Wolfgang Weisenstein

An Efficient Computational Model for Premixed Turbulent Combustion at High Reynolds Numbers Based on a Turbulent Flame Speed Closure

Theoretical background, details of implementation, and validation results for a computational model for turbulent premixed gaseous combustion at high turbulent Reynolds numbers are presented. The model describes the combustion process in terms of a single transport equation for a progress variable; turbulent closure of the progress variables source term is based on a model for the turbulent flame speed. The latter is identified as a parameter of prime significance in premixed turbulent combustion and determined from theoretical considerations and scaling arguments, taking into account physico-chemical properties and local turbulent parameters of the combustible mixture. Specifically, phenomena like thickening, wrinkling, and straining of the flame front by the turbulent velocity field are considered, yielding a closed form expression for the turbulent flame speed that involves, e.g., speed, thickness, and critical gradient of a laminar flame, local turbulent length scale, and fluctuation intensity. This closure approach is very efficient and elegant, as it requires only one transport equation more than the nonreacting flow case, and there is no need for costly evaluation of chemical source terms or integration over probability density functions. The model was implemented in a finite-volume-based computational fluid dynamics code and validated against detailed experimental data taken from a large-scale atmospheric gas turbine burner test stand. The predictions of the model compare well with the available experimental results. It has been observed that the model is significantly more robust and computationally efficient than other combustion models. This attribute makes the model particularly interesting for applications to large three-dimensional problems in complicated geometries.

Excitation of Thermoacoustic Instabilities by Interaction of Acoustics and Unstable Swirling Flow

Unstablethermoacousticmodeswereinvestigatedandcontrolledinanexperimentallow-emissionswirlstabilized combustor, in which theacousticboundary conditions weremodie ed to obtain combustion instability. Theacoustic boundary conditions of the exhaust system could be adjusted from almost anechoic (ree ection coefe cient jj rjj < 0:2) to open-end ree ection. Several axisymmetric and helical unstable modes were identie ed for fully premixed and diffusion-typecombustion. Theseunstablemodeswereassociated with e ow instabilities related to therecirculation wake-like region on the combustor axis and shear-layer instabilities at the sudden expansion (dump plane). The combustion structure associated with the different unstable modes was visualized by phase-locked images of OH chemiluminescence. The axisymmetric mode showed large variation of the heat release during one cycle, whereas the helical modes showed variations in the radial location of maximal heat release. The axisymmetric mode was the dominant one during unstable combustion. It was obtained by forcing a longitudinal low-frequency acoustic resonance. Helical modes could only be obtained when the axisymmetric mode was suppressed by using a nonree ectingboundarycondition.Aclosed-loopactivecontrolsystemwasemployedtosuppressthethermoacoustic pressure oscillations and to reduce NO x and CO emissions. Microphones were used to monitor the pressure oscillations during thecombustion process and provide input to thecontrol system. An acousticactuation was used to modulate the aire ow and thus affected the mixing process and the combustion. Upstream excitation modie ed the shear-layer structure and was shown to be superior to downstream excitation, which combined less effective shear-layer excitation with noise cancellation. Suppression levels of up to 5 dB in the pressure oscillations and a concomitant 24% reduction of NO x emissions were obtained in premixed combustion using an acoustic power of less than 0.002% of the combustion power. The control of the diffusion e ame was less effective, and NO x emissions increased at the phase that was most effective in suppressing the pressure oscillations. The differences between the behavior of the control system in the two combustion modes was caused by different levels of interaction between the combustion process and the shear layer.

Coherent structures in swirling flows and their role in acoustic combustion control

Interaction between flow instabilities and acoustic resonant modes and their effect on heat release were investigated and controlled in an experimental low-emission swirl stabilized combustor. Acoustic boundary conditions of the combustor were modified to excite combustion instability at various axisymmetric and helical unstable modes in a fully premixed combustion. The combustion unstable modes were related to flow instabilities in the recirculating wakelike region on the combustor axis and the separating shear layer at the sudden expansion (dump plane). Flow field measurements were performed in a water tunnel using a simulated combustor configuration. The water tunnel tests demonstrated the existence of several modes of flow instabilities in a highly swirling flow, modes which were shown to affect the combustion process. Mean and turbulent characteristics of the internal and external swirling shear layers were measured and unstable flow modes were identified. Instability modes during combustion were vis...

Structure and control of thermoacoustic instabilities in a gas-turbine combustor

Thermoacoustic instability was investigated and controlled in an experimental low-emission swirl stabilized combustor, in which the acoustic boundary conditions were modified to obtain combustion instability. Several axisymmetric and helical unstable modes were identified for fully premixed and partially premixed/diffusion combustion. These unstable modes were associated with flow instabilities related to the recirculation region on the combustor axis and shear layer instabilities at the sudden expansion (dump plane). The spatial locations of the intense combustion regions associated with the different unstable modes were visualized by phase locked images of OH chemiluminescence. The axisymmetric mode showed large variation of the heat release during one cycle, while the helical modes showed variations in the radial location of maximal heat release. A closed loop active control system was employed to suppress the thermoacoustic pressure oscillations and to reduce NO x emissions. Microphone and OH emission...

Control of thermoacoustic instabilities and emissions in an industrial-type gas-turbine combustor

Unstable thermoacoustic modes were investigated and controlled in an experimental low-emission swirl-stabilized combustor, in which the acoustic boundary conditions were modified to obtain combustion instability. The swirl-stabilized combustor was operated in premixed mode as well as with a diffusion-type flame. The combustion structure associated with the different unstable modes was visualized by phase-locked images of OH chemiluminescence. The axisymmetric mode showed large variation of the heat release during one cycle, while the helical modes showed variations in the radial location of maximal heat release. A closed-loop active control system was employed to suppress the thermoacoustic pressure oscillations and to reduce NOx emissions. Microphone and OH emission detection sensors were utilized to monitor the combustion process and provide input to the control system. An acoustic actuation was utilized to modulate the airflow and thus affect the mixing process and the combustion. Suppression levels of up to 5 dB in the pressure oscillations and a concomitant reduction of NOx emissions were obtained using an acoustic power of less than 0.002% of the combustor power. At the optimal control conditions, it was shown that the major effect of the control system was to reduce the coherence of the vortical structures that gave rise to the thermoacoustic instability.

Method and apparatus for controlling thermoacoustic vibrations in a combustion system

In a method of controlling thermoacoustic vibrations in a combustion system having a combustion chamber and a burner, the fluid shear layer forming in the region of the burner is acoustically excited. An apparatus for controlling thermoacoustic vibrations in a combustion system having a combustion chamber and a burner is distinguished by the fact that device(s) for the acoustic excitation of the working gas are arranged in the region of the burner.

Palladium-catalyzed combustion of methane: simulated gas turbine combustion at atmospheric pressure

Abstract Atmospheric pressure tests were performed in which a palladium catalyst ignites and stabilizes the homogeneous combustion of methane. Palladium exhibited a reversible deactivation at temperatures above 750°C, which acted to “self-regulate” its operating temperature. A properly treated palladium catalyst could be employed to preheat a methane/air mixture to temperatures required for ignition of gaseous combustion (ca. 800°C) without itself being exposed to the mixture adiabatic flame temperature. The operating temperature of the palladium was found to be relatively insensitive to the methane fuel concentration or catalyst inlet temperature over a wide range of conditions. Thus, palladium is well suited for application in the ignition and stabilization of methane combustion.

Method of controlling thermoacoustic vibrations in a combustion system, and combustion system

In a method of suppressing or controlling thermoacoustic vibrations which develop in a combustion system having a burner working in a combustion chamber due to the formation of coherent or vortex structures and a periodic heat release associated therewith, in which method the vibrations are detected in a closed control loop and acoustic vibrations of a certain amplitude and phase are generated as a function of the detected vibrations and induced in the combustion system, improved suppression is achieved in that, within the control loop, the amplitude of the generated acoustic vibrations is selected to be proportional to the amplitude of the detected vibrations.

Acoustic control of combustion instabilities and emissions in a gas-turbine combustor

Unstable thermoacoustic modes were investigated and controlled in an experimental low-emission swirl stabilized combustor, in which the acoustic boundary conditions were modified to obtain combustion instability. The swirl stabilized combustor was operated in premixed mode as well as with a diffusion type flame. A closed loop active control system was employed to suppress the thermoacoustic pressure oscillations and to reduce NOx emissions. Microphone and OH emission detection sensors mere utilized to monitor the combustion process and provide input to the control system. An acoustic actuation was utilized to modulate the air flow and thus affecting the mixing process and the combustion. Suppression levels of up to 5 dB in the pressure oscillations and a concomitant reduction of NOx emissions were obtained using an acoustic power of less than 0.002% of the combustor power. It was shown that the control system reduced the coherence of the vortical structures which gave rise to the thermoacoustic instability.

Flow-Acoustic Interactions as a Driving Mechanism for Thermoacoustic Instabilities

Unstable thermoacoustic modes were investigated and controlled in an experimental low-emission swirl stabilized combustor, in which the acoustic boundary conditions were modified to obtain combustion instability. The acoustic boundary conditions of the exhaust system could be adjusted from almost anechoic (reflection coefficient |r < 0.15) to open end reflection. Several axisymmetric and helical unstable modes were identified for fully premixed and diffusion type combustion. These unstable modes were associated with flow instabilities related to the recirculation wake-like region on the combustor axis and shear layer instabilities at the sudden expansion (dump plane). The combustion structure associated with the different unstable modes was visualized by phase locked images of OH chemiluminescence. The axisymmetric mode showed large variation of the heat release during one cycle, while the helical modes showed variations in the radial location of maximal heat release. The axisymmetric mode was the dominant one during unstable combustion. It was obtained by forcing a longitudinal low frequency acoustic resonance. Helical modes could only be obtained when the axisymmetric mode was suppressed by using a non-reflecting boundary condition. Closed loop active control system was employed to suppress the thermoacoustic pressure oscillations and to reduce NOx and CO emissions. Microphones were utilized to monitor the pressure oscillations during the combustion process and provide input to the control system. An acoustic actuation was utilized to modulate the airflow and thus affecting the mixing process and the combustion. Upstream excitation modified the shear layer structure, and was shown to be superior to downstream excitation which combined less effective shear layer excitation with noise cancellation. Suppression levels of up to 5 dB in the pressure oscillations and a concomitant reduction of NOx emissions were obtained in premixed combustion using an acoustic power of less than 0.002% of the combustion power. The control of the diffusion flame was less effective and NOx emissions increased at the phase which was most effective in suppressing the pressure oscillations. The differences between the behavior of the control system in the two combustion modes was due to different levels of interaction between the combustion process and the shear layer.