Axial Ventilation and Blade Row Effects on Transient Natural Convective Shutdown Cooling in a Gas Turbine

Abstract

As a large civil gas turbine is cooling down, natural convective flows cause components to cool asymmetrically – the bottom sector cools faster than the top. This can lead to a number of issues that have the potential to damage engine components and affect operability. The ability to predict this cooling cycle of a gas turbine has proven to be extremely difficult, owing to the complex nature of natural convective flow and its dependency on a considerable number of design parameters. An experimental and numerical investigation into the impact of axial ventilation (interaction between the annulus and the external air) and blade rows on the natural convective flow in a large civil gas turbine high-pressure compressor has led to some key discoveries. Axial ventilation caused a 70% increase in the peak top-to-bottom temperature difference in the cooling cycle, when compared to the baseline sealed case. The combinations of four blade rows and axial ventilation caused a 130% increase in peak temperature difference over the baseline case. Numerical simulations illuminated that the root cause of this was the cold air drawn into the lower section of the annulus led to a relatively high heat flux, coupled with a blockage effect on the natural draft in the upper section of the annulus. This study has highlighted the importance and inter-dependency of these effects in defining the level of rotor bow that is observed. Therefore, it is imperative that they are included and sufficiently captured in order for a shutdown.