In order to increase gas turbine efficiency without compromising operability, higher inlet temperatures are being employed along with reduced cooling flows. Lean burn combustion also leads to more uniform turbine inlet temperatures and higher heat load on vane endwalls. The need to optimise performance requires investigation into realistic aspects of turbine geometry, which have often been neglected in the past.

Gas turbine blades and vanes are typically manufactured with small clearances between adjacent vane and blade platforms, termed the midpassage gap. The midpassage gap reduces turbine efficiency and causes up to 20\\% additional heat load into the vane platform, as well as changing the distribution of endwall heat transfer and film cooling.

Despite this, there have been very few studies into the midpassage gap, and no comprehensive explanation of the fundamental flow phenomena of importance, or mitigating design measures has been found, or even a general estimate of its importance for a given design. Some literature exists on the effects of the gap on endwall heat transfer and cooling, but there has been no previously published work found on the flow within the gap and the fundamental parameters governing the gap flow.

Experiments were carried out in a high speed linear cascade using thin film heat transfer gauges, an IR camera, static pressure tappings and a five-hole probe traverse. A parametric measurement campaign was carried out to investigate the affect of gap width on heat transfer and aerodynamic losses. This was supported by a RANS CFD study.

Aerodynamic and heat transfer measurements are presented and a physical explanation of flow within the midpassage gap is given. A one-dimensional model is derived based on this and is shown to agree well with experiments and CFD. Using this, the effects of the gap can be quantified based only on geometric features. It is found that at present there are significant losses and a large proportion of heat load caused by the gap, but that with modified design this could be reduced to negligible levels.

Leakage flows into the gap are investigated analytically and with CFD, although experiments were carried out with zero leakage.

Recommendations are given for targets that turbine designers should work towards in reducing the adverse effects of the midpassage gap.

The demonstration case for this study was the nozzle guide vane of a large frame industrial gas turbine, but the analytical model developed is general and should apply to all rotor and stator blades which posses a similar mid-passage gap.

A method to estimate the effect of gap flow is presented, so that for any machine the significance of the gap may be assessed.


Aerodynamics and Heat Transfer Inside a Gas Turbine Mid-Passage Gap

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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