Combustion characteristics of a swirling inverse diffusion flame upon oxygen content variation

The combustion characteristics of a swirling inverse diffusion flame (IDF) upon variation of the oxygen content in the oxidizer were experimentally studied. The oxidizer jet was a mixture mainly composed of oxygen and nitrogen gases, with a volumetric oxygen fraction of 20%, 21% and 26%, and liquefied petroleum gas (LPG) was used as the fuel. Each set of experiment was conducted with constant oxygen content in the oxidizer. When the oxygen was varied, the changes in flame appearance, flame temperature, overall pollutant emission and heating behaviors of the swirling IDF were investigated. The swirling IDFs with different oxygen content in the oxidizer have similar flame structure involving a large-size and high-temperature internal recirculation zone (IRZ) which favors for thermal NO formation, and the thermal mechanism dominates the NO production for the swirling IDFs. The use of nitrogen-diluted air (with 20% oxygen) allowed the IDFs to operate at lower temperature with reduced NO_x formation, compared to the case of air/LPG combustion (with 21% oxygen). Meanwhile, an increase in CO emission is observed. With oxygen-enriched air (26% oxygen), the increase in temperature and EINO_x under lean conditions is more significant than under rich conditions. With 26% oxygen in the oxidizer stream, the IDF produces: (1) a shorter and narrowed navy-blue flame ring located closer to the burner exit, (2) highly luminous yellow flame extending into the central IRZ and above the blue flame ring, (3) a low CO emission, especially under lean conditions, (4) an increase in temperature at low Ф while a decrease in temperature at high Ф, and (5) an increase in EINO_x at all Ф. The heating test using the swirling IDFs in flame impingement heat transfer reveals that the heating rate can be monotonically increased as oxygen content in the oxidizer jet increases under the lean condition (Ф = 1.0). The oxygen enrichment does not contribute to the heating rate under the rich condition (Ф = 2.0), because for the non-premixed combustion of an IDF, the enrichment in oxygen means a lower oxidizer jet Reynolds number and thus less complete combustion occurs as a result of reduced amount of entrained ambient air.     

Effect of hydrogen enrichment on combustion characteristics of methane swirling and non-swirling inverse diffusion flame

Influence of hydrogen addition on appearance of swirling and non-swirling inverse diffusion flame (IDF) along with emissions characteristics are investigated experimentally. The combustion characteristics including flame length, axial and radial temperature variation, and noise level are analysed for hydrogen addition in methane by mass basis for constant energy input and by volume basis for constant volumetric fuel flow rate. Hydrogen addition in methane IDF produces shorter flame by compressing entrainment zone, mixing zone, reaction zone, and post-combustion zone. Hydrogen addition shift these zones towards fuel and air exit from the burner. Enrichment of methane with hydrogen on a mass basis up to 6% reduces CO emission considerably and increases NO_x emission moderately. Effect of H₂ addition on combustion and emission characteristics is more prominent in non-swirling IDF. Combustion noise is augmented with the hydrogen addition and the magnitude of sound level depends on the hydrogen concentration.     

Effects of nozzle length on flame and emission behaviors of multi-fuel-jet inverse diffusion flame burner

An experimental study was performed to investigate the effects of the nozzle length on the air-pollutant-emission and noise-radiation behaviors of a burner utilizing a multi-fuel-jet inverse diffusion flame (MIDF). Comparison of the experimental results obtained from two MIDF burners, one with a long nozzle and the other with a short nozzle, operating under the same air/fuel supply conditions (Re_air and Ф) shows rather significant differences in the flame appearance, flame centerline temperature, CO/CO₂ concentrations and the noise radiation. The nozzle length influences development of the jets and hence interaction between the air/fuel jets including their mixing process. The short nozzle produces a flame with a shorter base height and a smaller potential core due to the enhanced air/fuel mixing. It also leads to faster and more complete combustion at the inner reaction cone of the flame due to the stronger and faster air/fuel mixing. The nozzle length affects the CO and CO₂ concentrations, and higher peak values are obtained with the short-nozzle flame. Flame noise of the MIDF is defined as the noise radiation at different flame heights, which is of varying strength but of the same dominant frequency in the range of 250–700 Hz. The noise radiation from the inner reaction cone of the flame is stronger than that from the lower and upper parts of the flame, and the maximum noise radiation occurs when the total amounts of air and fuel in the combustion zone are at the stoichiometric air/fuel ratio. For all the experiments conducted in the present study, the MIDF produced by the long nozzle is always noisier than its counterpart and it is due to the increase of the low-frequency noise components.     

Investigation of OH1 chemiluminescence with lift-off characteristic in methane-oxygen inverse diffusion flame

OH¹ chemiluminescence is a reliable indication of the combustion state and can express characteristic parameters of flame. In this work, OH¹ chemiluminescence in the lift-off process of methane-oxygen inverse diffusion flame was investigated by spectrum diagnostic technology. The experimental results show that OH¹ chemiluminescence is mainly concentrated in the root of the flame and the fully developed region of the flame, and two OH¹ signal peaks appear obviously, which indicates that there are two different reaction regions during in the lift-off process. In addition, a simulation based on CFD combining with GRI-Mech 3.0 mechanism was performed about the OH¹ generation pathway of flame lift-off process. The simulation results show that the reaction of CH + O₂OH¹+CO (R2) is the main pathway to generate OH¹ in the root of the flame (zone I). Reaction H + O + M=OH¹+M (R1) is the dominant reaction in the fully developed region of the flame (zone II), and its reaction rate to generation OH¹ radical is significantly higher than reaction R2.     

Understanding the influence of burner structure on the stability and chemiluminescence of inverse diffusion flame

Inverse diffusion lift-off flame was widely used in industrial fields such as non-catalytic partial oxidation of methane. In order to investigate the stability and chemiluminescence characteristics of the inverse diffusion lift-off flame, the OH1 and CH1 radiation characteristics, lift-off height, and transition (attachment, lift-off and blow-out) of flames under different burner structure were discussed. The results showed that burner rim thickness and incidence angles would affect the stability of the inverse diffusion flame. When the thickness of the burner rim exceeded 0.5 mm, the flame would directly change from the attachment state to blow-out state as oxygen velocity increased. Additionally, the values of the blowout limit of the nozzle were inversely proportional to the rim thickness of the burner. Different incident angles would result in various shear angles, which would affect the flame structure. As the incidence angle decreased, the tangential velocity of flame increased and the flame tended to be more stable. When the lift-off flame generated, OH1 intensity and distribution showed a sudden change, and the OH1/CH1 peak intensity ratio of the flame appeared abrupt changes.     

Effects of fuel compositions on the heat generation and emission of syngas/producer gas laminar diffusion flame

Demand for the clean and sustainable energy encourages the research and development in the efficient production and utilisation of syngas for low-carbon power and heating/cooling applications. However, diversity in the chemical composition of syngas, resulting due to its flexible production process and feedstock, often poses a significant challenge for the design and operation of an effective combustion system. To address this, the research presented in this paper is particularly focused on an in-depth understanding of the heat generation and emission formation of syngas/producer gas flames with an effect of the fuel compositions. The heat generated by flame not only depends on the flame temperature but also on the chemistry heat release of fuel and flame dimension. The study reports that the syngas/producer gas with a low H_2:CO maximises the heat generation, nevertheless the higher emission rate of CO₂ is inevitable. The generated heat flux at H_2:CO = 3:1, 1:1, and 1:3 is found to be 222, 432 and 538 W m^-2 respectively. At the same amount of heat generated, H₂ concentration in fuel dominates the emission of NO_x. The addition of CH₄ into the syngas/producer gas with H_2:CO = 1:1 also increases the heat generation significantly (e.g. 614 W m^-2 at 20%) while decreases the emission formation. In contrast, adding 20% CO₂ and N₂ to the syngas/producer gas composition reduces the heat generation from 432 W m^-2 to 364 and 290 W m^-2, respectively. The role of CO₂ on this aspect, which is weaker than N₂, thus suggests CO₂ is preferable than N₂. Along with the study, the significant role of CO₂ on the radiation of heat and the reduction of emission are examined.     

Effect of hydrogen percentage and air jet Reynolds number on fuel lean flame stability of LPG-fired inverse diffusion flame with hydrogen enrichment

The flame type studied in this paper is a circumferential-fuel – jet inverse diffusion flame, and the fuel is liquefied petroleum gas enriched with hydrogen gas. Fuel lean flame stability limit regarding to the volumetric percentage of hydrogen and the air jet Reynolds number was investigated. There were three flame stable-related limits examined: local extinction limit, restore limit, and complete extinction limit. Global Energy Consumption Rate of fuel, fuel jet velocity, and overall equivalence ratio of the air/fuel mixture at the three stable-related limits were presented. Experimental results indicate that with hydrogen addition, the inverse diffusion flame can sustain burning with a lower global energy than without it. The most significant stabilization effect was obtained with 30% hydrogen addition for complete extinction limit and 30%–90% for local extinction limit. The corresponding fuel jet velocity at complete extinction limit also decreases with hydrogen addition. However, fuel jet velocities at local extinction limit and restore limit increase significantly, when hydrogen percentage is larger than 70%. Air jet Reynolds number does not show notable influence on Global Energy Consumption Rate or fuel jet velocity at the three stability limits. In addition, overall equivalence ratio, which is an important parameter of inverse diffusion flame combustion dropping dramatically with air jet Reynolds number when it is less than 2000.     

Heat transfer characteristics of an impinging inverse diffusion flame jet. Part II: Impinging flame structure and impingement heat transfer

This paper is the second part of the experimental study on exploring the feasibility of inverse diffusion flame (IDF) for impingement heating. The structures and heat transfer characteristics of an impinging IDF jet have been studied. Four types of impinging flame structure have been identified and reported. The distributions of the wall static pressure are measured and presented. The influences of the global equivalence ratio (), the Reynolds number of the air jet (Re_air), and the non-dimensional burner-to-plate distance (H/d_air), on the flame structure, and the local and averaged heat transfer characteristics, are reported and discussed. The highest heat transfer occurs when the tip of the flame inner reaction zone impinges on the plate. The heat transfer rate from the impinging IDF is found to be higher than that in the premixed flame jet due to the augmented turbulence level originated from the flame neck. This high heat transfer rate, together with its in-born advantage of no danger of flashback and low level of nitrogen oxides emission, demonstrates the blue, dual-structured, triple-layered IDF is a desirable alternative for impingement heating.     

Effects of fuel type and equivalence ratios on the flickering of triple flames

An experimental study has been conducted in axisymmetric, co-flowing triple flames with different equivalence ratios of the inner and outer reactant streams (2<?in<3 and 0??out<0.7). Different fuel combinations, like propane/propane, propane/methane or methane/methane in the inner and outer streams respectively, have been used in the experiments. The structures of the triple flames have been compared for the different fuel combinations and equivalence ratios. The conditions under which triple flames exhibit oscillation have been identified. During the oscillation, the non-premixed flame and the outer lean premixed flame flicker strongly, while the inner rich premixed flame remains more or less stable. The flickering frequency has been evaluated through image processing and fast Fourier transform (FFT) of the average pixel intensity of the image frames. It is observed that, for all the fuel combinations, the frequency decreases with the increase in the outer equivalence ratio, while it is relatively invariant with the change in the inner equivalence ratio. However, an increase in the inner equivalence ratio affects the structure of the flame by increasing the heights of the inner premixed flame and non-premixed flame and also enlarges the yellow soot-laden zone at the tip of the inner flame. A scaling analysis of the oscillating flames has been performed based on the measured parameters, which show a variation of Strouhal number (St) with Richardson number (Ri) as St ∝ Ri^0.5. The fuel type is found to have no influence on this correlation.     

Experimental study on the small‐scale diffusion flame of ethanol and the wall temperature field

An experimental investigation was carried out on the diffusion flames of liquid ethanol burning in air. Ceramic tubes with different inner diameters were used as burners. Three different flame structures at different flow rates were identified and different combustion regions were divided based on the experimental results. Some relevant factors which may affect the flame height and width were discussed. The outer surface wall temperature field of the tubes was measured in the combustion process. The results showed that both flame height and flame width all increased proportionally with an increasing flow rate in the steady flame region. Both flame height and flame width decreased with a decreasing tube inner diameter at the same flow rate. By decreasing the tube inner diameter, the quenching flow rate was decreased, the flow rate according to the produced periodic explosive flame decreased, and the flow rate range of the steady flames decreased. The outer wall temperature field presented an exponential distribution, and the wall temperature reached the greatest value at the outlet of the ceramic tube. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20282     

Effect of initial pressure,temperature and equivalence ratios on laminar combustion characteristics of hydrogen enriched natural gas

In this study, the flame propagation characteristics of premixed natural gas–hydrogen–air mixtures were studied in constant volume combustion bomb by using the high-speed schlieren photography system. The flame radius, laminar flame propagation speed and the flame stretch rate were obtained under different initial pressure, temperature, equivalence ratios and hydrogen fractions. Meanwhile, the flame stability and their influencing factors were obtained by analyzing the Markstein length and the flame propagation schlieren photos under various combustion conditions. The results show that the stretched laminar propagation speed increases with the increase of the initial temperature and hydrogen fraction of the mixture, and will decreases with the increase of the initial pressure. Meanwhile, according to the Markstein length and the flame propagation pictures, the flame stability decreases with the increase of the temperature and hydrogen fraction, and the slight flaws occurred at the early stage; at larger flame radius, the flame stability is more sensitive to the variation of the initial temperature and hydrogen fraction than to that of initial pressure and equivalence ratio.     

结构化载氧体颗粒化学链燃烧内扩散影响模拟分析

采用有限速率模型对CuO/Al_2O_3结构化载氧体颗粒化学链燃烧进行了数值模拟,考察了不同结构化载氧体颗粒模型和进气速度对载氧体化学反应内扩散的影响。结果表明,相比于球形载氧体,环形结构拉西环载氧体内扩散更为容易,反应快,温升高;相比于全活性载氧体,1/4活性和半活性载氧体气体扩散到活性物质距离短,反应更快;增加进气速度能够加速颗粒内扩散,改善反应速率。     

Effects of substituting fuel spray for fuel gas on flame stability in lean premixtures

We analyze flame propagation through a homogeneous three-component premixture composed of fuel gas, small fuel droplets, and air. This analytical study is carried out within the framework of a diffusional-thermal model with the simplifying assumption that both fuels—the fuel in the gaseous phase and the gaseous fuel evaporating from the droplets—have the same Lewis number. The parameter that expresses the degree of substitution of spray for gas is δ, the liquid loading, i.e., the ratio of liquid fuel mass fraction to overall fuel mass fraction in the fresh premixture. In this substitution of liquid fuel for gaseous fuel, the overall equivalence ratio is lean and is kept identical. We hence obtain a partially prevaporized spray, for which we analytically study the dynamics of the plane spray-flame front. The investigated model assumes the averaged distance between droplets to be small compared with the premixed flame thickness (i.e., small droplets and moderate pressure). Le, the Lewis number, Ze, the Zeldovich number, and δ are the main parameters of the study. Our stability analysis supplies the stability diagram in the plane {Le,δ} for various Ze values and shows that, for all Le, the plane front becomes unstable for high liquid loading. At large or moderate Lewis number, we show that the presence of droplets substantially diminishes the onset threshold of the oscillatory instability, making the appearance of oscillatory propagation easier. Oscillations can even occur for Le<1 when sufficient spray substitution is operated. The pulsation frequency occurring in this regime is a tunable function of δ. At low Lewis number, substitution of spray for gas leads to a more complex situation for which two branches can coexist: the first one still corresponding to the pulsating regime, the other one being related to the diffusive-thermal cellular instability.     

Comprehensive study of the evolution of an annular edge flame during extinction and reignition of a counterflow diffusion flame perturbed by vortices

The structure of a time-dependent methane/enriched-air flame established in an axisymmetric, laminar counterflow configuration is investigated, as the flame interacts with two counterpropagating toroidal vortices. Computationally, the time-dependent equations are written using a modified vorticity–velocity formulation, with detailed chemistry and transport, and are solved implicitly on a nonstaggered, nonuniform grid. Boundary conditions are chosen to create local extinction and reignition in the vicinity of the axis of symmetry. Experimentally, CO planar laser-induced fluorescence (PLIF), OH PLIF, and an observable proportional to the forward reaction rate (RR) of the reaction CO+OH→CO₂+H are measured. Particle image velocimetry (PIV) is used to characterize the velocity field of the vortical structures and to provide detailed boundary conditions for the simulations. Excellent agreement is found between model and experiments to the minutest morphological details throughout the interaction. The validated model is then used to probe the dynamics of the two-dimensional extinction process with high temporal resolution. During the initial phase of the interaction, the flame is locally extinguished by the two vortices. The resulting edge flame propagates outward as an extinction front, with a structure that does not depart significantly from that of a diffusion flame. The front recedes from the axis of symmetry with a negative propagation speed that reaches a value as large as six times that of the freely propagating laminar flame with the same reactant concentrations found at the stoichiometric surface. As the front propagates outward, it transitions to an ignition front, and it reaches a positive propagation speed comparable to that of the freely propagating laminar flame. During this transition, it develops a characteristic premixed “hook,” with a lean premixed branch, a stoichiometric segment that evolves into the remnant of the original primary diffusion flame, and a much weaker secondary diffusion flame resulting from a secondary peak in heat release in the original unperturbed diffusion flame. No evidence of a distinct rich premixed flame is found. The edge flame stabilizes at a radial location where the local gaseous speed equals the propagation speed of the front. When the local perturbation has decayed below the flame propagation speed, the flame edge starts reigniting the mixing layer as an ignition wave that propagates with an essentially frozen structure along the stoichiometric surface until the original diffusion flame structure is fully recovered. Implications for flamelet modeling of turbulent flames with local extinction are discussed.     

Determination of oxygen transport resistance in gas diffusion layer for polymer electrolyte fuel cells

Gas diffusion layer (GDL) plays an important role in the performance of membrane electrode assembly (MEA) in polymer electrolyte fuel cells. In this work, 2‐type MEAs were prepared by 2 different GDLs of 29 BC and 29‐WUT, and the performance were investigated using polarization curve methods. The performance of MEA with 29‐WUT was 120 mV higher than 29 BC at 1600 mA/cm². Electrochemical impedance spectroscopy (EIS) was applied to measure the mass transport resistance of 2‐type MEAs under normal running condition. The results of EIS showed that the mass transport resistance of 29 BC was 3.15 times higher than that of 29‐WUT at 1600 mA/cm². To clarify this phenomenon, limiting current methods were applied under diluting oxygen concentration, low humidity, and high flow rate conditions. The results of limiting current methods showed that both the total oxygen transport resistance and the molecular diffusion resistance in the GDL of 29 BC were larger than that of 29‐WUT due to the lower porosity of gas diffusion substrate in 29 BC . As a result, EIS can be well combined with limiting current methods to analyze oxygen transport resistance in GDLs of fuel cells.     

Lift-off of jet diffusion flame in sub-atmospheric pressures: An experimental investigation and interpretation based on laminar flame speed

This paper reveals lift-off behavior of jet diffusion flames in sub-atmospheric pressures less than 100 kPa, in view of that the current knowledge on this topic is limited for normal pressure conditions. Physically, the variation of ambient pressure may have significant influence on the lift-off behavior of jet diffusion flames due to the change of some critical parameters such as laminar flame speed. In this work, experiments are conducted in a large pressure-controllable chamber of 3 m (width) × 2 m (length) × 2 m (height) at different sub-atmospheric pressures of 60 kPa, 70 kPa, 80 kPa, 90 kPa as well as at normal pressure of 100 kPa. Axisymmetric turbulent jet diffusion flames are produced by nozzles with diameters of 4 mm, 5 mm and 6 mm using propane as fuel. It is revealed that the lift-off height increases as the pressure decreases and being much higher than that in normal pressure condition. The laminar flame speed with its dependency on pressure is introduced to interpret such behavior based on classic Kalghatgi model. It is found theoretically that the lift-off height has a power law dependency on pressure by P¹⁻ⁿ, where n is overall reaction order of the fuel which is usually larger than 1 indicating a negative power law function with pressure (for example p^−0.75 for propane as n = 1.75) as well verified by the experimental correlation. Finally, a global model is proposed by including such pressure dependency function into the Kalghatgi model, which is shown to well collapse the experimental results of lift-off heights of different sub-atmospheric pressures.     

Transient development of methane–air diffusion flame in a confined geometry with and without air‐preheat

The transient development of a methane–air diffusion flame is studied numerically in the present work in a confined geometry. An explicit finite difference based numerical model is developed for the solution of transient reacting flow with varying thermodynamic and transport properties. The effect of air preheat on the flow and temperature fields at different time planes are studied by comparing two cases with air temperatures of 300 and 400 K, respectively. The preheating of air is found to complicate the flow situation with the formation of additional vortices. Thermal stratification in the flow causes a radial stretching of the flame, which is more evident with higher preheat temperature. In the absence of any preheating a stable recirculation of ambient air is observed to extend from the exit plane into the domain adjacent to the wall. The mixing of ambient air results in gradual drooping in the bulk mean temperature of the flow. Such drooping nature in the bulk temperature distribution is not found with air preheat due to the absence of any stable recirculation. The complication in the initial flow transients delays the attainment of steady situation for the flame with preheat. Copyright © 2005 John Wiley & Sons, Ltd.     

Effects of CO addition on laminar flame characteristics and chemical reactions of H2 and CH4 in oxy-fuel (O2/CO2) atmosphere

An experimental study is conducted to investigate the effect of CO addition on the laminar flame characteristics of H₂ and CH₄ flames in a constant-volume combustion system. In addition, one-dimensional laminar premixed flame propagation processes at the same conditions are simulated with the update mechanisms. Results show that all mechanisms could well predict the laminar flame speeds of CH₄/CO/O₂/CO₂ mixtures, when Z_CO is large. For mixtures with lower CO, the experimental laminar flame speeds are always smaller than the calculated ones with Han mechanism. For mixtures with larger or smaller Z_CO2, GRI 3.0, San diego and USC 2.0 mechanisms all overvalue or undervalue the laminar flame speeds. When CO ratio in the CH₄/CO blended fuels increases, laminar flame speed firstly increases and then decreases for the CH₄/CO/O₂/CO₂ mixtures. For H₂/CO/O₂/CO₂ mixtures, San diego, Davis and Li mechanisms all undervalue the laminar flame speeds of H₂/CO/CO₂/CO₂ mixtures. Existing models could not well predict the nonlinear trend of the laminar flame speeds, due to complex chemical effects of CO on CH₄/CO or H₂/CO flames. Then, the detailed thermal, kinetic and diffusive effects of CO addition on the laminar flame speeds are discussed. Kinetic sensitivity coefficient is far larger than thermal and diffusive ones and this indicates CO addition influences laminar flame speeds mainly by the kinetic effect. Based on this, radical pool and sensitivity analysis are conducted for CH₄/CO/O₂/CO₂ and H₂/CO/O₂/CO₂ mixtures. For CH₄/CO/O₂/CO₂ mixtures, elementary reaction R38H + O₂ ↔ O + OH and R99 OH + CO ↔ H + CO₂ are the most important branching reactions with positive sensitivity coefficients when CO ratio is relative low. As CO content increases in the CH₄/CO blended fuel, the oxidation of CO plays a more and more important role. When CO ratio is larger than 0.9, the importance of R99 OH + CO ↔ H + CO₂ is far larger than that of R38H + O₂ ↔ O + OH. The oxidation of CO dominates the combustion process of CH₄/CO/O₂/CO₂ mixtures. For H₂/CO/O₂/CO₂ mixtures, the most important elementary reaction with positive and negative sensitivity coefficients are R29 CO + OH ↔ CO₂ + H and R13H + O₂(+M) ↔ HO₂(+M) respectively. The sensitivity coefficient of R29 CO + OH ↔ CO₂ + H is increasing and then decreasing with the addition of CO in the mixture. Chemical kinetic analysis shows that the chemical effect of CO on the laminar flame propagation of CH₄/CO/O₂/CO₂ and H₂/CO/O₂/CO₂ mixtures could be divided into two stages and the critical CO mole fraction is 0.9.     

Experimental analysis of the effects of porous wall on flame stability and temperature distribution in a premixed natural gas/air combustion

In the present analysis, the flame stabilization and temperature distribution within a premixed burner contain porous wall are studied experimentally. The effects of inner diameter, length, and pore density of the porous wall, thermal load, equivalence ratio, and the inlet velocity of the fuel‐air mixture on these are studied. The fuel used in this study is natural gas and the porous wall is SiC (silicon carbide) ceramic foam. The experimental results clearly indicate that the axial temperature along the porous wall increases when the inner diameter of the porous wall decreases and its length increases. The porous wall temperature with an inner diameter of 40 mm, length of 66 mm, and pore density of 30 PPI (pores per inch) has the highest temperature among the examined states. The results of studying the effect of the porous wall on flame stability show that the flame stability limit has a direct relationship with the length and pore density of porous wall and an inverse relationship with the inner diameter of the porous wall. Also, it is found that the porous wall has the highest temperature causes the maximum flame stability limit.     

Effects of water vapor addition to the air stream on soot formation and flame properties in a laminar coflow ethylene/air diffusion flame

The effects of adding water vapor to the air stream on flame properties and soot volume fraction were investigated numerically in a laminar coflow ethylene/air diffusion flame at atmospheric pressure by solving the fully elliptic conservation equations and using a detailed C₂ reaction mechanism including PAH up to pyrene and detailed thermal and transport properties. Thermal radiation was calculated using the discrete-ordinates method and a statistical narrow-band correlated-k based wide band model for the absorption coefficients of CO₂ and H₂O. Soot formation was modeled using a PAH based inception model and the HACA mechanism for surface growth and oxidation. Addition of water vapor significantly reduces radiation heat loss from the flame primarily through reduced soot loading and flame temperature. The added water vapor affects soot formation and flame properties through not only dilution and thermal effects, but also through chemical effect. The chemical effect is as significant as the dilution and thermal effects. The primary pathway for the chemical effect of water vapor is the reverse reaction of OH + H₂ ↔ H + H₂O. Our numerical results confirm that the reduced H radical concentration leads to lower PAH concentrations and consequently lower soot inception rates. In contrast, the radiation effect due to the added water vapor was found to have a minor influence on both flame structure and soot formation in the laminar diffusion flame investigated.