Extinction of laminar partially premixed flames

Flame extinction represents one of the classical phenomena in combustion science. It is important to a variety of combustion systems in transportation and power generation applications. Flame extinguishment studies are also motivated from the consideration of fire safety and suppression. Such studies have generally considered non-premixed and premixed flames, although fires can often originate in a partially premixed mode, i.e., fuel and oxidizer are partially premixed as they are transported to the reaction zone. Several recent investigations have considered this scenario and focused on the extinction of partially premixed flames (PPFs). Such flames have been described as hybrid flames possessing characteristics of both premixed and non-premixed flames. This paper provides a review of studies dealing with the extinction of PPFs, which represent a broad family of flames, including double, triple (tribrachial), and edge flames. Theoretical, numerical and experimental studies dealing with the extinction of such flames in coflow and counterflow configurations are discussed. Since these flames contain both premixed and non-premixed burning zones, a brief review of the dilution-induced extinction of premixed and non-premixed flames is also provided. For the coflow configuration, processes associated with flame liftoff and blowout are described. Since lifted non-premixed jet flames often contain a partially premixed or an edge-flame structure prior to blowout, the review also considers such flames. While the perspective of this review is broad focusing on the fundamental aspects of flame extinction and blowout, results mostly consider flame extinction caused by the addition of a flame suppressant, with relevance to fire suppression on earth and in space environment. With respect to the latter, the effect of gravity on the extinction of PPFs is discussed. Future research needs are identified.     

Tubular premixed and diffusion flames: Effect of stretch and curvature

Tubular flames are ideal for the study of stretch and curvature effects on flame structure, extinction, and instabilities. Tubular flames have uniform stretch and curvature and each parameter can be varied independently. Curvature strengthens or weakens preferential diffusion effects on the tubular flame and the strengthening or weakening is proportional to the ratio of the flame thickness to the flame radius. Premixed flames can be studied in the standard tubular burner where a single premixed gas stream flows radially inward to the cylindrical flame surface and products exit as opposed jets. Premixed, diffusion and partially premixed flames can be studied in the opposed tubular flame where opposed radial flows meet at a cylindrical stagnation surface and products exit as opposed jets. The tubular flame flow configurations can be mathematically reduced to a two-point boundary value solution along the single radial coordinate. Non-intrusive measurements of temperature and major species concentrations have been made with laser-induced Raman scattering in an optically accessible tubular burner for both premixed and diffusion flames. The laser measurements of the flame structure are in good agreement with numerical simulations of the tubular flame. Due to the strong enhancement of preferential diffusion effects in tubular flames, the theory-data comparison can be very sensitive to the molecular transport model and the chemical kinetic mechanism. The strengthening or weakening of the tubular flame with curvature can increase or decrease the extinction strain rate of tubular flames. For lean H₂-air mixtures, the tubular flame can have an extinction strain rate many times higher than the corresponding opposed jet flame. More complex cellular tubular flames with highly curved flame cells surrounded by local extinction can be formed under both premixed and non-premixed conditions. In the hydrogen fueled premixed tubular flames, thermal-diffusive flame instabilities result in the formation of a uniform symmetric petal flames far from extinction. In opposed-flow tubular diffusion flames, thermal-diffusive flame instabilities result in cellular flames very close to extinction. Both of these flames are candidates for further study of flame curvature and extinction.     

Stability characteristics of lifted turbulent partially premixed jet flames

The stability characteristics of partially premixed turbulent lifted methane flames have been investigated and discussed in the present work. Mixture fraction and reaction zone behavior have been measured using a combined 2-D technique of simultaneous Rayleigh scattering, Laser Induced Predissociation Fluorescence (LIPF) of OH and Laser Induced Fluorescence (LIF) of C₂H_x. The stability characteristics and simultaneous mixture fraction-LIPF-LIF measurements in three lifted flames with originally partially premixed jets at different mean equivalence ratio and Reynolds number are presented and discussed in this paper. Higher stability of partially premixed flames as compared to non-premixed flames has been observed. Lifted, attached, blow-out and blow-off regimes have been addressed and discussed in this work. The data show that the mixture fraction field on approaching the stabilization region is uniquely characterized by a certain level of mean and rms fluctuations. This suggests that the stabilization mechanism is likely to be controlled by premixed flame propagation at the stabilization region. Triple flame structure has been detected in the present flames, which is likely to be the appropriate model at the stabilization point.     

Important role of chemical interaction on flame extinction in downstream interaction between stretched premixed H2-air and CO-air flames

Important role of chemical interaction in flame extinction is numerically investigated in downstream interaction among lean (rich) and lean (rich) premixed as well as partially premixed H₂- and CO-air flames. The strain rate varies from 30 to 5917 s⁻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 presented. Highly stretched interacting flames are survived only within two islands in the flame stability map where partially premixed mixture consists of rich H₂-air flame, extremely lean CO-air flame, and a diffusion flame. Further increase in strain rate finally converges to two points. It is found that hydrogen penetrated from H₂-air flame (even at lean flame condition) participates in CO oxidation vigorously due to the high diffusivity such that it modifies the slow main reaction route CO + O₂ → CO₂ + O into the fast cyclic reaction route involving CO + OH → CO₂ + H. These chemical interactions force even rich extinction boundaries with deficient reactant Lewis numbers larger than unity to be slanted at high strain rate. Appreciable amount of hydrogen in the side of lean H₂-air flame also oxidizes the CO penetrated from CO-air flame, and this reduces flame speed of the H₂-air flame, leading to flame extinction. At extremely high strain rates, interacting flames are survived only by a partially premixed flame such that it consists of a very rich H₂-air flame, an extremely lean CO-air flame, and a diffusion flame. In such a situation, both the weaker H₂- and CO-air flames are parasite on the stronger diffusion flame such that it can lead to flame extinction in the situation of weakening the stronger diffusion flame. Important role of chemical interaction in flame extinction is discussed in detail.     

Flame structures and behaviors of opposed flow non-premixed flames in mesoscale channels

An opposed flow non-premixed flame (OFNPF) in a narrow channel was chosen as a model of a non-premixed flame in a mesoscale combustion space or micro-combustor. The stabilization limits and behaviors of methane-air flames and propane-air flames were compared for various experimental parameters such as flow velocity, nozzle distance, nozzle width, channel gap, and fuel dilution. Flames could be stabilized in a wide range of strain rates (0.9–150 s⁻¹) and dilution ratios (∼80% nitrogen at the fuel side). The flame extinction limits were classified into three types and their mechanisms were investigated: higher-strain-rate (HSR) extinction limit determined by the flame stretch, lower-strain-rate (LSR) extinction limit determined by the conductive or convective heat loss from the flame, and fuel-dilution-ratio (FDR) extinction limit determined by the decrease in the heat release rate from the flames. The HSR extinction limits in mesoscale channels could be explained with a modified strain rate, and the LSR extinction limits could be explained by employing a premixed quenching theory in which the heat loss through the dead space near the wall was considered as a major extinction mechanism. Finally, the variation of the extinction limits with the FDR in both the HSR and the LSR conditions could be explained with a modified global reaction rate in which the variations in flame temperature and species concentrations were reflected. This study provides an essential model for the stabilization and extinction of non-premixed flames in mesoscale combustion spaces.     

An a priori model for the effective species Lewis numbers in premixed turbulent flames

An a priori model for the effective species Lewis numbers in premixed turbulent flames is presented. This a priori   analysis is performed using data from a series of direct numerical simulations (DNS) of lean (?=0.4$?=0.4$) premixed turbulent hydrogen flames, with Karlovitz number ranging from 10 to 1562 (Aspden et al., 2011). The conditional mean profiles of various species mass fraction versus temperature are evaluated from the DNS and compared to unstretched laminar premixed flame profiles. The turbulent flame structure is found to be different from the laminar flame structure. However, the turbulent flame can still be mapped onto a laminar flame with an appropriate change in the Lewis numbers of the different species. A transition from “laminar” Lewis numbers to unity Lewis numbers as the Karlovitz number increases is clearly captured. A model for those effective Lewis numbers with respect to the turbulent Reynolds number is developed. This model is derived from a Reynolds-averaged Navier–Stokes (RANS) formulation of the reactive scalar and temperature balance equations. The dependency of the effective Lewis numbers on the Karlovitz number instead of the Reynolds number is discussed in this paper. Unfortunately, given that the ratio of the integral length to the laminar flame thickness is fixed throughout this series of DNS, a change in the Karlovitz number is equivalent to a change in the Reynolds number. Incorporating these effective Lewis numbers in simulations of turbulent flames would have several impacts. First, the associated laminar flame speed and laminar flame thickness vary by a factor of two through the range of obtained effective Lewis numbers. Second, the turbulent premixed combustion regime diagram changes because a unique pair of laminar flame speed and laminar flame thickness cannot be used, and a dependency on the effective Lewis numbers has to be introduced. Finally, a turbulent flame speed model that takes into account these effective Lewis numbers is proposed.     

Effects of Damköhler number on flame extinction and reignition in turbulent non-premixed flames using DNS

Results from a parametric study of flame extinction and reignition with varying Damköhler number using direct numerical simulation are presented. Three planar, non-premixed ethylene jet flames were simulated at a constant Reynolds number of 5120. The fuel and oxidizer stream compositions were varied to adjust the steady laminar extinction scalar dissipation rate, while maintaining constant flow and geometric conditions. Peak flame extinction varies from approximately 40% to nearly global blowout as the Damköhler number decreases. The degree of extinction significantly affects the development of the jets and the degree of mixing of fuel, oxidizer, and combustion products prior to reignition. The global characteristics of the flames are presented along with an analysis of the modes of reignition. It is found that the initially non-premixed flame undergoing nearly global extinction reignites through premixed flame propagation in a highly stratified mixture. A progress variable is defined and a budget of key terms in its transport equation is presented.     

Effect of flame stretch in downstream Interaction between premixed syngas-air flames

The effect of strain rate in downstream interactions between lean (rich) and lean (rich) premixed syngas flames with the fuel composition of 50% H₂ and 50% CO is numerically investigated by varying the strain rate in the range of 5∼500 s⁻¹. The flame stability maps for several strain rates are presented and main concerns are focused on the downstream interactions on the lean and rich extinction boundaries. The fuel composition of 50% H₂ and 50% CO with effective Lewis numbers larger than unity for both lean and rich extinction boundaries is chosen for grasping the important role of hydrogen with the deficient reactant Lewis numbers much smaller than unity. The results show that the lean extinction boundaries have the slanted shape, thereby leading to strong interactions; meanwhile the rich extinction boundaries at appropriately low strain rates are of square, indicating weak interactions. However, at highly strained interacting rich flames, the rich extinction boundaries show a slanted shape, thereby leading to strong interactions even for Lewis numbers much larger than unity. In such situations, thermal and chemical interactions are explained in detail. It is found that, in interacting flames, the excessive heat loss of the stronger flame partly to the weaker flame and mostly to the ambience is the mechanism of flame extinction.     

Suppression limits of low strain rate non-premixed methane flames

The suppression of low strain rate non-premixed flames was investigated experimentally in a counterflow configuration for laminar flames with minimal conductive heat losses. This was accomplished by varying the velocity ratio of fuel to oxidizer to adjust the flame position such that conductive losses to the burner were reduced and was confirmed by temperature measurements using thermocouples near the reactant ducts. Thin filament pyrometry was used to measure the flame temperature field for a curved diluted methane-air flame near extinction at a global strain rate of 20 s⁻¹. The maximum flame temperature did not change as a function of position along the curved flame surface, suggesting that the local agent concentration required for suppression will not differ significantly along the flame sheet. The concentration of N₂, CO_2, and CF₃Br added to the fuel and the oxidizer streams required to obtain extinction was measured as a function of the global strain rate. In agreement with previous measurements performed under microgravity conditions, limiting non-premixed flame extinction behavior in which the agent concentration obtained a value that insures suppression for all global strain rates was observed. A series of extinction measurements varying the air:fuel velocity ratio showed that the critical N₂ concentration was invariant with this ratio, unless conductive losses were present. In terms of fire safety, the measurements demonstrate the existence of a fundamental limit for suppressant requirements in normal gravity flames, analogous to agent flammability limits in premixed flames. The critical agent volume fraction in the methane fuel stream assuring suppression for all global strain rates was measured to be 0.841 ± 0.01 for N₂, 0.773 ± 0.009 for CO₂, and 0.437 ± 0.005 for CF₃Br. The critical agent volume fraction in the oxidizer stream assuring suppression for all global strain rates was measured as 0.299 ± 0.004 for N₂, 0.187 ± 0.002 for CO₂, and 0.043 ± 0.001 for CF₃Br.     

On the similitude between lifted and burner-stabilized triple flames: a numerical and experimental investigation

We have investigated lifted triple flames and addressed issues related to flame stabilization. The stabilization of nonpremixed flames has been argued to result due to the existence of a premixing zone of sufficient reactivity, which causes propagating premixed reaction zones to anchor a nonpremixed zone. We first validate our simulations with detailed measurements in more tractable methane–air burner-stabilized flames. Thereafter, we simulate lifted flames without significantly modifying the boundary conditions used for investigating the burner-stabilized flames. The similarities and differences between the structures of lifted and burner-stabilized flames are elucidated, and the role of the laminar flame speed in the stabilization of lifted triple flames is characterized. The reaction zone topography in the flame is as follows. The flame consists of an outer lean premixed reaction zone, an inner rich premixed reaction zone, and a nonpremixed reaction zone where partially oxidized fuel and oxidizer (from the rich and lean premixed reaction zones, respectively) mix in stoichiometric proportion and thereafter burn. The region with the highest temperatures lies between the inner premixed and the central nonpremixed reaction zone. The heat released in the reaction zones is transported both upstream (by diffusion) and downstream to other portions of the flame. Measured and simulated species concentration profiles of reactant (O₂, CH₄) consumption, intermediate (CO, H₂) formation followed by intermediate consumption and product (CO₂, H₂O) formation are presented. A lifted flame is simulated by conceptualizing a splitter wall of infinitesimal thickness. The flame liftoff increases the height of the inner premixed reaction zone due to the modification of the upstream flow field. However, both the lifted and burner-stabilized flames exhibit remarkable similarity with respect to the shapes and separation distances regarding the three reaction zones. The heat-release distribution and the scalar profiles are also virtually identical for the lifted and burner-stabilized flames in mixture fraction space and attest to the similitude between the burner-stabilized and lifted flames. In the lifted flame, the velocity field diverges upstream of the flame, causing the velocity to reach a minimum value at the triple point. The streamwise velocity at the triple point is ≈0.45 m s⁻¹ (in accord with the propagation speed for stoichiometric methane–air flame), whereas the velocity upstream of the triple point equals 0.7 m s⁻¹, which is in excess of the unstretched flame propagation speed. This is in agreement with measurements reported by other investigators. In future work we will address the behavior of this velocity as the equivalence ratio, the inlet velocity profile, and inlet mixture fraction are changed.     

NOx emissions in n-heptane/air partially premixed flames

NO_x emissions in n-heptane/air partially premixed flames (PPFs) in a counter-flow configuration have been investigated. The flame is computed using a detailed mechanism that combines the Held’s mechanism for n-heptane and the Li and Williams’ mechanism for NO_x. The combined mechanism contains 54 species and 327 reactions. Based on a detailed analysis, dominant mechanisms responsible for NOx formation and destruction in PPFs are found to be thermal, prompt, and reburn mechanisms. The dominant reactions associated with these mechanisms are also identified. The effects of strain rate (a_s) and equivalence ratio (φ) on NO_x emissions are characterized for conditions in which the flame contains two spatially separated reaction zones; a rich premixed zone on the fuel side and a non-premixed zone on the air side. For most conditions, except for relatively high level of partial premixing, the NO formation rate in the non-premixed zone is significantly higher than that in the rich premixed zone. Within the rich premixed zone, the contribution of thermal NO to total NO_x is higher than that of prompt NO, while in the non-premixed zone, the prompt NO is the major contributor. The behavior is related to the transport of acetylene from the rich premixed to the non-premixed zone, and higher concentrations of CH, O, and OH radicals in the latter zone. A notable result in this context is that the existence of CH does not automatically imply that prompt NO will form. The existence of O and OH is also necessary, in addition to CH, to form prompt NO. The relative contributions of thermal and prompt mechanisms to total NO_x are generally insensitive to variations in a_s, but show strong sensitivity to variations in φ. There is a NO_x destruction region sandwiched between the rich premixed and the non-premixed reaction zones. The NO_x destruction occurs mainly through the reburn mechanism. The NO_x emission index (EINOx) is computed as a function of φ and a_s. These results are qualitatively in accord with previous numerical and experimental results for methane-air PPFs.     

From diffusion to premixed flames in an H2/air opposed-jet burner: the role of edge flames

Edge flames obtained on a hydrogen/air non-premixed opposed-jet burner after the local extinction of the disk-shaped diffusion flame are investigated with 2-D direct numerical simulations using detailed chemical kinetics and transport. Over a large range of flowrates, edge flames were found to coexist with the well-known strongly burning diffusion flames corresponding to the upper branch of the S-shaped curve. The critical flowrates of the strong hysteresis associated with the transitions between the two solution branches were identified: re-establishment of the diffusion flame is controlled by the propagation of the edge flame and cannot be represented simply by the extinction scalar dissipation rate. It was also observed that in all the flow conditions simulated, the edge flame was able to consume all the supplied fuel by re-orienting itself, varying its flame surface area, or changing its structure. The latter was found to depend on the flow conditions (which strongly affects the degree of mixing ahead of the edge flame) and can take on different configurations ranging from a triple flame to an essentially premixed flame. Because of flame curvature and the preferential diffusion of hydrogen, the propagation speed of the edge flames was found to be higher than that of the corresponding planar premixed flames.     

Dual-pump CARS temperature and major species concentration measurements in counter-flow methane flames using narrowband pump and broadband Stokes lasers

Dual-pump coherent anti-Stokes Raman scattering (CARS) is used to measure temperature and species profiles in representative non-premixed and partially-premixed CH₄/O₂/N₂ flames. A new laser system has been developed to generate a tunable single-frequency beam for the second pump beam in the dual-pump N₂-CO₂ CARS process. The second harmonic output (∼532 nm) from an injection-seeded Nd:YAG laser is used as one of the narrowband pump beams. The second single-longitudinal-mode pump beam centered near 561 nm is generated using an injection-seeded optical parametric oscillator, consisting of two non-linear β-BBO crystals, pumped using the third harmonic output (∼355 nm) of the same Nd:YAG laser. A broadband dye laser (BBDL), pumped using the second harmonic output of an unseeded Nd:YAG laser, is employed to produce the Stokes beam centered near 607 nm with full-width-at-half-maximum of ∼250 cm⁻¹. The three beams are focused between two opposing nozzles of a counter-flow burner facility to measure temperature and major species concentrations in a variety of CH₄/O₂/N₂ non-premixed and partially-premixed flames stabilized at a global strain rate of 20 s⁻¹ at atmospheric-pressure. For the non-premixed flames, excellent agreement is observed between the measured profiles of temperature and CO₂/N₂ concentration ratios with those calculated using an opposed-flow flame code with detailed chemistry and molecular transport submodels. For partially-premixed flames, with the rich side premixing level beyond the stable premixed flame limit, the calculations overestimate the distance between the premixed and the non-premixed flamefronts. Consequently, the calculated temperatures near the rich, premixed flame are higher than those measured. Accurate prediction of the distance between the premixed and the non-premixed flames provides an interesting challenge for future computations.     

Field measurements of soot volume fractions in laminar partially premixed coflow ethylene/air flames

The conditions under which soot is formed vary widely and depend upon several factors, including pressure, temperature, fuel type, combustor geometry, and extent of premixing. Although it is known that partially premixed flames (PPFs) can become either more or less sooting than their nonpremixed or premixed counterparts, the impact of partial premixing on soot formation across a large equivalence ratio and flow range is still inadequately understood. Comprehensive experimental data are relatively sparse for this important configuration. Herein, we report on soot formation in various ethylene/air PPFs utilizing full-field light extinction. The dimensionless extinction coefficient Kext is an important calibrated constant for the determination of the soot volume fraction for this measurement technique. We find that a value of Kext=7.1 provides results that are in good agreement with benchmark literature data for a nonpremixed flame. We examined the soot microstructures for two flames established at ?=∞ (i.e., nonpremixed) and 5. In both cases, the primary particles were found to be nearly spherical. In case of the nonpremixed flame the average primary soot particle diameter was ∼35 nm, but for the ?=5 flame it was ∼20 nm. However, the parameter responsible for the value of Kext is the average aggregate size and not that of the primary particles. The aggregate sizes are similar for the two flames. We consider this as verification of a constant Kext value over the entire equivalence ratio range. The addition of air to the fuel stream produces an initial increase in the flame height. Further air addition gradually decreases the flame height, which is followed by a more rapid decrease with larger premixing. Likewise, the peak soot concentration first increases with small amounts of air addition (or partial premixing of the fuel stream) and reaches a maximum value at ?∼24. With further air addition, as ? decreases below a value of 20, the soot volume fraction considerably decreases.     

Numerical computations and optical diagnostics of unsteady partially premixed methane/air flames

The structures and dynamics of unsteady laminar partially premixed methane/air Bunsen flames are studied by means of numerical simulations, OH and CH PLIF imaging, and high speed chemiluminescence imaging employing a high framing speed intensified charge coupled device camera. The Bunsen burner has a diameter of 22 mm. Rich methane/air mixtures with an equivalence ratio of 1.5 are injected from the burner into atmosphere at different flow speeds ranging from 0.77 to 1.7 m/s, with Reynolds numbers based on the nozzle flow ranging from 1100 to 2500. The numerical simulations are based on a two-scalar flamelet manifold tabulation approach. Detailed chemistry is used to generate the flamelet manifold tabulation which relates the species concentrations, reaction rates, temperature and density to a distance function G and mixture fraction Z. Two distinct reaction zones are identified using CH and OH PLIF imaging and numerical simulations; one inner reaction zone corresponds to premixed flames on the rich side of the mixture and one outer reaction zone corresponds to mixing controlled diffusion flames on the lean side of the mixture. Under normal gravity conditions both the inner premixed flames and the outer diffusion flames are unsteady. The outer diffusion flames oscillate with a flickering frequency of about 15 Hz, which slightly increases with the burner exit velocity. The inner premixed flames are more random with much more small-scale wrinkling structures. Under zero gravity conditions the outer diffusion flames are stable whereas the inner premixed flames are unstable and highly wrinkled. It appears that the outer diffusion flames are governed by the Rayleigh-Taylor instability whereas the inner premixed flames are dictated by Landau-Darrieus instability. The two-scalar flamelet approach is shown to capture the basic structures and dynamics of the investigated unsteady partially premixed flames.     

Liftoff and extinction characteristics of fuel- and air-stream-diluted methane-air flames

Partial premixing of fuel and oxidizer is of common occurrence in fires. However, most previous studies dealing with flame extinction have focused on nonpremixed flames. In this experimental-numerical study, we examine the effectiveness of fuel-stream versus air-stream dilution for extinguishing laminar methane-air partially premixed (PPFs) and nonpremixed flames (NPF) using the chemically inert fire suppressant CO₂. Experimental measurements were made in lifted methane-air coflow flames, while both counterflow and coflow flames were simulated using a time-accurate implicit algorithm that incorporates detailed chemistry and includes radiation effects. Both measurements and simulations show that with fuel-stream dilution, PPFs stabilize at a higher liftoff height and blow out at a lower CO₂ dilution than NPFs. In contrast, with air-stream dilution, NPFs move to a higher liftoff height and blow out at a lower CO₂ dilution than PPFs. Despite different configurations, there is remarkable similarity in the extinction characteristics of coflow and counterflow flames with regard to the level of partial premixing and air- and fuel-stream dilution. The critical fuel-stream CO₂ mole fraction required for the extinction of both counterflow and coflow flames increases as ? is increased, i.e., as the level of partial premixing is reduced. Conversely, the critical air-stream CO₂ mole fraction decreases as ? is increased. Results also indicate a crossover value of ?≈2.0, corresponding to the stoichiometric mixture fraction of fs=0.5, such that flames (including NPFs) with fs<0.5 are more difficult to extinguish with fuel-stream dilution, since oxygen is the deficient reactant, whereas flames with fs>0.5 are more difficult to extinguish with air-stream dilution, since fuel is the deficient reactant for these flames.     

Effects of strain rate,turbulence, reactant stoichiometry and heat losses on the interaction of turbulent premixed flames with stoichiometric counterflowing combustion products

Intense strain, turbulence, heat transfer, and mixing with combustion products can affect premixed flames in practical combustion devices. These effects are systematically studied in turbulent premixed CH₄/N₂/O₂ flames using a reactant versus product counterflow system and independently varying bulk strain rate, turbulent Reynolds number, equivalence ratio of the reactant mixture, and temperature of the stoichiometric counterflowing combustion products. The flow field and the turbulent flames are investigated using particle image velocimetry (PIV) measurements and laser-induced fluorescence (LIF) imaging of OH. The OH-LIF images are used to identify the interface between the counterflowing streams, referred to here as the gas mixing layer interface (GMLI). The flame response for different flow conditions is compared in terms of the probability of localized extinction along the GMLI, the turbulent flame brush thickness, and flame position relative to the GMLI, by using an OH-LIF-based progress variable. The probability of localized extinction at the GMLI increases as the separation between the turbulent flame brush and the GMLI decreases. Flame fronts in the vicinity of the GMLI are more likely to extinguish as a result of heat losses, dilution of the reaction zone by the product stream, and large local strain rates. A higher probability of localized extinction at the GMLI is induced by either a larger bulk strain rate or a slower flame speed. As the turbulent Reynolds number increases, the corresponding increase in turbulent flame brush thickness enhances the interactions of the flame fronts with the GMLI. Heat losses are substantially less significant for cases in which the turbulent flame brush is sufficiently separated from the GMLI. For flames in close proximity to the GMLI, the effects of the product stream on the flame front differ for lean and rich reactant mixtures. These disparities are attributed in part to differences in the ignitibility of the reactant mixtures by the hot product stream.     

Hydrogen jet flames

A critical review and rethinking of hydrogen jet flame research is carried out. Froude number only based correlations are shown to be deficient for under-expanded jet fires. The novel dimensionless flame length correlation is developed accounting for effects of Froude, Reynolds, and Mach numbers. The correlation is validated for pressures 0.1–90.0 MPa, temperatures 80–300 K, and leak diameters 0.4–51.7 mm. Three distinct jet flame regimes are identified: traditional buoyancy-controlled, momentum-dominated “plateau” for expanded jets, and momentum-dominated “slope” for under-expanded jets. The statement “calculated flame length may be obtained by substitution the concentration corresponding to the stoichiometric mixture in equation of axial concentration decay for non-reacting jet” is shown to be incorrect. The correct average value for non-premixed turbulent flames is 11% by volume of hydrogen in air (range 8%–16%) not stoichiometric 29.5%. All three conservative separation distances for jet fire are shown to be longer than separation distance for non-reacting jet.     

Ultra-lean combustion through the backflow of burned gas in rotating counterflow twin premixed flames

Rotating counterflow twin premixed flames of methane–air were numerically simulated with detailed chemistry based on a similarity solution to explore the leanest extinction limit without preheating and to elucidate the mechanism of “ultra-lean” combustion. We focused on high rotation rate cases in which unrealistic backflow from infinity is allowed to occur since ultra-lean combustion was found to be realized in such a situation. It was found that the reaction zone is in the backflow zone, where the flame’s apparent burning velocity is negative, and that the flame zone width is much smaller than that of a 1-D planar premixed flame due to an inversion of the convexity directions of the profiles of temperature and main species concentrations. The decrease of the flame width seems to make the flame less extinguishable. The equivalence ratio of the leanest flame obtained neglecting radiative heat loss is 0.32, while that obtained with an optically thin radiation model is 0.42, which is still much leaner than the ratio of 0.49 for a 1-D planar premixed flame generated using the same radiation model.     

Multidimensional mathematical modeling and numerical investigation of co-flow partially premixed methane/air laminar flames

The goal of this paper is to analyze, by means of detailed numerical simulations, the influence of the partial premixing level and the adequacy of different mathematical submodels on the modeling of co-flow partially premixed methane-air laminar flames. Five levels of premixing of the primary inlet are considered from an equivalence ratio of Φ=∞ (non-premixed flame) to Φ=2.464. Main flame properties are provided, giving special emphasis to the analysis of pollutant formation. Different mathematical formulation aspects (several chemical mechanisms, radiation effects, mass transport models, and inlet boundary conditions) are tested and validated against experimental data available in the literature. Finite volume techniques over staggered grids are used to discretize the governing equations. A parallel multiblock algorithm based on domain decomposition techniques running with loosely coupled computers has been used to obtain a competitive ratio between computational cost and resources. To assess the quality of the numerical solutions presented in this article, a verification process based on the generalized Richardson extrapolation technique and on the grid convergence index (GCI) has been applied.