Methane-air Counterflow Diffusion Flames Research Articles

The effects of steam and water spray on NO formation in a methane–air counterflow diffusion flame

This paper presents a numerical investigation of the effects of steam and water mist/spray addition on NO formation in a methane–air counterflow diffusion flame. A detailed chemical mechanism (Glarborg et al. Modeling nitrogen chemistry in combustion, Prog. Energy Combust. Sci. 67 (2018), pp. 31–68) which contains 1397 reactions and 151 species has been employed to investigate the NO formation in a counterflow diffusion flame for a wide range of strain rates (50–400 s) in OPPDIF. The water spray/mist evaporation is represented as a first-order dynamic process with Arrhenius type dependence on temperature that can be expressed in a form similar to a first-order chemical reaction. Instead the process is represented as a first-order dynamic process by invoking a hypothetical chemical species and a surrogate reaction . This approach allows handling detail chemistry and droplet evaporation at a reasonable computational cost. This model is validated with CFD simulation results where droplets are modelled using the Discrete Phase Modelling approach. The effects of steam and water mist on the flame temperature and NO formation have been examined. It is found that steam/water mist inhibits NO formation by affecting both thermal and prompt pathways. It is seen that the percentage contribution of prompt NO in overall NO production significantly decreases with increasing steam dilution. Water mist causes an additional evaporative heat loss and results in a drop in percentage contribution of thermal NO in overall NO formation.

The time dependent investigation of methane-air counterflow diffusion flames with detailed kinetic and pollutant effects into a micro/macro open channel

In the present study, time-dependent numerical analysis of methane-air counterflow diffusion flame into a selected macro/micro open channel is investigated. The flame is simulated by multi and single-step reaction approaches into an open channel with a constant distance of 15 mm between air and the fuel inlet, and a hydraulic distance at the order of 0.1 mm. To solve the unsteady problem, a coupled pressure-velocity implicit division method is considered. The results show an acceptable agreement between numerical and experimental data that confirm the accuracy of the model. The results also revealed that the variation of the residence time to the inlet velocity is more sensitive than the inlet temperature. It is also found that at the larger inlet velocities, the flame is stabilized at a smaller value of hydraulic distance. This is a result of increasing the possibility of reactions between species. The generation rates of CO2, CO and H2O species are found to be nearly constant at t > 0.009s while for NO and NO2 species the rates remain unchanged at t > 0.013s and t > 0.016s, respectively.

Pressure effects on real-gas laminar counterflow

Two important problems are studied: the combustion of hydrocarbons at higher pressures and the burning of hydrocarbon/water-vapor mixtures both of which are relevant to many applications including diesel combustion and hydrate fuels. To study both of these problems, a numerical analysis of a steady laminar methane-air counterflow diffusion flame at high pressure is presented. The mathematical model is based on the well-known similar solution for counterflow with special considerations given to the high density and to detailed transport and chemistry. Modifications of transport properties and associated time scales with increasing pressure are considered. Real gas behavior is examined through the use of a cubic equation of state and an enthalpy departure function. A more complete version of the energy equation is presented. The effects on flame structure, location, and peak temperature are analyzed for a range of pressure from 1 to 100 atm. Assessment of the different high-pressure corrections indicate that introduction of the cubic equation of state is the most profound adjustment, while the correction of transport properties is the least significant. The use of the enthalpy departure function is important. The flame structure and heat-release rate are confirmed to follow previously identified correlations with the pressure-weighted strain rate. Extinction occurs when the mass fraction of H2O vapor in the methane stream is greater than 67% and the mixture impinges against air. Small differences in results occur between the classical chemical equilibrium employing partial pressures versus the non-ideal formulation that uses fugacities. The Soret effect and radiative heat losses are shown to be negligible, even at the highest pressures. An order of magnitude analysis shows that turbulence generation is practically inconsequential.

Numerical Simulation on the Extinguishment of Methane-Air Counterflow Diffusion Flame by Water Mist

Numerical Simulation on the Extinguishment of Methane-Air Counterflow Diffusion Flame by Water Mist

Influence factors of methane-air counterflow diffusion flame

This paper investigates the influences of pressures, velocities, and temperatures of gases at nozzles on the temperature of flame. Considering that temperature and species mass fractions are functions of axial co-ordinates, a quasi-1-D mathematic model in cylindrical co-ordinates for counterflow diffusion flame is built. The results show that the pressure, velocities, and temperatures of gases can affect the temperature distributions of methane-air counterflow diffusion flame, and that the influence of the variations of velocities at two nozzles on the movement of the starting reaction interface is most significant in these factors.

Numerical Study of Counterflow Diffusion Flame and Water Spray Interaction

This paper reports numerical investigation concerning the interaction of a laminar methane–air counterflow diffusion flame with monodisperse and polydisperse water spray. Commercial code ansys fluent with reduced chemistry has been used for investigation. Effects of strain rate, Sauter mean diameter (SMD), and droplet size distribution on the temperature along stagnation streamline have been studied. Flame extinction using polydisperse water spray has also been explored. Comparison of monodisperse and polydisperse droplet distribution on flame properties reveals suitability of polydisperse spray in flame temperature reduction beyond a particular SMD. This study also provides a numerical framework to study flame–spray interaction and extinction.

Inhibition of counterflow methane/air diffusion flame by water mist with varying mist diameter

In the present study, the effect of fine water mist on extinguishment of a methane–air counterflow diffusion flame was investigated to understand the underlying physics of fire extinguishment of highly stretched diffusion flame by water mist. Twin-fluid atomizers were used to generate polydisperse water mist of which Sauter mean diameters were 10, 20, 40, and 60μm. When water mist is not added, the critical stretch rate at extinguishment is 439s−1 as compared to the theoretical value of 460s−1. For the case with water mist addition, when the stretch rate is small enough, almost all the water mist evaporates within the flame zone. On the other hand, for high stretch rate case, large mist droplets pass through the flame zone and can reach the stagnation plane. However, no oscillatory motion was found around the stagnation plane. Critical stretch rate at extinguishment decreases monotonously with the mass fraction of water mist independently of the mist diameter within the range of D32 from 10μm to 60μm. On the other hand, with increase in the surface area parameter, the critical stretch rate at extinguishment decreases rapidly and becomes less sensitive at large surface area parameter, of which tendency is qualitatively in good agreement with theoretical predictions. For a constant surface area parameter, the critical stretch rate decreases with mist diameter because the mass fraction of water mist should increase in proportion to the mist diameter to keep the surface area parameter constant. When the water mist evaporates completely in the flame zone as in the present study, the mass fraction of the water mist is the dominant factor for fire extinguishment, rather than the surface area parameter. Therefore, an appropriate combination of stretch rate and water mist mass fraction should be provided to suppress effectively a given fire with a small amount of water mist.

Extinguishment of counterflow methane/air diffusion flame by polydisperse fine water droplets

In the present study, we investigated the effect of fine water droplets on extinguishment of a methane–air counterflow diffusion flame. Fine water droplets were generated by piezoelectric atomizer. Water droplet mass loading was varied by changing the number of activated piezoelectric units. Droplet characteristics were measured by a phase Doppler particle analyzer and temperature measurements were performed using a silica-coated fine thermocouple. With an increase in the non-dimensional fuel ejection parameter, the non-dimensional flame stand-off distance increases, but it is not affected by water droplets. When the stagnation velocity gradient is small enough, all the droplets are decelerated simultaneously along the stagnation stream line. However, non-equilibrium in droplet velocities appears when the stagnation velocity gradient is increased. Large droplets move faster than small ones when the Stokes number is larger than unity. On the other hand, thermophoretic velocities were negligibly small over all sizes of droplets as compared to convection velocity. When the surface area parameter is small, the critical velocity gradient at extinguishment decreases rapidly with increase in the surface area parameter and water droplets are significantly effective in fire suppression. However, the critical velocity gradient at extinguishment becomes less sensitive to the surface area parameter when it increases. The effect of humidity variation in the ambient air is not discernible. This fact suggests that the water vapor included in the ambient air is not so effective to reduce the maximum flame temperature and the critical velocity gradient at extinguishment. With increase of the water droplet mass loading, the maximum flame temperature decreases as well as the critical velocity gradient at extinguishment. However, the maximum flame temperature at the limit of extinguishment changes only slightly with increase of the water droplet mass loading and is of the order of 1570K.

Water Droplets Behavior in Extinguishing the Methane-Air Counterflow Diffusion Flame

The behavior of fine water droplets was investigated in a methane-air counterflow diffusion flame, specifically when the water droplets were added to the inlet air feed and used to extinguish the diffusion flame. The water droplets studied had a relatively broad size distribution (from 1 to 60 ?m), with the number mean diameter of 15 ?m and Sauter mean diameter of 25 ?m. With an increase in the velocity gradient, the flame approaches towards the stagnation plane and flame thickness decreases. The flame properties, such as flame location and flame thickness are insensitive to the water mass loading. Along the stagnation stream line, the velocity decreases towards a local minimum just before the flame front. In contrast, the velocity increases in the flame zone due to the thermal expansion, and then decreases towards the stagnation point. In the decelerating zone, the droplet mass flux decreases on approach to the flame zone due to a non-equilibrium in velocity of large droplets between liquid and gas phases. Furthermore, as the droplet-laden air approaches the flame front, the flow stream begins to diverge in the counterflow field. Since the small droplets move away from the burner axis, the divergence of the air flow acts to reduce the water droplet mass flux along the stagnation streamline. Large droplets moving faster than the air flow “catch up” to slower ones just before the flame front, and the mass flux of the droplets tends to increase, resulting in the droplets accumulating just before the flame zone. Measurements of the velocities of individual droplets show that large droplets move faster than the small droplets which are in equilibrium (in velocity) between liquid and gas phases. Droplet behavior was found to be controlled by the Stokes number. Equilibrium in velocity is re-established just in front of the flame zone. Closer to the flame front, evaporation occurs and the mass flux of the droplets decreases drastically. On the other hand, thermophoresis acts on extremely small droplets to move them along the steep temperature gradient and these droplets are forced from the high to low temperature regions against the convection velocity. The thermophoretic velocity was estimated in the present study to be compared with the measured convection velocity and it was concluded that the thermophoretic effect is negligibly small over all sizes of droplets.

Addition Effects of H2and H2O on Flame Structure and Pollutant Emissions in Methane–Air Diffusion Flame

Addition effects of H2 and H2O on flame structure and NOx emission behavior are numerically studied with detailed chemistry in methane–air counterflow diffusion flames. The discernible differences in flame structure and the behaviors of pollutant emissions such as CO, CO2, and NOx are compared among a pure methane flame, CH4–H2 flames, and CH4–H2–H2O flames. The important role of chemical effects of added H2O in flame structure and pollutant emissions is also discussed. It is seen that chemical effects of added H2O increase the maximum flame temperature for small H2O mole fraction, and this is relevant to the enhanced OH radical through the reaction step O + H2O → OH + OH. Emission indices of CO increase and then decrease after showing a maximum in the increase of methane mole fraction for CH4–H2 flames and in the increase of H2O mole fraction for CH4–H2–H2O flames, while those of CO2 increase monotonously. These behaviors are caused by the competition of the production through the reaction step HCO + H2O...

메탄-공기 확산화염에서 수소 첨가 효과에 관한 연구

Hydrogen-blending effects in flame structure and NO emission behavior are numerically studied with detailed chemistry in methane-air counterflow diffusion flames. The composition of fuel is systematically changed from pure methane to the blending fuel of methane-hydrogen through H2 molar addition up to 30%. Flame structure, which can be described representatively as a fuel consumption layer and a H₂-CO consumption layer, is shown to be changed considerably in hydrogen-blending methane flames, compared to pure methane flames. The differences are displayed through maximum flame temperature, the overlap of fuel and oxygen, and the behaviors of the production rates of major species. Hydrogen-blending into hydrocarbon fuel can be a promising technology to reduce both the CO and CO₂ emissions supposing that NOx emission should be reduced through some technologies in industrial burners. These drastic changes of flame structure affect NO emission behavior considerably. The changes of thermal NO and prompt NO are also provided according to hydrogen-blending. Importantly contributing reaction steps to prompt NO are addressed in pure methane and hydrogen-blending methane flames.

Hydrogen utilization as a fuel: hydrogen-blending effects in flame structure and NO emission behaviour of CH4–air flame

Hydrogen-blending effects in flame structure and NO emission behaviour are numerically studied with detailed chemistry in methane–air counterflow diffusion flames. The composition of fuel is systematically changed from pure methane to the blending fuel of methane–hydrogen through H2 molar addition up to 30%. Flame structure, which can be described representatively as a fuel consumption layer and a H2–CO consumption layer, is shown to be changed considerably in hydrogen-blending methane flames, compared to pure methane flames. The differences are displayed through maximum flame temperature, the overlap of fuel and oxygen, and the behaviours of the production rates of major species. Hydrogen-blending into hydrocarbon fuel can be a promising technology to reduce both the CO and CO2 emissions supposing that NOx emission should be reduced through some technologies in industrial burners. These drastic changes of flame structure affect NO emission behaviour considerably. The changes of thermal NO and prompt NO are also provided according to hydrogen-blending. Importantly contributing reaction steps to prompt NO are addressed in pure methane and hydrogen-blending methane flames. Copyright © 2006 John Wiley & Sons, Ltd.

A simple one-step chemistry model for partially premixed hydrocarbon combustion

This work explores the applicability of one-step irreversible Arrhenius kinetics with unity reaction order to the numerical description of partially premixed hydrocarbon combustion. Computations of planar premixed flames are used in the selection of the three model parameters: the heat of reaction q, the activation temperature T a , and the preexponential factor B. It is seen that changes in q with equivalence ratio ϕ need to be introduced in fuel-rich combustion to describe the effect of partial fuel oxidation on the amount of heat released, leading to a universal linear variation q ( ϕ ) for ϕ > 1 for all hydrocarbons. The model also employs a variable activation temperature T a ( ϕ ) to mimic changes in the underlying chemistry in rich and very lean flames. The resulting chemistry description is able to reproduce propagation velocities of diluted and undiluted flames accurately over the whole flammability limit. Furthermore, computations of methane–air counterflow diffusion flames are used to test the proposed chemistry under nonpremixed conditions. The model not only predicts the critical strain rate at extinction accurately but also gives near-extinction flames with oxygen leakage, thereby overcoming known predictive limitations of one-step Arrhenius kinetics.

이산화탄소로 희석된 메탄-공기 확산화염의 에지화염 불안정성

Experiments in low strain rate methane-air counterflow diffusion flames diluted with CO₂ have been conducted to investigate the flame extinction behavior and edge flame oscillation in which flame length is less than the burner diameter and thus lateral conductive heat loss in addition to radiative loss could be remarkable at low global strain rates. The critical mole fraction at flame extinction is examined in terms of velocity ratio and global strain rate. It is seen that flame length is closely relevant to lateral heat loss, and this affects flame extinction and edge flame oscillation considerably. Lateral heat loss causes flame oscillation even at fuel Lewis number less than unity. Edge flame oscillations are categorized into three: a growing-, a harmonic- and a decaying-oscillation mode. Onset conditions of the edge flame oscillation and the relevant modes are examined with global strain rate and CO₂ mole fraction in fuel stream. A flame stability map based on the flame oscillation modes is also provided at low strain rate flames.

Edge flame instability in low-strain-rate counterflow diffusion flames

Experiments in low-strain-rate methane–air counterflow diffusion flames diluted with nitrogen have been conducted to study flame extinction behavior and edge flame oscillation in which flame length is less than the burner diameter and thus lateral conductive heat loss, in addition to radiative loss, could be high at low global strain rates. The critical mole fraction at flame extinction is examined in terms of velocity ratio and global strain rate. Onset conditions of the edge flame oscillation and the relevant modes are also provided with global strain rate and nitrogen mole fraction in the fuel stream or in terms of fuel Lewis number. It is observed that flame length is intimately relevant to lateral heat loss, and this affects flame extinction and edge flame oscillation considerably. Lateral heat loss causes flame oscillation even at fuel Lewis number less than unity. Edge flame oscillations, which result from the advancing and retreating edge flame motion of the outer flame edge of low-strain-rate flames, are categorized into three modes: a growing, a decaying, and a harmonic-oscillation mode. A flame stability map based on the flame oscillation modes is also provided for low-strain-rate flames. The important contribution of lateral heat loss even to edge flame oscillation is clarified finally.

Numerical study of the effects of radiative heat losses on diffusion flames

The effects of thermal radiation are numerically investigated for a methane-air counterflow diffusion flame, using ‘detailed’ chemistry. The radiative losses from combustion products (CO2 and H2O) were considered by using a thin gas approximation. The results show a significant effect of radiative losses causing extinction at low strain rates. On the basis of the radiative losses from gaseous combustion products, an extinction limit was found to be 0.7 s−1. The presence of soot will move this limit to higher strain rates. The radiation effects are relatively less at moderate and high strain rates, where they may cause a reduction in the peak temperatures by ∼ 10 per cent. In addition to decreasing peak temperatures and combustion products, the radiative losses also reduce the flame width. The results show the importance of including detailed chemical mechanism in correctly predicting the extinction limit and the influence of radiative losses on flame structure.

Effects of Oxygen Concentration on PAHs Formation in a Counterflow Diffusion Flame

The effects of the oxygen concentration on polycyclic aromatic hydrocarbons (PAHs) and soot formation in a methane air counterflow diffusion flame are investigated experimentally. The oxygen concentration in the oxidizer is varied in the range from 0.18 to 0.22 (mole). PAHs measurement is performed by a gas chromatograph mass spectrometer (GC/MS). It is found that PAHs concentrations in the flame increase along with the increase of the oxygen concentration. This result shows that the increase of the oxygen concentration will promote soot formation in diffusion flames. The responses of Xi/XC6H6, which is the mole fraction ratio of the species i to benzene, to the change of the oxygen concentration are also examined to elucidate PAHs formation mechanism. In the naphthalene (2 rings) formation process, H-abstraction-C2H2-addition (HACA) mechanism would be more dominant than Marinov's model, in which the combination of 5-membered ring radicals contributes to PAlls formation. On the other hand, in phenanthrene (3 rings) formation process, not only HACA mechanism, but also Marinov's model can be responsible for its formation.

QUANTITATIVE LASER-INDUCED FLUORESCENCE MEASUREMENTS AND MODELING OF NITRIC OXIDE IN HIGH-PRESSURE (6–15 ATM) COUNTERFLOW DIFFUSION FLAMES

Laser-induced fluorescence (LIF) measurements of NO concentration ([NO]) have been obtained along the centerline of methane–air counterflow diffusion flames at 6 to 15 atm. This study is an extension of our previous work involving measurements of [NO] in similar flames at two to five atm, wherein we had used a counterflow premixed flame for calibration. For the flames studied here, a method based on computed overlap fractions is developed to calibrate [NO] measurements at higher pressures. The linear LIF measurements of [NO], which are corrected for variations in the electronic quenching rate coefficient, are compared with numerical predictions from an opposed-flow flame code utilizing two Gas Research Institute (GRI) chemical kinetic mechanisms (versions 2.11 and 3.0). The effect of radiative heat loss on code predictions is accounted for by using an optically thin radiation model. The revised GRI mechanism (version 3.0) offers a significant improvement in prompt-NO predictions for these flames compared to the older version (2.11), especially at pressures below eight atm. However, a consistent discrepancy remains in the comparisons, particularly at peak NO locations for pressures lower than six atm. The measurements display a continuing trend of decreasing NO concentration with increasing pressure at 6–15 atm as expected for flames dominated by prompt NO. The discrepancy between measurements and predictions decreases with rising pressure so that the revised GRI mechanism predicts [NO] with reasonable accuracy at pressures above six atm.

Chemical effect of diluents on flame structure and NO emission characteristic in methane-air counterflow diffusion flame

The dilution effect of air stream according to agent type on flame structure and NO emission behaviour is numerically simulated with detailed chemistry in CH4/air counterflow diffusion flame. The volume percentage of diluents (H2O, CO2, and N2) in air stream is systematically changed from 0 to 10. The radiative heat loss term, based on an optically thin model, is included to clearly describe the flame structure and NO emission behaviour especially at low strain rates. The effect of dilution of air stream on the decrease of maximum flame temperature varies as CO2>H2O>N2, even if heat capacity of H2O is the highest. It is also found that the addition of CO2 shows the tendency towards the reduction of flame temperature in both the thermal and chemical sides, while the addition of H2O enhances the reaction chemically and restrains it thermally due to a super-equilibrium effect of the chain carrier radicals caused by the breakdown of H2O in high-temperature region. The comparison of the nitrogen chemical reaction pathway between the cases of the addition of CO2 and H2O clearly displays that the addition of CO2 is much more effective to reduce NO emission. Copyright © 2002 John Wiley & Sons, Ltd.

Laser-Induced Fluorescence Measurements and Modeling of Nitric Oxide in High-Pressure Counterflow Diffusion Flames

Quantitative laser-induced fluorescence (LIF) measurements of NO concentration ([NO]) have been obtained along the centerline of prompt-NO dominated, methane-air counterflow diffusion flames at two to five atm, Global strain rates of 20, 30 and 40 s−1 were investigated at each pressure, with the addition of a 15 s−1 case at three and four atm. Linear LIF measurements of [NO] are corrected for variations in the electronic quenching rate coefficient by using major species profiles generated by an opposed-flow flame code and quenching cross-sections for NO available from the literature. Corrected linear LIF measurements of [NO] are compared with numerical predictions from the opposed-flow flame code by utilizing the GRI (version 2.11) mechanism for the NO kinetics. The effect of radiative heat loss on code predictions is accounted for by using an optically thin radiation model. A modest decrease in predicted temperature owing to radiative heat loss causes a significant decrease in predicted [NO], indicating the temperature sensitivity of the prompt-NO kinetics. Comparisons between [NO] measurements and predictions show that the GRI mechanism underpredicts prompt-NO by a factor of two to three at all pressures. The underprediction peaks at 2 to 3 atm, and decreases with pressure from 3 to 5 atm. Although the GRI mechanism does not display this trend, predictions with a modified rate coefficient for the prompt-NO initiation reaction give qualitative agreement with the experimentally observed variation. However, modifying the prompt-NO initiation reaction is not sufficient to account for the differences between measurements and predictions, thus indicating a need for refinement of the CH chemistry.