In the recent years, as the technical developments in the field of GT related technology are more and more driven by regulations on environmental pollution control, a whole series of different industrial evolution and innovation lines are investigated so to make combustion processes ever “cleaner”. Among those, there is for sure the adoption of lean and ultra lean combustion processes to be pursued by means of air-fuel premixing combustion technologies. Within this scenario, at DIMSET/SCL (Savona Combustion Laboratory, Dept. of Thermal Machines, Energy Systems and Transportation, Univ. of Genoa) since several years research activities are carried out, mainly within the frame of EC-funded Research Programmes (ICLEAC, MUSCLES, TLC, H2-IGCC) and cooperation with industrial companies of the energy sector (Ansaldo Energia S.p.A.) and aero-propulsion (Avio Group) sectors. Research activities can take advantage of a close integration between experimental facilities, such as several reactive and non-reactive dedicated burner test-rigs, instrumented with LDV, PDA and PIV laser-based equipment, as well as of in-house continuously improved reactive Navier-Stokes solvers for combustor analysis (NastComb solver) and design (TPM method). The paper deals with the stability characterisation of the different combustion-processes taking place within several GT power plants, namely, the heavy duty AE64-3A heavy duty gas turbine (Ansaldo Energia), already present on the market, the so-called Liquid and Gas Rapid Pre-Mix burners, LRPM and GRPM, designed at DIMSET/SCL and still prototypical, and the Avio-designed LPP (Lean Premixed Prevaporised) burner, for aero-engine applications. The research has been addressed at in-depth characterising the stability behaviour of the burner’s operation. In particular, those aspects have been investigated deemed of greatest importance in affecting a stable performance profile, such as swirlers’ design, burner’s internal aerodynamics, premixing duct configuration, fuel typology and injection modalities, etc. The paper gives a synoptic view both of the research approaches (experimental, instrumental, numerical analysis and design) jointly pursued by DIMSET/SCL team in investigating the combustion instability, as well as of the obtained results, which help in pointing out those burner design and operational parameters which appear as most critical in affecting instability insurgence and self-sustainment.

The research here presented is focused on the laser based experimental characterisation of an Ansaldo Energia burner equipping the Heavy Duty gas turbine (HD). The component is a partially premixed, swirl stabilized burner, adopting a central axial swirler surrounded by a mixed-flow, radially inward one. The burner can be fed with gaseous and liquid fuels by adopting three different injection modalities: diffusion, premixed and pilot injections. The experimental campaigns were carried out at DIMSET/SCL, the Savona Combustion Laboratory, within a joint research initiative between Ansaldo Energia and DIMSET (University of Genoa), on a full scale burner-combustor assembly, by scaling the base load operational conditions to ambient pressure under a Mach number similitude. The research activities performed have been addressed at achieving a detailed set of experimental data adequate to obtain a complete unsteady flow field characterisation in terms of velocity components’ radial distributions together with their local turbulent and periodical fluctuations within the combustor primary zone. In this way, the inner recirculation region at the burner exit can be neatly identified. Furthermore, the main fluid-dynamical parameters of the turbulent flow have been calculated, in terms of turbulent kinetic energy, turbulence intensity, Reynolds stresses and swirl number in order to characterise in detail the burner-combustor assembly from a fluid-dynamics point of view. The said investigations being performed with different operational and geometrical settings and properly managed in order to allow further burner developments at the technological/industrial level. In parallel, the research activities have also pursued the target of performing a thorough velocity fluctuation analysis, to be correlated with possible combustion instabilities, in order to attain a deeper comprehension of phenomena typically affecting gas turbine combustors, such as thermo-acoustical instabilities (humming). The velocity fluctuations have been investigated with particular reference to their inception locations within the burner: it turned out that they are typically related to the presence of the two different swirlers, which induce peculiar interactions between two different flow structures, each one presenting its own dynamical characteristics.

The present paper reports the results of an experimental investigation on the unsteady flow in a Lean Premixed Prevaporized (LPP) burner for aeronautical applications. The experiments were focused on two main aspects: understanding the effect of the fuel spray on the unsteady air flow field and characterizing the fuel spray under unsteady flow conditions in terms of velocity and spatial distribution of the fuel droplets. The experimental campaign was performed with laser-based instrumentation (LDV, PDA and PIV) on a large-scale model of the LPP burner with air preheating and fuel injection in order to allow detailed measurements of the two-phase unsteady flow. The gas flow field is dominated by a spiral vortex breakdown phenomenon, which results in a complex unsteady flow configuration and an extended recirculation zone near the axis of the burner. The fuel droplets flow field is strongly correlated to the gas flow field. By comparing the results of the present experimental campaign with results obtained without fuel spray, there is evidence of a positive effect of the spray on the air flow field. The spray effect results in a reduction of the recirculation phenomenon in the exit section of the LPP burner. At the LPP burner exit a general satisfactory degree of vaporization is obtained. However, at the periphery of the premixing duct outlet section, a significant concentration of larger droplets of not yet vaporized fuel is present, due to the secondary air blast disintegration of the liquid film formed on the internal surface of the premixer tube. This phenomenon is responsible for lack of homogeneity of the fuel distribution in time and space at the premixer duct exit.