Partial Premixing Research Articles

Study on Suppression of PAHs Formation through Partial Premixing Approach

Study on Suppression of PAHs Formation through Partial Premixing Approach

Concentrations of Nitric Oxide in Laminar Counterflow Methane/Air Diffusion Flames

Results of nearly 20 different measurements of profiles of concentrations of oxides of nitrogen in steady counterflow methane-air flames are studied and compared with predictions of four different chemical-kinetic mechanisms. The conditions include air-side strain rates between about 10 and 200 s - 1 , pressures between 1 and 15 atm, air temperatures between about 300 and 560 K, partial premixing of air on the fuel side down to an equivalence ratio of 1.5, and dilution of the fuel with nitrogen down to a fuel mole fraction of 0.25, all for a fuel temperature of about 300 K. Although most of the experimental results were taken from the literature, two new measurements were made, employing a newly devised NO x -scrubber technique to correct chemiluminescent NO x -analyzer results for interference from other species, principally hydrocarbons on the fuel side containing two or more carbon atoms. The results show that differences between measured and predicted NO x concentrations tend to be smaller on the fuel-lean side of the flame than they are on the fuel-rich side, where typical discrepancies reach values on the order of 50%. Whereas the Milan mechanism, the San Diego mechanism, and the Gas Research Institute (GRI) mechanism version 2.11 usually produce reasonable agreement with the experimental data, the newer GRI mechanism version 3.0 tends to overpredict NO x under most of the conditions investigated. Because the prompt mechanism is the principal source of NO x in these flames, this might be associated in some way with the tendency of the GRI mechanisms to overpredict acetylene concentrations by about a factor of two, an overprediction that is demonstrated to occur by comparisons with additional experimental data on concentration profiles of intermediate hydrocarbons.

Effect of partial premixing on the sooting structure of methane flames

An experimental investigation of the sooting structure of diluted methane–oxygen counterflow flames is reported for partial premixing in the following two nonpremixed flame configurations: Case 1: Nonpremixed flame on the oxidizer side of the stagnation plane, Case 2: Nonpremixed flame on the fuel side of the stagnation plane. Effects of both fuel-side and oxidizer-side partial premixing for Cases 1 and 2 were investigated in a low-strain-rate (∼6–8 s −1) counterflow flame. Computations using OPPDIF code were in excellent agreement with the measured concentrations of major species and [OH]. Distribution of measured soot volume fraction and particle sizes are presented along with measured distributions of C 2 hydrocarbon species. Soot loading can increase or decrease depending on (a) the level of partial premixing, (b) the side of partial premixing (fuel side or oxidizer side), and (c) the nonpremixed flame configuration. Of particular interest is the trend for fuel-side partial premixing of Case 1, where the peak soot loading, the peak soot particle diameter, and the thickness of the soot zone initially decrease and then increase with progressive partial premixing. The trends presented are discussed based on chemical, dilution, and flow-field effects of partial premixing on soot growth in counterflow flames. Unlike previous literature, which focused on soot inception, this work emphasizes the role of partial premixing on soot growth by taking into account the changes in the temperature–time history of soot particulates in addition to the previously reported “chemical” and “dilution” effects.

Flame Radiation and Soot Emission From Partially Premixed Methane Counterflow Flames

Motivated by heat transfer and environmental concerns, a study of flame radiation and soot particulate emission is reported for partial premixing in low strain-rate (<20s−1) methane counterflow flames. Temperature, OH concentration, and soot volume fraction distributions were measured along the stagnation streamline for progressive addition of oxygen to methane. These measurements along with an optically thin model for soot and gas radiation were used to study the effect of partial premixing on flame radiation and soot emission. It was found that with progressive partial premixing, the peak soot loading and the thickness of the soot zone first decreased and then increased, and while the gas radiation was enhanced, the gas radiative fraction (gas radiation per unit chemical energy release) showed a systematic decrease. The net radiative fraction (soot+gas), however, first decreased and then increased. A configuration with the soot zone spatially entrapped between the premixed and non-premixed reaction zones was experimentally found. This flame configuration has the potential to enhance radiative heat transfer while simultaneously reducing soot and NOx emissions.

Joint scalar transported PDF modeling of nonpiloted turbulent diffusion flames

A transported joint probability density function (JPDF) approach closed at the joint scalar level has been applied to investigate two nonpiloted CH 4/H 2/N 2 turbulent ( Re ≃ 15200 and 22800) jet diffusion flames. The flames have been studied experimentally at the Deutsches Zentrum für Luft- und Raumfahrt (DLR) and at Sandia National Laboratories and are well characterized experimentally through extensive velocity and scalar measurements. The flames offer the opportunity of computational investigations of turbulence–chemistry interactions in CH 4/H 2 flames in the absence of both partial premixing with air and with a smaller stoichiometric mixture fraction ( Z st = 0.167 ) than in the corresponding piloted Sandia flames. The two flames also offer different levels of local extinction. Comparatively few theoretical studies have been performed of these flames and the present work provides an assessment of the ability of the transported PDF approach to reproduce their detailed thermochemical structure. The chemical closure is obtained through a systematically reduced C/H/O/N mechanism featuring 16 independent, 4 dependent, and 28 steady-state scalars. The velocity field is computed using the second moment closure of Speziale et al. and molecular mixing is modeled using the modified Curl's model. It is shown that velocity and scalar fields are generally well reproduced for 10 ⩽ x / D ⩽ 80 though uncertainties in boundary conditions have an impact closer to the burner exit.

Liftoff characteristics of partially premixed flames under normal and microgravity conditions

An experimental and computational investigation on the liftoff characteristics of laminar partially premixed flames (PPFs) under normal (1-g) and microgravity (μ-g) conditions is presented. Lifted methane–air PPFs were established in axisymmetric coflowing jets using nitrogen dilution and various levels of partial premixing. The μ-g experiments were conducted in the 2.2-s drop tower at the NASA Glenn Research Center. A time-accurate, implicit algorithm that uses a detailed description of the chemistry and includes radiation effects is used for the simulations. The predictions are validated through a comparison of the flame reaction zone topologies, liftoff heights, lengths, and oscillation frequencies. The effects of equivalence ratio, gravity, jet velocity, and radiation on flame topology, liftoff height, flame length, base structure, and oscillation frequency are characterized. Both the simulations and measurements indicate that under identical conditions, a lifted μ-g PPF is stabilized closer to the burner compared with the 1-g flame, and that the liftoff heights of both 1-g and μ-g flames decrease with increasing equivalence ratio and approach their respective nonpremixed flame limits. The liftoff height also increases as the jet velocity is increased. In addition, the flame base structure transitions from a triple- to a double-flame structure as the flame liftoff height decreases. A modified flame index is developed to distinguish between the rich premixed, lean premixed, and nonpremixed reaction zones near the flame base. The 1-g lifted flames exhibit well-organized oscillations due to buoyancy-induced instability, while the corresponding μ-g flames exhibit steady-state behavior. The effect of thermal radiation is to slightly decrease the liftoff heights of both 1-g and μ-g flames under coflow conditions.

Concentration measurements of selected hydrocarbons in methane/air partially premixed flames using gas chromatography

Concentrations of selected hydrocarbons (viz. CH 4, C 2H 6, C 2H 4 and C 2H 2) have been measured in CH 4/air partially premixed flames for different primary mixture equivalence ratio ( ϕ) and primary jet velocity using gas chromatography. Visual observations of the flames are also made using digital photography. An axi-symmetric, co-flowing burner is placed on a traversing stage and a quartz probe is used for the collection of the gas sample by placing it at appropriate locations with respect to the burner. The double flame structure is observed with the increase in the premixing air with the fuel. At a high premixing air, the inner flame turns from blue to a greenish one. Hydrocarbons are detected only inside the inner flame structure of the partially premixed flames at all equivalence ratios studied and the concentrations of the non-fuel hydrocarbons are much less than in the non-premixed flames at most of the locations. The peak C 2H 6 and C 2H 4 concentrations are found to be the maximum in a partially premixed flame with an intermediate ϕ, while peak C 2H 2 concentration continually decreases with the decrease in ϕ. The partial premixing decreases the radial transport of species at lower elevations mainly because of the high axial velocity of the primary jet. At higher elevations within the inner flame the radial distribution of C 2H 4 and C 2H 2 concentrations show the peak value to exist at the centerline.

The evolution of incipient soot particles in an inverse diffusion flame of ethene

Motivated by the application of internally air-assisted flares and recent anomalous data that black carbon (BC) emissions changes are not monotonic with the amount of air added inside the fuel stream, a burner was designed to study the flow, emissions, and stability aspects of the existence of both normal and inverse jet diffusion flames in close proximity. Since the radius of curvature of a burner affects all aspects of combustion, such as dynamics, stability, flame structure, and emissions, a slot burner configuration, inspired by the Wolfhard-Parker burner, was adopted. This multi-slot burner consists of five parallel rectangular slots (i.e., the central slot for the inner air, sandwiched between two fuel slots, and surrounded by two outer air slots). When operated with laminar fuel and air flows, this burner produced flame sheets at each fuel-air mixing layer with open optical access to all the flows. Flow fields were characterized using two-dimensional two-component particle image velocimetry, while simultaneous single-lens reflex photography was used to establish the overall height and the location of the flames. Emission measurements specifically targeted BC, a precursor of soot, and oxides of nitrogen (NOx). The experimental test conditions involved constant flows of outer air and propane, and variable inner air flow. At one extreme, i.e., zero inner air flow, two flame bases were produced at the interfaces between the fuel and outer air streams. These flames merged to produce a normal diffusion flame. With the addition of inner air, two more flame bases appeared (referred to as the inner flame) and formed an inverse diffusion flame. With increasing inner air flow, the inner flame transitioned from a closed-tip flame, to an open-tip flame, and eventually to a lifted flame. With increasing inner air flow, the NOx emissions remained constant while the BC emissions, which were 3 g/kg-fuel with no inner air, rose by an order of magnitude as it became an open-tip flame. Only when the inner flame finally lifted off due to the inner air flow, did the BC emissions collapse to near zero. Phenomenological models associated with the importance of partial premixing were proposed to explain this collapse, thereby generalizing this finding to other combustion systems when attempting to reduce BC emissions through secondary internal air addition.

Structure of partially premixed n-heptane–air counterflow flames

To avoid the complexities associated with the droplet/vapor transport and nonuniform evaporation processes, a fundamental investigation of liquid fuel combustion in idealized configurations is very useful. An experimental–computational investigation of prevaporized n-heptane nonpremixed and partially premixed flames established in a counterflow burner is described. There is a general agreement between various facets of our nonpremixed flame measurements and the literature data. The partially premixed flames are characterized by a double flame structure. This becomes more distinct as the strain rate decreases and partial premixing increases, which also increases the separation distance between the two reaction zones. The peak partially premixed flame temperature increases with increasing premixing of the fuel stream. The peak CO 2 and H 2O concentrations are relatively insensitive to partial premixing. The CO and H 2 peak concentrations on the premixed side increase as the fuel-side equivalence ratio decreases. These species are transported to the nonpremixed reaction zone where they oxidize. The C 2 species have peaks in the premixed reaction zone. The concentrations of olefins are ten times larger than those of the corresponding paraffins. The oxidizer is present in partially premixed flames throughout the combustion system and there are no regions characterized by simultaneous high temperature and high fuel concentration. As a result, pyrolysis reactions leading to soot formation are greatly diminished.

An experimental study of partially premixed flame sound

The occurrence of oscillating combustion and combustion instability has led to resurgence of interest in the causes, mechanisms, suppression, and control of combustion noise. Noise generated by enclosed flames is of greater practical interest but is more complicated than that by open flames, which itself is not clearly understood. Studies have shown that different modes of combustion, premixed and non-premixed, differ in their sound generation characteristics. However, there is lack of understanding of the region bridging these two combustion modes. This study investigates sound generation by partially premixed flames. Starting from a non-premixed flame, air was gradually added to achieve partial premixing while maintaining the fuel flow rate constant. Methane, ethylene, and ethane partially premixed flames were studied with hydrogen added for flame stabilization. The sound pressure generated by methane partially premixed flames scales with M 5 compared to M 3 for turbulent non-premixed methane flames. Also, the sound pressure generated by partially premixed flames of ethane and ethylene scales as M ∼4.5. With progressive partial premixing, spectra level increases at all frequencies with a greater increase in the high-frequency region compared to the low-frequency region; flames develop a peak and later a constant level plateau in the low frequency region. The partially premixed flames of methane, ethylene, and ethane generate a similar SPL as a function of equivalence ratio when the fuel volume flow rate is matched. However, when fuel mass flow rate is matched, the ethane and ethylene flames produce a similar SPL, which is lower than that produced by the methane flame.

Characteristics and structure of inverse flames of natural gas

Characteristics and structure of nominally non-premixed flames of natural gas are investigated using a burner that employs simultaneously two distinct features: fuel and oxidiser direct injection, and inverse fuel and oxidiser delivery. At low exit velocities, the result is an inverse diffusion flame that has been noted in the past for its low NO x emissions, soot luminosity, and narrow stability limits. The present study aimed at extending the burner operating range, and it demonstrated that the inverse flame exhibits a varying degree of partial premixing dependent on the discharge nozzle conditions and the ratio of inner air jet and outer fuel jet velocities. These two variables affect the flame length, temperature distributions, and stability limits. Temperature measurements and Schlieren visualisation show areas of enhanced turbulent mixing in the shear region and the presence of a well-mixed reaction zone on the flame centreline. This reaction zone is enveloped by an outer diffusion flame, yielding a unique double-flame structure. As the fuel–air equivalence ratio is decreasing with an increase in the inner jet velocity, the well-mixed reaction zone extends considerably. These findings suggest a method for establishing a flame of uniform high temperature by optimising the coaxial nozzle geometry and flow conditions. The normalised flame length is decreasing exponentially with the air/fuel velocity ratio. Measurements demonstrate that the inverse flame stability limits change qualitatively with varying degree of partial premixing. At the low premixing level, the flame blow-out is a function of the inner and outer jet velocities and the nozzle conditions. The flame blow-out at high degree of partial premixing occurs abruptly at a single value of the inner air jet velocity, regardless of the fuel jet velocity and almost independent of the discharge nozzle conditions.

Fuel effects on NOx emissions in partially premixed flames

The requirement to significantly reduce NOx and particulate emissions while maintaining combustor performance is one of the main drivers for internal combustion engine research. Partially premixing and using fuel blends represent two promising approaches for reducing both the NOx and the particulate emissions from flames. This paper reports on the results of a numerical investigation on the effects of using different fuels on NOx emissions in counterflow partially premixed flames. The fuels investigated include methane, n-heptane, and their blends with hydrogen. The methane flame is computed using the GRI-3.0 mechanism, while the n-heptane flame is computed by combining the Held et al. oxidation mechanism with the Li and Williams NOx mechanism. Results indicate that, with regard to their NOx characteristics, partially premixed flames can be grouped into two distinct regimes, namely a double-flame regime, characterized by high levels of partial premixing and/or low strain rates ( a s ) with two physically separated reaction zones, and a merged-flame regime, characterized by low levels of partial premixing and/or high a s with nearly merged reaction zones. In the first regime, NOx characteristics of both methane and n-heptane flames are strongly affected by changes in equivalence ratio ( ϕ) and strain rate, while in the second regime, they exhibit a relatively weak dependence on ϕ and a s . In addition, the n-heptane and methane flames established under identical conditions exhibit widely different NOx emission behavior in the first regime but qualitatively similar behavior in the second regime. Major differences include (i) significantly higher NO level and NOx emission index, (ii) much wider double-flame regime with regard to ϕ and a s , (iii) dominance of the prompt mechanism over the thermal mechanism in the entire partially premixed regime, and (iv) noticeable reduction in NOx emission with hydrogen addition for n-heptane flames compared to methane flames. These differences are attributable to the different fuel pyrolysis/oxidation chemistry of the two fuels, as the consumption of n-heptane occurs mainly through the C 2 path, while that of methane occurs mainly through the C 1 path. As a result, the amounts of C 2H 2 and, consequently, of CH radicals formed in n-heptane flames are significantly higher than those in methane flames and are responsible for the observed differences in NOx characteristics of the two fuels.

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 K ext is an important calibrated constant for the determination of the soot volume fraction for this measurement technique. We find that a value of K ext = 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 ϕ = ∞ (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 ϕ = 5 flame it was ∼20 nm. However, the parameter responsible for the value of K ext 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 K ext 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 ϕ ∼ 24 . With further air addition, as ϕ decreases below a value of 20, the soot volume fraction considerably decreases.

Parametric and statistical investigation of the behavior ofBaBlifted flame over a turbulent free-jet structure

Partially premixed combustion is involved in many practical applications, due to partial premixing of combustible and oxidant gases before ignition, or due to local extinctions, which lead to mixing of reactants and burned gases. To investigate some features of flames in stratified flows, the stabilization processes of lifted turbulent jet flames are studied. This work offers a large database of liftoff locations of flames stabilized on turbulence-free jets for different fuels and nozzle diameters studied over their flame stability domains. Methane, propane, and ethylene flames are investigated for nozzle diameters of 2, 3, 4, and 5 mm. Blowout velocities are measured and compared with an approach based on large-scale structures of the jet. The axial and radial locations of the flame base are measured by planar laser-induced fluorescence (PLIF) of the OH radical through high sampling (at least 5000 points). From this large database the average locations of the flame base are analyzed for the fuels investigated. The pdfs exhibit an evolution of their shapes according to the region of the turbulent jet where the flame stabilizes (potential core, transition to turbulence, or fully developed turbulence regions). This dependence is probably due to the interaction of the flame with the jet structures. This is confirmed by the comparison between the amplitude of the height fluctuations and the local size of the large-scale structures deduced from particle image velocimetry measurements and self-similarity laws for velocity. The results show the flame can be carried over a distance equal to the local diameter of the jet within the region of fully developed turbulence for propane and ethylene, and over a slightly larger distance for methane.

Parametric and statistical investigation of the behavior of a lifted flame over a turbulent free-jet structure

Partially premixed combustion is involved in many practical applications, due to partial premixing of combustible and oxidant gases before ignition, or due to local extinctions, which lead to mixing of reactants and burned gases. To investigate some features of flames in stratified flows, the stabilization processes of lifted turbulent jet flames are studied. This work offers a large database of liftoff locations of flames stabilized on turbulence-free jets for different fuels and nozzle diameters studied over their flame stability domains. Methane, propane, and ethylene flames are investigated for nozzle diameters of 2, 3, 4, and 5 mm. Blowout velocities are measured and compared with an approach based on large-scale structures of the jet. The axial and radial locations of the flame base are measured by planar laser-induced fluorescence (PLIF) of the OH radical through high sampling (at least 5000 points). From this large database the average locations of the flame base are analyzed for the fuels investigated. The pdfs exhibit an evolution of their shapes according to the region of the turbulent jet where the flame stabilizes (potential core, transition to turbulence, or fully developed turbulence regions). This dependence is probably due to the interaction of the flame with the jet structures. This is confirmed by the comparison between the amplitude of the height fluctuations and the local size of the large-scale structures deduced from particle image velocimetry measurements and self-similarity laws for velocity. The results show the flame can be carried over a distance equal to the local diameter of the jet within the region of fully developed turbulence for propane and ethylene, and over a slightly larger distance for methane.

Nonpremixed flame control with microjets

The hydrodynamic control of buoyant nonpremixed flames is investigated by injecting high-momentum fluid through a central microjet. The resulting flame characteristics are mapped for jets of different strengths. The flame height decreases linearly with an increase in the microjet Froude number as the flow changes from a buoyancy-dominated to a momentum-controlled regime. The flame luminosity is reduced by injecting stronger microjets. The jets alter the flame structure by establishing strong entrainment of the ambient air from the quiescent surroundings. The introduction of an inert species as the microjet fluid has a similar qualitative effect as air. Microjet assistance is as effective as partial premixing for reducing the flame height and luminosity.

Leading-Edge Reaction Zones in Lifted-Jet Gas and Spray Flames

An investigation of the leading edge characteristics in lifted turbulent methane-air (gaseous) and ethanol-air (spray) diffusion flames is presented. Both combustion systems consist of a central nonpremixed fuel jet surrounded by low-speed air co-flow. Non-intrusive laser-based diagnostic techniques have been applied to each system to provide information regarding the behavior of the combustion structures and turbulent flow field in the regions of flame stabilization. Simultaneous sequential CH-PLIF/particle image velocimetry and CH-PLIF/Rayleigh scattering measurements are presented for the lifted gaseous flame. The CH-PLIF data for the lifted gas flame reveals the role that ``leading-edge'' combustion plays as the stabilization mechanism in gaseous diffusion flames. This phenomenon, characterized by a fuel-lean premixed flame branch protruding radially outward at the flame base, permits partially premixed flame propagation against the incoming flow field. In contrast, the leading edge of the ethanol spray flame, examined using single-shot OH-PLIF imaging and smoke-based flow visualization, does not exhibit the same variety of leading-edge combustion structure, but instead develops a dual reaction zone structure as the liftoff height increases. This dual structure is a result of the partial evaporation (hence partial premixing) of the polydisperse spray and the enhanced rate of air entrainment with increased liftoff height (due to co-flow). The flame stabilizes in a region of the spray, near the edge, occupied by small fuel droplets and characterized by intense mixing due to the presence of turbulent structures.

NO x 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.

Structure and Extinction of Heptane/Air Partially Premixed Flames

A numerical investigation of the structure and extinction of n-heptane/air partially premixed e ames in a countere ow cone guration is reported. The e ame is computed using a detailed reaction mechanism consisting of 275 elementary reactions and 41 species. The mechanism is validated by comparing its predictions with measurements for both the premixed and nonpremixed e ames. The partially premixed e ame structure is characterized by twostage burning or two spatially distinct, but synergistically coupled, reaction zones, namely, a rich premixed zone on the fuel side and a nonpremixed zone on the air side. Two major pathways characterize the n-heptane/air chemistry in these two reaction zones. The fuel is completely consumed in the premixed reaction zone, with ethylene and acetylene being the major intermediate species. Interactions between the two zones involve the transport of heat from the nonpremixed to the premixed reaction zone and the transport of CO, H 2, and C2H2 from the premixed to the nonpremixed reaction zone. The e ame response to variations in equivalence ratio and strain rate is characterized. Increasing the equivalence ratio and/or strain rate to a critical value leads to merging of the two reaction zones. Further increase in equivalence ratio leads to a nonpremixed e ame, whereas that in strain rate leads to e ame extinction. The partially premixed combustion regime is obtained by computing the critical strain rate for merging as a function of the equivalence ratio. The e ame structure is characterized in terms of a modie ed mixture fraction (conserved scalar ), and laminar e amelet proe les are provided. Stretch-induced extinction is also investigated. Results indicate that as the level of partial premixing is increased, the e ame extinction occurs at increasingly higher strain rates, implying that partially premixed e ames are less prone to extinction compared to nonpremixed e ames.

The effect of Lewis number on radiative extinction and flamelet modeling

This study investigates the influence of Lewis number on radiative extinction and flamelet modeling. The interaction of Lewis number with different transient effects, such as fluctuating reactant concentrations, fluctuating reactant temperatures, and variable partial premixing, are considered. The results underscore the importance of including the effect of non-unity Lewis numbers and their interaction with chemistry and unsteadiness in improving the predictive capability of flamelet combustion modeling approach, and in precise determination of radiation-induced extinction limits. An increase of Lewis pushes the radiation-induced extinction limit, which occurs at low strain rates, toward higher values of strain rates.