Francesco Di Sabatino, Ph.D. Candidate, Supervised by Professor Deanna Lacoste
Gas turbine engines are an efficient and flexible way of generating electrical power or propelling aircraft. More stringent regulations on pollutant emissions have been imposed to these engines throughout the years, especially in terms of nitrogen oxides (NOx). A very promising combustion technology to reduce NOx emissions and increase efficiency is the lean premixed combustion (LPC). The main drawback of LPC is its intense flame dynamics. Thermoacoustic instabilities, lean blow-off and lean instabilities are examples of such dynamical phenomena that can seriously damage or even lead to failure of gas turbines.
This work investigates the response of lean premixed swirl flames to acoustic perturbations at atmospheric and elevated pressures, for different values of equivalence ratio and various fuels. Understanding this response is a prerequisite for solving problems of thermoacoustic instabilities. For analyzing the flame response to acoustic fluctuations, the flame transfer functions are obtained from the heat release and velocity fluctuations measurements. Moreover, the flow and flame dynamics are analyzed for relevant conditions using phase-locked particle imaging velocimetry, images of the flame chemiluminescence, and planar laser-induced fluorescence of OH radicals. Furthermore, the effects of non-thermal plasma discharges on the lean blow-off and stability limits of premixed swirl flames are discussed at pressure up to 5 bar. To this end, electrical measurements of the plasma discharges and direct imaging of the flames are performed.
The results of these analyses show that the pressure has a strong effect on the flame response to acoustic fluctuations as well as on the flame-plasma interactions. Specifically, increasing the pressure, or changing equivalence ratio and fuel, affects mainly the flame vortex roll-up that is the dominant mechanism controlling the response of swirl flames to acoustic perturbations. Regarding the flame-plasma interactions, it is observed that the non-thermal plasma discharges can efficiently extend the lean blow-off and stability limits to lower equivalence ratios even at elevated pressures, underlining the potential applicability of plasma discharges at conditions relevant for gas turbines.