Numerical investigation of the flow over a model transonic turbine blade tip

Abstract

Direct numerical simulations are used to investigate the unsteady flow over a model turbine blade tip at engine scale Reynolds and Mach numbers. The DNS are performed with an in-house multi-block structured compressible Navier-Stokes solver. The particular case of a transonic tip flow is studied since previous work has suggested compressibility has an important effect on the turbulent nature of the separation bubble at the inlet to the tip-casing gap and subsequent flow reattachment. The flow is simulated over an idealized tip geometry where the tip gap is represented by a constant area channel with a sharp inlet corner to represent the pressure side edge of the turbine blade. The effects of free-stream disturbances, cross-flow and pressure-side boundary-layer on the tip flow aerodynamics and heat transfer are studied. For 'clean' in-flow cases we find that even at engine scale Reynolds numbers the tip flow is intermittent in nature, i.e. neither laminar nor fully turbulent. The breakdown to turbulence occurs through the development of spanwise streaks with wavelengths around 15 − 20% of the gap height. Multi-dimensional linear stability analysis confirms the two-dimensional base state to be most unstable with respect to spanwise wavelengths of 25% of the gap height. The linear stability analysis also shows that addition of cross-flows with 25% of the streamwise gap exit velocity increase the stability of the tip flow. This is confirmed by the DNS which also show that the turbulence production is significantly reduced in the separation bubble. For the case when free-stream disturbances are added to the inlet flow, viscous dissipation and the rapid acceleration of the flow at the inlet to the tip-casing gap causes significant distortion of the vorticity field and reductions of turbulence intensity as the flow enters the tip gap. The DNS results also suggest that the assumption of the Reynolds analogy and a constant recovery factor are not accurate, in particular in regions where the skin friction approaches zero while significant temperature gradients remain, such as in the vicinity of flow reattachment.

Investigation of the formation and evolution of over-tip shock waves in the pressure-driven tip leakage flow by time-resolved schlieren visualization

“…The main objective of this study is to investigate the formation and evolution of over-tip shock waves in the pressure-driven tip leakage flow, resulting in a better understanding of the high un- the literature. 3,12,26,27 In this study, we do not focus on a specific position along the blade chord, because the position of the strongest TLF varies along the chord in different applications. This position tends to be near the leading edge in compressors/fans, 28 at the mid-chord for propeller rotors, 29 and near the mid-to-trailing-edge chord for turbines.…”

“…This leads to pressure-driven tip leakage flows and related flow-flow interactions, such as the rolling-up of tip leakage vortices (TLVs). TLFs are profoundly important for the performance of rotors because of their significant influence on rotor efficiency loss, 1,2 heat transfer and corrosion fatigue, 3 flow instability 4 and cavitation. 5 The loss introduced by TLFs constitutes about one-third of the total stage loss, 1 and about half of this loss occurs in the gap region.…”

Compliance COTPA 2003 and other supportive law for tobacco free district Kanpur Nagar in State of Uttar Pradesh

“…The imposed disturbance follows a spatial chessboard pattern in streamwise velocity and temporal fluctuations in wall-normal and spanwise velocities. 65 It can simulate streamwise vortices convecting at the local velocity while remaining divergence-free, mimicking grid turbulence. The equations used for generating perturbations in u, v and w components of velocity are prescribed as:…”

Unsteady Structure of Compressor Tip Leakage Flows

“…Previous high-fidelity simulations have focused on the tip leakage vortex [6,7], its interaction with the passage flow [8] and the effect of gap size on this physics [9,10]. Separation bubbles in turbine gaps are found to break down to turbulence through the development of spanwise streaks 15-20% of the tip gap height [11] and this breakdown alters the tip gap flow significantly. Compressor tips are narrower and Mach numbers lower, and therefore the separation bubble physics are likely to be different.…”