of all these advancements splittered or tandem rotors have not been used on rotors and hence the lack of use is the motivation for the current study.
It is our contention that both the location of the splittered blade as well as the tip section profile, of both the main blade and splittered blade, can be optimized to mitigate tip leakage and end wall flows. Since computational analysis has matured to the level where design optimization can be performed with such variables as airfoil shape and spacing of the main and splitter blades. As clearance-to-span ratios increase in high-pressure ratio compressors, so too will the chord-to-clearance ratios. Hence the logical conclusion to attempt to overcome the associated tip clearance loss with strategically placed partial blades within the main passage. Further mitigation of tip-leakage losses will be investigated with casing treatments. Since the optimization of the blade profiles, particularly in the tip region, will be performed with computational fluid dynamics the blade shapes will no doubt be of three-dimensional shape including lean and sweep.
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