Transonic Turbine Blade Tip Aero-Thermal Performance With Different Tip Gaps: Part I—Tip Heat Transfer

Abstract

A closely combined experimental and CFD study on a transonic blade tip aero-thermal performance at engine representative Mach and Reynolds numbers (Mexit = 1, Reexit = 1.27×106) is presented in this and its companion paper (Part II). The present paper considers surface heat transfer distributions on tip surfaces, and on suction and pressure side surfaces (near-tip region). Spatially-resolved surface heat transfer data are measured using infrared thermography and transient techniques within the Oxford University High Speed Linear Cascade research facility. The Rolls-Royce PLC HYDRA suite is employed for numerical predictions for the same tip configuration and flow conditions. The CFD results are generally in good agreement with experimental data, and show that the flow over a large portion of the blade tip is supersonic for all three tip gaps investigated. Mach numbers within the tip gap become lower as the tip gap decreases. For the flow regions near the leading edge of the tip gap, surface Nusselt numbers decrease as the tip gap decreases. Opposite trends are observed for the trailing edge region. Several ‘hot spot’ features on blade tip surfaces are attributed to enhanced turbulence thermal diffusion in local regions. Other surface heat transfer variations are attributed to flow variations induced by shock waves. Flow structure and surface heat transfer variations are also investigated numerically when a moving casing is present. The inclusion of moving casing leads to notable changes to flow structural characteristics and associated surface heat transfer variations. However, significant portions of the tip leakage flow remain transonic with clearly identifiable shock wave structures.

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

“…The change of the gap dependence behavior is well captured by both the experiment and CFD. Similar cross-over behavior was reported for a flat tip by Zhang et al [29]. …”

“…[28]. Further high-speed work led to similar findings on a flat tip by Zhang et al [29], and a winglet tip by O'Dowd et al [30]. Zhang and He [31] further illustrated a tip-shaping technique that would locally accelerate flow to reduce the local heat transfer, based on a concept instigated by He et al [32].…”

“…A common limit to the previous moving casing studies is that they were mainly at low speed conditions. Although there are some preliminary indications of the relative moving casing effects in some recent publications of the authors' group [29], [30], there is a clear lack of analyses for systematic identification and understanding of the impact of relative casing movement and underlined mechanisms.…”

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

“…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

“…Backed up by the experimental results, their virtual Schlieren visualizations in a CFD analysis clearly demonstrated that the stripe variations of heat flux correspond to those flow features associated with the over-tip shock system. 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.…”