Combustor non-uniformities convection in a HP turbine operating in design and off-design conditions

Achieving low NOx emissions in gas turbines is fundamental for air quality. In order to meet pollutant emission regulations, modern combustors exploit lean premixed combustion to limit flame temperature, and hence NOx formation. However, this results in a strong unsteady heat release rate which produces unsteady temperature fluctuations, commonly known as entropy waves (EWs). Moreover, combustion air is swirled to get it thoroughly mixed with fuel, thus preventing local hot spots that can lead to significant NOx formation. These non-uniformities, i.e. swirled entropy waves, generated inside combustion chamber have strong impact on turbine stage aerodynamics and so it’s crucial to investigate their convection through the stage. In the past years some studies have been already done, but some aspects have still to be addressed. This work aims at studying swirled entropy waves convection through an axial turbine stage working in both design and off-design conditions, by means of an experimental campaign carried out on the high-speed test rig at Politecnico di Milano. The facility features a combustor simulator called Entropy Wave Generator (EWG) equipped with a swirl generator. This device injects the disturbance axially upstream of the stator and two different clocking positions are investigated to study the effects of injection position. Along the turbine stage three measuring sections are defined (stator inlet, stator outlet and rotor outlet) and measurements are performed thanks to temperature and pressure probes, whose features are deeply described. The experimental results show that the injected swirl profile influences remarkably the aerodynamics and that EWs magnitude undergoes attenuation as they cross the stage.

Per ragioni di inquinamento atmosferico, `e necessario limitare l’emissione di NOx nelle turbine a gas. I requisiti imposti dalle autorit`a regolatorie possono essere soddisfatti grazie ai moderni combustori, che sfruttano una combustione magra premiscelata limitando cos`ı la temperatura di fiamma e quindi la formazione dei NOx. Tuttavia questa soluzione comporta un rilascio di calore instabile che causa fluttuazioni di temperatura nel flusso, comunemente chiamate entropy waves (EWs). Inoltre all’aria `e impartito un moto di swirl cos`ı da favorire la perfetta miscelazione di aria e combustibile, evitando cos`ı la formazione di punti a temperatura pi`u alta in camera di combustione che favorirebbero la formazione di NOx. Le disuniformit`a nel flusso causate dalle entropy waves e dallo swirl condizionano l’aerodinamica all’interno dello stadio di turbina ed `e pertanto importante studiarne gli effetti. La campagna di test si `e svolta nel Laboratorio di Fuidodinamica delle Macchine del Politenico di Milano sul banco prova denominato High-Speed Closed-Loop. Sono state utilizzate differenti sonde per caratterizzare il flusso a monte statore, valle statore e valle rotore.