Active Control of Thermoacoustical Instabilities.

This dissertation presents some advances in active control of thermoacoustic instabilities in combustion chambers. Large-size gaseous and liquid fueled swirl stabilized combustors were used during the studies. Active control was implemented using different types of actuators. Proportional (loudspeakers and fuel valves) and discrete actuators (open-close automotive fuel injectors) were investigated. Acoustic and fuel modulation control were successfully applied. In large-scale combustors, flame stabilization techniques such as swirl add three dimensional characteristics to the flow. Moreover, the induced turbulence creates highly nonlinear interactions in the system. Thus, in order to capture these characteristics nonlinear partial differential equations have to be used. Alternatively, the main dynamics of the combustion process can be modeled experimentally. This approach was chosen. Time and frequency domain linear identification techniques were used for this purpose. Several model-based control strategies such as LQG, Hinfinity Disturbance Rejection and Hinfinity Loop-Shaping techniques were tested experimentally with success. A simple controller whose parameters were optimized on-line is also introduced. An evolution algorithm was developed to perform its parameter optimization achieving good convergence to optimal values. The improvements with these proposed control techniques over classical phase-delay control are demonstrated experimentally. A new control configuration was suggested from heat-release visualizations of the flame. In this new configuration, control actuation is directly focused onto the main area of heat-release in the flame front. Consequently, a more efficient actuation is achieved. It is shown that with just a small amount of modulated fuel, phase-delay control can substantially attenuate the pressure oscillations. Finally, during the development of Hinfinity controllers, there were cases where the stability of the resulting controllers restricted the closed-loop performance. A control design strategy to solve the Hinfinity Strong Stabilization problem is then presented. The proposed design strategy pursues to overcome the conservativeness of existing formulations. Examples show its potential for future applications

Modelling and control of combustion instabilities with anchored laminar ducted flames

This thesis deals with the derivation of new semi-analytical methods for the modelling of combustion instabilities in anchored laminar flame combustors. In a first part, through an analysis of the motion of the acoustic discontinuity in a ducted flame model, it shows that the movement of the flame induced discontinuity can lead to stability changes. For unstable combustors, it can also affect the amplitude of limit cycle oscillations. In a second part, the problems that are encountered when attempting to obtain the transfer functions for linearly unstable systems from within limit cycle are demonstrated. Indeed, under these circumstances, both the phase and amplitude of the unstable mode need to be corrected. Whilst the correction to the phase can easily be determined, the correction to the gain cannot, supporting the need for robust model based controllers or adaptive control methods which do not require system identification. Lastly, this thesis presents the derivation and implementation of the first asymptotic-based mathematical models which account for the flame motion, hydrodynamic field and acoustic field in an anchored ducted flame setup. This modelling exploits the difference in length scales associated with the flame, hydrodynamic field and acoustic waves. Unlike ducted flame models which omit the hydrodynamic field, this allows us to capture instability mechanisms such as Rayleigh-Taylor, or Darrieus-Landau instabilities, in the context of anchored laminar flames. This is done for two simplified configurations: a weakly conical flame shape, and a conical flame shape case with small mean heat release.Open Acces

Active control of spray combustion

Effect of a forced dilution air jet on air-fuel spray mixing and emissions has been investigated. Temperature measurements have been made at a number of forcing frequencies in the range of 100-1100 Hz and blowing ratios between 6-15. Open-loop flame response to forcing has also been acquired by recording pressure and heat release spectra. The results show that the mean temperature field inside the flame can be altered due to jet modulation. Significant effects are observed by forcing at locations close to the dump plane. Enhancements in temperature of the order of 100–200 ˚C, and reduction in pattern factor of the order of 40% were observed. Substantial reductions in nitric oxide emissions can be obtained over a range of flow conditions. More rigorous burning can be obtained due to enhanced fuel air mixing. A multi-resolution technique is utilized to analyze temperature fields to decompose the response of different hierarchical scales to forcing. Forcing is found to have most impact on large-scale structures that are in the order of characteristic jet length scale. Bulk mixing is not the only factor that determines pollutant emissions level. Consequently, there exist select frequencies, which affect both emissions and mixing positively. An artificial intelligence based extremum-seeking algorithm is introduced to optimize the combustor behavior. The second part of this dissertation deals with syngas combustion. Stability of a pre-mixed gas turbine combustor is quite sensitive to fuel composition. Behavior of a premixed confined hydrogen enriched methane flame is studied with regard to thermo-acoustic instability induced flashback, emissions, flammability limits and acoustics over a range of conditions. Hydrogen addition extends the flammability limits and enables lower emissions levels to be achieved. Contrarily, increased RMS pressure fluctuation levels, and higher susceptibility to flashback is observed with increasing hydrogen volume fraction inside the fuel mixture. In addition, a semi-analytical model has been utilized to capture the flame holding and flashback dynamics utilizing G-equation. A limit cycle behavior in the flame front movement is observed due to a non-linearity in the feedback term. Experiments including phase locked radical imaging and PLIF measurements have been performed at varying fuel composition

Numerical and experimental investigation of a gasturbine model combustor with axial swirler

This thesis presents the simulations of and experiments with the so-called CECOST burner. The CECOST burner is a gas turbine model combustor equipped with an axial swirler and placed in a laboratory for the lean premixed turbulent combustion of a range of fuels at atmospheric pressure. The operation with natural gas, methane, hydrogen-enriched methane and syngas from black-liquor gasification was investigated experimentally and numerically. The CECOST burner is modular and adaptive, which enables the separate investigation of different parameters. A wide section of the CECOST swirler is manufactured from quartz to enable optical access. The flow field in the combustion chamber was measured by particle image velocimetry (PIV). High-speed imaging of the chemiluminescence signal in the bandwidth of hydroxyl radical relaxation was performed to document the intensity distribution for flames throughout the operating range of the CECOST burner. The lower (lean blow out) and upper (flashback) fuel-air equivalence ratio limits of stable operation were determined. The flashback of the flame upstream into the mixing section was captured by high-speed imaging. In the stable operating range, planar laser-induced fluorescence of hydroxyl radicals (OH-PLIF) was recorded to assess the turbulent flame structure.Reynolds-averaged Navier-Stokes (RANS) simulations were performed to calculate the steady isothermal flow in the CECOST burner for parameter studies. The measured flow data was used to validate the geometry and numerical discretisation of the computational model of the burner. Large eddy simulation (LES) was carried out to investigate the combusting flow with temporal resolution and high accuracy