Weiqun Geng

Optimization of Rate Coefficients for Simplified Reaction Mechanisms with Genetic Algorithms

Abstract A general procedure for determining optimum rate coefficients of simplified kinetic mechanisms is presented. The optimization’s objective is to match heat release or net species production rates of the simplified and an underlying detailed kinetics mechanism. A genetic algorithm is employed to carry out the matching procedure with a minimum requirement of human effort and expertise. Applications of optimized two- and three-step schemes to lean-premixed laminar methane flames show very promising results: Profiles of temperature and main species and also the peak values of intermediate species match those obtained with the detailed kinetic mechanism very well; flame speeds have been reproduced with good accuracy. The optimized mechanisms have proven to be numerically robust and efficient; the flexibility and ease of use make the approach presented here particularly appropriate for the numerical modeling of combustion in situations of technical interest.

Combustion Control by Extended EV Burner Fuel Lance

Flame stabilization in a swirl-stabilized combustor occurs in an aerodynamically generated recirculation region which is a result of vortex breakdown. The characteristics of the recirculating flow are dependent on the swirl number and on axial pressure gradients. Coupling to downstream pressure pulsations is also possible. In order to fix the position of the recirculation zone, an extended fuel lance was inserted into the burner. An additional benefit of the extended lance was to enable secondary fuel injection directly into the recirculation zone where the flame is stabilized. Tests were conducted with and without secondary fuel injection. The measurements included optimization of the location of the extended lance in the mixing chamber and variation of the amount of secondary fuel injection at different equivalence ratios and output powers. Flow visualizations showed that stabilization of the recirculation zone was achieved. The effect of the extended lance on pressure and heat release oscillations and on emissions of NOx , UHC and CO was investigated. The results were confirmed in high pressure single burner pressure tests and in a full scale land-based test gas-turbine. The lance has been successfully implemented in engines with sufficient stability margins and good operational flexibility. This paper shows the careful development process from lab scale tests to full scale engine tests until the implementation into the field engines.© 2002 ASME

An Annular Combustor Natural Gas Ignition Model Derived From Atmospheric Sector Experiments

Results from ignition and cross-ignition tests performed on an atmospheric 60°-sector test rig equipped with three EV-type burners are presented. Based on these results a model was developed for an annular combustor, which calculates the primary ignition and burner-burner cross-ignition limits for the combustor in terms of burner operation variables (equivalence ratio and pilot fuel ratio) using a generally applicable methodology described in the paper. Key ingredients of the model are the description of mixture flammability and a mixing model representing the ignition relevant mixing behaviour of the burners in the annular combustor. Ignition and cross-ignition are observed to occur, if the mixture equivalence ratio determined from the mixing model is above the flammability limits calculated for the particular operating conditions. Even in the case of cross iginition across an externally piloted or switched-off burner, the model reproduces the experimental cross-ignition limits, confirming that the basic physics have been captured.Copyright

Impact of Cooling Air Injection on the Combustion Stability of a Premixed Swirl Burner Near Lean Blowout

In most dry low NOx combustor designs of stationary gas turbines the front panel impingement cooling air is directly injected into the combustor primary zone. This air partially mixes with the swirling flow of premixed reactants from the burner and reduces the effective equivalence ratio in the flame. However, local unmixedness and the lean equivalence ratio are supposed to have a major impact on combustion performance. Overall goal of this investigation is to answer the question whether the cooling air injection into the primary combustor zone has a beneficial effect on combustion stability and NOx emissions or not. The flame stabilization of a typical swirl burner with and without front panel cooling air injection is studied in detail under atmospheric conditions close to the lean blowout limit (LBO) in a full scale single burner combustion test rig. Based on previous isothermal investigations a typical injection configuration is implemented for the combustion tests. Isothermal results of experimental studies in a water test rig adopting high speed planar laser-induced fluorescence (HSPLIF) reveal the spatial and temporal mixing characteristics for the experimental setup studied under atmospheric combustion. This paper focuses on the effects of cooling air injection on both flame dynamics and emissions in the reacting case. To reveal dependencies of cooling air injection on combustion stability and NOx emissions, the amount of injected cooling air is varied. OH*-chemiluminescence measurements are applied to characterize the impact of cooling air injection on the flame front. Emissions are collected for different cooling air concentrations, both global measurements at the chamber exit and local measurements in the region of the flame front close to the burner exit. The effect of cooling air injection on pulsation level is investigated by evaluating the dynamic pressure in the combustor. The flame stabilization at the burner exit changes with an increasing degree of dilution with cooling air. Depending on the amount of cooling only a specific share of the additional air participates in the combustion process.Copyright

On the Use of Thermoacoustic Analysis for Robust Burner Design

Advanced thermoacoustic analysis is now routinely used in gas turbine combustor development. A thermoacoustic approach based on a combination of numerical analysis (CFD and three-dimensional acoustics), acoustic network models, and dedicated measurements of acoustic flame response is well accepted across the industry. However, its application to specific combustor upgrade or development programs in “prediction mode” as opposed to “analysis mode” remains a challenge. This is mainly due to the large sensitivity of the complex methodology to key inputs, such as flame transfer functions, that can be only predicted in the burner design phase. This paper discusses an example where we made an effort to apply the thermoacoustic approach in predictive mode. The example refers to the upgrade of a first generation diffusion burner with a partially premix burner to achieve low emissions. Thermoacoustic instabilities were predicted as a limiting factor for combustor operation and thus a design parameter was identified to perform the thermoacoustic combustor tuning at engine level. A particular challenge of this development program was that no test rig was available. Therefore, the new premix burner was directly installed into a field engine where it was successfully tested.

Using Thermoacoustic Analysis for Robust Burner Design

Advanced thermoacoustic analysis is now routinely used in gas turbine combustor development. A thermoacoustic approach based on a combination of numerical analysis (CFD and three-dimensional acoustics), acoustic network models and dedicated measurements of acoustic flame response is well accepted across the industry. However, its application to specific combustor upgrade or development programs in “prediction mode” as opposed to “analysis mode” remains a challenge. This is mainly due to the large sensitivity of the complex methodology to key inputs, such as flame transfer functions, that can be only predicted in the burner design phase. This paper discusses an example where we made an effort to apply the thermoacoustic approach in predictive mode. The example refers to the upgrade of a first generation diffusion burner with a partially premix burner to achieve low emissions. Thermoacoustic instabilities were predicted as a limiting factor for combustor operation and thus a design parameter was identified to perform the thermoacoustic combustor tuning at engine level. A particular challenge of this development program was that no test rig was available. Therefore, the new premix burner was directly installed into a field engine where it was successfully tested.Copyright

A CFD Methodology for Assessment and Improvement of Aerodynamic Stability of a Premixed Swirler Burner

A methodology is presented in this paper how to assess and to improve burner aerodynamic stability of a premixed swirl burner by means of CFD. Steady-state RANS has been widely used for complex industrial applications for decades because of its good reproducibility of mean flow and acceptable turn-around time. With affordability of high performance cluster it becomes feasible to extend steady-state RANS to unsteady-state LES for industrial applications. It is especially beneficial for swirl burners to assess burner stability directly because swirling flow is in fact an unsteady-state phenomenon, which RANS cannot fully capture. This paper shows an industrial practice for how to improve burner design using both RANS and LES. The former helps find potential problems in the flow field, e.g. flow separation. The latter quantifies their impact on flow stability/turbulent fluctuations, e.g. helical modes generated by coupling of flow disturbances and swirling flow. Improvement measures were worked out to supress such flow fluctuations and to enhance stability of burner aerodynamics.Another critical issue for burner design, reverse flow within the burner, was also discussed because it is a potential risk. When a flame instantaneously enters the burner backwards it can be stabilized in the recirculation zone and damage hardware. The risky region was eliminated by enhancing axial momentum of the air inflow.The strong turbulent fluctuations within the burners interfere with burner stability significantly and lead to a bad flashback resistance in macroscopic view [1]. In the second part of the paper, atmospheric tests verify the improvement of burner stability via improved flashback resistance and show an upgrading of aerodynamic behaviour via reduced burner pressure drop.Copyright

Influence of the Inflow Confinement on the Flashback Limits of a Premixed Swirl Burner

This work presents a study of the effect of the inflow condition on the flame flashback performance of a gas turbine burner. A generic swirl burner for basic combustion research on engine scale is investigated both under atmospheric conditions in a combustion test rig and numerically to reveal the impact of inflow conditions on the burner stability. Flashback resistance is examined with highly reactive hydrogen fuel and numerical studies with isothermal large eddy simulations (LES) are performed to investigate transient flow field data. Earlier publications showed excellent flashback resistance of a down scaled burner version of similar design, which was tested in a rig with strongly restricted cross sectional inflow area. An influence of the test rig setup on the flashback limits was not expected. However, the results presented in the paper reveal that the inflow conditions at the swirler and the distribution of axial velocity inside the swirler are crucial for flame stability. The inflow conditions upstream of the swirler were modified to redistribute the axial velocity field inside the swirler. Velocity fluctuations both inside the swirler and downstream of the burner outlet were reduced and consequently the susceptibility to perturbations in the flow field. This measure prevents the formation and propagation of local zones of negative axial velocity upstream of the flame position and increases the robustness of the flow field. After modification of the inflow condition the excellent flashback limit data of the down scaled burner was fully reproduced.Copyright

The Effect of Cooling Air on the Air Fuel Distribution of a Silo Combustor

Lean blow out (LBO) has a big impact on emission formation at part load of gas turbines, where flame temperature is low and flame stabilization is an issue. With improved combustion behavior at LBO conditions the operation flexibility of a silo gas turbine can be increased within the scope of retrofitting. In multi burner arrangements a part of the preheated air designated for combustion is used for impingement cooling of the burner front panel and subsequently injected into the primary combustion zone. In this region of flame stabilization air and unburned fuel as well as burned products are mixed to sustain stable combustion. The object of this study is to determine the level of dilution of the flow field by the cooling air with the focus on the conditions below LBO that can impair flame stability. The question addressed in this paper is how mixing of the front panel cooling air with the incoming reactants and the combustion products in multi burner arrangements can be computed in a numerically efficient way. As test case for the methodology the local distribution of cooling air in a silo combustor is presented. In this numerical study mixing processes of air-fuel mixture and cooling air as well as aerodynamic interaction of adjacent burners in a multi burner systems are investigated using isothermal Reynolds Averaged Navier Stokes (RANS) simulations. Former published single burner water channel experiments and Large Eddy Simulations (LES) [1] serve as a baseline. Single burner RANS simulations are done and compared to measurement and LES to validate the velocity and scalar fields. A Schmidt number variation is used to modify the mixing process in the RANS single burner calculations. Based on the LES the single burner is modified to address the multi burner conditions and calculated with LES and RANS. Finally the multi burner system is computed with the settings applied in the single burner configuration. Using the symmetry of the investigated burner matrix an efficient methodology is implemented that allows computation of one sixth of a silo combustor. The results expose a strong burner-burner interaction of the recirculation zones and in contrast to the single burner configuration regions of concentrated cooling air.Copyright

Reduction of NOx Emissions in Alstom GT11NM Engines: Development, Validation and Engine Operation Experience of the EV-Alpha Burner

The first Alstom GT11N engine was introduced to the market at the end of the eighties. In order to keep our base fleet engines (aged fleet) attractive for dispatch and therefore for operation, one of the key issues of the service business is the development of upgrade packages. For the GT11N fleet an emission reduction package was worked out in recent years with the target of single digit NOx at base load (<10 ppm NOx @15%O2). The purpose of this paper is to present the performed development work, starting from the R&D work with the CFD optimization of the mixing quality, going to the atmospheric combustion tests and finally to the engine validation tests on site.The first section of the paper focuses on the performed R&D work, mainly on the improvement of the gas/air mixing quality of the EV burner. For the down selection of the most promising configurations the calculated unmixedness at different places of the burner and the hottest flame zones were analyzed and evaluated. The second part of the paper focuses on the results of the atmospheric burner combustion tests. The last part of the paper is focusing on the verification phase and on the achieved results during the engine validation tests on site. The EV-alpha burners were implemented during an A-inspection. Before the implementation a reference measurement with the standard EV burners was carried out. This section gives an overview of the performed engine tests and achieved emissions at base load and part load, about flame stability behaviour and transient operation results.With the implementation of the EV-alpha burner the target “single digit NOx” could be achieved and the combustion stability kept at comparable levels.Copyright