Research project: The effects of high speed flows on transonic turbine tip heat transfer and efficiency

Within aero-engines gas-turbines, the turbine blade tips can require a significant amount of the cooling air and can experience significant degradation throughout the engine life-cycle due to the high mechanical and thermal stresses in the tip region. The high thermal loads are largely due to the tip-leakage flow, which forms in the gap between the moving rotor blade tips and the casing of the machine. Turbine tip flows are also responsible for up to 30% of stage losses for newly installed engines. In-service, the blade-tips are eroded due to chemical attack, fatigue and creep which are all exacerbated by the high gas temperatures, and this causes further performance degradation. Shroudless high-pressure turbine blades can have peak Mach numbers within the tip-gap greater than 1.5. Despite this, many experimental investigations of tip leakage flows have been undertaken in low-speed conditions and transonic effects have until recently not been widely considered. Supersonic effects tend to occur over the aft region of the tip where the blade loading is highest (see Fig 1). This area of the tip is important because the blade thickness in this region is small which means complex tip geometries are often not feasible and this zone is typically difficult to cool. As the flow enters the tip gap, the flow separates from the pressure side corner. The separation accelerates the flow to supersonic conditions like a converging-diverging nozzle, thus as the separation begins to reattach the change in area forces the mainstream flow to accelerate. This in-turn aids the reattachment. Compression waves form due to the curvature of the reattaching boundary-layer. These coalesce into an oblique shock which reflects from the tip and casing, leading to shock/boundary-layer interactions which can cause further boundary-layer separation. The reattachment process of the pressure-side edge separation bubble is important because it will influence the heat transfer over the remainder of the tip.

Recent projects have shown that compressibility plays an important role on both the heat transfer and loss. In a series of collaborative projects with Rolls-Royce we have shown that supersonic flows in the tip gap can significantly reduce tip heat load [1] and choking of the tip leakage flow can also help to reduce tip leakage loss [2] (see Fig 2). Recently, with support from the UK Turbulence Consortium, we have been performing Direct Numerical Simulations of transonic turbine tip flows to study the influence of free-stream turbulence on the tip heat transfer (see Fig. 3), and to also assess the reliability of current turbulence models in predicting heat transfer [3, 4].