Lean blowout (LBO) of a partially premixed swirl flame is studied using chemiluminescence imaging and simultaneous stereo-PIV and OH-PLIF measurements at repetition rates up to 5 kHz. The flame, which is operated with methane and air in a gas turbine model combustor at atmospheric pressure, features a pronounced precessing vortex core (PVC) at the inner shear layer. In the first part of the study, the stabilization mechanism of the flame close to LBO is investigated. The fields of velocity and OH show that near LBO there are essentially two regions where reaction takes place, namely the helical zone along the PVC and the flame root around the lower stagnation point. The zone along the PVC is favorable to the flame due to low strain rates in the vortex center and accelerated mixing of burned and fresh gas. The flame root, which is located close to the nozzle exit, is characterized by an opposed flow of hot burned gas and relatively fuel-rich fresh gas. Due to the presence of high strain rates, the flame root is inherently unstable near LBO, featuring frequent extinction and reignition. The blowout process, discussed in the second part of the study, starts when the extinction of the flame root persists over a critical length of time. Subsequently, the reaction in the helical zone can no longer be sustained and the flame finally blows out. The results highlight the crucial role of the flame root, and suggest that well-aimed modifications of flow field or mixture fraction in this region might shift the LBO limit to leaner conditions.

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Dynamics of lean blowout of a swirl-stabilized flame in a gas turbine model combustor

Combustion dynamics of a low-swirl combustor

A complete, active control system has been developed to permit turbine engine combustors to operate safely closer to the lean-blowout (LBO) limit, even in the presence of disturbances. The system uses OH chemiluminescence and a threshold-based identification strategy to detect LBO precursor events. These nonperiodic events occur more frequently as the LBO limit is approached. When LBO precursors are detected, fuel entering the combustor is redistributed between a main flow and a small pilot, so as to increase the equivalence ratio near the stabilization region of the combustor. This moves the effective LBO limit to leaner mixtures, thus increasing the safety margin. The event-based control system was demonstrated in an atmospheric pressure, methane-air, swirl-stabilized, dump combustor. The NOx emissions from the piloted combustor were found to be lower than those from the unpiloted combustor operating at the same safety margin and same nominal velocity field. The controller minimizes the NOx at constant total power by keeping the pilot fuel fraction at the lowest value needed to limit the number of precursor events to an acceptable level.

Active Control of Lean Blowout for Turbine Engine Combustors

The diagnostic techniques for simultaneous velocity and relative OH distribution, simultaneous temperature and relative OH distribution, and three component velocity mapping are described. The data extracted from the measurements include statistical moments for inflow fluid dynamics, temperature, conditional velocities, and scalar flux. The work is a first step in the development of a detailed large eddy simulation (LES) validation database for a turbulent, premixed flame. The low-swirl burner used in this investigation has many of the necessary attributes for LES model validation, including a simplified interior geometry; it operates well into the thin reaction zone for turbulent premixed flames, and flame stabilization is based entirely on the flow field and not on hardware or pilot flames.

Simultaneous PIV/OH-PLIF, Rayleigh thermometry/OH-PLIF and stereo PIV measurements in a low-swirl flame

A new experimental method is described that provides high-speed movies of turbulent premixed flame wrinkling dynamics and the associated vorticity fields. This method employs cinema stereoscopic particle image velocimetry and has been applied to a turbulent slot Bunsen flame. Three-component velocity fields were measured with high temporal and spatial resolutions of 0.9 ms and 140 μm, respectively. The flame-front location was determined using a new multi-step method based on particle image gradients, which is described. Comparisons are made between flame fronts found with this method and simultaneous CH-PLIF images. These show that the flame contour determined corresponds well to the true location of maximum gas density gradient. Time histories of typical eddy–flame interactions are reported and several important phenomena identified. Outwardly rotating eddy pairs wrinkle the flame and are attenuated at they pass through the flamelet. Significant flame-generated vorticity is produced downstream of the wrinkled tip. Similar wrinkles are caused by larger groups of outwardly rotating eddies. Inwardly rotating pairs cause significant convex wrinkles that grow as the flame propagates. These wrinkles encounter other eddies that alter their behavior. The effects of the hydrodynamic and diffusive instabilities are observed and found to be significant contributors to the formation and propagation of wrinkles.

Measurements of turbulent premixed flame dynamics using cinema stereoscopic PIV

A new approach for low emission gas turbines was investigated by modifying a typical production high-swirl injector (HSI) for gas turbine combustors to operate in a novel low-swirl mode (LSI). This LSI (S = 0.5) was configured in the laboratory and investigated by particle image velocimetry at firing rates of 9–87 kW (1

A comparison of the flowfields and emissions of high-swirl injectors and low-swirl injectors for lean premixed gas turbines

The low-swirl injector (LSI) is a simple and cost-effective lean premixed combustion method for natural-gas turbines to achieve ultralow emissions (<5 ppm NO x and CO) without invoking tight control of mixture stoichiometry, elaborate active tip cooling, or costly materials and catalysts. To gain an understanding of how this flame stabilization mechanism remains robust throughout a large range of Reynolds numbers, laboratory experiments were performed to characterize the flowfield of natural-gas flames at simulated partial load conditions. Also studied was a flame using simulated landfill gas of 50% natural gas and 50% CO 2 . Using particle image velocimetry, the nonreacting and reacting flowfields were measured at five bulk flow velocities. The results show that the LSI flowfield exhibits similarity features. From the velocity data, an analytical expression for the flame position as function of the flowfield characteristics and turbulent flame speed has been deduced. It shows that the similarity feature coupled with a linear dependency of the turbulent flame speed with bulk flow velocity enables the flame to remain relatively stationary throughout the load range. This expression can be the basis for an analytical model for designing LSIs that operate on alternate gaseous fuels such as slower burning biomass gases or faster burning coal-based syngases.

Laboratory Studies of the Flow Field Characteristics of Low-Swirl Injectors for Adaptation to Fuel-Flexible Turbines

The low-swirl injector (LSI) is a simple and cost-effective lean premixed combustion method for natural-gas turbines to achieve ultra-low emissions (< 5 ppm NOx and CO) without invoking tight control of mixture stoichiometry, elaborate active tip cooling or costly materials and catalysis. To gain an understanding of how this flame stabilization mechanism remains robust throughout a large range of Reynolds numbers, laboratory experiments were performed to characterize the flowfield of natural gas flames at simulated partial load conditions. Also studied was a flame using simulated landfill gas of 50% natural gas and 50% CO2 . Using Particle Image Velocimetry (PIV), the non-reacting and reacting flowfields were measured at five bulk flow velocities. The results show that the LSI flowfield exhibits similarity features. From the velocity data an analytical expression for the flame position as function of the flowfield characteristics and turbulent flame speed have been deduced. It shows that the similarity feature coupled with a linear dependency of the turbulent flame speed with bulk flow velocity enable the flame to remain relatively stationary throughout the load range. This expression can be the basis for an analytical model for designing LSIs that operate on alternate gaseous fuels such as slower burning biomass gases or faster burning coal-based syngases.Copyright © 2006 by ASME

The continued development of a low swirl injector for ultra-low NOx gas turbine applications is described. An injector prototype for natural gas operation has been designed, fabricated and tested. The target application is an annular gas turbine combustion system requiring twelve injectors. High pressure rig test results for a single injector prototype are presented. On natural gas, ultra-low NOx emissions were achieved along with low CO. A turndown of approximately 100°F in flame temperature was possible before CO emissions increased significantly. Subsequently, a set of injectors was evaluated at atmospheric pressure using a production annular combustor. Rig testing again demonstrated the ultra-low NOx capability of the injectors on natural gas. An engine test of the injectors will be required to establish the transient performance of the combustion system and to assess any combustor pressure oscillation issues.Copyright © 2006 by ASME

Full Scale Testing of a Low Swirl Fuel Injector Concept for Ultra-Low NOx Gas Turbine Combustion Systems

A side-by-side comparison between the MKS On-Line MultiGas™ analyzer (MGA) and conventional continuous emissions monitoring systems (CEMS) was performed under various test rig operating conditions at Solar Turbines Incorporated (Solar). The CEMS used an array of single gas analyzers (SGA’s) to measure NOX , CO, CO2 , O2 and UHC (unburned hydrocarbons). The MGA, which uses Fourier transform infrared (FTIR) spectrometry to make its measurements, simultaneously analyzed the exhaust stream for ten compounds: NO, NO2 , CO, CO2 , H2 O, CH4 , H2 CO (formaldehyde), N2 O, SO2 , and NH3 . Both dry and wet samples were collected using the MGA. The only calibration grade gas required was nitrogen, which was used as a zero (or purge) gas. Overall, the comparison between the CEMS and the MGA was very favorable, with the NO emissions within 0.1 ppmV (∼5%) and CO2 emissions within about 6% of each other. Larger differences were observed for NO2 and CO, but were most likely caused by sensor sensitivity range and sample handling problems with the in-house conventional CEMS.Copyright © 2003 by ASME

Waseem Nazeer

Papers

A comparison of the flowfields and emissions of high-swirl injectors and low-swirl injectors for lean premixed gas turbines

Laboratory Studies of the Flow Field Characteristics of Low-Swirl Injectors for Adaptation to Fuel-Flexible Turbines

Laboratory Studies of the Flow Field Characteristics of Low-Swirl Injectors for Adaptation to Fuel-Flexible Turbines

Full Scale Testing of a Low Swirl Fuel Injector Concept for Ultra-Low NOx Gas Turbine Combustion Systems

Evaluation of the MKS On-Line FTIR MultiGas™ Analyzer for Gas Turbine Applications