Flame chemiluminescence from CO2- and N2-diluted laminar CH4 /air premixed flames

An experimental and numerical investigation of the chemiluminescence signals from OH∗ , CH∗ , and CO2 ∗ is conducted for laminar premixed conical CH4/air flames diluted with CO2 or N2. Experiments are con- ducted either at fixed equivalence ratio or fixed adiabatic flame temperature. An ICCD camera, equipped with different narrow bandpass filters, is used to record flame images at 307 nm (OH∗), 430 nm (CH∗), and 455 nm (CO2∗). A spectrometer is also used to correct the OH∗ and CH∗ emissions from the CO2∗ broadband background emission. Measured chemiluminescence intensities are then compared to one dimensional freely-propagating-flame direct simulations accounting for the chemistry of the excited radicals. Simulations predict accurately the OH∗ chemiluminescence intensity, independently of the diluent nature and concentration. Correction for the CO2∗ background has a weak influence on the recorded OH∗ signal. Predictions of CH∗ emissions are also in good agreement with experimental data if the CO2∗ back- ground intensity is subtracted from intensity measurements. Measured and calculated CO2∗ emissions lead to acceptable results using a simplified chemistry mechanism for CO2∗ and an heuristic model for its emission intensity. Finally, it is shown that CO2 dilution modifies chemiluminescence intensity couples and particularly the OH∗/CO2∗ intensity ratio. These ratios regularly decrease with CO2 dilution, a feature which is reproduced by the simulations. It is then shown that the ratio OH∗/CO2∗ is well suited to infer the CO2 diluent concentration in diluted CH4/air flames, a method which appears not feasible for sensing N2 in N2-diluted CH4/air flames

Combustion state monitoring of premixed heating appliances with flame ionization current and chemiluminescence

The development of a combustion operating point control system has received interests from gas boiler manufacturers in order to ensure optimized performances despite variations of natural gas composition. Fully premixed burners are widely used in these boilers and the equivalence ratio is a key parameter to control. Experiments are carried out in this study for CH4/air fuel blends diluted by N2 or CO2 or enriched by H2, C2H6, or C3H8. Two low cost solutions for monitoring the equivalence ratio based on the flame ionization current and the chemiluminescence signal are investigated on a laboratory laminar conical flame. The behavior of the ionization current with changes of gas velocity, fuel composition, and equiva- lence ratio is shown to be mainly related to the size of the dead space between the flame base and burner rim. In a second stage, changes of the chemiluminescence signal is studied and the use of CH*/OH* intensity ratio as an equivalence ratio indicator is verified. Numerical 1-D premixed flame simulations are performed to support the experimental results on the flame chemiluminescence. Finally, a third equivalence ratio sensing method is proposed, coupling both techniques and the three strategies are compared

Flame Describing Function analysis of spinning and standing modes in an annular combustor and comparison with experiments

This article reports a numerical analysis of combustion instabilities coupled by a spinning mode or a standing mode in an annular combustor. The method combines an iterative algorithm involving a Helmholtz solver with the Flame Describing Function (FDF) framework. This is applied to azimuthal acoustic coupling with combustion dynamics and is used to perform a weakly nonlinear stability analysis yielding the system response trajectory in the frequency-growth rate plane until a limit cycle condition is reached. Two scenarios for mode type selection are tentatively proposed. The first is based on an analysis of the frequency growth rate trajectories of the system for different initial solutions. The second consid- ers the stability of the solutions at limit cycle. It is concluded that a criterion combining the stability analysis at the limit cycle with the trajectory analysis might best define the mode type at the limit cy- cle. Simulations are compared with experiments carried out on the MICCA test facility equipped with 16 matrix burners. Each burner response is represented by means of a global FDF and it is considered that the spacing between burners is such that coupling with the mode takes place without mutual interac- tions between adjacent burning regions. Depending on the nature of the mode being considered, two hypotheses are made for the FDFs of the burners. When instabilities are coupled by a spinning mode, each burner features the same velocity fluctuation level implying that the complex FDF values are the same for all burners. In case of a standing mode, the sixteen burners feature different velocity fluctua- tion amplitudes depending on their relative position with respect to the pressure nodal line. Simulations retrieve the spinning or standing nature of the self-sustained mode that were identified in the exper- iments both in the plenum and in the combustion chamber. The frequency and amplitude of velocity fluctuations predicted at limit cycle are used to reconstruct time resolved pressure fluctuations in the plenum and chamber and heat release rate fluctuations at two locations. For the pressure fluctuations, the analysis provides a suitable estimate of the limit cycle oscillation and suitably retrieves experimental data recorded in the MICCA setup and in particular reflects the difference in amplitude levels observed in these two cavities. Differences in measured and predicted amplitudes appear for the heat release rate fluctuations. Their amplitude is found to be directly linked to the rapid change in the FDF gain as the velocity fluctuation level reaches large amplitudes corresponding to the limit cycle, underlying the need of FDF information at high modulation amplitudes

Combustion dynamics of annular systems.

New results on the dynamics of annular combustors during ignition and combustion instabilities will be reviewed. Ignition dynamics is consid- ered first by examining experiments carried out in a system comprising a plenum feeding premixed gaseous reactants through multiple swir- ling injectors and an annular combustor formed by two concentric transparent quartz walls allowing full optical access to the flame. The analysis focuses on the “light-round” process during which the flame spreads from one injector to the next eventually leading to established flames on each injector. The transparent lateral walls allow a full view of the flame propagation from a spark igniter located in the neighborhood of one injector. High speed imaging is used to examine flame displace- ment and deduce the ignition delay yielding a full light around of the annular combustor. Changes associated to operation with spray flames are then discussed. The second part of this article is concerned with combustion instabilities of annular systems coupled by azimuthal modes. This type of oscillation has received considerable attention in recent years because the underlying coupling is often observed in the advanced premixed combustion architectures used in modern gas tur- bines. Recent studies have allowed a detailed examination of the dynamics of annular devices comprising multiple swirling injectors. Experiments on annular systems and single sector configurations pro- vide new insight on the coupling process between acoustics and unsteady combustion. Results for self-sustained combustion oscillations coupled by azimuthal modes are presented for operation with gaseous premixed reactants and with spray flames

Chemiluminescence based operating point control of domestic gas boilers with variable natural gas composition

The current control strategy of domestic gas boilers optimizes the heating efficiency and pollutant gas emissions assuming a fixed fuel composition. However, larger and more frequent variations of gas composition are ex- pected in the European natural gas network. New control systems capable of monitoring and regulating the flame equivalence ratio in real time are therefore necessary. The present work investigates one equivalence ratio sensing strategy by analyzing the flame chemiluminescence signal from a laminar premixed burner widely used in domestic gas boilers. The OH∗ chemiluminescence intensity is first identified as a reliable equivalence ratio indicator, valid for different natural gas composition scenarios with CH4 diluted by N2 or CO2 or enriched by H2, C2H6 or C3H8. These fuel mixtures are representative of biogas and hydrogen enriched natural gas. A demonstrative control loop is developed based on the OH∗ signal measured by a photomultiplier tube mounted with a bandpass optical filter. The system is tested by changing the equivalence ratio set value and the fuel composition. It is shown to be capable of regulating the equivalence ratio of these natural gas fuel blends with a good precision. The differences observed between the real and target values for the equivalence ratio are in most cases lower than 0.01. Only fuel blends with C3H8 lead to slightly larger errors of 0.03

Nonlinear thermoacoustic mode synchronization in annular combustors

Nonlinear coupling between azimuthal and axisymmetric modes in annular combustors is studied analyti- cally. Based on the thermoacoustic wave equation, a model featuring three nonlinearly coupled oscillators is derived. Two oscillators represent the dynamics of an azimuthal mode, and the third accounts for the axisym- metric mode. A slow-time system for the evolution of the mode amplitudes and phases is obtained through the application of the method of averaging. The averaged system is shown to accurately reproduce the solu- tions of the full oscillator model. Analysis of this five-dimensional dynamical system shows that a standing azimuthal mode may synchronize with an axisymmetric mode, provided that their individual resonance fre- quencies and growth rates are similar. This phase-coupled two-mode oscillation corresponds to the so-called slanted mode, observed in recent experiments involving an annular model combustion chamber. Quantitative conditions for the occurrence of mode synchronization are derived in terms of the growth rate ratio and a frequency detuning parameter. The analysis results are found to be consistent with experimental observations of the slanted mode

Flame and spray dynamics during the light-round process in an annular system equipped with multiple swirl spray injectors

A successful ignition in an annular multi-injector combustor follows a sequence of steps. The first injector is ignited; two arch-shaped flame branches nearly perpendicular to the combustor backplane form; they propagate, igniting each injection unit; they merge. In this paper, characterization of the propagation phase is performed in an annular combus- tor with spray flames fed with liquid n-hepane. The velocity and the direction of the arch- like flame branch are investigated. Near the backplane, the flame is moving in a purely azi- muthal direction. Higher up in the chamber, it is also moving in the axial direction due to the volumetric expansion of the burnt gases. Time-resolved particle image velocimetry (PIV) measurements are used to investigate the evaporating fuel droplets dynamics. A new result is that, during the light-round, the incoming flame front pushes the fuel droplets in the azimuthal direction well before its leading point. This leads to a decrease in the local droplet concentration and local mixture composition over not yet lit injectors. For the first time, the behavior of an individual injector ignited by the passing flame front is examined. The swirling flame structure formed by each injection unit evolves in time. From the igni- tion of an individual injector to the stabilization of its flame in its final shape, approxi- mately 50ms elapse. After the passage of the traveling flame, the newly ignited flame flashbacks into the injector during a few milliseconds, for example, 5 ms for the conditions that are tested. This could be detrimental to the service life of the unit. Then, the flame exits from the injection unit, and its external branch detaches under the action of cooled burnt gases in the outer recirculation zone (ORZ