Flame structure analysis and flamelet progress variable modelling of strained coal flames

Strained two-phase pulverised coal flames in a counterflow configuration are investigated numerically. Three operating conditions with different coal-to-primary-air ratios and inlet velocities were evaluated in order to establish different flame regimes. At first, the two-phase flow of the fully resolved reference cases is calculated solving the transport equation for the species and directly evaluating the reaction rates. Different flame structures are identified using the heat release rate and the chemical explosive mode as markers, showing that complex structures with a combination of lean premixed and non-premixed flames can be observed in strained counterflow coal flames. In addition to the fully resolved simulation, the suitability of the Flamelet-Progress Variable (FPV) model is investigated. Both premixed and non-premixed tables are employed. At first, the suitability of the look-up tables is evaluated by means of an a priori analysis, using the fully resolved simulations as reference solutions, showing that the non-premixed flamelet table correctly predicts the structure of the strained coal flames, while the premixed table shows sensible deviations in terms of temperature and species, especially at rich conditions. Finally, the a posteriori analysis shows that the fully coupled FPV model with a non-premixed flamelet look-up table can accurately predict strained coal flames.     

Spectroscopic,calibration and RET issues for linear laser-induced fluorescence measurements of nitric oxide in high-pressure diffusion flames

Two different strategies are compared for linear laser-induced fluorescence (LIF) measurements of nitric oxide concentration ([NO]) in counter-flow diffusion flames at high pressures via the A-X(0,0) system. Excitation of NO via a rovibronic transition at 226.03 nm is found to be slightly better compared to a previously utilized excitation wavelength of 225.58 nm. An indirect approach based on the computed spectral overlap fraction is verified and applied to calibrate [NO] measurements in counter-flow diffusion flames at high pressures. A five-level model for NO molecular dynamics is presented and utilized to investigate the effects of rotational energy transfer (RET) on linear LIF measurements of [NO] at pressures up to 15 atm. The results indicate that rotational relaxation effects are essentially negligible under high-pressure conditions at low laser fluences, and thus they need not be accounted for when measuring [NO] using linear LIF. The calibration technique is validated by direct comparisons to [NO] measurements made at pressures up to 5 atm via another calibration method, based on doping NO in counter-flow premixed flames at the same pressure. Using this calibration technique, LIF measurements of [NO] are obtained in a series of counter-flow diffusion flames at pressures up to 15 atm. These measurements are found to be in excellent agreement with previously reported measurements of [NO] in similar flames. PACS 07.35.+k; 33.20.Sn; 42.62.Fi     

Understanding cool flames and warm flames

Low-temperature flames such as cool flames, warm flames, double flames, and auto-ignition assisted flames play a critical role in the performance of advanced engines and fuel design. In this paper, an overview of the recent progresses in understanding low-temperature flames and dynamics as well as their impacts on combustion, advanced engines, and fuel development will be presented. Specifically, at first, a brief review of the history of cool flames is made. Then, the recent experimental studies and computational modeling of the flame structures, dynamics, and burning limits of non-premixed and premixed cool flames, warm flames, and double flames are presented. The flammability limit diagram and the temperature-dependent chain-branching reaction pathways, respectively, for hot, warm, and cool flames at elevated temperature and pressure will be discussed and analyzed. After that, the effect of low temperature auto-ignition of auto-igniting mixtures at high ignition Damköhler numbers at engine conditions on the propagation of cool flames, warm flames, and double flames as well as turbulent flames will be discussed. Finally, a new platform using low temperature flames for the development and validation of chemical kinetic models of alternative fuels will be presented. Discussions of future research of the dynamics and control of low temperature flames under engine conditions will be made.     

Effectiveness of flame suppressants on cool flames and hot flames

While the effectiveness of various flame suppressants such as bromotrifluoromethane and trimethylphosphate on hot flames has been relatively well studied over the years, such suppressants have not been examined in the context of low-temperature cool flames. This investigation solves this issue by exploring the extinction limits of six suppressants on both hot flames and cool flames in the counterflow geometry using n-dodecane as the fuel. In contrast to hot flames, it is found both experimentally and numerically that cool flames are relatively impervious to chemically based suppressants such as bromotrifluoromethane; these suppressants are essentially diluents at low temperatures. Detailed examination of the computed flame structure reveals that the reactions composing the catalytic cycles that interfere with hydrogen radical and hydroxyl radical production in hot flames are orders of magnitude lower in cool flames. Furthermore, mildly flammable suppressants such as trimethylphosphate and 2‑bromo-3,3,3-trifluoropropene are observed to ignite under the conditions necessary to initiate cool flames, which limits measurements of the cool flame extinction limits. This premature oxidation is not predicted by kinetic models describing the suppressant chemistry.     

Large eddy simulation of Cambridge bluff-body coal (CCB2) flames with a flamelet progress variable model

In the present study, we report the first large eddy simulation (LES) study of the Cambridge CCB2 coal flames, one of the target flames in the Workshop on Measurement and Simulation of Coal and Biomass Conversion, with an extended flamelet progress variable (FPV) model. The extended FPV model is based on two mixture fractions considering the volatiles and char off-gases. The normalized total enthalpy is used for the interphase heat transfer modelling. Turbulence-chemistry interaction is treated with an assumed probability density function approach. The results show that the present LES can generally capture the flow field and particle distribution, while there are considerable deviations in the OH prediction due to the boundary treatment of using a mixture of volatiles and carrier gas to replace the methane-containing mixtures in the primary and pilot flow. It indicates that for such gas-assisted coal flames, the pilot fuel stream needs to be rigorously considered in the flamelet tabulation that could be resolved by extending the two-mixture-fraction model into a three-mixture-fraction model. The instantaneous Lagrangian particles histories show that the increasing of coal load has a negligible effect on devolatilization, but delays the char conversion.     

A DNS study on temporally evolving jet flames of pulverized coal/biomass co-firing with different blending ratios

Biomass co-firing within the existing pulverized coal boiler is thought as a practical near-term way of biomass utilization, while its detailed combustion characteristics and pollutant formation have not yet been fully understood. In the present study, we report a Carrier-phase Direct Numerical Simulation study coupled with detailed mechanism to provide a deep insight into the coal/biomass co-firing (CBCF) jet flames under different blending ratios. It is found that compared with the pure coal flame, the CBCF could (i) prompt the volatiles ignition, produce higher H₂O and similar CO₂ mass fractions at blending ratios of 20% and 40%, and obviously reduce the gas temperature and CO₂ mass fraction at the blending ratio of 50%; (ii) prompt the coal devolatilization and char burnout at blending ratios of 20% and 40%, while the char burnout is reduced when blending ratio is 50% due to the local enrichment of large particles and lack of oxygen; (iii) reduce the thermal, prompt, NNH and N₂O-intermediate routes of NO formation, but show limited effect on the NO-reburning route of NO destruction, therefore, resulting in an obvious NO reduction.     

Aspects of oscillating flames

Photography and chemieluminescence from CH radicals have been used to identify the reaction zones and quantify the areas and shapes of kerosene-fuelled flames with swirl numbers of 0.7 and 0.8 and an overall equivalence ratio of 0.25. The air flow was oscillated at a frequency of 350 Hz and the results suggest that the oscillations caused a sequence of vortex rings at the burner exit and that these distorted the reaction zone and increased its area in the near burner region leading to an overall shorter flame. For the swirl number of 0.7, the flame was lifted and the oscillations led to an increase in the average lift off length whereas the higher swirl number caused an attached flame with and without oscillations. The stretch rate, evaluated from the variation of the flame area in time, was higher for the lifted flame suggesting that lift off was caused by local extinction.     

Stability of inclined planar flames as a local approximation of weakly curved flames

To evaluate the effect of vorticity usually generated by curved flames on the flame stability, laminar premixed planar flames inclined in the gravitational field is asymptotically examined. The flame structure is resolved by a large activation energy asymptotics and a long wave approximation. The coupling between hydrodynamics and diffusion processes is included and near-unity Lewis number is assumed. The results show that as the flame is more inclined from the horizontal plane it shows more unstable characteristics due to not only the decrease of the stabilizing effect of gravity but also the increase of the destabilizing effect of rotational flow. Unlike the planar flame propagating downward with the right angle to the upstream flow, the obtained dispersion relation involves the Prandtl number and shows the destabilizing effect of viscosity. The analysis predicts that the phase velocity of unstable wave depends on the Lewis number as well as the flame angle and, especially for unity Lewis number, it is the same with tangential velocity at the reaction zone. For relatively short wave disturbances, still much larger than flame thickness, the most unstable wavelength is nearly independent on the flame angle and the flame can be stabilized by gravity and diffusion mechanism.     

Temperature distribution of magnesium flames

Applied Physics A - Methods used in the past for calculating the radiation properties of flames with diffusing particles were employed in the investigation of oxygen/city gas flames containing...     

Numerical studies of flames in wide tubes: stability limits of curved stationary flames

Flame dynamics in wide tubes with ideally adiabatical and slip walls is studied by means of direct numerical simulations of the complete set of hydrodynamical equations including thermal conduction, fuel diffusion, viscosity, and chemical kinetics. Stability limits of curved stationary flames in wide tubes and the hydrodynamic instability of these flames (the secondary Darrieus-Landau instability) are investigated. The stability limits found in the present numerical simulations are in a very good agreement with the previous theoretical predictions. It is obtained that close to the stability limits the secondary Darrieus-Landau instability results in an extra cusp at the flame front. It is shown that the curved flames subject to the secondary Darrieus-Landau instability propagate with velocity considerably larger than the velocity of the stationary flames.     

The computation of laminar flames

Hydrocarbon fuels will remain a major source of energy well into the second half of the 21st century and, despite dire warnings about their limited supply, known resources have actually increased over the past decade. Nevertheless, finite supplies and increasing demand will exert pressure on the efficient use of these fuels, especially if their price continues to climb. Specifically, electricity generation and propulsion will continue to rely heavily upon the burning of hydrocarbon fuels for many years to come. Although an understanding of combustion in practical combustors is essential to the goals of reducing pollution and increasing energy efficiency, three-dimensional models of these systems with detailed transportation fuel chemistry and complex transport are beyond our current computational capabilities. Instead, one can study flames with complex chemistry in simpler laminar configurations to provide insight into the chemical and physical processes occurring in many engineered systems. In this paper, we trace the development of mathematical models and computational methods for laminar flame problems, with a particular emphasis on numerical algorithms that enable their coupled solution. While most of the focus is on steady systems, we also discuss issues related to time-dependent flames.     

Particle synthesis in flames

From the view point of combustion science, the fundamentals of particle formation in a flame environment are discussed. The principles of converting a gas phase precursor dopant into particles and also their growth are addressed. Various experimental methods and examples for the synthesis of particles of both single and mixed oxides are reviewed. First attempts to tune the stoichiometry of oxide particles by varying the combustion parameters of premixed flames are demonstrated. Some aspects of modelling and the problems which still need to be solved are illustrated by means of the particles’ population balance equation.     

Emission characteristics of hydrogen-oxygen flames

The emission spectra of hydrogen-oxygen flames are discussed. High temperature measurements of the radiative characteristics of the OH group in flames are described. The spectral absorption coefficients and the integral intensities of the hydroxyl bands are analyzed. A method for determining the nonequilibrium radiative cooling of hydrogen-oxygen flames using experimental data, their temperatures, and nonequilibrium coefficients is described and the results are examined.     

Adjoint-based sensitivity analysis of flames

Simulation of chemically reacting flows using detailed chemistry introduces a large number of chemistry model parameters. While not all significantly affect the target outcomes of a simulation, the parameters that do are not always known a priori. In order to improve simulations for specified target outcomes, termed quantities of interest (QoIs), the sensitivity of these QoIs to the model parameters are needed. However, evaluating the sensitivities is computationally expensive, especially for complex fuels that may involve many parameters. For these simulations, the forward sensitivity method requires the solution of an additional number of governing equations proportional to the number of parameters. Here, an adjoint sensitivity approach is formulated where the computational cost scales as the number of QoIs and not the number of parameters. Specifically, adjoint equations are derived for laminar, incompressible, variable density reacting flow and applied to hydrogen flame simulations. From the solution of the corresponding adjoint equations, sensitivity of the QoIs to chemistry model parameters is calculated. The one-dimensional simulation results show that the adjoint sensitivity results closely match those of forward sensitivity methods, thus providing validation of the adjoint method. The two-dimensional simulation results indicate the most sensitive parameters for two QoIs, flame tip temperature and NOx emission. For these tests, the adjoint method reduces computational expense compared to forward sensitivity methods by a factor proportional to the number of QoIs over the number of parameters, here 2/172. Such savings can be more drastic for cases that involve complex fuels, such as combustion of jet fuel, requiring thousands of chemistry model parameters. Further, this sensitivity information can be used in development of experiments by pointing out which are the critical chemistry model parameters.     

Interferometric visualization of jet flames

This paper presents visualizations of reacting, round jets of the premixed and nonpremixed type realized by using interferometry and, complementarily, direct photography. The available interferometer, proposed by Carlomagno (1986), employs low-cost components and is flexible and robust to geometrical misalignments, allowing the drawbacks limiting the application of traditional interferometric systems to be overcome. Several flames are produced by varying the non-dimensional, governing parameters (Reynolds number, equivalence ratio, Grashof number). The results discussion is organized considering laminar, transitional and turbulent flows. In the steady, laminar case, in view of the radial symmetry of the fringes pattern, the temperature field is reconstructed by the interferograms. The structure of the transitional and turbulent combusting jets, primarily determined by shear layer destabilization mechanisms and large-scale vortices formation due to buoyancy, is analyzed and differences with isothermal flows are pointed out. In turbulent regime, studied only for premixed combustion case, qualitative insights into the structure of the reaction zone as a function of the equivalence ratio and turbulence properties in the incoming fresh mixture are also deduced.     

Direct comparison of self-excited instabilities in mesoscale multinozzle flames and conventional large-scale swirl-stabilized flames

The present experimental investigation demonstrates important trends and offers physical insights into self-excited combustion instabilities in mesoscale multinozzle flames composed of sixty small injectors. Here we focus on the response of a prototypical micromixer-type injector assembly, fabricated using an additive manufacturing technique, in comparison with the behavior of conventional large-scale swirl-stabilized flames. Our results highlight that the development of self-excited instabilities in unconventional mesoscale flames is fundamentally different from that in large-scale swirl flames, in terms of the onset of instabilities, nonlinear modal dynamics, and amplitude/frequency of limit cycle oscillations under the same operating conditions. These differences are attributable to the alteration in local flow/flame structures and the resulting flame-to-flame/flame-wall interaction mechanisms. An integrated analysis of large datasets reveals that the two interacting swirl-stabilized flames tend to couple strongly with a low-frequency L1 mode at about 220 Hz, whereas the sixty-injector small-scale flames are capable of triggering multiple higher-frequency instabilities at ～ 310, ～ 470, and ～ 600 Hz. That is, the use of the micromixer-type injector assembly in a lean-premixed system causes the occurrence of combustion instabilities to shift toward a higher equivalence ratio. However, due to the absence of a large recirculation zone near the primary reaction region, the combustion system equipped with the small-scale multinozzle injectors was found to suffer from lean blowoff phenomena at low equivalence ratio.     

Pulverized coal swirl flames in oxy-fuel conditions: Effects of oxidizer O2 concentration on flow field and local gas composition

In an experimental study the effects of varied oxygen concentrations in the oxidizer gas on resulting flow fields, combustion products and general behavior of pulverized coal swirl flames under oxy-fuel conditions have been investigated. Experiments were carried out in a small scale down-fired cylindrical combustion chamber equipped with an annular swirl burner. Studied flames had a constant power output of 40 kW_th and O₂/CO₂ oxidizer gas mixtures with O₂ concentrations ranging from 23 to 33 vol%. Detailed two-dimensional flow field measurements are obtained from laser Doppler anemometry (LDA). Velocity profiles (Mean and RMS) have been obtained for all conditions investigated and serve as basis for identification of flow field characteristics. Velocity RMS values are provided as supplementary material. To complement flow field measurements, in-flame gas composition measurements were also conducted using a sampling probe combined with infrared gas absorption analysis via Fourier-transform infrared (FTIR) spectrometry. The results obtained show increased velocities, particularly along the main vortex for flames with increased oxygen contents, while lower velocities are found to occur inside the recirculation regions. The opposite occurs with lower O₂ concentrations, showing significantly reduced velocities in the main vortex, but stronger recirculation than the high oxygen counterparts. This effect is attributed to a modification of the swirl level introduced by the expansion of product gases. Measured NO and CO in-flame concentrations showed significant variations under different O₂ concentrations in the oxidizer.