Partially Premixed Flames Research Articles

Structure of Unsteady Partially Premixed Flames and the Existence of State Relationships

In this study, we examine the structure and existence of state relationships in unsteady partially premixed flames (PPFs) subjected to buoyancy-induced and external perturbations. A detailed numerical model is employed to simulate the steady and unsteady two-dimensional PPFs established using a slot burner under normal and zero-gravity conditions. The coflow velocity is parametrically varied. The methane-air chemistry is modeled using a fairly detailed mechanism that contains 81 elementary reactions and 24 species. Validation of the computational model is provided through comparisons of predictions with nonintrusive measurements. The combustion proceeds in two reaction zones, one a rich premixed zone and the other a nonpremixed zone. These reaction zones are spatially separated, but involve strong interactions between them due to thermochemistry and scalar transport. The fuel is mostly consumed in the premixed zone to produce CO and H2, which are transported to and consumed in the nonpremixed zone. The nonpremixed zone in turn provides heat and H-atoms to the premixed zone. For the range of conditions investigated, the zero-g partially premixed flames exhibit a stable behavior and a remarkably strong resistance to perturbations. In contrast, the corresponding normal-gravity flames exhibit oscillatory behavior at low coflow velocities but a stable behavior at high coflow velocities, and the behavior can be explained in terms of a global and convective instabilities. The effects of coflow and gravity on the flames are characterized through a parameter VR, defined as the ratio of coflow velocity to jet velocity. For VR ≤ 1 (low coflow velocity regime), the structures of both 0- and 1-g flames are strongly sensitive to changes in VR, while they are only mildly affected by coflow in the high coflow velocity regime (VR > 1). In addition, the spatio-temporal characteristics of the 0- and 1-g flames are markedly different in the first regime, but are essentially similar in the second regime. A more significant difference in the first regime between these flames is the presence of a flow instability that manifests itself through the self-excited oscillations of the 1-g flame and the concomitant flickering of the nonpremixed reaction zone. For VR ≤ 1, as the coflow velocity is increased, the oscillation amplitude decreases and the oscillation frequency increases, both of which are in accord with previous experimental and computational results concerning 1-g jet diffusion flames. The modified conserved scalar approach is found to be effective in characterizing the flame structure and developing state relationships for both steady and unsteady partially premixed flames. This is demonstrated by the fact that the temperature as well as the major and minor species profiles follow similar state relationships in terms of the modified mixture fraction for the 0- and 1-g flames, even though these flames have markedly different spatio-temporal characteristics.

The effect of chemical structure of dimethyl ether (DME) on NOx formation in nonpremixed counterflow flames

To clarify the effect of chemical structure of Dimethyl ether(DME) on NOx formation in nonpremixed counterflow flame, DME flame was investigated numerically to compare the flame structures and NOx emissions with C2H6 and Mixed-fuel. Numerically, the governing equations were solved using the Oppdif code coupled with CHEMKIN package, and DME flames were calculated by Kaiser’s mechanism, while the C2H6 flames and Mixed-fuel flames were calculated by the C3 mechanism. These mechanisms were combined with the modified Miller-Bowman mechanism for the analysis of NOx. Numerical results of nonpremixed counterflow flames show that the EINO of DME nonpremixed flame is low as much as 50 % of the C2H6 nonpremixed flame. The cause of EINO reduction is attributed mainly to the characteristics of partial premixed flame due to the existence of oxygen atom in DME and partly to the O-C bond in DME, instead of C-C bond in hydrocarbon fuels.

Fundamental Studies of NOx Emission Characteristics in Dimethyl Ether (DME)/Air Non-premixed Flames

The NOx emission characteristics of dimethyl ether (DME) in laminar co-axial jet and counterflow non-premixed flames were investigated using experimental and numerical approaches, respectively. The flame structure and NOx emissions of DME were compared to those of C2H6, which has equivalent methyl structures but lacks an oxygen atom. Experimental results showed that, in the co-axial jet case, the combustion of DME had the characteristics of a partial premixed flame. Additionally, it had a shorter flame and lower NOx emissions compared to the C2H6 flame. It is thus concluded that the major cause of low NOx emissions from DME co-axial jet flames may be the short flame length because of the lower stoichiometric air/fuel ratio. The activation of reburning NO chemistry because of the characteristics of the partially premixed flame may also play a role. The computational results of the DME counterflow non-premixed flame revealed that the EINO decreased by approximately 50% relative to that of the C2H6 flame. Although the overall NOx reaction path of the DME flame is similar to that of the C2H6 flame, it is concluded that the DME non-premixed flame has a distinct NO reduction mechanism. This is associated with reburning NO chemistry in the fuel-rich region because of fast pyrolysis and oxidation reactions in comparison to that of the C2H6 flame.

Extinction of methane/air counterflow partially premixed flames

Flame interactions of partially premixed flames (PPFs) were experimentally and numerically studied using counterflow configurations with CH4/air mixtures under atmospheric condition. The main aims of this study were to describe the flame interactions in terms of variations in global strain rate (K) and to understand flame extinction. Two premixed-fuel streams were characterized by equivalence ratios (ϕlean and ϕrich). Experiments were conducted under the following conditions: ϕlean+ϕrich=2, 0.5<ϕlean<0.7 (i.e., 1.3<ϕrich<1.5), and several K values. To describe the interactions, twin flames (TFs), also examined under corresponding ϕ, were used as a criterion, because the ϕ values were within the flammability limits. Experimental results of PPFs showed a different behavior than those of TFs: first, three different luminous layers were observed; second, thin single-layered flames, with a nearly constant flame width, were observed near extinction for several combinations of ϕ; third, PPFs could exist for significantly higher K than TFs. Numerical results supported the experimental results and showed that two reaction paths, CH4+OH→CH3+H2O and CH3+O→CO+H2+H, were remarkably intensified by flame interaction of PPF near extinction; the significance of the reaction paths was determined. In addition, it was found that excess CO production plays an important role in extinction of PPFs.

Prediction of flame formation in highly preheated air combustion

Fundamental information about the ignition position and shape of a flame in highly preheated air combustion was obtained, and the suitability of the suggested reduced kinetic mechanism that reflects the characteristics of the highly preheated air combustion was demonstrated. Flame lift height and flame length with variations of premixed air temperature and oxygen concentration were measured by CH* chemiluminescence intensity, and were computed with a reduced kinetic mechanism. Flame attached near a fuel nozzle started to lift when preheated air temperature became close to auto-ignition temperature and/or oxygen concentration reduced. The flame lift height increased but the flame length decreased with decreasing preheated air temperature and flame length reversed after a minimum value. Calculated results showed good agreement with those of experiment within tolerable error. Flame shape shifted from diffusion flame shape to partial premixed flame shape with increasing lift height and this tendency was also observed in the computation results.

Effect of fuel type on the extinction of fuel and air stream diluted partially premixed flames

Previous investigations have demonstrated that the roles of fuel stream dilution (FSD) and air stream dilution (ASD) in suppressing CO 2-diluted methane flames are strongly influenced by the level of partial premixing. Herein, we compare this influence for both counterflow and coflow laminar non-premixed and partially premixed flames (PPFs) established with various fuels, including methane, ethylene and acetylene. We find that ethylene and acetylene flames are more difficult to extinguish than those burning methane. For methane, FSD is more effective in suppressing PPFs while ASD suppression is more effective for non-premixed flames (NFs). In contrast, FSD suppression is typically more effective than with ASD for the corresponding ethylene and acetylene flames irrespective of the level of partial premixing. ASD is ineffective in suppressing the investigated coflow ethylene and acetylene flames. For counterflow flames, FSD is more effective in suppressing PPFs while ASD is more effective in suppressing NFs irrespective of the fuel burned but the range of equivalence ratios for the relative effectiveness of FSD and ASD depends strongly on the fuel. There is a linear correlation between the transition equivalence ratio at which the effectiveness switches from FSD to ASD, and the rich flammability limit for all three fuels.

DME/Air 비예혼합화염의 NOx 생성특성

The NOx emission characteristics of DME in laminar coaxial jet and counterflow nonpremixed flames were investigated using experimental and numerical approaches, respectively. The flame structure and NOx emission of DME were compared with those of C₂H? and C₃H?. The DME flame was calculated using the Kaiser’s mechanism, while the C₂H? and C₃H? flames were calculated using the C₃ mechanism. These mechanisms were combined with the modified Miller-Bowman mechanism for the analysis of NOx. Experimental results show in coaxial jet flame that DME flame has the characteristics of partial premixed flame and the flame length decreases up to 1/3 than that of C₃H? in the same condition of fuel mass flowrate. Then, the NOx emission of DME decreases to 40% approximately, comparing with that of C₃H?. In the calculated results of counterflow nonpremixed flame, DME flame shows the EINO decreases up to 50% approximately than those of C₂H? and C₃H? flames when the equivalent fuels are consumed per unit mass and time. Although the overall NOx reaction path of DME is similar with other hydrocarbon fuels, it can be identified that DME flame has a distinct NO reduction mechanism due to the reburning NO chemistry in fuel rich region. From these results, we can conclude that the different NOx emission characteristics of DME flame with other hydrocarbon fuels are attributed to not the temperature increase and the activation of NO reactions due to O atom in DME fuel but the rapid processes of pyrolysis/oxidation.

Suppression of fuel and air stream diluted methane–air partially premixed flames in normal and microgravity

The effects of fuel and air stream dilution (ASD) with carbon dioxide on the suppression of normal and microgravity laminar methane–air partially premixed coflow jet flames were experimentally and numerically investigated. Experiments were conducted both in our normal-gravity laboratory and at the NASA Glenn Research Center 2.2 s drop tower. Measurements included flame topology and liftoff heights of diluted flames, critical diluent mole fractions for flame blowout, and the radiant heat loss from flames. The flames were also simulated using an axisymmetric unsteady numerical code that utilizes detailed chemistry and transport models. In addition, counterflow flame simulation results were used to examine similitude between the counterflow and coflow flame suppression, and further characterize the effectiveness of fuel stream versus ASD on flame extinction. A smaller relative fuel stream dilution (FSD) extinguishes partially premixed flames (PPFs) with increasing premixing as compared to dilution of the air stream. Conversely, smaller ASD is required to extinguish PPFs as they become less premixed and approach nonpremixed (NP) behavior. Fuel stream diluted PPFs and air stream diluted NP flames extinguish primarily through a reactant dilution effect while fuel stream diluted NP flames and air stream diluted PPF are extinguished primarily by a thermal cooling effect. Normal gravity flames lift off and blow out with a smaller diluent mole fraction than microgravity flames. The difference between the fuel and ASD effectiveness increases as the gravitational acceleration is reduced. Radiation heat losses are observed to increase with increasing diluent mole fraction and decreasing gravity.

A NUMERICAL INVESTIGATION OF METHANE AIR PARTIALLY PREMIXED FLAMES AT ELEVATED PRESSURES

A numerical investigation of high pressure, counterflow methane-air partially premixed flames (PPFs) is presented to characterize the effect of pressure on flame structure. Four different mechanisms, namely GRI-2.11, GRI-3.0, the San Diego, and the C2 mechanisms have been examined. The mechanisms have been validated by comparing the predicted laminar flame speeds with experimental data. While there is excellent agreement between the measured and predicted laminar flame speeds based on these mechanisms, there are noticeable discrepancies amongst the mechanisms for the prediction of high-pressure PPFs. The GRI 3.0 mechanism is used to examine the detailed PPF structure at elevated pressures. At low pressures, the PPF exhibits a typical double flame structure that consists of rich premixed and nonpremixed reaction zones, and the separation distance between the reaction zones decreases with increasing pressure. However, for pressure above 10 atm, the separation distance becomes nearly independent of pressure. In addition, a critical pressure is observed, above which the PPF structure exhibits anomalous behavior, characterized by the presence of endothermic reactions and the production of CO in the region between the two reaction zones. A rate of production analysis revealed that the anomalous behavior is attributable to the following reactions: H + H2O → OH + H2, CH2CO + M → CH2 + CO + M, and HCO + M → H + CO + M. The critical pressure at which this behavior is first observed increases with the increase in equivalence ratio and/or strain rate. Thermal radiation is found to have a negligible effect on the flame structure for the range of conditions investigated.

A numerical investigation of flame liftoff, stabilization, and blowout

The effects of fuel stream dilution on the liftoff, stabilization, and blowout characteristics of laminar nonpremixed flames (NPFs) and partially premixed flames (PPFs) are investigated. Lifted methane-air flames were established in axisymmetric coflowing jets. Because of their flame suppression characteristics, two predominantly inert agents, CO2 and N2, were used as diluents. 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 using measurements of the reaction zone topologies and liftoff heights of both NPF and PPF. While an undiluted PPF is stabilized at the burner rim, characterized by significant radical destruction and heat loss to the burner, the corresponding undiluted NPF is lifted and stabilized in a low-velocity region extending from the wake of the burner. Detailed comparison of diluted NPF with PPF reveals that the base structures of both the flames are similar and exhibit a double flame structure in the near-field region, where the flame stabilization depends on a balance between the reaction rate and the scalar dissipation rate, which could also be interpreted as a balance between the edge-flame speed undergoing its local scalar dissipation rate and the local flow velocity. As diluent concentration is increased, the flames become weaker, move downstream along the stoichiometric mixture fraction line, and stabilize at a location where they can find a local flow field that has a lower scalar dissipation rate. Further increase of the diluent concentration moves the flames further downstream into the far-field region, where both the NPF and PPF exhibit a triple flame structure, and the flame stabilization mechanism also involves a balance between the triple flame speed and local flow velocity. The PPFs, however, shift to a higher liftoff height and blow out at a lower diluent concentration compared to the NPF, which can withstand larger amounts of dilution. In addition, both NPF and PPF are stabilized at lower liftoff heights and blowout at a lower diluent concentration, when they are diluted with N2 compared to that with CO2. The observed effects of fuel stream dilution and partial premixing on flame liftoff and blowout can be explained using the existing flame stabilization theories.

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.

Gravity effects on partially premixed flames: an experimental-numerical investigation

While premixed and nonpremixed microgravity flames have been extensively investigated, the corresponding literature regarding partially premixed flames (PPFs) is sparse. We report the first experimental investigation of burner-stabilized microgravity PPFs. Partially premixed flames with multiple reaction zones are established in microgravity on a Wolfhard–Parker slot burner in the 2.2 s drop tower at the NASA Glenn Research Center. Microgravity measurements include flame imaging, and thermocouple and radiometer data. Detailed simulations are also used to provide further insight into the steady and transient response of these flames to variations in g. The flame topology and interactions between the various reaction zones are strongly influenced by gravity. The flames widen substantially in microgravity. During the transition from normal to microgravity, the flame structure experiences a fast change and another relatively slower transient change. The fast response is due to the altered advection as the value of g is reduced, while the slow response is due to the changes in the diffusive fluxes. The radiative heat loss from the flames increases in microgravity. A scaling analysis based on a radiation Damköhler number is able to characterize the radiation heat loss.

EFFECT OF FUEL BLENDS ON POLLUTANT EMISSIONS IN FLAMES

ABSTRACT Fuel blending represents a promising approach for reducing both NOx and particulate emissions from flames. This paper reports a fundamental investigation on the effects of blending hydrogen with different fuels (methane and n-heptane) on the structure and emission characteristics of counterflow nonpremixed and partially premixed flames (PPF). The emission behavior is characterized in terms of the concentrations and emission indices of various pollutant species as a function of hydrogen content in the blend. Results indicate that hydrogen blending has a much more favorable effect on emissions in heptane flames than in methane flames. With hydrogen addition in methane/hydrogen blends, the emission index of C2H2 (which is an important soot precursor) is reduced, CO remains unchanged, and NO increases slightly. In heptane/hydrogen blends, however, emission indices of all three species (NO, C2H2, and CO) decrease significantly with hydrogen addition. The behavior is attributed to two factors. First, for the same strain rate, the addition of hydrogen decreases carbon content in the fuel, which reduces the amount of CO and C2H2 formed. Because C2H2 is the major source of CH that leads to the formation of prompt NO, and because prompt NO is the dominant contributor to total NO in heptane flames, hydrogen addition leads to a dramatic decrease in NO emission in these flames. Second, the addition of hydrogen changes combustion chemistry due to the higher reactivity of H2 and the higher concentrations of H and OH radicals, which increase the CO and C2H2 oxidation rates. In addition, this enhances the C1 path for methane oxidation and decomposition of n-heptane to C3–C6 species and, thus, decreases the formation of C2H2. The net result is a decrease in the emission of all three pollutant species, although the effect is significantly higher for n-heptane flames. For PPFs, the addition of hydrogen increases the physical separation between the two reaction zones—namely, the rich premixed and the nonpremixed zones (i.e., the flames become broader). The emission of NO, C2H2, and CO species is also affected favorably due to hydrogen addition, but the effect is less significant for PPFs compared to that for nonpremixed flames.

Effect of pressure on counterflow H 2–air partially premixed flames

A computational investigation of high-pressure hydrogen–air partially premixed flames (PPFs) is reported to characterize the effect of pressure on the flame structure, and the relevance of reaction limits for these flames. The flames are computed using the Mueller mechanism consisting of 19 elementary reactions and 9 species. Although the mechanism has been validated during previous investigations, additional validations are provided at high pressure. The PPF structure is characterized by two spatially distinct reaction zones, namely a rich premixed zone on the fuel side and a nonpremixed zone on the air side. In both reaction zones, consumption of reactants occurs primarily through reactions H + O 2 ↔ OH + O (R1), H 2 + O ↔ OH + H (R2), H 2 + OH ↔ H 2O + H (R3), and H + O 2 + M ↔ HO 2 + M (R9). As pressure increases, it decreases the physical separation between the two reaction zones. This can be attributed to the effects of pressure on (i) flame speed associated with the rich premixed zone, which moves this zone further downstream and (ii) mass diffusivity which moves the nonpremixed zone further upstream (toward the fuel nozzle). At higher pressures, however, these effects are significantly reduced, and the flame maintains its twin-flame structure even at very high pressures. Three reaction limits are identified for these flames. While the chemical structure of the nonpremixed zone is characterized by the first reaction limit in the range of pressure investigated ( p = 1 to 40 atm), that of the rich premixed zone is characterized by transition from first to second limit, and then from second to third limit, as pressure is increased. This implies that H 2–air PPFs can exploit the advantages of the two reaction zones; each dominated by different reaction limits or chain reactions. Thermal radiation is found to have a negligible effect on the flame structure, while the Soret effect is found to cause transition between the reaction limits at lower pressure.

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.

A numerical study of H 2–air partially premixed flames

A numerical investigation of the structure of hydrogen/air partially premixed flames (PPFs) in a counterflow configuration is reported. The partially premixed flame structure is characterized by two spatially distinct reaction zones. Unlike hydrocarbon/air PPFs, the fuel is partially consumed in the rich premixed zone, where H, O, and OH are the major intermediate species, with the remaining fuel being consumed in the nonpremixed zone. In both the reaction zones, consumption of reactants occur primarily through reactions H+ O 2↔ OH+ O ( R1) , H 2+ O↔ OH+ H ( R2) , H 2+ OH↔ H 2 O+ H ( R3) , and H+ O 2+ M↔ HO 2+ M ( R9) . Maximum heat release occurs in the rich premixed zone through reactions R9 and R3. Interactions between the two reaction zones involve the transport of heat from the nonpremixed to the premixed reaction zone and the transport of H 2 from the premixed to the nonpremixed zone. The flame response to variations in equivalence ratio ( φ) and strain rate ( a s) is characterized. Increasing φ and/or a s causes the two reaction zones move closer, and eventually merge with each other. Further increase in φ leads to a nonpremixed flame, while that in a s leads to stretch-induced extinction. Finally, differences between the structures of hydrogen PPFs and hydrocarbon-fuel PPFs are highlighted.

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.

Temperature measurements in steady axisymmetric partially premixed flames by use of rainbow schlieren deflectometry.

We focus on the utility of rainbow schlieren as a tool for measuring the temperature of axisymmetric partially premixed flames (PPFs). Methane-air PPFs are established on a coannular burner. The flames involve two spatially distinct reaction zones, one in an inner premixed region that has a curved tip and a spatially planar wing portion and another that involves an outer nonpremixed zone in which intermediate species burn in air. Schlieren images are found to visualize clearly these PPF characteristics through light deflection by steep refractive-index gradients in the two reaction zone fronts. The temperature distributions of two flames established at fuel-rich mixture equivalence ratios of phi(r) = 1.5 and 2.0, with bulk-averaged velocities, Vreac = 60 cm s(-1) and Vair = 50 cm s(-1), are inferred from color schlieren images, and a measurement error analysis is performed. Errors arise from two sources. One lies in the process of inferring the temperature from the refractive-index measurement by making assumptions regarding the local composition of the flame. We have shown through simulations that the average temperature deviations due to these assumptions are 1.7% for the phi(r) = 1.5 flame and 2.3% for the phi(r) = 2.0 flame. Another source involves the local uncertainty in the measurement of the transverse ray displacement at the filter plane that is used to determine the refractive index and thereafter the flame temperature. We have ascertained that a maximum error of 4.3% in the temperature determination can be attributed to this local measurement uncertainty. This investigation demonstrates the capability of the schlieren technique for providing not only qualitative displays of the PPFs but also full-field-of-view temperature measurements that are accurate, spatially resolved, and nonintrusive.

Effect of varying composition on temperature reconstructions obtained from refractive index measurements in flames

Optical methods (such as holographic interferometry, speckle photography, speckle shearing interferometry, moiré deflectometry, rainbow Schlieren deflectometry and Talbot interferometry, and so forth) have the potential for accurately measuring the entire temperature field associated with multidimensional flames, which may be difficult to do using other techniques. These interferometric or deflectometric techniques first determine the refractive index in flames, and thereafter infer the temperature distribution. The relationship between the refractive index and temperature is obtained by using a state equation and the Gladstone-Dale relation. However, a potential source of error arises since the local composition of the flame being studied is usually unknown. In a previous investigation, we examined the occurrence of this error by assuming the local flame composition to be the same as that of air at the local temperature. This was examined through one-dimensional simulations of counterflow flames. We found that while calculating the temperature from the measured refractive index this assumption could lead to significant errors for some flames and to minimal errors in other flames. This investigation quantifies those errors in the context of two-dimensional flames in both planar and axisymmetric geometries. It is found that the refractive index values of a mixture are nearly identical with those of the refractive index of air for partially premixed flames (PPFs). The maximum error lies in the range of 6.3 to 10.7% for one-dimensional (1-D) counterflow PPFs, and between 6.1 to 8.0% for two-dimensional (2-D) planar and axisymmetric PPFs (for equivalence ratios in the range of 1.5 ≤ φr ≤ 2.0). For nonpremixed flames, however, the maximum error can have values up to 33.8% and 34.5% for the 1-D and 2-D configurations, respectively. Therefore, the accurate inference of the temperature of nonpremixed flames from the measured refractive index distribution requires that an alternative approach be developed. We have developed an interpolation method that reduces the maximum error from 34.5% to 9.8% for these flames, and is associated with even smaller errors in most other regions of these flames.

Flame stretch effects on partially premixed flames

Flame stretch is an important characteristic of flame behavior that arises due to flow motion and nonuniformity, and flame curvature. Various investigators have obtained different expressions for flame stretch by employing dissimilar analytical approaches for diverse flame configurations. In this investigation, we consider two such formulations, one which is “exact” and the other that assumes a simplified unidirectional flow, and apply both relations to characterize the behavior of the rich premixed reaction zone in partially premixed flames (PPFs). Steady two-dimensional partially premixed methane–air slot burner flames are established by introducing a rich fuel–air mixture from the inner slot and air from the two outer slots. The flames contain two distinct spatially separated reaction zones. In order to characterize the flame stretch, the corresponding flame deformation must be first measured. The experimental diagnostics include the use of C ∗ 2-chemiluminescence images, holographic interferometry, and particle image velocimetry. The flame structure depends upon the heat release, temperature, and fluid velocity (therefore, momentum) associated with a particular burner or flame configuration. We focus attention on stretch effects on the inner premixed reaction zone of the PPFs, which exhibits a highly curved portion near its tip and planar portions along its sides. By applying the two formulations, we have inferred unstretched flame speeds and found these to be in good agreement with previously measured results. The methodology also allows us to infer “effective flame speeds” for rich methane–air mixtures with compositions that lie beyond the rich flammability limits for which no data is available in the literature. In general, both formulations provide satisfactory results, although the exact relation is a more accurate representation, as is to be expected. Unidirectional flow can be assumed for relatively larger flames in which the flow behind the flamefront is relatively unperturbed by the preheat zone. As we follow the flame curvature, both the sign and magnitude of the corresponding values of the stretch rate change in a systematic manner. This suggests that the dominating effect in these flames occurs more due to flame curvature and less due to flow nonuniformity effects. In the curved region, the deduced “unstretched” flame speed S uc o is greater than S u o . The measured values of Ma are of O(10 −1).