Flat Flame Research Articles

Mechanisms performance and pressure dependence of hydrogen/air burner-stabilized flames

The kinetic mechanism of hydrogen combustion is the most investigated combustion system. This is due to extreme importance of the mechanism for combustion processes, i.e. it is present as a sub-mechanism in all mechanisms for hydrocarbon combustion systems. Therefore, detailed aspects of hydrogen flames are still under active investigations, e.g. under elevated pressure, under conditions of different heat losses intensities and local equivalence ratios etc. For this purpose, the burner stabilized flame configuration is an efficient tool to study different aspects of chemical kinetics by varying the stand-off distance, pressure, temperature of the burner and mixture compositions. In the present work, a flat porous plug burner flame configuration is revisited. A hydrogen/air combustion system is considered with detailed molecular transport including thermo-diffusion and with 8 different chemical reaction mechanisms. Detailed numerical investigations are performed to single out the role of chemical kinetics on the loss of stability and on the dynamics of the flame oscillations. As a main outcome, it was found/demonstrated that the results of critical values, e.g. critical mass flow rate, weighted frequency of oscillations and blow-off velocity, with increasing the pressure scatter almost randomly. Thus, these parameters can be considered as independent and can be used to improve and to validate the mechanisms of chemical kinetics for the unsteady dynamics.

Numerical and Experimental Investigation of Hydrogen Enrichment Effect on the Combustion Characteristics of Biogas

In the present work, the combustion characteristics like adiabatic flame temperature (AFT) and laminar burning velocity (LBV) of methane (CH 4 ) diluted with carbon dioxide (CO 2 ), representing biogas, is investigated in detail. The laboratory prepared biogas samples containing CH 4 and CO 2 were also enriched with hydrogen (H 2 ) to realize the change in their combustion behaviour. The experiments were conducted on flat flame burners based on heat flux method at 1 bar, 298 K and at stoichiometric and off-stoichiometric conditions. The experimental results were also compared with the numerical predictions of ANSYS Chemkin-Pro ® with full GRI Mech. 3.0 reaction mechanism. The results revealed that the presence of CO 2 in biogas dominates on richer mixtures, i.e., the CO 2 dilution affects the combustion characteristics of richer biogas mixtures more strongly than for leaner or stoichiometric mixtures. The simulated results of hydrogen-enriched biogas showed that the slope of the laminar burning velocity (LBV) curve for biogas containing the highest percentage of CO 2 sharply rises with about 33% H 2 , whereas the slope of the mixture with least CO 2 and more CH 4 , sharply changes its nature around 40% H 2 -enrichment. This signifies that even with a small amount of H 2 in biogas, may be a suitable option to improve its combustion characteristics. Some preliminary correlations for H 2 -enriched biogas were also derived to estimate the effect of H 2 presence in biogas fuels with higher Hydrogen concentrations

Experimental and Numerical Study of Ethyl Valerate Flat Flames at Low Pressure

ABSTRACTThree flat flames of ethyl valerate (C7H14O2) at different equivalence ratios, 0.81, 0.95, and 1.31, have been stabilized at 55 mbar and analyzed using gas chromatography. Based on the experimental results, a detailed kinetic model of ethyl valerate combustion, containing 211 species and 1368 reactions, has been elaborated. The model predictions agree well with the experimental results for the temperature range of 1100–2000 K. In the lean flame, the main decomposition pathways leads to C7H14O2 → CH3(CH2)3COOCHCH3 → CH3CHO → CH3CO → CH3 → CH2O → HCO. For the stoichiometric flame, the ethyl valerate consumption pathway produces C7H14O2 → C4H9COOH → CH3CHCH2CH2COOH → C4H7COOH → CH2COOH → CH2CO → CH3 → CH2O → HCO. In the rich flame, from ethyl valerate to pentenoic acid (C4H7COOH), the decomposition pathway is the same as in the stoichiometric flame. The pentenoic acid leads to the formation of C2H4 via C4H6 and C3H6. Finally, the new model has been tested on the experimental data obtained in a jet-stirred reactor at high pressure (10 atm) and low temperatures (560–1160 K). The mechanism predicts fairly well all the species, except H2 and C2H2. Future work should improve the mechanism to extend its validity range up to high pressure and low temperature range.

An experimental study on the effect of a turbulence generating plate in low swirl combustor

Turbulence generator (Called turbulence generating plate or perforated plate) of a swirler in low-swirl combustion plays an important role in generating a stable lifted flame with an isotropic turbulence intensity and forming a uniform flow field. According to the previous experimental studies, the typical porosities of the generator in a low-swirl combustor have been reported between 25 % and 40 %. This study focused to confirm these results in terms of various porosities and hole patterns of a turbulence generator in a lab-scale based low swirl combustor. The pressure drop, the flame shapes, the velocity profiles and the emission performances were experimentally investigated to confirm the flame characteristics of low swirl combustion. The outer diameter(2R c) of the turbulence generator was taken to 2R c = 20 mm, and the porosity and the diameter of the circular hole pattern were varied in ranges of 15 - 35 % and 1.5 - 4.5 mm, respectively. The results show that as the porosity decreases, the occurrence of attached flame is encouraged, the lean flammable limit is extended, the flow rate through the turbulence generator decreases, the swirl intensity increases. The central velocity increases, so that the V-shaped flame changes into a flat flame. Also, the minimum velocity in the central low velocity zone increases and the flame liftoff height increases. As the axial velocity increases and the swirl intensity decreases, the effect of outer recirculation decreases, thereby, the temperature of the outer recirculation zone decreases and the exhaust gas temperature increases. Therefore, the NOx emission slightly increases because the flame temperature is increased due to the reduction of outer recirculation influence. As the hole pattern of the turbulence generator becomes a configuration with having more holes for the same porosity, the stable flame region is widened. Also, the velocity uniformity of the central low velocity zone is improved and the turbulence intensity is decreased. These results confirm that the factors of flame stabilization of low swirl combustor are velocity uniformity and turbulence intensity of the turbulence generator.

A small porous-plug burner for studies of combustion chemistry and soot formation.

We have developed and built a small porous-plug burner based on the original McKenna burner design. The new burner generates a laminar premixed flat flame for use in studies of combustion chemistry and soot formation. The size is particularly relevant for space-constrained, synchrotron-based X-ray diagnostics. In this paper, we present details of the design, construction, operation, and supporting infrastructure for this burner, including engineering attributes that enable its small size. We also present data for charactering the flames produced by this burner. These data include temperature profiles for three premixed sooting ethylene/air flames (equivalence ratios of 1.5, 1.8, and 2.1); temperatures were recorded using direct one-dimensional coherent Raman imaging. We include calculated temperature profiles, and, for one of these ethylene/air flames, we show the carbon and hydrogen content of heavy hydrocarbon species measured using an aerosol mass spectrometer coupled with vacuum ultraviolet photoionization (VUV-AMS) and soot-volume-fraction measurements obtained using laser-induced incandescence. In addition, we provide calculated mole-fraction profiles of selected gas-phase species and characteristic profiles for seven mass peaks from AMS measurements. Using these experimental and calculated results, we discuss the differences between standard McKenna burners and the new miniature porous-plug burner introduced here.

An investigation of pyrolysis and ignition of moist leaf-like fuel subject to convective heating

The burning of a thin rectangular-shape moist fuel element, representing a living leaf subject to convective heating, was investigated computationally. The setup resembled a previous bench-scale experimental setup (Pickett et al., Int. J. Wildland Fire19, 2010, 153-162), where a freshly harvested horizontally oriented manzanita (Arctostaphylos glandulosa) leaf was held over a flat flame burner and burned by its convective heating. Computations were performed by FDS coupled with an improved version of Gpyro3D. This improvement was concerned with the calculation of the mean porosities in the computational cells to account for the net volume reduction that the condense phase experiences within the computational cells during moisture evaporation and pyrolysis. The dry mass was assumed to consist of cellulose, hemicellulose and lignin undergoing the pyrolysis reactions proposed by Miller and Bellan (Combust. Sci. Technol.126, 1997, 97-137) for biomass. The reaction scheme was initially validated against published experimental and computational TGA results. Then, the burning of leaf-like fuels with three initial fuel moisture contents (40%, 76%, 120%), selected as per the range of experimentally measured values, was modeled. The time evolutions of the normalized mass were good for the modeled fuels with 76% and 120% FMCs and fair for the one with a 40% FMC, as compared to the experimental burning results of four manzanita leaves with unspecified FMCs. The computed ignition time was also in good agreement with the measurement. The computed burnout time was somewhat shorter than the measurement. Modeling revealed the formation of unsteady flow structures, including vortices and regions with high strain rates, near the fuel that acted as a bluff body against the stream of the burner exit. These structures played a significant role in the spatial distribution of gas phase temperature and species around the fuel, which in turn, had an impact on the ignition location. Fuel moisture content primarily affected the temperature response of the fuel and solid phase decomposition.

An experimental water line list at 1950 K in the 6250–6670 cm[formula omitted] region

An absorption spectrum of H216O at 1950 K is recorded in a premixed methane/air flat flame using a cavity-enhanced optical frequency comb-based Fourier transform spectrometer. 2417 absorption lines are identified in the 6250–6670 cm−1 region with an accuracy of about 0.01 cm−1. Absolute line intensities are retrieved using temperature and concentration values obtained by tunable diode laser absorption spectroscopy. Line assignments are made using a combination of empirically known energy levels and predictions from the new POKAZATEL variational line list. 2030 of the observed lines are assigned to 2937 transitions, once blends are taken into account. 126 new energy levels of H216O are identified. The assigned transitions belong to 136 bands and span rotational states up to J=27.

Determination of rate parameters based on NH2 concentration profiles measured in ammonia-doped methane–air flames

The experiments of Rahinov et al. (Combust. Flame, 145 (2006) 105–116) measuring NH2 concentration profiles in methane–air laminar flat flames doped with ammonia were re-evaluated. The flames were simulated with the FlameMaster code using a modified NOx combustion mechanism of the CRECK Modeling Group. Based on local sensitivity analysis results, Arrhenius parameters A, n, E of reaction steps NH2 + H = NH + H2 and NH3 + OH = NH2 + H2O were selected for optimization, which took into account not only the experimental data of Rahinov et al., but also related direct measurements and theoretical determinations as optimization targets. The optimized mechanism described the measured concentration profiles better than the original one, while the new rate parameter values were within the prior uncertainty limits obtained from the evaluation of literature data. The optimization process also provided new posterior uncertainty limits, which are within the prior uncertainty limits.

Combustion characteristics of well-dispersed boron submicroparticles and plasma effect

Boron is an attractive high-energy fuel additive. But it could not burn efficiently in practical systems due to its high ignition temperature and slow burning velocity. Finding methods to enhance the combustion of boron is desired. This work focused on the combustion characteristics of boron submicroparticles with and without plasma discharges in a hot environment supported by CH4/N2/O2 flat flame based on the optical diagnostics. The boron submicroparticles were dispersed by the nebulization method to control the agglomeration. The well-dispersed boron flame exhibited two different burning modes, depending on the ambient temperature. As the ambient temperature was above 1520 K, the boron flame showed definitely two-stage characteristics where the upstream of particle flow was yellow, corresponding to the first-stage flame, while the downstream was green and diffusive, corresponding to the second-stage flame. The first-stage and second-stage burn times were respectively in the range of 0.46–1.08 ms and 0.92–1.87 ms, as the ambient temperature decreased from 1752 K to 1520 K. The chemical kinetics-controlled mechanism was confirmed by the nearly linear size dependence of the burn time (d1 law). Nevertheless, as the ambient temperature was below 1520 K, the boron submicroparticles were partially burned or oxidized, exhibiting a mildly orange stream. This mild boron flame could be enhanced using a plasma discharge. The ignition delay time was shortened from 3.06 ms to 0.77 ms when the discharge was introduced at the ignition delay stage. The two-stage combustion characteristics occurred when the discharge was introduced at the combustion stage.

Experimental observation of pulsating instability under acoustic field in downward-propagating flames at large Lewis number

According to previous theory, pulsating propagation in a premixed flame only appears when the reduced Lewis number, β(Le-1), is larger than a critical value (Sivashinsky criterion: 4(1 +3) ≈ 11), where β represents the Zel'dovich number (for general premixed flames, β ≈ 10), which requires Lewis number Le > 2.1. However, few experimental observation have been reported because the critical reduced Lewis number for the onset of pulsating instability is beyond what can be reached in experiments. Furthermore, the coupling with the unavoidable hydrodynamic instability limits the observation of pure pulsating instabilities in flames. Here, we describe a novel method to observe the pulsating instability. We utilize a thermoacoustic field caused by interaction between heat release and acoustic pressure fluctuations of the downward-propagating premixed flames in a tube to enhance conductive heat loss at the tube wall and radiative heat loss at the open end of the tube due to extended flame residence time by diminished flame surface area, i.e., flat flame. The thermoacoustic field allowed pure observation of the pulsating motion since the primary acoustic force suppressed the intrinsic hydrodynamic instability resulting from thermal expansion. By employing this method, we have provided new experimental observations of the pulsating instability for premixed flames. The Lewis number (i.e., Le ≈ 1.86) was less than the critical value suggested previously.

A Study of Two Dimensional Tomography Reconstruction of Temperature and Gas Concentration in a Combustion Field Using TDLAS

Based on tunable diode laser absorption spectroscopy (TDLAS), two-dimensional (2D) distribution reconstructions of gas concentration and temperature are realized using an algebraic reconstruction technique (ART). The influence of the beam distribution and grid size on combustion field reconstruction is investigated to attain optimal reconstruction results with a limited number of beams. Under limited optical-path numbers, it shows that a better spatial resolution is attainable only when the laser beam paths are vertical and parallel to the symmetry axis of the combustion field. Furthermore, experiments with 16 beam paths using one and two flat flame combustion fields are carried out in different fuel-air equivalence ratios under room temperature. The results are in agreement with the simulation results, and the time resolution is less than 1 s.

Comparison of Measurement Techniques for Temperature and Soot Concentration in Premixed, Small-Scale Burner Flames

Optical and intrusive measurement techniques for temperature and soot concentration in hot reacting flows were tested on a small-scale burner in fuel-rich, oxygen-enriched atmospheric flat flames produced to simulate the environment inside an entrained flow reactor. The optical techniques comprised two-color pyrometry (2C-PYR), laser extinction (LE), and tunable diode laser absorption spectroscopy (TDLAS), and the intrusive methods included fine-wire thermocouple thermometry (TC) and electrical low pressure impactor (ELPI) particle analysis. Vertical profiles of temperature and soot concentration were recorded in flames with different equivalence and O2/N2 ratios. The 2C-PYR and LE data were derived assuming mature soot. Gas temperatures up to 2200 K and soot concentrations up to 3 ppmv were measured. Close to the burner surface, the temperatures obtained with the pyrometer were up to 300 K higher than those measured by TDLAS. Further away from the burner, the difference was within 100 K. The TC-derived t...

Simultaneous measurement of the adiabatic flame velocity and overall activation energy using a flat flame burner and a flame asymptotic model

This work presents a method for the simultaneous measurement of the adiabatic flame velocity and the overall apparent activation energy from measurements taken at a single unburned gas temperature. This is achieved using a McKenna flat flame burner and measuring the heat loss from the flame region for different mixture flow velocities. Then, the measurements of laminar flame velocity in the presence of heat loss are curve-fitted to a flame asymptotic model assuming one-step reaction, providing the estimated adiabatic flame velocity and the corresponding overall activation energy. Care is taken to establish clear cut conditions to reduce the effect of flame instabilities in the measurements. Then, an orthogonal regression method is used for the curve fitting and to precisely assess the uncertainties in the adiabatic flame velocity and overall activation energy. The method was tested with mixtures of methane and air, for equivalence ratio between 0.75 and 1.05, 298K, 1bar. The values of adiabatic flame velocity and apparent overall activation energy obtained at stoichiometric mixture were 37.8±0.4cm/s and 271±4kJ/mol, respectively, which are in good agreement with values from the literature, including estimates using GRIMech 3.0.

Assessment of radiation correction methods for bare bead thermocouples in a combustion environment

The use of bare bead thermocouples for temperature measurement in combustion environments is widespread due to the low cost, robustness and ease of use of such a type of device. It is however well known that temperatures monitored with thermocouples can be significantly affected by radiation errors. This in turn implies to properly estimate the energy losses induced by this phenomenon so as to get a consistent estimate of the true temperature of the investigated medium. The present paper thus aims to rule on the relative efficiency of four different compensation techniques including the electrical compensation (EC), the reduced radiative error (RRE), the extrapolation and the multi-element ones. To do so, an original set of temperature measurements carried out in a fully characterized flat flame reactor has been acquired. The comparison of the above-listed correction approaches has moreover been supported by comprehensive sensitivity analyses dealing with the influence of parameters like the composition of the surrounding atmosphere, the local velocity, the Nusselt number correlation or the emissivity of the thermocouple junction on the predicted radiation corrections. The main conclusions drawn through this study coupling experimental approaches and theoretical calculations show that EC and RRE methods can lead to globally converging trends provided that some thermo-physical parameters integrated into the calculation of the RRE correction coefficient are correctly assessed. The extrapolation approach tends to predict radiation losses similar to those derived from the EC and RRE techniques while the multi-element strategy significantly diverges from the other ones probably due to its very important sensitivity to raw temperature values.

Effects of hydrogen addition on laminar flame speeds of methane, ethane and propane: Experimental and numerical analysis

The main purpose of this study is to investigate the effects of hydrogen addition on the laminar flame speeds of methane, ethane and propane. In this work, a flat flame method was used to measure the laminar flame speed in a counter-flow configuration combined with particle image velocimetry (PIV) system. The results indicate that with the increase of hydrogen amount, the laminar flame speeds of methane, ethane and propane increase linearly approximately. In addition, as hydrogen is increased, the flame speed of methane has the maximum increasing amplitude among them, which indicates that methane is more sensitive to hydrogen addition in flame speed than the other two fuels.Simulation analysis finds that the reaction R1: H + O2 ⇔ OH + O can promote the flame speeds of these three kinds of gaseous fuel obviously, and with the increase of hydrogen amount, the promoting effect is more obviously. Therefore, the main reason why hydrogen addition could increase flame speed is that the increase of H radical prompts reaction R1 to proceed in the forward direction. Comparing the flames of methane, ethane and propane mixed with hydrogen, it was found that the promotion of reaction R1 to the methane/hydrogen mixtures flame speed is strongest, and its free radicals concentration in flame increase more obviously. Therefore, hydrogen addition has a greater effect on the flame speed of methane than on that of ethane and propane.

Effects of potassium and calcium on the early stages of combustion of single biomass particles

The main objective of this work is to evaluate the effects of potassium (K) and calcium (Ca) on the early stages of combustion of single biomass particles. The biomass used was grape pomace, sieved in the size range of 200–250µm. With the pre-treatments of demineralization and impregnation, a total of 12 different samples were obtained: raw grape pomace, demineralized grape pomace and impregnated grape pomace with 0.1, 0.5, 0.82, 3 and 6wt% of K, and 0.1, 0.5, 1.08, 3 and 6wt% of Ca. The tests were performed in an optical flat flame McKenna burner able to produce a confined laminar flow of combustion products, in which single particles were air-injected upward through a central hole. The equivalence ratio and the thermal input of the burner were adjusted to yield two operating conditions in the working zone: condition T1 with a mean temperature of 1575K and a mean dry O2 concentration of 5.4vol%, and condition T2 with a mean temperature of 1775K and a mean dry O2 concentration of 5.2vol%. A CMOS high-speed camera was used to record the early stages of the combustion process, particularly the ignition and the volatiles combustion events. The collected images were processed to calculate the ignition delay time and the volatiles combustion time. The results obtained showed that the demineralization pre-treatment used increased both the ignition delay time and the volatiles combustion time of the single biomass particles. The K impregnation pre-treatment led to a decrease in the ignition delay time as the concentration of K increased, while the Ca impregnation pre-treatment did not have a significant impact on the ignition delay time. Both impregnation pre-treatments decreased the volatiles combustion time as the concentration of K or Ca increased. Finally, the impregnation with K and Ca had a more significant impact on the volatiles combustion time than in the ignition delay time.

Laminar burning velocities of low calorific and hydrogen containing fuel blends

The laminar burning velocity is a fundamental property of a reactive fuel-oxidizer mixture, varying with composition, pressure and initial temperature. These values are important for validation of reaction mechanisms and the specific design of industrial burners. There are several experimental methods to measure laminar burning velocity, e.g. the bunsen flame method, the spherically expanding flame method, the stagnation flame method and the flat flame burner method, which also includes the heat-flux burner method. The accuracy of the different methods could be enhanced over the last years but there are still uncertainties of up to 30 % depending on method, boundary condition and fuel composition. Furthermore, for a lot of fuels, especially fuel blends, there is a lack of data.In this study, the heat-flux burner method was applied to measure laminar burning velocities of low calorific fuels and hydrogen containing fuel blends. These fuel blends are of major interest since the hydrogen concentration in the gas grid system could in future to a significant value. For example, within the “power-to-gas” concept, some new technologies arise to use renewable energy to produce hydrogen and take it as “fuel”. On the other hand, the change of the gas composition also changes the combustion properties, for example burning velocity, heating value and ignition delay. Therefore, different low calorific fuels and hydrogen containing fuel are tested within a range of equivalence ratios from 0.7 to 1.4 for initial temperatures of 298 K up to 363 K for atmospheric conditions.

Laser-Induced Fluorescence Investigations for Temperature Measurements in a Carbon Dioxide Flow

The objective of the presented work was the investigation of two-photon laser-induced fluorescence excitation spectra of carbon monoxide (CO) molecules (B-X) in a carbon dioxide () flow with an enthalpy of . The flow was generated with an arcjet-driven plasma wind tunnel. For validation, different stoichiometric premixed flat flames and a low-pressure cold-gas cell filled with CO have been examined. For the excitation of the Q-branch of the transition, the laser was tuned from 230.04 to 230.11 nm, with a step width of 0.0005 nm. Simulated spectra were calculated in advance with a simulation tool called NOCO-Spectra and showed good agreement with the experimental data. Finally, the rotational temperature of CO in the flow was evaluated by comparison of the CO-excitation spectra and the calculated spectra using a correlation automated rotational fitting procedure.

Hydrogen/air burner-stabilized flames at elevated pressures

In spite of the progress made in understanding of the kinetics of hydrogen combustion there is a demand for further investigation of the detailed aspects of the flame-wall interaction under elevated pressure and under conditions of different wall and gas temperatures, local equivalence ratios etc. The burner stabilized flame configuration can be efficiently used to study different aspects of chemical kinetics by varying the stand-off distance, pressure, temperature of the burner and mixture compositions. Moreover, the analysis of the unsteady dynamics of the burner stabilized flames can give a deep insight into transient processes in flames. In the current study, a flat porous plug burner flame configuration is revisited and investigated numerically to study combustion systems at high pressures, low temperatures and at near stationary limits. A hydrogen/air combustion system is considered with detailed molecular transport including thermo-diffusion and with variety of different chemical reaction mechanisms. The loss of stability, dynamics of the flame oscillations, effects of the mixture composition and of the burner temperature are in the focus of the study. A dependence of critical values of the mass flow rate on system parameters is investigated in detail. In particular, it was found that the results of critical values for different mechanisms with increasing the pressure scatter almost randomly i.e. these parameters can be considered as independent.

Investigation of the size of the incandescent incipient soot particles in premixed sooting and nucleation flames of n-butane using LII, HIM, and 1 nm-SMPS

ABSTRACTLaser-induced incandescence (LII) measurements were conducted to explore the ability of LII to detect small soot particles of less than 10 nm in two sooting flat premixed flames of n-butane: a so-called nucleation flame obtained at a threshold equivalence ratio Φ = 1.75, in which the incipient soot particles undergo only minor soot surface growth along the flame, and a more sooting flame at Φ = 1.95. Size measurements were obtained by modeling the time-resolved LII signals detected using 1064 nm laser excitation. Spectrally-resolved LII signals collected in the nucleation flame display a similar blackbody-like behavior as mature soot. Soot particle temperature was determined from spectrally-resolved detection. LII modeling was conducted using parameters either relevant to those of mature soot or derived from fitting the modeled results to the experimental LII data. Particle size measurements were also carried out using (1) ex situ analysis by helium-ion microscopy (HIM) of particles sampled thermo...