Simultaneous Measurements of Temperature and CO Concentration in Stagnation Stabilized Flames

An impinging jet burner was developed to investigate flame-wall interactions (FWI) using laser based diagnostics. CO concentrations were measured with two-photon laser-induced fluorescence (LIF) in combination with coherent anti-Stokes Raman spectroscopy (CARS) gas phase temperature measurements. Besides being the principal factor in chemical kinetics, temperature data is required to correct the CO LIF data for various factors like density variation, quenching and variation in the Boltzmann population. Phosphor thermometry was used to determine surface temperatures of the wall and to estimate the heat flux. In an parameter study Reynolds numbers and fuel equivalence ratio were varied.     

Enhancement of evaporation and combustion of liquid fuels through porous media

Existing designs of most conventional liquid fuel burners have relied solely on spray atomizers, with a large amount of very fine droplets forming in a relatively large combustion chamber, resulting in a relatively low combustion intensity. Against this background, a novel down-flow compact porous burner system was developed for burning kerosene without the need of using a spray atomizer. Successive development on this burner research is important in view of the need to create energy by an efficient device based on simple technology. The focus has been on the introduction of the packed bed emitter installed downstream of the porous burner. The evaporation process and combustion phenomena that occurred are described through the coupled interaction of the solid phase (porous burner), the liquid phase (kerosene) and the gas phase. Enhancement of evaporation and combustion are evaluated through the measured thermal structures in terms of temperature distribution along the burner length and emission characteristics at the burner exit. Stable combustion with low emission of pollutants was realized even though the combustion flame was confined in-between the porous burner and the packed bed emitter with an increase in the back-pressure. The effects of various parameters including heat input and equivalence ratio on the combustion characteristics were clarified to confirm improvement in mixing of the fuel vapor/air mixture and turn-down ratio of the burner. The effect of the introduced packed bed emitter with suitable bed length and its installation location is investigated as an efficient method for enhancement of evaporation and combustion of the liquid fuels without a spray atomizer. Future applications of this type of burner system are suggested.     

Investigation of porous media combustion by coherent anti-Stokes Raman spectroscopy

High efficiency, marginal pollutant emissions and low fuel consumption are desirable standards for modern combustion devices. The porous burner technology is a modern type of energy conversion with a strong potential to achieve these standards. However, due to the solid ceramic framework investigation of the thermodynamic properties of combustion, for example temperature, is difficult. The combustion process inside the ceramic structure of a porous burner was experimentally investigated by coherent anti-Stokes Raman spectroscopy (CARS). In this work, we present measurements using dual-pump dual-broadband CARS (DP-DBB-CARS) of temperature and species concentrations inside the reaction and flue gas zone of a porous media burner. Improvements to the setup and data evaluation procedure in contrast to previous measurements are discussed in detail. The results at varied thermal power and stoichiometry are presented. In addition, measurements at a range of radial positions inside a pore are conducted and correlated with the solid structure of the porous foam, which was determined by X-ray computer tomography.     

Development of a high performance flexible porous burner(FPMB) with an adjustable cooling effect

A high performance flexible porous medium burner that can burn gaseous and liquid fuel with different type of flames(premixed and non-premixed) is proposed. The merit of the combustion within porous medium is that heat is recirculated from the combustion gas to porous medium at upstream wherein vaporization is taken place(in case of liquid fuel) or preheated(in case of gaseous fuel) before mixing with the combustion air followed by combustion within another porous medium at downstream. In a former version of the high performance flexible porous medium burner, the upstream porous medium is incorporated with a cooling system using the combustion air as a coolants to prevent thermal decomposition of fuels and thus the burner clogging caused by carbon deposit within the porous medium can be avoided. However, the cooling effect cannot be properly controlled such that the boiling point of the liquid fuel is maintained at suitable value irrespective of the volume flow rate of the combustion air,which is linearly varied with the firing rate of the burner. In particular at the lean burn condition, where high air flow rate is required with high cooling effect with porous medium. This can result in the porous medium temperature lower than the corresponding boiling point of the liquid fuel and thus evaporation of the fuel is failed and the combustion is ceased. Therefore, method of controlling the cooling air flow rate in the porous medium is proposed and studied in order to appropriately control the porous medium temperature and maintain it at above the boiling point irrespective of the combustion conditions. In this research, experimental and computation analysis are used to design the flexible porous burner(FPMB),with adjustable cooling effect. The result shows that, the new design of FPMB which has temperature in the upstream porous medium is higher than boiling point and lower than thermal decomposition temperature of fuel(kerosene) at all conditions and can be operated at a wide range of equivalence ratio without fuel decomposition and fuel non-vaporization problem.     

Modeling of Liquid Fuel Injection, Evaporation and Mixing in a Gas Turbine Burner Using Large Eddy Simulations

The interaction of turbulence, temperature fluctuation, liquid fuel transport, mixing and evaporation is studied by using Large Eddy Simulations (LES). To assess the accuracy of the different components of the methods we consider first isothermal, single phase flow in a straight duct. The results using different numerical methods incorporating dynamic Sub-Grid-Scale (SGS) models are compared with DNS and experimental data. The effects of the interactions among turbulence, temperature fluctuation, spray transport, evaporation and mixing of the gaseous fuel are studied by using different assumptions on the temperature field. It has been found that there are strong non-linear interactions among temperature-fluctuation, evaporation and turbulent mixing which require additional modeling if not full LES is used. The developed models and methods have been applied to a gas turbine burner into which liquid fuel is injected. The dispersion of the droplets in the burner is described. This revised version was published online in July 2006 with corrections to the Cover Date.     

Large-Eddy Simulation of a Jet-in-Hot-Coflow Burner Operating in the Oxygen-Diluted Combustion Regime

Large-eddy simulations of moderate and intense low-oxygen dilution (MILD) combustion of a jet-in-hot-coflow (JHC) burner are performed. This burner configuration consists of three streams, providing fuel, oxygen-diluted coflow, and air to the burner. To account for the mixing between the three reactant streams, a three-stream flamelet/progress variable (FPV) formulation is utilized. This model was previously applied to a condition corresponding to the upper range of MILD-combustion, and the objective of this contribution is to further investigate this model in application to highly diluted operating conditions. Comparisons of mean and conditional results show that the model accurately captures effects of increasing oxygen-depletion on the flame-structure and heat-release, and predictions for temperature and species mass fractions are in overall good agreement with experiments.     

Structures and stabilization of low calorific value gas turbulent partially premixed flames in a conical burner

Experiments are carried out on partially premixed turbulent flames stabilized in a conical burner. The investigated gaseous fuels are methane, methane diluted with nitrogen, and mixtures of CH₄, CO, CO₂, H₂ and N₂, simulating typical products from gasification of biomass, and co-firing of gasification gas with methane. The fuel and air are partially premixed in concentric tubes. Flame stabilization behavior is investigated and significantly different stabilization characteristics are observed in flames with and without the cone. Planar laser induced fluorescence (LIF) imaging of a fuel-tracer species, acetone, and OH radicals is carried out to characterize the flame structures. Large eddy simulations of the conical flames are carried out to gain further understanding of the flame/flow interaction in the cone. The data show that the flames with the cone are more stable than those without the cone. Without the cone (i.e. jet burner) the critical jet velocities for blowoff and liftoff of biomass derived gases are higher than that for methane/nitrogen mixture with the same heating values, indicating the enhanced flame stabilization by hydrogen in the mixture. With the cone the stability of flames is not sensitive to the compositions of the fuels, owing to the different flame stabilization mechanism in the conical flames than that in the jet flames. From the PLIF images it is shown that in the conical burner, the flame is stabilized by the cone at nearly the same position for different fuels. From large eddy simulations, the flames are shown to be controlled by the recirculation flows inside cone, which depends on the cone angle, but less sensitive to the fuel compositions and flow speed. The flames tend to be hold in the recirculation zones even at very high flow speed. Flame blowoff occurs when significant local extinction in the main body of the flame appears at high turbulence intensities.     

Ionic strontium fluorescence as a method for flow tagging velocimetry

A flow tagging technique based upon ionic fluorescence in strontium is investigated for applications to velocity measurements in gas flows. The method is based upon a combination of two laser based spectroscopic techniques, i.e. resonantly-enhanced ionisation and laser-induced ionic fluorescence. Strontium is first ionised and then planar laser-induced fluorescence is utilised to give 2D `bright images' of the ionised region of the flow at a given time delay. The results show that this method can be used for velocity measurements. The velocities were measured in two types of air–acetylene flames – a slot burner and a circular burner yielding velocities of 5.1 ± 0.1 m/s and 9.3 ± 0.2 m/s, respectively. The feasibility of the method for the determination of velocities in faster flows than those investigated here is discussed. Received: 5 November 1998/Accepted: 19 January 2000     

LES of Jets in Cross Flow and Its Application to a Gas Turbine Burner

LES computations of jets in cross flow (JICF) were performed. Experimental investigations reported in literature are reproduced with good agreement concerning the momentum field and the mixing of a passive scalar. The results validate the ability of the present LES approach to compute fuel injection of the type JICF. LES computations of fuel injection in an industrial gas turbine burner are presented. This revised version was published online in July 2006 with corrections to the Cover Date.     

Investigation of the Syngas Flame Characteristics at Elevated Pressures Using Optical and Laser Diagnostic Methods

The effect of pressure on the characteristics of syngas flames is investigated under gas turbine relevant conditions using planar laser induced fluorescence of OH radicals and OH^* chemiluminescence imaging. An optically accessible combustor fitted with a swirl burner was operated with two different syngas mixtures, preheated air at 700?K, and pressures ranging from 5 to 20?bars. The thermal load varied from 15 to 25?kW/bar at an equivalence ratios 0.5. The OH-PLIF measurements show that the flames under all conditions exhibited two reaction fronts, one at the shear layer between the inner recirculation zone and the fuel inlet, and one between the fuel inlet and the air nozzle. The more or less continuous reaction front at low pressure turned into a highly corrugated flame front at higher pressures, with isolated regions of ignition and extinction. The probability density distribution of the flame curvature for the mixtures studied showed that the inner and outer flame responded differently to the pressure increase, with the mean curvature magnitude also depending on the mixture composition and thermal load. The measurements clearly shows the limitations associated with the use of OH^* chemiluminescence images as a marker for the heat release rate especially in case of syngas mixtures.     

PIV, 2D-LIF and 1D-Raman measurements of flow field,composition and temperature in premixed gas turbine flames

Several laser diagnostic measurement techniques have been applied to study the lean premixed natural gas/air flames of an industrial swirl burner. This was made possible by equipping the burner with an optical combustion chamber that was installed in the high-pressure test rig facility at the DLR Institute of Combustion Technology in Stuttgart. The burner was operated with preheated air at various operating conditions with pressures up to p = 6 bar and a maximum thermal power of P = 1 MW.The instantaneous planar flow field inside the combustor was studied with particle image velocimetry (PIV). Planar laser induced fluorescence (PLIF) of OH radicals on a single-shot basis was used to determine the shape and the location of the flame front as well as the spatial distribution of reaction products. 1D laser Raman spectroscopy was successfully applied for the measurement of the temperature and the concentration of major species under realistic gas turbine conditions.Results of the flow field analysis show the shape and the size of the main flow regimes: the inflow region, the inner and the outer recirculation zone. The highly turbulent flow field of the inner shear layer is found to be dominated by small and medium sized vortices. High RMS fluctuations of the flow velocity in the exhaust gas indicate the existence of a rotating exhaust gas swirl. From the PLIF images it is seen that the primary reactions happened in the shear layers between inflow and the recirculation zones and that the appearance of the reaction zones changed with flame parameters. The results of the multiscalar Raman measurements show a strong variation of the local mixture fraction allowing conclusions to be drawn about the premix quality. Furthermore, mixing effects of unburnt fuel and air with fully reacted combustion products are studied giving insights into the processes of the turbulence–chemistry interaction.     

气体示踪法研究燃烧器横向湍流混合扩散特性

用气体示踪法对侧边风燃烧器弱旋流一次风和侧边直流二次风横向湍流混合扩散特性进行了详细研究，讨论了各截面混合物浓度分布规律及混合强度分布规律，分析对燃料着火的影响，同时利用湍流运动方程反算气体湍流扩散系数     

Flamelet-Modeling of Inverse Rich Diffusion Flames

An experimental and numerical investigation of a confined laminar inverse diffusion flame (IDF) with pure oxygen as oxidizer and carbon dioxide diluted methane as fuel with a global stoichiometry of partial oxidation processes (equivalence ratio of 2.5) is presented. The present burner setup allows studying both the flame and the post-flame zone in a simplified geometry considering typical operating conditions as found in large-scale gasifiers. This partial oxidation flame setup is characterized by very high temperatures close to the stoichiometric oxidation zone due to oxy-fuel combustion, whereas lower temperatures and slow endothermic post-flame conversion reactions with long residence times are found in the fuel rich post-flame region. The scope of this paper is to investigate different modeling approaches suitable for both regimes by comparing the simulation results to detailed experimental data. Planar OH laser-induced fluorescence (OH-LIF) was performed for measuring the hydroxyl radical in the reaction zone and the results are compared to CFD calculations. Based on this comparison, the necessary level of detail of diffusion flux modeling, which includes Soret and Dufour effects, is analyzed and established. Finally, steady and unsteady non-premixed flamelet approaches based on a single mixture fraction are used in order to study their applicability for both the oxidation and post-flame zone. Significantly different time scales are obtained using different flamelet paths. Their influence on the results is investigated in the steady flamelet and the Lagrangian flamelet approach.     

Rheological studies of coal-oil mixtures

Summary Samples of stable coal-oil mixtures were prepared with coal concentrations ranging from 30–50% by weight. Extensive rheological data were obtained using capillary and cone-plate viscometers for samples of coal-oil mixtures and fuel-oil which served as a reference fluid. Viscosity measurements show coal-oil mixtures to be shear-thinning suspensions, i.e., the viscosity decreases moderately with increasing shear rates. In the concentration range of 30–40% coal, the coal-oil mixture shows a predominantly Newtonian behavior. For mixtures with coal concentrations higher than 40%, a yield point was observed. The study of these samples with the rotating-rod viscometer indicated a migration of coal particles away from the rotating rod with no noticeable rod climbing, thus no evidence of normal stress effects. The rheological data can be represented by any of the three two-parameter models: Power Law, Bingham Plastic, or Casson, with the last two models being more realistic and consistent with the observations.With 9 figures     

Influence of Co-flow on Flickering Diffusion Flame

The influence of air co-flow on flickering methane diffusion flame was studied experimentally using the image processing technique and the proper orthogonal decomposition (POD) analysis. The flickering of the flame is characterized by the mean height, the oscillation amplitude and the Strouhal number, which are measured by the digital image analysis of the diffusion flame. The experiments are carried out for various combinations of burner diameters, fuel velocities and co-flow velocities. With increasing the velocity ratio of the co-flow to the fuel flow, the oscillation amplitude is decreased and the Strouhal number is increased slightly in proportional to the inverse Froude number, while the frequency jump occurs in the low co-flow velocity ratio. These results are commonly observed in all the burners of different diameters, while the critical co-flow velocity ratio to suppress the flickering is found to be increased with increasing the burner diameters due to the influence of Froude number. The POD analysis of the flickering flame shows that the flickering energy is dominant in the first two POD modes and they are axisymmetric except for the zero co-flow velocity case and fully suppressed case. The correlation of POD coefficients in the first two fluctuating POD modes suggests the suppression of large-scale structure of flickering due to the influence of co-flow.     

Computational investigation of hydrodynamics,coal combustion and NOx emissions in a tangentially fired pulverized coal boiler at various loads

This work presents a computational investigation of hydrodynamics, coal combustion and NOx emissions in a tangentially fired pulverized coal boiler at different loads (630, 440 and 300 MW; relative loads of 100%, 70% and 48%) to clarify the effect of load change on the furnace processes. A computational fluids dynamics model was established; the flow field, temperature profile, species concentration and NOx emissions were predicted numerically; and the influence of burner tilt angles was evaluated. Simulation results indicate that a decrease in boiler load decreases the gas velocity, attenuates the airflow rotations, and increases the tangent circle size. The high-temperature zone and flame moved toward the side walls. Such behaviors impair air–fuel mixing, heat transfer and steady combustion in the furnace. In terms of species concentrations, a decrease in boiler load increased the O₂ content, decreased the CO content, and decreased the char burnout rates only slightly. A change in boiler load from 630 to 440 and 300 MW increased the NOx emissions from 202 to 234 and 247 mg/m³, respectively. Burner tilt angles are important in coal combustion and NOx emissions. A burner angle of –15° favors heat transfer and low NOx emissions (<185 mg/m³) for the current tangentially fired boiler.     

Eulerian and Lagrangian Large-Eddy Simulations of an evaporating two-phase flow

Large-Eddy Simulations (LES) of an evaporating two-phase flow in an experimental burner are performed using two different solvers, CDP from CTR-Stanford and AVBP from CERFACS, on the same grid and for the same operating conditions. Results are evaluated by comparison with experimental data. The CDP code uses a Lagrangian particle tracking method (EL) while the code AVBP can be coupled either with a mesoscopic Eulerian approach (EE) or with a Lagrangian method (EL). After a validation of the purely gaseous flow in the burner, liquid-phase dynamics, droplet dispersion and fuel evaporation are qualitatively and quantitatively evaluated for three two-phase flow simulations. They are respectively referred as: CDP-EL, AVBP-EE and AVBP-EL. The results of the three simulations show reasonable agreement with experiments for the two-phase flow case. To cite this article: J.M. Senoner et al., C. R. Mecanique 337 (2009).     

LES of a Multi-burner Annular Gas Turbine Combustor

In this study, Large Eddy Simulation (LES) has been used to predict the flow, mixing and combustion in both a single burner laboratory gas turbine combustor and in an 18 burner annular combustor, having identical cross sections. The LES results for the single burner laboratory combustor are compared with experimental data for a laboratory model of this combustor, and with other LES predictions, with good agreement. An explicit finite volume based LES model, using the mixed subgrid model together with a partially stirred reactor model for the turbulence chemistry interactions, is used. For the annular combustor, with the swirlers parameterized by jet inflow boundary conditions, we have investigated the influence of the a-priori unknown combustor exit impedance, the influence of the swirler characteristics and the fuel type. The combustion chemistry of methane–air and n-decane–air combustion is modeled by a two-step reaction mechanism, whereas NOx is separately modeled with a one-step mechanism. No experimental data exists for the annular combustor, but these results are compared with the single burner LES and experimental results available. The combustor exit impedance, the swirler- and fuel characteristics all seem to influence the combusting flow through the acoustics of the annular combustor. To examine this in greater detail time-series and eigenmodes of the combustor flow fields are analyzed and comparisons are made also with results from conventional thermoacoustic eigenmode analysis, with reasonable agreement. The flow and pressure distributions in the annular combustor are described in some detail and the mechanisms by which the burners interact are outlined.     

An experimental investigation of the interaction of swirl flow with partially premixed disk stabilized propane flames

The present work describes the experimental investigation of reacting wakes established through fuel injection and staged premixing with air in an axisymmetric double cavity arrangement, formed along three concentric disks, and stabilized in the downstream vortex region of the afterbody. The burner assembly is operated with a co-flow of swirling air, aerodynamically introduced upstream of the burner exit plane, to allow for the study of the interaction between the resulting partially premixed recirculating afterbody flames with the surrounding swirl. At low swirl the primary afterbody disk stabilizes the partially premixed annular jet in the downstream reacting wake formation region. As swirl increases, a system of two successive vortices emerges along the axis of the developing wake; the primary afterbody vortex is cooperating with an adjacent, swirl induced, central recirculation zone and this combination further promotes turbulent mixing in the hot wake.Complementary measurements of the counterpart isothermal turbulent velocity fields provided important information on the near wake aerodynamics under the interaction of the variable swirl and the double cavity produced annular jet stabilized by the afterbody. Under reacting conditions, measurements of turbulent velocities, temperatures and statistics together with an evaluation of the exhaust emissions were performed using LDV, thin digitally-compensated thermocouples and gas analyzers. A selected number of lean and ultra-lean flames were investigated by regulating the injected fuel and the air supply ratio, while the influence of the variation of the imposed swirl on wake development, flame characteristics and emission performance was documented for constant fuel injections. The differences and similarities between the present partially premixed stabilizer and other types of axisymmetric configurations are also highlighted and discussed.     

Local curvature measurements of a lean,partially premixed swirl-stabilised flame

A swirl-stabilised, lean, partially premixed combustor operating at atmospheric conditions has been used to investigate the local curvature distributions in lifted, stable and thermoacoustically oscillating CH₄-air partially premixed flames for bulk cold-flow Reynolds numbers of 15,000 and 23,000. Single-shot OH planar laser-induced fluorescence has been used to capture instantaneous images of these three different flame types. Use of binary thresholding to identify the reactant and product regions in the OH planar laser-induced fluorescence images, in order to extract accurate flame-front locations, is shown to be unsatisfactory for the examined flames. The Canny-Deriche edge detection filter has also been examined and is seen to still leave an unacceptable quantity of artificial flame-fronts. A novel approach has been developed for image analysis where a combination of a non-linear diffusion filter, Sobel gradient and threshold-based curve elimination routines have been used to extract traces of the flame-front to obtain local curvature distributions. A visual comparison of the effectiveness of flame-front identification is made between the novel approach, the threshold binarisation filter and the Canny-Deriche filter. The novel approach appears to most accurately identify the flame-fronts. Example histograms of the curvature for six flame conditions and of the total image area are presented and are found to have a broader range of local flame curvatures for increasing bulk Reynolds numbers. Significantly positive values of mean curvature and marginally positive values of skewness of the histogram have been measured for one lifted flame case, but this is generally accounted for by the effect of flame brush curvature. The mean local flame-front curvature reduces with increasing axial distance from the burner exit plane for all flame types. These changes are more pronounced in the lifted flames but are marginal for the thermoacoustically oscillating flames. It is concluded that additional fuel mixture fraction and velocimetry studies are required to examine whether processes such as the degree of partial-premixedness close to the burner exit plane, the velocity field and the turbulence field have a strong correlation with the curvature characteristics of the investigated flames.