Syngas-air Flames Research Articles

Effect of copper foam location on syngas-air explosion in a closed duct

Experiments were conducted in a closed duct filled with copper foam to study the premixed syngas-air flame propagation behaviors. A copper foam with a thickness of 20 mm is installed at different distance from the ignition end (CFLs) respectively. The stoichiometric syngas-air with different hydrogen volume fractions in syngas (α(H2)s) is employed. Results show that the setting of copper foam does not affect the flame evolution process before the flame tip reaches the copper foam. The flame shape observed when the flame tip reaches the copper foam is closely related to the distance between the copper foam and igniter. The presence of copper foam can contribute to aggravate the chaos of the flame front. In most cases, the flame accelerates after passes through the copper foam only except for α(H2) = 10%, along with rise and oscillation of the flame velocity and overpressure. The flame acceleration is enhanced with the increase of α(H2) but decrease of CFL. In general, the copper foam may be more dangerous when it is closer to the ignition end and the α(H2) is high.

Flame propagation mode transition of premixed syngas-air mixtures in a closed duct

Knowledge regarding the transformation of the flame propagation dynamics is of importance to predict the safety issues of explosion disasters. The current work experimentally investigated the premixed syngas-air flame propagating in a 2 m duct with a set of obstacles, focusing on the flame propagation mode transition with the change of hydrogen volume fraction (φ). Three modes were identified as the φ increases: steady flame propagation, oscillating flame propagation, and end-gas autoignition. The steady flame propagation mode occurred when φ = 0, where the slow oxidation of pure CO determines its propagation feature. The flame became unsteady and oscillated violently with a slight addition of hydrogen. Flame oscillation resulted from its interaction with pressure wave, and the strength of the pressure wave directly impacted the severity of oscillation. The autoignition appeared when φ ≥ 0.4, resulting from the secondary reflected pressure wave interacted with the region that the thermodynamic state was highly disturbed. What is more, the autoignition kernel migrated from the region ahead of the flame front to the end-wall region when φ ≥ 0.6. The reason is that syngas with higher φ has a lower temperature threshold for explosion and a shorter ignition delay.

An experimental study on the oscillation of the propagating syngas-air flame in a duct

In this paper, premixed syngas-air flame propagating from the open end to the closed end were experimentally investigated. The effects of equivalence ratios, 0.8 ≤ Ф ≤ 1.2, and hydrogen volume fractions, 10% ≤ α(H2) ≤ 90%, on flame deformation and oscillation had been discussed in detail. The tulip-like flame was observed because of the large pressure gradient. Results indicate that the pressure wave plays an important role in the flame deformation and oscillation. The flame oscillates as hydrogen volume fraction varies. There are two oscillation modes. When the flame oscillates as mode Ⅰ, the flame first oscillates smoothly, then the oscillation is gradually enhanced, and finally the oscillation decays. The interaction of flame and pressure waves continuously stimulates the flame deformation and oscillation, finally the violent flame folding emerges in the later stage. When the flame oscillates as mode Ⅱ, the flame just oscillates violently in the early stage.

Validation of leading point concept in RANS simulations of highly turbulent lean syngas-air flames with well-pronounced diffusional-thermal effects

While significant increase in turbulent burning rate in lean premixed flames of hydrogen or hydrogen-containing fuel blends is well documented in various experiments and can be explained by highlighting local diffusional-thermal effects, capabilities of the vast majority of available models of turbulent combustion for predicting this increase have not yet been documented in numerical simulations. To fill this knowledge gap, a well-validated Turbulent Flame Closure (TFC) model of the influence of turbulence on premixed combustion, which, however, does not address the diffusional-thermal effects, is combined with the leading point concept, which highlights strongly perturbed leading flame kernels whose local structure and burning rate are significantly affected by the diffusional-thermal effects. More specifically, within the framework of the leading point concept, local consumption velocity is computed in extremely strained laminar flames by adopting detailed combustion chemistry and, subsequently, the computed velocity is used as an input parameter of the TFC model. The combined model is tested in RANS simulations of highly turbulent, lean syngas-air flames that were experimentally investigated at Georgia Tech. The tests are performed for four different values of the inlet rms turbulent velocities, different turbulence length scales, normal and elevated (up to 10 atm) pressures, various H2/CO ratios ranging from 30/70 to 90/10, and various equivalence ratios ranging from 0.40 to 0.80. All in all, the performed 33 tests indicate that the studied combination of the leading point concept and the TFC model can predict well-pronounced diffusional-thermal effects in lean highly turbulent syngas-air flames, with these results being obtained using the same value of a single constant of the combined model in all cases. In particular, the model well predicts a significant increase in the bulk turbulent consumption velocity when increasing the H2/CO ratio but retaining the same value of the laminar flame speed.

A Three-step Global Kinetic Mechanism for Predicting Extinction Strain Rate of Syngas-air Nonpremixed Flames

ABSTRACT A systematic survey of chemical kinetic mechanisms (both simplified and global) shows an absence of a robust kinetic mechanism that can accurately predict characteristics of syngas-air nonpremixed flames. In this work, a three-step global kinetics mechanism is proposed for predicting the extinction strain rate (ag ) of syngas-air nonpremixed flames. The available experimental ag data-set from the Tsuji type burner was used to evaluate the performance of six syngas kinetics models (four simplified and two global). For this, 2D-planar computations were performed to obtain ag values using four CO/H2 ratios (1, 3, 5, and 29). The results showed that all the simplified kinetics mechanisms overpredicted the ag values in the range of 16–224% for the four CO/H2 ratios. In the case of the global kinetics models namely, CFFR and WO, except for CO/H2 ratio of 29, CFFR underpredicted ag values by 22–55%. WO underpredicted ag by 10–44% for CO/H2 = 1 and overpredicted ag by 19–58% for CO/H2 = 3 and 5. For CO/H2 = 29, WO overpredicted ag values by 151–205%. For CO/H2 = 1, WO underpredicted ag due to two reasons, (1) double accounting of CO hydroxyl oxidation path and, (2) suppression of H2 oxidation pathways in four-step global JL mechanism, a source for the construction of WO mechanism. From the overall perceptive, WO has performed better compared to other mechanisms. Hence, WO was chosen for further optimization using experimental ag data as a target. Sensitivity analysis in terms of ag and maximum flame temperature (TF ) was performed for WO using two CO/H2 ratios (1 and 5). It was found that for CO/H2 = 1 and 5, ag is most sensitive to Rxn1 (H2 + 0.5O2↔ H2O) and Rxn2 (CO + 0.5O2 → CO2). Results from sensitivity analysis, 0D (PSR) computations, and 2D computations have shown that the improper quantification of CO and H2 oxidation pathways leads to inaccurate ag prediction by the WO model. Based on these new insights, a modified three-step WO syngas kinetics model (WO-Opt) is proposed for predicting the extinction strain rate of syngas-air nonpremixed flames. The WO-Opt model was validated against ag experimental data. Compared to the WO model, ag predictions obtained from the modified WO-Opt model were in good agreement with the syngas-air nonpremixed flames experiments.

Effects of Differential Diffusion on the Stabilization of Unsteady Lean Premixed Flames Behind a Bluff-Body

Two-dimensional direct numerical simulations were conducted to investigate the effects of differential diffusion on flame stabilization and blow-off dynamics of lean premixed hydrogen–air and syngas–air flames stabilized on a meso-scale bluff-body in a square channel. The unity Lewis number for all species was imposed to isolate the effects of differential diffusion. Four sets of simulation cases were conducted. Two different inflow temperature with unity Lewis number were applied to examine distinct levels of hydrodynamic instability. Each unity Lewis number case was compared with the non-unity Lewis number case to investigate how differential diffusion affects the overall flame responses, instabilities, and blow-off mechanism. For all cases, the overall flame dynamics were observed in several distinct modes as the inflow velocity approaches blow-off limit. One of the primary effects of unity Lewis number was an increased level of hydrodynamic instability due to the lower flame temperature and thus a lower density ratio. The lower gas temperature also led to a weakening of the re-ignition of the quenched local mixture by the product gas entrainment. The combined effects were manifested as suppression of the re-ignition events, leading to a revised conclusion that the ultimate blow-off behavior at high velocity conditions are mainly controlled by the onset of local extinction.

Effects of obstacle position and hydrogen volume fraction on premixed syngas-air flame acceleration

There have been many studies concerning on the influence of obstacles on gas explosion, but the understanding of some phenomena is still not clear, such as explosion overpressure or the change of flame velocity with the position of obstacles. This work aims to figure out the coupling effects of obstacle position (L1, L2, L3 from near to far) and hydrogen volume fraction (φ) on syngas-air flame acceleration and overpressure variation in a closed duct. Detailed flame structure evolution in the acceleration process was visualized by a high-speed camera, and overpressure was recorded by pressure transducers. Results show that the effect of obstacles moving backward is not always monotonous on the flame acceleration, which affects the maximum flame speed and maximum overpressure. Such as when φ = 0.5, the flame acceleration that is reflected as the flame propagation speed normalized by the laminar flame speed, ranks as L2 > L3 > L1. Because the obstacles strengthened turbulence on the tulip- or distorted tulip-shaped flame in L2. While when φ = 0.1, the flame acceleration ranks as L1 > L2 > L3 since the effect of turbulence is reduced. It is found that maximum overpressure is related to both flame acceleration and local overpressure. Local overpressure is the transient overpressure before the flame front touches obstacles. Obstacles moving backward is beneficial for local overpressure accumulation. Hence, the maximum overpressure increases with obstacle position moving backward when φ ≥ 0.1. This study clarifies the role of obstacle position and hydrogen volume fraction on flame acceleration, which may help to guide the application of syngas in a complex environment.

A comparative investigation of premixed flame propagation behavior of syngas-air mixtures in closed and half-open ducts

A comparative investigation has been developed in conjunction with high-speed cameras and piezoelectric gauge to gain deep insight into the propagation behavior of premixed syngas-air flames. Experiments were carried out in two constant rectangular ducts, one closed duct (C-D) and one half-open duct (HO-D). One important finding is that the flame behavior in the C-D is different than that in the HO-D. The repeatable distorted tulip flame forms in the C-D while an unrepeatable distorted tulip shape forms in the HO-D. The repeated pressure-flame interactions result in a repetitive process of the tulip distortion. The downstream opening end of the duct has a significant influence on the flame propagation characteristics after the flame surface reaches the lateral walls. The opening end results in a higher maximum flame tip velocity, a lower overpressure and a longer plane formation time in the HO-D. Meanwhile, the position of the plane flame formation decreases as hydrogen fraction increases in the C-D while the opposite is true in the HO-D. Furthermore, the pressure build-up in both ducts are examined, and the maximum flame tip velocity is of importance for distorted tulip flame formation.

Laminar syngas–air premixed flames in a closed rectangular domain: DNS of flame propagation and flame/wall interactions

The propagation of a lean laminar premixed syngas–air flame is investigated numerically in a confined rectangular domain with isothermal walls at a temperature that is lower than that of the unburned mixture. Initiated by a hot kernel, the flame propagates towards the cold walls, setting in motion the initially quiescent mixture and compressing the gases, so that propagation proceeds under varying flow and thermodynamic conditions. The complex flow field changes structure and its effect on the local propagation characteristics is quantified by following the local displacement speed together with the local flow velocity and stretch rate. The flame first quenches head-on at the horizontal walls and then propagates towards the lateral walls affected by side-wall quenching. During the final stage, thermodiffusive instabilities are triggered along the front whose thickness has become half of the initial mainly because of the increased pressure. The temporal evolution of the quenching distances, the wall heat flux distribution and the heat transfer to the cold walls are quantified during the whole process. Except for the stages of the initial flame kernel growth and the final consumption of the fuel in the near-wall region, the fuel is consumed at an almost constant rate.

Laminar burning velocity and ignition delay time for premixed isooctane–air flames with syngas addition

The effects of blending syngas in different proportions to isooctane on the laminar burning velocity and ignition delay time of the fuel–air mixture have been studied in SI engine relevant conditions. The syngas is assumed to be composed of 50% H2 and 50% CO. Simulations have been carried out using a skeletal mechanism containing 143 species and 643 reaction steps. It has been found that the blending of syngas augments the laminar burning velocity of isooctane due to increase of the thermal diffusivity of the reactant mixture and alteration in the chemistry of the flame reactions. For the mixture of 30% isooctane/70% syngas, the laminar burning velocity and the ignition delay time values are very close to those corresponding to pure isooctane. Additionally, the effects of exhaust gas recirculation have been explored for the 30% isooctane/70% syngas–air flame. It is seen that the reduction in laminar burning velocity due to the dilution by the recirculated exhaust gas can be compensated by an increase in the unburnt gas temperature. The effect of the exhaust gas dilution on the ignition delay time of 30% isooctane/70% syngas–air mixture has been found to be negligible.

Effects of the Equivalence Ratio and Reynolds Number on Turbulence and Flame Front Interactions by Direct Numerical Simulation

Direct numerical simulation (DNS) of a two-dimensional premixed turbulent syngas flame (50% H2 and 50% CO) was performed to investigate turbulence–flame interactions. An in-house DNS code, based on the low-Mach-number version of Navier–Stokes equations, was used to predict the influences of the equivalence ratio and Reynolds number on the turbulence–flame interactions using the constant volume enclosure method. The reaction between syngas and air was modeled with the Davis mechanism, which includes 14 species and 38 elemental reactions, and the species had distinct Lewis numbers. Syngas–air flames at combinations of four equivalence ratios and three Reynolds numbers were investigated. The flame structures at turbulence time scales were analyzed to demonstrate the influence of the turbulence on the flame structure, such as localized deformation of the flame front and isolated pockets of flame. The phenomenon was based on real effects detected by planar laser-induced fluorescence imaging of premixed syngas ...

Numerical simulations of steady perforated-plate stabilized Syngas air pre-mixed flames

Numerical investigations of steady, laminar premixed Syngas-air flames are presented in this paper. Three-dimensional simulations were performed to examine the impact of operating conditions on steady state characteristics of perforated burner flame. A detailed H2CO reaction mechanism having 12 species and 38 reactions was used for combustion modelling. The three dimensional simulation results are validated against the 1D flat flame result using PREMIX. Effects of inlet velocity, fuel composition and equivalence ratio on flame stability were examined. A clearly identified recirculation zone was present above the top surface of the burner plate in the case of 50% H2-50% CO Syngas mixture. The strength recirculation zone was diminishing with the increase in percentage of hydrogen in the Syngas mixture, and the flame has stabilized closer to the top surface of the burner plate. The flame stand-off distance is found to decrease with increase in inlet velocity. Effect of increase in H2 fraction in Syngas has less effect on flame height at higher H2 fractions.

Experimental investigation of the stability limits of premixed syngas-air flames at two moderate swirl numbers

This article presents an investigation of the swirl number effects on the stability limits of various syngas compositions in an atmospheric premixed variable-swirl burner. Lean blowout (LBO) and flashback (FB) experiments were performed using fuel mixtures containing varying amounts of H2, CO and CH4 at two swirl numbers. Reducing the swirl number from 0.66 to 0.53, reduced the flashback propensity of various syngas/air mixtures but it did not affect the LBO limits considerably. The flow-field in the combustor was studied at S = 0.66 and 0.53 using high-speed particle image velocimetry (PIV) for a mixture of H2/CH4 (50:50) at various equivalence ratios. At both swirl numbers, for the non-reacting flow and low equivalence ratios the flow-field consisted of an inner recirculation zone at the entrance of the combustor, an annular high velocity zone and an outer recirculation zones between the high-velocity zone and the bounding walls. Increasing the equivalence ratio towards the flashback limit, had varying effects on the flow-field, depending on swirl number. At S = 0.66, increasing the equivalence ratio did not have a significant effect on the general features of the flow-field. At S = 0.53 the flow-field consisted of an inner recirculation zone at low equivalence ratios, but as the equivalence ratio was increased, the high velocity zone extended radially towards the center and the recirculation zone disappeared from the flow-field. The high-speed OH*-chemiluminescence images recorded at the onset of flashback revealed significant differences between S = 0.66 and S = 0.53 in the flame stabilization mechanism prior to flashback and flame propagation in the premixing tube. Considering the velocity measurements together with the OH*-chemiluminescence images, it was concluded that at S = 0.66 flashback was caused by CIVB mechanism whereas at S = 0.53 flashback was initiated by the competition between the flame speed and flow velocity in the core flow.

The Scalar Structure of Turbulent Oxy-fuel Non-premixed Flames

This study addresses the scalar structure of turbulent oxy-fuel syngas non-premixed jet flame in the context of direct numerical simulation (DNS) and detailed chemistry. The main objective is to identify the influence of the Reynolds number on oxy-fuel flame structure, and to clarify the differences of scalar structures between oxy-fuel syngas and syngas-air flames at a similar higher Reynolds number. Two oxy-syngas flames at Reynolds numbers of 3000 and 6000 and one syngas-air flame at Reynolds number of 6000 were simulated. These studies show that the existence of CO2 in the oxidant has a profound effect on scalar structure of oxy-fuel syngas flame compared to syngas-air flame.

Flame extinction in interacting CO-air and syngas-air premixed flames

Chemical interaction on flame extinction in downstream interaction of syngas-air and CO-air premixed flames was investigated. Flame stability diagrams mapping the extinction limits for various fuel concentrations as functions of the strain rate were developed for the downstream interactions between syngas-air and CO-air premixed flames. Of particular concern was the impact of the chemical interaction on the flame extinction characteristics. The results showed that the lean extinction boundary was precipitously slanted for extremely small amount of syngas, finally causing the extinction boundary to open up in further increase of strain rate. There existed a critical syngas concentration below which the flame could not be sustained and above which the extinction boundary was double-valued. On the upper branch, hydrogen was produced via the reaction steps CO + H + M → HCO + M and HCO + H → H2 + CO near the highly rich CO-air premixed flame to participate in CO-oxidation. For highly stretched interacting flames, only a flammable island existed. The pure syngas-air diffusion flame was extinguished at a critical strain rate. Further increase of strain rate forced the flammable island to be shrunk and finally to be a point. For highly stretched interacting flames, a minimum CO concentration existed below which the flame could not be survived. In such flames, the diffusion flame can act as a parasite to the lean CO-air premixed flame through sharing hydrogen penetrated from the rich syngas-air premixed flame. Such chemical interactions were examined in detail to describe the flame extinction characteristics.

Research on Cellular Instabilities of Lean Premixed Syngas Flames under Various Hydrogen Fractions Using a Constant Volume Vessel

An experimental study of the intrinsic instabilities of H2/CO lean (φ = 0.4 to φ = 1.0) premixed flames at different hydrogen fractions ranging from 0% to 100% at elevated pressure and room temperature was performed in a constant volume vessel using a Schlieren system. The unstretched laminar burning velocities were compared with data from the previous literature and simulated results. The results indicate that excellent agreements are obtained. The cellular instabilities of syngas-air flames were discussed and critical flame radii were measured. When hydrogen fractions are above 50%, the flame tends to be more stable as the equivalence ratio increases; however, the instability increases for flames of lower hydrogen fractions. For the premixed syngas flame with hydrogen fractions greater than 50%, the decline in cellular instabilities induced by the increase in equivalence ratio can be attributed to a reduction of diffusive-thermal instabilities rather than increased hydrodynamic instabilities. For premixed syngas flames with hydrogen fractions lower than 50%, as the equivalence ratio increases, the cellular instabilities become more evident because the enhanced hydrodynamic instabilities become the dominant effect. For premixed syngas flames, the enhancement of cellular instabilities induced by the increase in hydrogen fraction is the result of both increasing diffusive-thermal and hydrodynamic instabilities.

Near-field local flame extinction of oxy-syngas non-premixed jet flames: A DNS study

An investigation of the local flame extinction of H2/CO oxy-syngas and syngas-air nonpremixed jet flames was carried out using three-dimensional direct numerical simulations (DNS) with detailed chemistry by using flamelet generated manifold chemistry (FGM). The work has two main objectives: identify the influence of the Reynolds number on the oxy-syngas flame structure, and to clarify the local flame extinction of oxy-syngas and syngas-air flames at a higher Reynolds number.Two oxy-syngas flames at Reynolds numbers 3000 and 6000 and one syngas-air flame at Reynolds number 6000 were simulated. The scattered data, probability density function distributions and fully burning probability provide the local flame characteristics of oxy-syngas and syngas-air nonpremixed jet flames. It is found that the H2/CO oxy-syngas flame burns well compared to the syngas-air flame and the high Reynolds number causes more flow straining, resulting in higher scalar dissipation rates which lead to lower temperatures and eventually local flame extinction. The oxy-syngas flames burns more vigorously than the syngas-air flame with the same adiabatic flame temperature of approximately 2400K.

Multi-zone modeling and simulation of syngas combustion under laminar conditions

The use of laminar models has been generally accepted, as quench distances in engines and the distances obtained for laminar flame quench calculations could be well correlated. In this work a multi-zone model is presented in order to derive the quenching distance and heat flux in laminar syngas–air flames based on recent developments in the science of combustion. Three typical mixtures of H2, CO, CH4, CO2 and N2 have been considered as representative of the syngas coming from wood gasification, and its laminar combustion is made in a static spherical vessel. The model is validated for the methane–air case and then applied to syngas–air mixtures in order to estimate the heat flux to the walls and quenching distances. Two wall heat transfer models are implemented and compared. The classical Woschni model based on the hypotheses of forced convection and the Rivère model based on kinetic theory of gases. Conclusion could be drawn that the Rivère heat transfer model is capable to better reproduce the heat flux to the walls. Heat flux through the walls is higher for stoichiometric syngas–air mixtures which follows the same behavior of the pressure inside the combustion vessel. Quenching distance of syngas–air mixtures decreases with the heat flux increase, which is consistent with earlier studies. This model could be very useful in predicting the physical conditions of quenching especially for estimation of the quenching distance where the measurement is not possible such as in engines. However, the estimation given should be understood as an order of magnitude, because in turbulent conditions the flame–wall interaction results in lower Peclet numbers than in the laminar case.

Downstream interaction between stretched premixed syngas–air flames

Effect of strain rate on flame extinction is numerically investigated in downstream interaction among lean (rich) and lean (rich) premixed as well as partially premixed (50% H2+50% CO)–air flames. The strain rate varies from 30 to 4671s−1 until interacting flames cannot be sustained anymore. Flame stability diagrams mapping lower and upper limit fuel concentrations for flame extinction as a function of strain rate are explored. It is shown that significant increase of strain rate makes even the upper rich extinction boundary be slanted due to strong chemical interaction. The lower extinction boundary can be also extended to rich fuel concentrations over the stoichiometric mixture condition when strain rate significantly increases. The lower and upper extinction boundaries, mainly caused by the conductive heat loss from the stronger flame to ambience, become narrower and narrower in increasing strain rate. The results also show that the extinction boundaries with positive flame speed are extended and then reduced in increasing strain rate, thereby leading to an island of extinction boundary and subsequently being changed into a point on the symmetric fuel concentration line. The detailed explanations on those flame extinction characteristics in the stability diagrams are made through analysis of the flame structures in increase of strain rate. The mechanism of flame extinction is also proposed and discussed in detail.

Effects of preferential diffusion on downstream interaction in premixed H2/CO syngas–air flames

Effects of strain rate and preferential diffusion of H2 on flame extinction are numerically explored in interacting premixed syngas–air flames with the fuel compositions of 50% H2 + 50% CO and 30% H2 + 70% CO. Flame stability diagrams mapping lower and upper limit fuel concentrations at flame extinction as a function of strain rate are examined. Increasing strain rate reduces the boundaries of both flammable lean and rich fuel concentrations and produces a flammable island and subsequently even a point, implying that there exists a limit strain rate over which interacting flame cannot be sustained anymore. Even if effective Lewis numbers are slightly larger than unity on the lean extinction boundaries, the shape of the lean extinction boundary is slanted even at low strain rate, i.e. ag = 30 s−1 and is more slanted in further increase of strain rate, implying that flame interaction on lean extinction boundary is strong and thus hydrogen (as a deficient reactant) Lewis number much less than unity plays an important role of flame interaction. It is also shown that effects of preferential diffusion of H2 cause flame interaction to be stronger on lean extinction boundaries and weaker on rich extinction boundaries. Detailed analyses are made through the comparison between flame structures with and without the restriction of the diffusivities of H2 and H in symmetric and asymmetric fuel compositions. The reduction of flammable fuel compositions in increase of strain rate suggests that the mechanism of flame extinction is significant conductive heat loss from the stronger flame to ambience.