Pilot Burner Research Articles

Numerical Simulation with an Extinction Database for Use with the Eddy Dissipation Concept for Turbulent Combustion

A Local Extinction approach for treating chemical reaction kinetics within the Eddy Dissipation Concept (EDC) has been examined. It applies a database of pre-calculated chemical time scales, which contains the influence of chemical kinetics that is otherwise time-consuming to calculate. The approach was evaluated against experimental data for two piloted diffusion flames (Sandia/TNF Flame D and Flame E) and a piloted lean-premixed jet burner (PPJB). Results were also compared to the EDC with Fast Chemistry and with full Detailed Chemistry (GRI-Mech 3.0). All validation simulations were carried out using a standard k − e turbulence model and the open-source CFD-toolbox OpenFOAM. The Local Extinction approach showed significantly better results than the Fast Chemistry approach while having a comparably small computational cost. For Flame D and the PPJB, the reactions along centerline and in the mixing layer near the nozzle, the reactions were reduced. For Flame E, the Local Extinciton model predicted some lift off of the flame. The Detailed Chemistry approach gave the best predictions compared to the experimental data, however the calculation effort was orders of magnitude higher.

Towards Comprehensive Coal Combustion Modelling for LES

Large eddy simulations of pulverised coal combustion (PCC-LES) stabilised on a laboratory-scale piloted jet burner are carried out. The joint simulation effort of three research groups at Freiberg University (FG), Imperial College (IC) and Stuttgart University (ST) is presented, and the details of the comprehensive coal combustion models and their numerical implementation in three different computer programs are discussed. The (standard) coal sub-models and parameters used by the different groups are unified wherever possible. Differences amongst the groups are a different code basis and an Eulerian treatment of the coal particles by IC, while FG and ST use the Lagrangian framework for particle transport. The flow modelling is first validated for the corresponding non-reacting case and all LES calculations accurately capture the experimental trends. Velocity field statistics for the PCC case are in good accordance with the experimental evidence, but scalar statistics illustrate the complexity of coal combustion modelling. The results show notable differences amongst the groups that cannot only be attributed to the different treatment of the particle phase, and they highlight the difficulty to assess and interpret the quality of specific modelling approaches, and a need for further work by the research community. The present study is the first to compare three originally independent transient coal simulations and a step towards comprehensive PCC-LES.

Experimental Investigation of Turbulent Boundary Layer Flashback Limits for Premixed Hydrogen-Air Flames Confined in Ducts

This article discusses the results of experimental investigation of turbulent boundary layer flashback limits for premixed hydrogen-air flames confined in ducts. A tube burner experiment was set-up to double-check the findings of the channel rig. Unconfined flashback experiments were carried out by stabilizing the flame on top of the pilot burner in free atmosphere. A confined flame configuration was achieved by simply fixing a ceramic ring with a diameter higher by 4 mm on top of the pilot burner. Flashback measurements with unconfined flame holding neatly reproduced literature values for fully premixed, atmospheric H2–air mixtures and turbulent flow. The results of unconfined and confined tube burner experiments were plotted. The results showed that the drastic decrease of wall flashback stability for confined flames was the very same for both, tube and channel.

Two-dimensional direct numerical simulation of spray flames – Part 2: Effects of ambient pressure and lift, and validity of flamelet model

The effect of ambient pressure on spray flames is investigated by means of two-dimensional direct numerical simulation (DNS), and the validity of an extended flamelet/progress-variable approach (EFPV) is examined under the high-pressure condition. The DNS is performed not only for a simple jet spray flame with a pilot burner but also for a lifted recirculation spray flame without any pilot burner at ambient pressures of 0.1 and 0.5MPa. n-decane (C10H22) is used as liquid spray fuel, and the evaporating droplets’ motions are tracked by the Lagrangian method. The results show that the behaviors of jet and lifted recirculation spray flames are strongly affected by ambient pressure. The effects of the change of the ambient pressure on these spray flame behaviors can be well captured by EFPV and EFPV coupled with G-equation model (EFPV-G), respectively.

High-Speed Fuel/Hydroxyl Radical Imaging in a Gas Turbine Pilot Burner

High-Speed Fuel/Hydroxyl Radical Imaging in a Gas Turbine Pilot Burner

High resolution imaging of flameless and distributed turbulent combustion

Planar laser-induced fluorescence (PLIF) and Rayleigh scattering measurements were used for the study of turbulence/combustion interactions in distributed reaction regimes including flameless or MILD combustion. A novel laboratory scale burner (Distributed and Flameless Combustion Burner – DFCB) was used to reach uniquely high Karlovitz numbers, presently reported up to 14,400. It consists of a highly turbulent piloted high speed jet burner with a vitiated coflow. Six cases are reported whereas two of them (leaner cases) led to an invisible reacting zone, though still emitting light in the UV and near infrared range. Simultaneous OH/CH 2O PLIF image with 50 μm spatial resolution were achieved to capture the variation of intermediate species in the reaction layer. When complemented with temperature images obtained by Rayleigh scattering measurement, it provided insights of the reaction front structures as well as measures of the flame brush thicknesses. In particular, variations in the jet velocity highlighted the influence of turbulent mixing (hence turbulence/chemistry interaction) on the flame structures as depicted by the formation of relatively large pools of CH 2O. Further, variations in the jet stoichiometry impacted on the reaction zone visibility but only marginally on the intensity and moderately on the overall shape of the OH and CH 2O signals.

A numerical investigation of the flame stability in porous burners employing various ceramic sponge-like structures

In order to optimize the porous burner for the application as a pilot burner of stationary gas turbines aiming to reduce NO x emissions a fundamental study investigating the influence of the thermo-physical properties of the porous structure on the flame stabilization in a porous burner was conducted. This work presents a numerical study of the stability of one-dimensional laminar premixed flame in porous inert ceramic sponge structure. A set of steady computations are considered, using a numerical model that takes into account solid and gas energy equations as well as detailed chemistry. The model considers additionally the enhancement of both, thermal and species diffusivity by the flow dispersion, whereas the dispersion coefficients of the investigated structures have been determined from three-dimensional flow simulations using MRI (magnet resonance imaging) and CT (computer tomography) data to regenerate the real sponge structures. Hence, it was possible to calculate a thickened flame front as it was detected in experiments, too. The computations were conducted for different operational conditions and different burner configurations in respect to geometrical and material properties of the porous inert media. The numerical predictions showed very good agreement with the corresponding experimental stability data. The obtained numerical results were used for the formulation of a simple stability model based on the Pe number that enables a prediction of the lean blow-off limits in the combustion systems employing porous burner concept.

Results with a bench scale downdraft biomass gasifier for agricultural and forestry residues

A small scale fixed bed downdraft gasifier system to be fed with agricultural and forestry residues has been designed and constructed. The downdraft gasifier has four consecutive reaction zones from the top to the bottom, namely drying, pyrolysis, oxidation and reduction zones. Both the biomass fuel and the gases move in the same direction. A throat has been incorporated into the design to achieve gasification with lower tar production. The experimental system consists of the downdraft gasifier and the gas cleaning unit made up by a cyclone, a scrubber and a filter box. A pilot burner is utilized for initial ignition of the biomass fuel. The product gases are combusted in the flare built up as part of the gasification system. The gasification medium is air. The air to fuel ratio is adjusted to produce a gas with acceptably high heating value and low pollutants. Within this frame, different types of biomass, namely wood chips, barks, olive pomace and hazelnut shells are to be processed. The developed downdraft gasifier appears to handle the investigated biomass sources in a technically and environmentally feasible manner. This paper summarizes selected design related issues along with the results obtained with wood chips and hazelnut shells.

Study on incineration technology of oil gas generated during the recovery process of oil spill

The objective of this study is to design, set up and operate an incinerator system capable of providing clean exhaust and safety control for burning oil gas generated during the recovery process of oil spill in Taiwan. In this study, we successfully develop a vertical-type incinerator, which consists of five oil gas burners with entrained primary air, a pilot burner, and an auxiliary burner. The incinerator system is equipped with necessary control units in order to achieve safe, easy, fast, and efficient operation. Flame appearance, flue gas temperature and CO emission of the incinerator system for burning oil gas are reported and discussed. Under the long-term operation, it is found that the new designed incinerator is satisfactory for burning oil gas with low supply pressure at various compositions and supply rates during the recovery process of oil spill. It is noteworthy that the results obtained herein are of great significance to provide a good guidance for those who need to design, set up and operate an incinerator system providing clean exhaust and safety control for burning oil gas generated during the recovery process of oil spill in a polluted site with a large area.

Influential parameters of nitrogen oxides emissions for microturbine swirl burner with pilot burner

Swirl burners are the most common type of device in wide range of applications, including gas turbine combustors. Due to their characteristics, swirl flows are extensively used in combustion systems because they enable high energy conversion in small volume with good stabilization behavior over the wide operating range. The flow and mixing process generated by the swirl afford excellent flame stability and reduced NOx emissions. Experimental investigation of NOx emission of a purposely designed micro turbine gas burner with pilot burner is presented. Both burners are equipped with swirlers. Mixtures of air and fuel are introduced separately: through the inner swirler - primary mixture for pilot burner, and through the outer swirler - secondary mixture for main burner. The effects of swirl number variations for the both burners were investigated, including parametric variations of the thermal power and air coefficient. It was found that the outer swirler affects the emission of NOx only for the air coefficient less than 1.4. The increase of swirl number resulted in decrease of NOx emission. The inner swirler and thermal power were found to have negligible effect on emission.

D112 層流境界層内を伝播する天然ガスハイドレート火炎の特性(OS-11:火災安全(I))

Flame spreading over natural gas hydrate in a laminar boundary layer of air is experimentally investigated. The hydrate surface temperature when the pilot burner is ignited (T_<s,ig>) is varied between -90℃ and -20℃ and free stream velocity U_∞ is set at 0.4 ,0.7 or 1.1m/s. Two types of flame spreading is observed. One is the low speed linear spreading due to the heat transfer from the leading flame edge to the hydrate surface. This type of the flame spreading is observed when T_<s,ig> is relatively low and U_∞ is relatively high. The other is the high speed oscillating spreading. The leading flame edge spreads toward the upstream end of the hydrate with very high speed but suddenly disappear. This fast spreading and disappearance repeatedly occurs and finally the stable flame is formed. This type of the flame spreading is observed when T_<s,ig> is relatively high and U_∞ is relatively low.

Influence of carbon dioxide content in the biogas to nitrogen oxides emissions

Fuels derived from biomass are an alternative solution for the fossil fuel shortage. Usually this kind of fuels is called low calorific value fuels, due to the large proportion of inert components in their composition. The most common is carbon dioxide, and its proportion in biogas can be different, from 10 up to 40%, or even more. The presence of inert component in the composition of biogas causes the problems that are related with flame blow off limits. One of the possibilities for efficient combustion of biogas is the combustion in swirling flow including a pilot burner, aimed to expand the borders of stable combustion. This paper presents an analysis of the influence of the carbon dioxide content to the nitrogen oxides emissions. Laboratory biogas was used with different content of CO2 (10, 20, 30 and 40%). Investigation was carried out for different nominal powers, coefficients of excess air and carbon dioxide content. With increasing content of carbon dioxide, emission of nitrogen oxides was reduced, and this trend was the same throughout the whole range of excess air, carried out through measurements. Still, the influence of carbon dioxide content is significantly less than the influence of excess air. The coefficient of excess air greatly affects the production of radicals which are essential for the formation of nitrogen oxides, O, OH and CH. Also, the results show that the nominal power has no impact on the emission of nitrogen oxides.

Створення та підготовка до експериментальної експлуатації пілотних пальників енергетичного котлоагрегата для пилоподібного антрациту підвищеної зольності

Створення та підготовка до експериментальної експлуатації пілотних пальників енергетичного котлоагрегата для пилоподібного антрациту підвищеної зольності

Piloted jet flames of CH 4/H 2/air: Experiments on localized extinction in the near field at high Reynolds numbers

Measurements of temperature and major species concentrations, based on the simultaneous line-imaged Raman/Rayleigh/CO-LIF technique, are reported for piloted jet flames of CH 4/H 2 fuel with varying amounts of partial premixing with air (jet equivalence ratios of ϕ j = 3.2, 2.5, 2.1 corresponding to stoichiometric mixture fraction values of ξ st = 0.35, 0.43, 0.50, respectively) and varying degrees of localized extinction. Each jet flame is operated at a fixed and relatively high exit Reynolds number (60,000 or 67,000), and the probability of localized extinction is increased in several steps by progressively decreasing the flow rate of the pilot flame. Dimensions of the piloted burner, originally developed at Sydney University, are the same as for previous studies. The present measurements complement previous results from piloted CH 4/air jet flames as targets for combustion model calculations by extending to higher Reynolds number, including more steps in the progression of each flame from a fully burning state to a flame with high probability of local extinction, and adding the degree of partial premixing as an experimental parameter. Local extinction in these flames occurs close to the nozzle near a downstream location of four times the jet exit diameter. Consequently, these data provide the additional modeling challenge of accurately representing the initial development of the reacting jet and the near-field mixing processes.

Observation of Microexplosions in Spray Flames of Light Oil-Water Emulsions (3rd Report, Influence of the Diameter of Dispersed Water Droplets on the Spray Flame Structure)

Microexplosions of light oil-water emulsified fuel droplets were successfully documented using a high-speed video camera with laser illumination. The local frequency of explosion occurrence, temperature profile and exhaust gas emission were measured in spray flames of water-in oil type emulsion formed using an air-assist atomizer with a ring pilot burner. Those results indicate that the flame structure changes as the water droplet diameter in the emulsion fuel was varied, even if the fuel components and their fractions were same. When fuel includes water droplets, whose median diameter was around 75μm, HC and CO emissions were reduced as compared to those for the fuel of smaller water droplet content. It is probable that if the water droplet diameter is uniform, avalanching microexplosions occur at certain local region in the flame, and the water content vaporizes almost at once and extinguishes the flame. It leads to HC and CO emission increase. When the water droplet diameters are large, atomizer atomizes those; therefore, emulsion droplets include various size of water droplet in the flame. Consequently, the avalanching microexplosion occurrence is avoided, and HC and CO emissions are reduced.

Optical Diagnostics Applied to a Gas Turbine Pilot Burner

This paper presents the application of several optical measurement techniques for the characterization of the combustion behavior of the pilot burner of a gas turbine combustor developed according to the lean premixed prevaporized (LPP) concept. The vaporization of the kerosene fuel (Jet-A) and its consequent mixing with air has been visualized using combined Laser-Induced Fluorescence (LIF) and Mie scattering measurements. In addition the feasibility of using Laser-Induced Incandescence (LII) for soot characterization has also been investigated. Furthermore, spontaneous flame emission (i.e. chemiluminescence and thermal radiation) have been measured providing information on the overall characteristics of the combustion. The LIF/Mie measurements can also be used to estimate the length and time scales for evaporation and mixing with air of Kerosene (Jet-A) for the LP(P)4 pilot burner.

An Experimental Study of Flame Characteristics of Jet Diffusion Flames in Cylindrical Furnaces (1st Report, Effect of Inner Diameter of Furnace on NOx Emission Properties)

The combination of a burner and a combustion chamber is an important factor controlling flame characteristics. However, to our knowledge, this factor has yet to be investigated systematically. In the present study, coaxial jet diffusion flames in cylindrical combustion chambers have been studied in terms of inner diameters of the combustion chambers, global equivalence ratios, and turbulence in airflow. A fuel nozzle is composed of a stainless steel tube having an inner diameter (i.d.) of 2 mm with a coaxial pilot burner of 3.19 mm i.d., surrounded by two air coaxial tubes of 12 mm i.d. and 30 mm i.d., respectively. The inner and outer air tubes are for higher and lower airflows, respectively, and the turbulence in the airflow is changed by the velocity difference. The main fuel is propane. Hydrogen is used for the pilot flame, with a volumetric fuel ratio of 0.3. Each wall of the combustion chamber is made of a heat-resistant glass Pyrex tube so that each flame can be visualized. The inner diameter of the furnace is varied in order to investigate the effect of furnace size on the flame characteristics. The increase in the diameter of the combustion chamber has been found to enhance the exhaust gas self-recirculation, because the NOx emission decreases. The increase in turbulence in the airflow strengthens the entrainment of the exhaust gas transported upstream by the recirculation vortex. The increase in the global equivalence ratio from 0.2 to 0.8 in the present study decreases the oxygen concentration of the exhaust gas and leads to diluted combustion through the exhaust gas self-recirculation. A proper combination of these factors has been found to yield a low NOx combustion.

A flame stability diagram for piloted non-premixed oxycombustion of low calorific residual gases

Residual gases having a low calorific value are by-products of many industrial combustion systems. Subsequently burning those exhaust mixture, as blast furnace gases (BFG), is a direct way to increase the overall efficiency of processes. Flame stability is a crucial issue when burning such low calorific fuels and the choice of a piloted burner with pure oxygen as oxidizer appears as an interesting option. To progress in this direction, a new concept of piloted burner is discussed in which oxycombustion of a CH 4/BFG mixture is studied over a large range of global Net Calorific Value. From the results, the parameters controlling the major flame properties are determined and an attempt is made to draw a generic stability map to delineate between various combustion regimes. The burner consists of four coaxial jets. Starting from the axis of symmetry, it is composed of an oxygen (O 2i) center jet, surrounded by a synthetic BFG flow, followed by a CH 4 jet, and oxygen (O 2e) at the periphery. The objective of such geometry is to stabilize low calorific BFG combustion with an external CH 4–O 2e pilot flame. The flame structure is studied according to the proportion of residual BFG gases injected. CH ∗ chemiluminescence imaging is performed. Three types of flames are observed and organized in stability diagrams for such piloted non-premixed burner. In Type 1, two concentric flames are attached at the burner nozzle. In Type 2, the internal BFG flame is lifted. In Type 3, a single annulus flame burning a fuel mixture of CH 4 and BFG with the external oxygen is observed. Characteristic numbers, as ratio of jets velocities and Damköhler numbers are found to control transition between the various types of flames. They are used to build the stability diagrams.

CH and C2 radicals characterization in natural gas turbulent diffusion flames

This paper reports the construction of an axisymmetric nonpremixed piloted jet burner, with well-defined initial and boundary conditions, known as the Delft burner, to assess turbulence-chemistry interaction in non-premixed turbulent flames. Detailed experimental information is described, involving hot-wire anemometry, thin-wire thermocouples and chemiluminescence visualization measurements. Radial profile of the axial mean velocity indicates excellent agreement between flow patterns developed within Delft installation and the one described herein. Chemiluminescence emissions from CH and C2 free-radicals were acquired with a CCD camera. Tomography reconstruction analysis was utilised to compare radical emissions and temperature spatial distributions. There was a strong dependence between temperature and CH/C2 emissions. This is an indication that these radicals can be used in flame front studies.

Exhaust Gas Recirculation in Gas Turbines for Reduction of CO2 Emissions; Combustion Testing with Focus on Stability and Emissions

Exhaust gas recirculation can be applied with the intention of reducing CO2 emissions. When a fraction of the exhaust gas is injected in the entry of a gas turbine, the amount of CO2 in the exhaust gas not being recirculated will be higher and less complicated to capture. However, with this change in combustion air composition, especially the reduced concentration of oxygen, the combustion process will be affected. The lower oxygen concentration decreases the stability and the increased amount of CO2, H2O and N2 will decrease the combustion temperature and thus, the NOx emissions. Testing has been performed on a 65 kW gas turbine combustor, to investigate the effect of adding N2, CO2 and O2 in the combustion process, with focus on stability and emissions of NOx. Results show that adding N2 and CO2 decreases the NOx emissions, whereas O2 addition increases the NOx emissions. The tests have been performed both in a diffusion flame (pilot burner) and a premixed flame (main burner), and for additives being injected with the fuel or with the air stream. Addition into the fuel stream is proven to affect the NOx emissions the most. The stability limits of the flames are indicated with respect to mass-based additive-to-fuel ratios.