Transonic Turbine Blade Tip Aero-Thermal Performance With Different Tip Gaps: Part II—Tip Aerodynamic Loss

Abstract

Blade tip aerodynamic loss results from experimental and numerical investigations are presented for engine representative conditions downstream of a blade row with an exit Mach number Mexit of 1.0, and an exit Reynolds number Reexit of 1.27×106 (based on axial chord). These results are presented for three different tip gaps of 0.5, 1.0, and 1.5 percent relative to engine-equivalent blade span. Experimental data are obtained by traversing a specially-made and calibrated three-hole pressure probe as well as a single-hole probe one axial chord downstream of the blade within the Oxford High Speed Linear Cascade research facility. Three-dimensional RANS CFD numerical predictions are conducted using the Rolls-Royce HYDRA numerical prediction code for steady flow with the Spalart-Allmaras (SA) turbulence model. Included are detailed distributions of stagnation pressure losses, and pitch-wise flow angle. Local total pressure data and mass-averaged total pressure loss coefficients show that the strength of the tip leakage vortex decreases as the tip gap decreases. Magnitudes of the pitch-wise flow angle increase within over-tip leakage vortices, as these vortices become stronger and the tip leakage flow increases. The most important difference between experimental and numerical results is in relation to the passage vortex signatures, which are more apparent for all three tip gap values within the numerical results. The effects of relative casing motion and tip clearance are also examined and discussed, and show that the relative casing movement has a relatively small impact on the size of the over-tip leakage vortex at the medium (1.0% of span) and large (1.5% of span) tip gaps, with more noticeable impact at the smallest tip gap (0.5% of span).

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

“…Most recently, the over tip choking and shock structure were reported for a shroudless flat tip (Zhang et al [26]), and even a modern winglet tip design (O'Dowd et al [27]). Similar results have also been reported by Shyam et al [28,29] in their numerical study for a highly The present computational analysis using an extensively developed CFD code which has been recently validated for transonic blade tip configurations [25], [26], [31] should shed some light on this primary issue. Furthermore, one may ask, are there any advantageous implications if a leakage flow is made choked?…”

“…Some preliminary evaluations of the influence of a moving casing have been reported by Zhang et al [26], O'Dowd et al [31] for the same and similar transonic blade tip configurations. Their results suggest that while a moving wall can reduce the size of the transonic region, a significant part of tip (over 40 percent) still remains transonic.…”

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

“…The stagnation pressure, stagnation temperature and flow angle are specified at inlet, and the static pressure at exit. [31]. HYDRA numerical predictions give overall good agreement, as shown in Fig 2b. Generally, the sizes and magnitudes of the over tip leakage loss core are similar in both Fig.…”

Aerothermal Performance of Shroudless Turbine Blade Tips with Relative Casing Movement Effects

“…The uncertainty associated with the aerodynamic loss has been conducted by O'Dowd et al [36]. Again, since the testing facility and apparatus used are identical, the estimated uncertainty is also valid here.…”