Abstract
The combustor-turbine interface in a gas turbine is characterised by complex, highly unsteady flows. In a combined experimental and large eddy simulation (LES) study including realistic combustor geometry, the standard model of secondary flows in the nozzle guide vanes (NGV) is found to be oversimplified. A swirl core is created in the combustion chamber which convects into the first vane passages. Four main consequences of this are identified: variation in vane loading; unsteady heat transfer on vane surfaces; unsteadiness at the leading edge horseshoe vortex, and variation in the position of the passage vortex. These phenomena occur at relatively low frequencies, from 50–300 Hz. It seems likely that these unsteady phenomena result in non-optimal film cooling, and that by reducing unsteadiness designs with greater cooling efficiency could be achieved. Measurements were performed in a high speed test facility modelling a large industrial gas turbine with can combustors, including nozzle guide vanes and combustion chambers. Vane surfaces and endwalls of a nozzle guide vane were instrumented with 384 high speed thin film heat flux gauges, to measure unsteady heat transfer. The high resolution of measurements was such to allow direct visualisation in time of large scale turbulent structures over the endwalls and vane surfaces. A matching LES simulation was carried out in a domain matching experimental conditions including upstream swirl generators and transition duct. Data reduction allowed time-varying LES data to be recorded for several cycles of the unsteady phenomena observed. The combination of LES and experimental data allows physical explanation and visualisation of flow events.
Highlights
In order to reduce emissions of harmful nitrogen oxides (NOx), most modern industrial gas turbines use lean premixed, swirl stabilised combustion
This paper presents the results of an investigation into these combustor-turbine interactions, using a swirler to model the aerodynamics of a combustor, though without hot streaks or reactive flow
This combined experimental and computational study shows some of the many complex flow interactions which are often overlooked in simplified studies
Summary
In order to reduce emissions of harmful nitrogen oxides (NOx), most modern industrial gas turbines use lean premixed, swirl stabilised combustion. This lowers the peak flame temperature, which prevents the formation of NOx. the presence of the swirler produces large scale flow structures in the combustion chamber which provide a highly varying inflow condition to the nozzle guide vanes (NGV). The NGV faces the highest temperatures within the turbine, so a detailed knowledge of flow phenomena and heat transfer is required to optimise cooling design. The swirl core generated in the combustion chamber moves at a low frequency This results in a time-varying loading on the vanes. The pressure side leg of the horseshoe vortex may move across the passage towards the suction side, becoming merged with the passage vortex, but at other times remains attached to its own pressure side, in which case the endwall boundary layer may still separate and roll up into another passage vortex regardless
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