Stephen Peluso

The Effect of Confinement on the Structure and Dynamic Response of Lean-Premixed, Swirl-Stabilized Flames

The effect of flame-wall interaction on the forced response of a lean-premixed, swirl-stabilized flame is experimentally investigated by examining flames in a series of three combustors, each with a different diameter and therefore a different degree of lateral confinement. The confinement ratios tested are 0.5, 0.37 and 0.29 when calculated using the diameter of the nozzle relative to the combustor diameter. Using both flame images and measured flame transfer functions, the effect of confinement is investigated and generalized across a broad range of operating conditions. The major effect of confinement is shown to be a change in flame structure in both the forced and unforced cases. This effect is captured using the parameter Lf,CoHR/Dcomb, which describes the changing degree of flame-wall interaction in each combustor size. The measured flame transfer function data, as a function of confinement, is then generalized by Strouhal number. Data from the two larger combustors is collapsed by multiplying the Strouhal number by the confinement ratio to account for the flow expansion ratio and change in convective velocity within the combustor. Trends at the transfer function extrema are also assessed by examining them in the context of confinement and by using flame images. A change in the fluctuating structure of the flame is also seen to result from an increase in confinement.Copyright

The Three-Dimensional Structure of Swirl-Stabilized Flames in a Lean Premixed Multinozzle Can Combustor

Flame structure can have a significant effect on a combustors static stability (resistance to blowoff) and dynamic stability (combustion instability) and therefore is an important aspect of the combustion process that must be taken into account in the design of gas turbine combustors. While the relationship between flame structure and flame stability has been studied extensively in single-nozzle combustors, relatively few studies have been conducted in multinozzle combustor configurations typical of actual gas turbine combustion systems. In this paper, a chemiluminescence-based tomographic reconstruction technique is used to obtain three-dimensional images of the flame structure in a laboratory-scale five-nozzle can combustor. Analysis of the 3D images reveals features of the complex, three-dimensional structure of this multinozzle flame. Effects of interacting swirling flows, flame–flame interactions, and flame–wall interactions on the flame structure are also discussed.

Flame Area Fluctuation Measurements in Velocity-Forced Premixed Gas Turbine Flames

Fluctuations in the heat release rate that occur during unstable combustion in lean premixed gas turbine combustors can be attributed to velocity and equivalence ratio fluctuations. For a fully premixed flame, velocity fluctuations affect the heat release rate primarily by inducing changes in the flame area. In this paper, a technique to analyze changes in flame area using chemiluminescence-based flame images is presented. The technique decomposes the flame area into separate components which characterize the relative contributions of area fluctuations in the large scale structure and the small scale wrinkling of the flame. The fluctuation in the wrinkled area of the flame which forms the flame brush is seen to dominate its response in the majority of cases tested. Analysis of the flame area associated with the large scale structure of the flame resolves convective perturbations that move along the mean flame position. Results are presented that demonstrate the application of this technique to both single-nozzle and multi-nozzle flames.Copyright

3-D Chemiluminescence Imaging of Unforced and Forced Swirl-Stabilized Flames in a Lean Premixed Multi-Nozzle Can Combustor

A tomographic image reconstruction technique has been developed to measure the 3-D distribution of CH* chemiluminescence of unforced and forced turbulent premixed flames. Measurements are obtained in a lean premixed, swirl-stabilized multi-nozzle can combustor. Line-of-sight images are acquired at equally spaced angle increments using a single ICCD camera. 3-D images of the flames are reconstructed by applying a filtered back projection algorithm to the acquired line-of-sight images. Methods of viewing 3-D images to characterize the structure, dynamics, interaction and spatial differences of multi-nozzle flames are presented. Accuracy of the reconstruction technique is demonstrated by comparing reconstructed line-of-sight images to measured line-of-sight downstream-view images of unforced flames. The effect of the number of acquired projection images on the quality of the reconstruction is assessed. The reconstructed 3-D images of the unforced multi-nozzle flames show the structure of individual flames as well as the interaction regions between flames. Forced flame images are obtained by phase-synchronizing the camera to the forcing cycle. The resulting 3-D reconstructions of forced flames reveal the spatial and temporal response of the multi-nozzle flame structure to imposed velocity fluctuations, information which is essential to identifying the underlying mechanisms responsible for this behavior.Copyright

Comparison Between Self-Excited and Forced Flame Response of an Industrial Lean Premixed Gas Turbine Injector

An experimental study was conducted to compare the relationship between self-excited and forced flame response in a variable-length lean premixed gas turbine (LPGT) research combustor with a single industrial injector. The variable-length combustor was used to determine the range of preferred instability frequencies for a given operating condition. Flame stability was classified based on combustor dynamic pressure measurements. Particle velocity perturbations in the injector barrel were calculated from additional dynamic pressure measurements using the two-microphone technique. Global CH* chemiluminescence emission was used as a marker for heat release. The flame’s response (i.e. normalized heat release fluctuation divided by normalized velocity fluctuation) was characterized during self-excited instabilities. The variable-length combustor was then used to tune the system to produce a stable flame at the same operating condition and velocity perturbations of varying magnitudes were generated using an upstream air-fuel mixture siren. Heat release perturbations were measured and the flame transfer function was calculated as a function of inlet velocity perturbation magnitude. For cases in this study, the gain and phase between velocity and heat release perturbations agreed for both self-excited and forced measurements in the linear and nonlinear flame response regimes, validating the use of forcing measurements to measure flame response to velocity perturbations. Analysis of the self-excited flame response indicates the saturation mechanism responsible for finite limit amplitude perturbations may result from nonlinear driving or damping processes in the combustor.© 2011 ASME

Structure of Flames in Flame Interaction Zones

Two identical burners with variable burner separations are used to investigate the dynamics of interacting flames. The presence of adjacent flames in large scale combustion devices influences the structure and dynamics of the flames, and understanding the sensitivity of flames to these interactions is vital for local and global flame characterization. The behavior of the flame in the interaction zone is dependent on a number of operating parameters, including burner separations, inlet bulk flow velocities, and flame stabilization or flame shape. High-repetion-rate CH* chemiluminescence and OH-planar laser-induced flourescence measurements are performed to obtain the flame structure and flame-front locations. These measurement techniques allow for the determination of how varying fluid-dynamic and geometric parameters change the behavior of these flames in the flame interaction zones. We also quantify global metrics like flame surface density, global consumption speed, and flame curvature PDFs to understand the impact of flame interaction.

The 3-D Structure of Swirl-Stabilized Flames in a Lean Premixed Multi-Nozzle Can Combustor

Flame structure is an important aspect of the combustion process which must be considered in the design of gas turbine combustors as it can have a significant effect on the combustor’s static stability (blowoff) and dynamic stability (combustion instability). The relationship between flame structure and flame stability has been studied extensively in single-nozzle combustors. However, relatively few studies have been conducted in multi-nozzle combustor configurations typical of actual gas turbine combustion systems. In this paper, a chemiluminescence-based tomographic reconstruction technique is used to obtain three-dimensional images of the flame structure in a laboratory-scale five-nozzle can combustor. The images reveal the complex three-dimensional structure of this multi-nozzle flame, as well as, the effects of interacting swirling flows, flame-flame interactions and flame-wall interactions on flame structure.Copyright