Overtip Choking and Its Implications on Turbine Blade-Tip Aerodynamic Performance

Zhang, Q.; He, L.

doi:10.2514/1.b34112

Cited by 58 publications

(18 citation statements)

References 28 publications

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“…For most of the tip surface, y -h values are around 1-2. The nonslip wall condition without a "wall function" is employed for more consistent results in tip heat transfer predictions, based With the computational setup mentioned above, satisfactory agreements between experimental and CFD results have been reported by a series of previous publications, including Zhang et al [19,20] on the flat tip heat transfer, Zhang and He [25] on the flat tip aerodynamics, and O'Dowd et al [21] on the winglet tip heat transfer. Among these studies, the wall-gas temperature ratio was maintained at around 0.92 (near adiabatic).…”

Section: Computational Methods and Validationmentioning

confidence: 58%

Impact of Wall Temperature on Turbine Blade Tip Aerothermal Performance

Zhang

2014

Journal of Engineering for Gas Turbines and Power

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Currently the aerodynamics and heat transfer over a turbine blade tip tend to be analyzed separately with the assumption that the wall thermal boundary conditions do not affect the over-tip-leakage (OTL) flow field. There are some existing correlations for correcting the wall temperature effect on heat transfer when scaled to engine realistic conditions. But they were either developed to account for the temperature dependence of fluid properties largely empirically, or based on a boundary-layer model. It would be difficult (if not impossible) to define a boundary layer in many parts of a realistic blade passage with marked three-dimensional (3D} end wall and secondary flows (including those within a blade tip and around it). The questions to be asked here are: is the OTL aerodynamics significantly affected by the wall thermal condition? And if it is, how can we count this effect consistently in turbine blade tip design and analysis using modern CFD methods? In the present study the problem has been examined for typical high-pressure turbine blade tip configurations. An extensively developed RANS code (HYDRA) is employed and validated against the experimental data from a high speed linear cascade testing rig. The numerical analysis reveals that the wall-gas temperature ratio could greatly affect the transonic OTL flow field and there is a strong two-way coupling between aerodynamics and heat transfer. The feedbacks of the thermal boundary condition to aerodynamics behave differently at different flow regimes over the tip, clearly indicating a highly localized dependence of the convective heat transfer coefficient (HTC) upon wall temperatures. This implies that to use HTC for blade metal temperature predictions without resorting a fully conjugate solution, the temperature dependence needs to be corrected locally. A nonlinear correction approach has been adopted in the present work, and the results demonstrate its effectiveness for the transonic turbine tip configurations studied.

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Section: Computational Methods and Validationmentioning

confidence: 58%

Impact of Wall Temperature on Turbine Blade Tip Aerothermal Performance

Zhang

2014

Journal of Engineering for Gas Turbines and Power

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“…Qualitative differences of loss mechanism and tip heat transfer between highspeed and low-speed conditions were recognized [18][19][20][21]. More specifically, for the transonic part of the over tip flow, turbulent diffusion is shown to have a distinctively different impact on tip heat transfer than for its subsonic counterpart [18,19,21,22].…”

Section: Introductionmentioning

confidence: 96%

Effects of Inlet Turbulence and End-Wall Boundary Layer on Aerothermal Performance of a Transonic Turbine Blade Tip

Zhang

Rawlinson

2014

Journal of Engineering for Gas Turbines and Power

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Most of the previous researches of inlet turbulence effects on blade tip have been carried out for low speed situations. Recent work has indicated that for a transonic turbine tip, turbulent diffusion tends to have a distinctively different impact on tip heat transfer than for its subsonic counterpart. It is hence of interest to examine how inlet turbulence flow conditioning would affect heat transfer characteristics for a transonic tip. The present work is aimed to identify and understand the effects of both inlet freestream turbulence and end wall boundary layer on a transonic turbine blade tip aerothermal performance. Spatially-resolved heat transfer data are obtained at aerodynamic conditions representative of a high-pressure turbine, using the transient infrared thermógraphy technique with the Oxford High-Speed Linear Cascade research facility. With and without turbulence grids, the turbulence levels achieved are 7%-9% and 1%, respectively. On the blade tip surface, no apparent change in heat transfer was observed with high and low inlet turbulence intensity levels investigated. On the blade suction suiface, however, substantially different local heat transfer distributions for the suction side near tip surface have been observed, indicating a strong local dependence of the local vortical flow structure on the freestream turbulence. These experimentally observed trends have also been confirmed by CFD examinations using the Rolls-Royce HYDRA. A further CFD analysis suggests that the level of inflow turbulence alters the balance between the passage vortex associated secondaiy flow and the over tip leakage (OTL) flow. Consequently, an enhanced inertia of near wall fluid at a higher inflow turbulence weakens the cross-passage flow. As such, the weaker passage vortex leads the tip leakage vortex to move further into the mid passage, with the less spanwise coverage on the suction surface, as consistently indicated by the heat transfer signature. Different inlet end wall boundary layer profiles are employed in the computational study with HYDRA. All CFD results indicate the inlet boundary layer thickness has little impact on the heat transfer over the tip surface as well as the pressure side near-tip suiface. However, noticeable changes in heat transfer are obsei-vedfor the suction side near-tip surface. Similar to the inlet turbulence effect, such changes can be attributed to the interaction between the passage vortex and the OTL flow.

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“…Zhang and He [16] found that the flow over the flat tip chocked in a high-pressure turbine cascade. They suggested that the aerodynamic performance of the tip leakage flow can be optimized by using this chock mechanism.…”

Section: Introductionmentioning

confidence: 98%

Thermal Performance of Transonic Cooled Tips in a Turbine Cascade

Zhou

2015

Journal of Propulsion and Power

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Transonic high-pressure turbines are promising candidates for future aircraft engines. Recent studies in transonic turbine cascades showed that shock waves appeared over the uncooled flat tip, which affects its heat transfer. However, the effects of coolant injection on the thermal performance of transonic cooled tips are not clear. The current study aims to understand the thermal performance of a cooled flat tip and a cooled cavity tip in a transonic turbine cascade. The effects of tip film cooling are studied at five coolant pressure ratios (which are the stagnation pressure ratios of the coolant inlet and the cascade inlet) from 0.8 to 1.2. The flow physics over the tip, the heat transfer coefficient, the film-cooling effectiveness, and the heat flux of the tip are investigated. The effects of relative endwall motion are considered.= thermal conductivity of the air, W∕m · K Ma = Mach number P = static pressure, Pa P 0 = stagnation pressure, Pa Q = overall heat flux, W q = local heat flux per unit area, W∕m 2 K Re = Reynolds number; ρVC∕μ T = temperature, K T aw;c = adiabatic wall temperature (ratio of stagnation temperature of coolant to main flow is 0.6), K T g;1 = hot-gas recovery temperature (total temperature at coolant inlet is same as inlet flow), K T w = wall temperature, K T 0 = stagnation temperature, K U = velocity of endwall, m∕s V = velocity, m∕s V x;1 = axial velocity at cascade inlet, m∕s x = coordinate in axial direction y = coordinate in tangential direction η = cooling effectiveness; T aw;c − T aw;1 ∕T c − T aw;1 μ = dynamic viscosity, Pa · s ρ = density, kg∕m 3 σ = contraction coefficient (unblocked height at the vena contracta/tip gap τ) φ = flow coefficient; V x;1 ∕U Subscripts c = coolant 1 = cascade inlet freestream 2 = cascade exit (45%C x downstream of cascade)

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Overtip Choking and Its Implications on Turbine Blade-Tip Aerodynamic Performance

Abstract: This is the accepted version of the paper.This version of the publication may differ from the final published version. Permanent repository link

Cited by 58 publications

References 28 publications

Impact of Wall Temperature on Turbine Blade Tip Aerothermal Performance

Impact of Wall Temperature on Turbine Blade Tip Aerothermal Performance

Effects of Inlet Turbulence and End-Wall Boundary Layer on Aerothermal Performance of a Transonic Turbine Blade Tip

Thermal Performance of Transonic Cooled Tips in a Turbine Cascade

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