Opposed Jet Burner Research Articles

Opposed jet burner and Tsuji burner for representative laminar flamelet with differential molecular diffusion for non-premixed combustion modeling

Modeling differential molecular diffusion in turbulent non-premixed combustion remains challenging for flamelet models. Laminar flamelet is a key building block of flamelet models for turbulent combustion. One significant challenge that has not been adequately addressed is the representativity of laminar flamelet for the characteristics of differential molecular diffusion in turbulent combustion. Laminar flamelet is typically generated based on two conceptual burner configurations, the opposed jet burner and the Tsuji burner. The two burners are commonly viewed to be equivalent for the description of laminar flamelet structures. In this work, a major difference between them is revealed for the first time when they are used to represent differential molecular diffusion. The traditional opposed jet burner yields almost fixed equal diffusion locations defined as the points with equal values of mixture fractions based on different elements in the mixture fraction space. The Tsuji burner, with a slight extension, can produce a continuous variation of the equal diffusion locations in the mixture fraction space. This variation of the equal diffusion locations is shown to be an important feature of non-premixed combustion, as demonstrated in a laminar jet mixing layer, a turbulent jet mixing layer, and a turbulent jet non-premixed flame. Theoretical analysis is conducted based on an idealized opposed jet mixing layer to identify the major cause of the difference between the two burners. It is found that the inlet diffusion caused the major difference in differential molecular diffusion in the two burners. The curvature difference, another main difference between the two burners, however, is not found to cause any major difference in differential molecular diffusion in the two burners. Based on the finding, a modified opposed jet burner with disabled inlet diffusion is introduced and is shown to be able to reproduce the feature of differential molecular diffusion observed in the Tsuji burner. Both burners, after properly generalized with suitable boundary conditions and additional parameters (like the velocity ratio of fuel and oxidizer), are expected to be suitable and equivalent choices for the generation of representative laminar flamelets that can be used eventually in flamelet modeling of differential molecular diffusion in turbulent non-premixed combustion.Novelty and significance statement: (1) A significant difference between the opposed jet burner and the Tsuji burner is revealed for generating representative laminar flamelet with differential molecular diffusion (under specific conditions); (2) The flamelet generated from the Tsuji burner is shown to be representative of differential molecular diffusion found in practically relevant mixing and combustion problems; (3) The treatment of inlet diffusion is identified to be the major cause of the difference observed between the two burners; (4) Modifications are introduced to the opposed jet burner to reconcile the two different burners for producing representative flamelet that can be used in the modeling of differential molecular diffusion in turbulent non-premixed combustion.

Studies on flow field, and instabilities in a large diameter opposed jet burner

This work reports studies on flow fields, and instabilities exhibited by opposed jets at equal momenta, for Reynolds numbers (Re) ranging from 178 to 5000 using a large diameter counterflow jet setup. Flow instability investigations were conducted over a range of aspect ratios (α) too. This study identifies the regions of bi-stability and those of oscillatory stagnation plane offsets identified by Re. Experiments on flow field were carried out using particle image velocimetry technique, and the paper presents the axial and radial velocity profiles at various locations as well as their gradients. A decreasing trend in stagnation plane displacements with the Reynolds number was observed. The experiments in comparison with the past literature suggest a possible dependence of the stagnation plane displacements on a nozzle-exit diameter. The trends in maximum stagnation plane displacements (δmax), as well as the critical Reynolds numbers (Recr) with aspect ratios (α), are analyzed and compared. The flow field studies reveal the need for two dimensional axisymmetric simulations with realistic velocity boundary conditions to predict opposed jet flow phenomena accurately. Reacting flow instability studies were also carried out for equal momenta using methane-air and ethylene–air flames at various aspect ratios. The results show an enhanced bistability for ethylene–air flames over methane–air flames.

Extinction strain rates of premixed ammonia/hydrogen/nitrogen-air counterflow flames

Chemical energy vectors will play a crucial role in the transition of the global energy system, due to their essential advantages in storing energy in form of gaseous, liquid, or solid fuels. Ammonia (NH3) has been identified as a highly promising candidate, as it is carbon-free, can be stored at moderate pressures, and already has a developed distribution infrastructure. As a fuel NH3 has poor combustion properties that can be improved by the addition of hydrogen, which can be obtained energy-efficiently by partially cracking ammonia into hydrogen (H2) and nitrogen (N2) prior to the combustion process. The resulting NH3/H2/N2 blend leads to significantly improved flame stability and resilience to strain-induced blow-out, despite similar laminar flame properties compared to equivalent methane/air flames. This study reports the first measurements of extinction strain rates, measured using the premixed twin-flame configuration in a laminar opposed jet burner, for two NH3/H2/N2 blends over a range of equivalence ratios. Local strain rates are measured using particle tracking velocimetry (PTV) and are related to the inflow conditions, such that the local strain rate at the extinction point can be approximated. The results are compared with 1D-simulations using three recent kinetic mechanisms for ammonia oxidation. By relating the extinction strain rates to laminar flame properties of the unstretched flame, a comparison of the extinction behaviour of CH4 and NH3/H2/N2 blends can be made. For lean mixtures, NH3/H2/N2-air flames show a significant higher extinction resistance in comparison to CH4/air. In addition, a strong non-linear dependence between the resistance to extinction and equivalence ratio for NH3/H2/N2 blends is observed.

On Flame Extinction and Free-Floating Limits Using a Large Diameter Opposed Jet Burner

ABSTRACT This paper presents studies on flame extinction and associated flame structure using a large diameter (40 mm) opposed jet burner for the first time. The extinction experiments were carried out at various nozzle separation distance (L) to diameter (D) ratios (L/Ds) for methane-air non-premixed flames, while all similar experiments in the past literature are with smaller burner diameters (<25 mm). The extinction studies were used to determine the free-floating limits of the burner, and a detailed analysis of the effect of nozzle exit diameter and Applied Stress Rates (ASRs) on the free-floating limits are reported by comparing with similar works in the past. The Particle Image Velocimetry (PIV) technique was used to determine the axial velocity profiles and local extinction strain rates (Kl) of the flames. Based on the experiments, a correlation for the free-floating separation distance from the stagnation plane (zFF) is proposed, which was found to be proportional to the square of the ASRs. The reported free-floating limits in the literature using various diameters agree with the observed data trend. This solves the ambiguity regarding the free-floating limits of opposed jet burners and provides a general trend of the free-floating separation distance (LFF) even in the off-extinction operation regime. 2D axisymmetric simulations were carried out using realistic boundary conditions on the OpenFOAM® platform to predict flame extinction and, consequently, global flame extinction strain rates (Kg). The model predicts the Kg values and also its trend with L/Ds accurately. The model also captures the flame structure before extinction for various L/Ds.

Laser-induced breakdown spectroscopy for local equivalence ratio measurement in opposed jet methane-air flames

Laser-induced breakdown spectroscopy (LIBS) has been applied to both non-reacting and reacting opposed jet flows to evaluate the measurement accuracy of instantaneous, local air-fuel ratio. However, the LIBS signal is much weaker in flames than in the corresponding non-reacting gases and more work is required to investigate the potential of this technique for instantaneous measurements in reacting flows and quantify measurement uncertainties. The present study reported local equivalence ratio measurements of methane/air flames in an opposed-jet burner, which allowed independent control of flow rate and air-fuel ratio. Images and emission spectra of laser-induced plasma were investigated simultaneously using both an Intensified CCD (ICCD) and a spectrometer with a high degree of spatial and temporal resolution. The influences of the camera delay time, exposure time and laser pulse energy on the LIBS measurements were investigated as well. The dependence of spectral intensity ratios of H/O and C2/CN on air-fuel ratio was quantified over a wide range of conditions extending from pure air to pure fuel. It was found that H/O and C2/CN intensity ratios depend monotonically on the mole fraction of methane in the ranges of 0.0–0.8 and 0.3–1.0 respectively. The presence of a flame within the laser beam led to significant measurement deterioration relative to the corresponding non-reacting flows. This could lead to increased measurement uncertainty, and was therefore corrected by increasing the laser pulse energy and applying a proposed data processing method; the proposed correction method was able to reduce the equivalence ratio measurement uncertainty to be within 10% for mixtures with methane mole fractions lower than 50% and within 15% for mixtures with higher methane mole fraction. The established correlations between the intensity ratios of H/O and C2/CN with local air-fuel ratio allowed measurement of the spatial gradient of air-fuel ratio across opposed jet diffusion flames. The LIBS measurements of air-fuel ratio in non-premixed flames were finally compared successfully with CHEMKIN simulations, which demonstrates the ability of LIBS to accurately measure the spatial gradient of the air-fuel ratio.

Modelling and validation of biomass combustion in a screw burner

A combined 1D-fuel-bed and 3D-CFD model for the simulation of an innovative 35 kW screw burner has been developed and validated with experiments. The 1D-fuel-bed model is a further development of a previously presented walking column approach and accounts for the special transport conditions in a screw burner. A new reduced chemical mechanism combined with the Eddy Dissipation Concept (EDC) combustion model is used in the subsequent 3D-CFD simulation. Detailed and reduced mechanisms are first compared in an opposed jet burner and then an improved reduced chemistry model is used in CFD calculations to investigate the influence of different primary and secondary air settings and to be validated with experimental data from a 35 kW prototype burner. The model enables a reasonable prediction of the trends of variations of the excess air ratio in the primary zone (primary λ) and of the global excess air on the emissions and the heat output and is therefore suitable for the design of biomass combustion systems with a moving fuel bed.

Experimental and numerical study of laminar flame extinction for syngas and syngas-methane blends

ABSTRACTExtinction strain rates of syngas and syngas–methane blends at atmospheric pressure were investigated in laminar counter-flow diffusion flames as a function of nitrogen dilution through both experiment and two-dimensional axisymmetric full-domain numerical simulations. Three representative compositions of syngas were examined along with two syngas–methane blends for a range of fuel mole fractions. An opposed-jet burner configuration with straight-tube fuel and air nozzles was used in the experiments and an advanced, computational, solution algorithm was used to obtain the corresponding simulation results. Very good agreement was found between predicted and experimentally observed flame structure as well as global strain rate extinction limits for all syngas fuel and syngas–methane blends over the full range of flow rates and fuel mole fractions considered. Additionally, the local strain rate near extinction was shown to correlate with the global strain rate and an explanation of the differences between these two values was provided by the predicted changes in the nozzle exit plane velocity profiles as a function of flow rate. In particular, non-zero gradients in the axial center-line velocity were predicted at the nozzle exit plane for higher flow rates due to the pressure field created by the opposed-jet flows. The syngas extinction limits as a function of fuel composition were examined. While the amount of hydrogen was found to have a dominant effect on the extinction strain limits, methane and carbon monoxide were also found to induce early extinction and increase reactivity, respectively, if present in sufficiently large quantities.

Combined experimental and numerical study of ethanol laminar flame extinction

ABSTRACTExtinction strain rates of ethanol at atmospheric pressure were investigated in laminar counter-flow diffusion flames as a function of nitrogen dilution through both experiment and two-dimensional axisymmetric full domain numerical analysis. An opposed-jet burner configuration with straight-tube fuel and air nozzles and having two different design nozzle exit flow profiles was used in the experiments. An advanced numerical solution algorithm, which makes use of a block-based parallel implicit finite-volume scheme, was used to obtain the corresponding simulation results. A comparison of the numerical results for two different chemical kinetic mechanisms helped to identify key factors affecting the flame extinction. While the predicted values over estimated the experimentally observed global strain rate extinction limits, the general trends and nozzle flow profiles were well captured. The local strain rates near extinction were also examined numerically and shown to be independent of the nozzle velocity flow profile and a fundamental property of the reactants. The experimental data from the present study were also compared to previous measurements and good agreement was demonstrated. The predicted nozzle exit and axial velocity flow profiles arising from the two-dimensional analysis were examined and limitations of conventional one-dimensional counter-flow analyses were highlighted.

Effect of Oxycombustion Diluents on the Extinction of Nonpremixed Methane Opposed-Jet Flames

ABSTRACTThis study investigates the differences between oxycombustion and conventional air combustion through methane flame extinction measurements and calculations. We report global extinction strain rates for methane with O2/N2, O2/CO2, and O2/CO2/H2O flames for a range of calculated stoichiometric adiabatic flame temperatures. Tests were performed in a subatmospheric opposed-jet burner, with pre-vaporized water. For a given oxidant diluent, global extinction strain rates increase strongly with increasing flame temperature and with pressure. At a given temperature, the effect of oxidant composition depends on whether dissociation is included in computing the stoichiometric adiabatic flame temperature. When dissociation is included, O2/CO2 and O2/CO2/H2O flames are roughly 30% harder to extinguish than O2/N2 flames. When dissociation is not included, the oxidant composition has a minimal effect. Calculations with the CHEMKIN suite of programs yield the same ranking of extinction strain rate for the different additives, when stoichiometric adiabatic flame temperature is matched with dissociation included.

Turbulent transport in premixed flames approaching extinction

The turbulent transport in premixed flames approaching extinction has been characterised in terms of statistical properties using an opposed jet burner featuring fractal grid generated turbulence. The burner was used in a symmetric twin flame configuration featuring pre-vaporised cyclopentane and JP-10 (exo-tetrahydrodicyclopentadiene) at equivalence ratios of 0.75 and 0.85. The choice of fuels follows from the practical importance of JP-10 as an aviation fuel with cyclic C5 compounds appearing as breakdown products. The bulk velocity was set to 3.0m/s resulting in a turbulent Reynolds number of 120. The obtained data includes conditional velocity statistics, flame curvature information and scalar fluxes. The conversion between conventionally and Favre averaged statistics follows the Bray–Moss–Libby theory and the assumption that finite reaction interface thickness effects can be neglected. The impact of deviations from the latter on statistics is explored and results suggest that the effect is modest for interface thicknesses less than 20% of the turbulent flame brush. The experimental data obtained is sufficient to enable terms up to and including triple correlations to be evaluated in closed form. The work also clearly illustrates the rapid transition from non-gradient to gradient turbulent transport as the extinction limit is approached.

Experimental investigation of the morphology and stability of diffusion and edge flames in an opposed jet burner

The stability of methane/air and hydrogen/air flames in an axisymmetric counterflow burner was investigated experimentally for different burner geometries, degrees of fuel dilution, and combinations of flow velocities. Both planar diffusion flames and edge flames were observed, and the transitions between these flame types were studied. The experimental results confirmed previously published numerical predictions on diluted hydrogen/air flames: the existence of two distinct stable flame types; the possibility of switching between the two flame types by perturbing the flames, e.g., by suitably changing a flow velocity; and the strong hysteresis for the transition from one flame type to the other. Flame stability diagrams were compiled which delineate the range of fuel and air flow velocities for which the planar diffusion flame and the toroidal edge flame are stable. The lower boundary curve for the edge flame stability exhibits a characteristic minimum at a well-defined value of the fuel velocity. For fuel velocities lower than this value, the transition between the edge and the diffusion structure is reversible, and the flames exhibit bistable behavior. For higher fuel velocities, the decrease of air velocity leads to the extinction of the edge flame. An investigation of both the cold and the reactive flow field identified bistable behavior for the flow field as well. Except for very low flow rates, the stagnation plane stabilizes in two positions, close to either of the two nozzles. Detailed numerical simulations of hydrogen flames capture the essentials of this behavior. The observed flame extinction results from the interaction of the flame dynamics with the dynamics of the flow field.

Flame Structure of Turbulent Premixed Flames in Opposed Jet Burner

Flame Structure of Turbulent Premixed Flames in Opposed Jet Burner

Stretch rate of tubular premixed flames

Following Seshadri and Williams solution describing the flow field for the opposed jet burner, the analytical solution is given for the flow field of two other burners: the opposed tubular burner and the tubular burner. Under plug flow boundary conditions, it is shown that the stretch rate at the stagnation surface of the opposed tubular burner is k = π V / ( R 2 − R 1 ) for the case of equal velocities and equal densities (i.e., ρ 1 = ρ 2 and V 1 = − V 2 = V ). For the tubular burner, the stretch rate at the center of the burner is k = π V / R 2 . The comparison of the numerical simulation and analytical solution is carried out to verify the analytical solution.

506 対向流バーナに形成される乱流予混合火炎の構造観察(GS-6.8 燃焼(2))

506 対向流バーナに形成される乱流予混合火炎の構造観察(GS-6.8 燃焼(2))

Remeshed smoothed particle hydrodynamics for the simulation of laminar chemically reactive flows

We present an extension of the remeshed smooth particle hydrodynamics (RSPH) method for the simulation of chemically reactive flows. The governing conservation equations are solved in a Lagrangian fashion, while particle locations, which are distorted by the flow, are periodically re-initialized (remeshed) on a grid. The RSPH implementation is employed for the simulation of a hydrogen/air opposed-jet burner with detailed chemistry and transport. The effect of particle number (resolution), compressibility (Mach number) and outflow boundary condition (length of the domain) on the solution are considered. The RSPH computational results are compared with numerical results obtained by a spectral element implicit scheme and by a one-dimensional code. It is shown that RSPH provides a flexible and accurate alternative for the numerical simulation of chemically reacting flows.

From diffusion to premixed flames in an H 2/air opposed-jet burner: the role of edge flames

Edge flames obtained on a hydrogen/air non-premixed opposed-jet burner after the local extinction of the disk-shaped diffusion flame are investigated with 2-D direct numerical simulations using detailed chemical kinetics and transport. Over a large range of flowrates, edge flames were found to coexist with the well-known strongly burning diffusion flames corresponding to the upper branch of the S-shaped curve. The critical flowrates of the strong hysteresis associated with the transitions between the two solution branches were identified: re-establishment of the diffusion flame is controlled by the propagation of the edge flame and cannot be represented simply by the extinction scalar dissipation rate. It was also observed that in all the flow conditions simulated, the edge flame was able to consume all the supplied fuel by re-orienting itself, varying its flame surface area, or changing its structure. The latter was found to depend on the flow conditions (which strongly affects the degree of mixing ahead of the edge flame) and can take on different configurations ranging from a triple flame to an essentially premixed flame. Because of flame curvature and the preferential diffusion of hydrogen, the propagation speed of the edge flames was found to be higher than that of the corresponding planar premixed flames.

Transient phenomena during diffusion/edge flame transitions in an opposed-jet hydrogen/air burner

Time-accurate direct numerical simulations with detailed chemistry and transport were used to studytransient effects during the transitions between different flame structures in an axisymmetric hydrogen/air opposed-jet burner. By starting from a steady flame and impulsively changing the flowrates, we simulated two different scenarios: (1) extinction of diffusion flames around the axis of symmetry and formation of a propagating edge flame that ultimately leads to a ring-shaped edge flame stabilizing away from the axis of symmetry, and (2) propagation of ring-shaped premixed flames along the stoichiometric isoline through a weakly stratified mixture in the direction normal to the direction of propagation, eventually closing the flame hole and re-establishing a disk-shaped diffusion flame. A front moving to quench or extend the diffusion flames is observed at the initial and final stages of cases 1 and 2, respectively. After extinction of or before re-establishment of the diffusion tail, the flames (as a whole) move in a quasi-steady manner with a propagating velocity relative to the fresh reactants that is close to the premixed laminar flame speed. No negatively propagating edge flame was found during extinction: before the transition to a strongly burning edge flame, the diffusion flame edge is convected outward with the local flow velocity. During re-ignition, the edge flame propagates all the way to the axis, and no homogeneous re-ignition was observed. Close to the axis it attains a propagation velocity that is almost twice the laminar flame speed.

1204 対向流場に形成された非予混合乱流火炎の観察(GS-6 燃焼(乱流・拡散・火炎))

1204 対向流場に形成された非予混合乱流火炎の観察(GS-6 燃焼(乱流・拡散・火炎))

Injection of single and multiple vortices in an opposed-jet burner

A thorough understanding of turbulent reacting flows is essential to the continued development of practical combustion systems. Combustor codes can be validated using data such as those generated in this study of a vortex interacting with a nonpremixed, opposed-jet hydrogen-air flame. When experimental results are compared with model predictions, the underlying flowfield must be matched carefully. Since the vortex-injection process used in the present experiments can result in many types of vortices, including multiple vortices, restrictions on the experimental operation of the burner are required as well as careful vortex characterization. Vortex-characterization data are acquired using digital, two-color particle-image velocimetry (PIV), and the hydroxyl (OH) layer produced by the flame is imaged using planar laser-induced fluorescence (PLIF). The PIV and OH PLIF measurements are performed simultaneously. Good agreement with previous numerical-modeling predictions is obtained when experiments and computations are performed using similar vortex conditions.

Numerical Study of Opposed-Jet H2/Air Diffusion Flame - Vortex Interactions

We consider the interaction of vortices of different size and strength (vorticity) and a diffusion flame of N2-diluted H2 and air stabilized on an opposed-jet burner. In our direct numerical simulations, which take into account the effects of detailed chemistry and transport, we demonstrate the effects of flame curvature of opposite orientations by placing a vortex on either the air or the fuel side of the diffusion flame. When the flame curvature is convex towards the fuel stream, the flame burns more intensely even at a scalar dissipation rate, X, close to the critical (extinction) value, X c of the flat one-dimensional diffusion flame (for the same composition of the reactant streams). On the other hand, if the flame curvature is convex towards the air stream, the flame weakens and in some cases extinguishes even when the local X is significantly lower than X c. Depending on the curvature orientation, the extinction scalar dissipation rate can vary considerably. This observation raises questions on the common use of a single extinction scalar dissipation rate in many turbulent diffusion flame simulations. Our results indicate that the role of curvature in the transient flame-vortex interaction is similar to what was observed in previous studies of steady curved flames. We also observe that the reignition process following local extinction is two-dimensional. The reignition process observed in our simulations may not be described well by flamelet models (steady or transient) that are based on one-dimensional formulations.