Response of Spray and Heat Release to Forced Air Flow Fluctuations in a Gas Turbine Combustor at Elevated Pressure

Kurzfassung

The driving mechanism of pulsations in gas turbine combustors depends on a complex interaction between flow field, chemistry, heat release and acoustics. In aeroengines with airblast nozzles, the response of the fuel, in terms of liquid fuel flux, atomization and evaporation to a fluctuating aerodynamical and thermochemical environment needs to be considered as well. The objective of this work was to investigate the response of the heat release and the liquid fuel distribution to forced periodic modulations of the primary air flow through a diffusion type burner using airblast atomizers. A second focus was on the investigation of the pressure effect and the influence of the nozzle exit geometry on fuel placement and its interaction with the heat release region. The spatial distribution of spray density and heat release at three different modulation frequencies was studied using simultaneous phase-resolved OH chemiluminescence for heat release visualization, and planar Mie scattering of kerosene in two airblast nozzles with different exit geometries. Experiments were performed for a variety of pressures, preheat temperatures, AFR values and modulation frequencies. The purpose of these experiments was to study the potential effect of the prefilmer on the impedance of the system, and its dependence on operating parameters. For one set of operating conditions, the dependence of the phase shift between plenum and combustor pressures on modulation frequency was studied as well. It was observed that the exit geometry of the nozzle has a considerable effect on the flame shape and fuel distribution, but also on the modulation depth of the response to forcing of the air flow. Superposition of fuel and heat release distributions shows that the expanding and contracting fuel spray cone during a period of the air flow modulation drives the region of the heat release, by establishing favourable flammability conditions at varying locations during the oscillation. Instantaneous and phase-averaged images of spray and heat release distribution were recorded at eight phase angles with 45° separation, to cover a full period of the oscillation. Within this phase resolution, no phase shift between spatially integrated fuel flux and heat release could be observed. If there is a changing impedance of the fuel flow caused by a potential storage effect on the prefilmer, it is too small to cause a phase shift variation with forcing frequency within the phase resolution of this experiment.