Gilles Cabot

EXPERIMENTAL STUDY OF STABILITY, STRUCTURE AND CH∗ CHEMILUMINESCENCE IN A PRESSURIZED LEAN PREMIXED METHANE TURBULENT FLAME

The present experimental study investigates the main parameters governing structure and stability of a natural gas premixed lean flame, close to conditions encountered in gas turbines. A scale-model of a gas turbine combustion chamber has been equipped with a quartz tube in order to visualize flame and to perform measurements of CH* chemiluminescence. The combustion chamber can be pressurized up to 0.4 MPa, and combusting air temperature can be preheated up to 673K. The influence of swirl number, equivalence ratio, pressure, combusting air temperature, and flow rate are studied. Stability domain and possible operating range depend on swirl number. For a given swirler and for fixed conditions of pressure and temperature, different flame regimes are encountered and flame regime limits depend only on equivalence ratio. Moreover blow-off limit can be strongly decreased by increasing combusting air temperature whereas pressure increase has no significant influence. The evolution of mean global flame CH* emission as function of pressure, temperature, and equivalence ratio is found to be similar to that observed by other authors in a laminar flame, except close to the blow-off limit.

Flame Stabilization by Hot Products Gases Recirculation in a Trapped Vortex Combustor

New regulations regarding NOx emissions are forcing manufacturers to develop advanced research and technology strategies. Ultra-lean combustion is considered as an attractive solution; however, it generally produces combustion instabilities in swirl-stabilized burners. This work provides experimental results for a new burner technology based on two concepts: the trapped vortex combustor (TVC) and the ultra-compact combustor (UCC). Methane/air flame stabilization was achieved by generating hot product recirculation, with a rich pilot flame located in an annular cavity, and by flame holders located in the main flow slightly upstream of the cavity. In addition, azimuthal gyration could be added to the main flow to reproduce the suppression of the last diffuser stage, which increased the velocity and modified the mixing between the cavity and the mainstream due to centrifugal forces. The combustor characterization was performed by coupling several optical diagnostics, pollutant emissions, and pressure measurements (for both cold and reactive conditions) at atmospheric pressure. An understanding of the combustion dynamics was achieved through phase averaged PIV/CH* images. The analysis highlighted the importance of the stabilization process of a double vortex structure inside the cavity and the presence of reactive gas close to the upstream cavity wall. These conditions were improved by a high cavity equivalence ratio and a high main airflow rate. The addition of swirl considerably increased the flame stability.Copyright

Analysis of the Flame Structure in a Trapped Vortex Combustor Using Low and High-Speed OH-PLIF

Operating with lean combustion has led to more efficient “Low-NOx” burners but has also brought several technological issues. The burner design geometry is among the most important element as it controls, in a general way, the whole combustion process, the pollutant emissions and the flame stability. Investigation of new geometry concepts associating lean combustion is still under development, and new solutions have to meet the future pollutant regulations. This paper reports the experimental investigation of an innovative staged lean premixed burner. The retained annular geometry follows the Trapped Vortex Combustor concept (TVC) which operates with a two stage combustion chamber: a main lean flame (1) is stabilized by passing past a vortex shape rich-pilot flame (2) located within a cavity. This concept, presented in GT2012-68451 and GT2013-94704, seems to be promising but exhibits combustion instabilities in certain cases, then leading to undesirable level of pollutant emissions and could possibly conduct to serious material damages. No precise information have been reported in the literature about the chain of reasons leading to such an operation. The aim of this paper is to have insights about the main parameters controlling the combustion in this geometry. The flame structure dynamics is examined and compared for two specific operating conditions, producing an acoustically self-excited and a stable burner. Low and high-speed OH-PLIF laser diagnostics (up to 10 kHz) are used to have access to the flame curvature and to time-resolved events. Results show that the cavity jets location can lead to flow-field oscillations and a non-constant flame’s heat release. The associated flame structure, naturally influenced by turbulence is also affected by hot gases thermal expansion. Achieving a good and rapid mixing at the interface between the cavity and the main channel leads to a stable flame.Copyright

Determination of Criteria for Flame Stability in an Annular Trapped Vortex Combustor

Development of lean premixed (LP) combustion is still a challenge as it results in considerable constraints for the combustor design. Indeed, new combustors using LP combustion are more prone to flashback, blow-off or even thermo-acoustic instabilities. A detailed understanding of mechanisms leading to such extreme conditions is then crucial to reduce pollutant emissions, widen the range of operating conditions, and reduce design time. This paper reports the experimental study of an innovative LP trapped vortex combustor (TVC). The TVC concept uses a recirculating rich flow trapped in a cavity to create a stable flame that continuously ignites a main lean mixture passing above the cavity. This concept gave promising performances but some workers highlighted the existence of combustion instabilities for some operating conditions. Detailed studies have therefore been carried out in order to understand the occurrence of these drastic operating conditions. Results showed that the cavity flow dynamics in conjunction with the location of the interfacial mixing zone (between the cavity and the mainstream) were the driving forces to obtain stable combustion regimes. The goal of this work has been to take advantage of these detailed recommendations to determine stability maps, trends, and dimensionless parameters which could be easily used as early design rules. For this reason, the study introduced a simple and robust criterion, based on the global pressure fluctuation energy. The latter was used to distinguish stable and unstable modes. An aerodynamic momentum flux ratio and a chemical stratification ratio (taken between the cavity and the mainstream) were defined to scale all measurements. Results indicated that the mainstream velocity was critically important to confine the cavity and to prevent combustion instabilities. Remarkably, this trend was verified and even more pronounced for larger cavity powers. In addition, flame stabilization above the cavity resulted in the existence of specific stratification ratios, in order to obtain a soft gradient of gas composition between the rich and lean regions. Finally, a linear relation between the mainstream and cavity velocities became apparent, thereby making possible to simply predict the combustor stability.

Spray ignition and local flow properties in a swirled confined spray-jet burner: experimental analysis

The authors acknowledge financial support from ANR under the project TIMBER ANR-14-CE23-0009.

Thermal Structure of Laminar Methane/Air Flames: Influence of H2 Enrichment and Reactants Preheating

The Rayleigh scattering technique has been applied to a V-shaped laminar flame in order to investigate the effect of reactant temperature on the thickness and thermal structure of an H2-enriched laminar methane-air flame. A systematic comparison of experimental and numerical results obtained with the GRI3.0 chemical mechanism is provided. First, the effects of reactant temperature on the pure methane-air flame are presented. A decrease in flame thickness and higher temperature gradients in the flame front are observed, and the maximum temperature gradient is shifted toward lower progress variable values. The preheat zone is substantially modified by the reactant preheating. Globally, the numerical predictions are in good agreement with the experimental results and validate the measurements. In addition, the effects of H2 enrichment on the thermal methane/air flame thickness are presented for a reactant temperature of 300 K. We show that the main role of H2 enrichment is to increase the temperature gradient and to shift toward lower values of the thermal progress variable c the location of the maximum temperature gradient. The reactivity of the mixture is strongly increased. At constant equivalence ratio, the burnt gas temperature is not modified by the H2 enrichment, and the analysis can be done either in c or T space. Finally, the effect of reactant preheating on H2/CH4/air flames (40% H2 dilution in volume) is investigated. The flame thickness is found to be lowered when reactant temperature is increased, but unlike the preheated methane-air flame, the localization of the maximum temperature gradient is not found to be markedly shifted. The reactivity of the mixture is rather controlled by the H radicals produced by H2 dilution than the reactant preheating.