Peak OH Research Articles

Effects of CO and H2 addition on OH* chemiluminescence characteristics in laminar rich inverse diffusion flames

Optical diagnostic technique based on chemiluminescence emissions is a promising alternative flame monitoring method to indicate the combustion regime and related characteristic parameters. In this work, laminar rich inverse diffusion flames were numerically studied to elucidate the effects of CO and H2 addition on OH* chemiluminescence characteristics. A jet flame simulation based on CFD was carried out to determine the effects of CO and H2 addition on OH* chemiluminescence distribution, and a 1-D counterflow diffusion flame calculation by CHEMKIN-PRO was performed to analyze the effects of CO and H2 addition on the formation characteristic of OH* chemiluminescence. The results show that OH* chemiluminescence is mainly concentrated at the central axis of the downstream region in inverse diffusion flames. With the increase of oxy-fuel equivalence ratio, the intensity of OH* chemiluminescence gradually decreases, and location of peak OH* intensity gradually moves from the downstream flame to the upstream. When the oxy-fuel equivalence ratio is fixed, the increase of the CO content in the fuel will reduce the H concentration in the flame, thereby reducing the concentration of OH* radicals. But the increase of the H2 addition would only accelerate the burning velocity of the flame without having a significant impact on the concentration of OH* radicals. When the flow rates of fuel and oxygen are fixed, the distribution and formation characteristics of OH* chemiluminescence are affected by the synergetic impact of fuel component and oxy-fuel equivalence ratio. The effect of H2 addition plays the most important role in the upstream of flame, and CO addition mainly affects the OH* radical production in the downstream region.

Experimental study of the influence of dimethyl methylphosphonate on methane/air coflow diffusion flames using OH-PLIF

Experiments were conducted to evaluate the inhibition efficiency of dimethyl methylphosphonate (DMMP) on methane/air coflow diffusion flames. An optical diagnostic system (hydroxyl planar laser-induced fluorescence) was used to measure the OH radical distribution in the flames. The minimum extinguishing concentration (MEC) of CO2, as a function of DMMP addition, was measured to characterize the inhibition efficiency of DMMP. Results show that, when the concentration of DMMP is higher than 0.6%, DMMP’s marginal inhibition efficiency reduces to almost zero. It is found that the normalized peak OH concentrations keep unchanged with a value of 0.9 for flames at conditions near extinction limit. This is firstly observed based on experimental measurements. Furthermore, using DMMP alone cannot extinguish the flame even with 4.59% DMMP addition. When the inhibition efficiency of DMMP gets saturated, a large amount of phosphorus-containing molecules agglomerate around the flame. These particles act as sinks for the active phosphorus-containing species with a high inertial force, which are hard to diffuse horizontally into the flame. Therefore, it is concluded that the generation of the particles is a reason for the saturation phenomenon of DMMP in extinguishing the coflow diffusion flames.

Transported PDF simulation of turbulent CH4/H2 flames under MILD conditions with particle-level sensitivity analysis

Transported probability density function (TPDF) simulation with sensitivity analysis has been conducted for turbulent non-premixed CH4/H2 flames of the jet-into-hot-coflow (JHC) burner, which is a typical model to emulate moderate or intense low oxygen dilution combustion (MILD). Specifically, two cases with different levels of oxygen in the coflow stream, namely HM1 and HM3, are simulated to reveal the differences between MILD and hot-temperature combustion. The TPDF simulation well predicts the temperature and species distributions including those of OH, CO and NO for both cases with a 25-species mechanism. The reduced reaction activity in HM1 as reflected in the peak OH concentration is well correlated to the reduced oxygen in the coflow stream. The particle-level local sensitivities with respect to mixing and chemical reaction further show dramatic differences in the flame characteristics. HM1 is less sensitive to mixing and reaction parameters than HM3 due to the suppressed combustion process. Specifically, for HM1 the sensitivities to mixing and chemical reactions have comparable magnitude, indicating that the combustion progress is controlled by both mixing and reaction in MILD combustion. For HM3, there is however a change in the combustion mode: during the flame initialization, the combustion progress is more sensitive to chemical reactions, indicating that finite-rate chemistry is the controlling process during the autoignition process for flame stabilization; at further downstream where the flame has established, the combustion progress is controlled by mixing, which is characteristic of nonpremixed flames. An examination of the particles with the largest sensitivities reveals the difference in the controlling mixtures for flame stabilization, namely, the stoichiometric mixtures are important for HM1, whereas, fuel-lean mixtures are controlling for HM3. The study demonstrates the potential of TPDF simulations with sensitivity analysis to investigate the effects of finite-rate chemistry on the flame characteristics and emissions, and reveal the controlling physio-chemical processes in MILD combustion.

Effect of Toluene Addition on the PAH Formation in Laminar Coflow Diffusion Flames of n-Heptane and Isooctane

Characterization of soot formation is becoming more important for gasoline surrogate fuels, requiring a deeper understanding of the effect of toluene ratio on PAHs formation. In this work, the different size PAHs distribution in laminar diffusion flames was measured by PLIF technique, and the chemiluminescences of OH and CH radicals were recorded by ICCD coupled with band-pass filters. The effect of toluene addition to n-heptane and isooctane on the flame and PAHs formation was comparatively studied. Then, the chemical kinetic of PAHs was analyzed using the CHEMKIN Diffusion Flame model. The experimental results showed that the cool flame area ranks as n-heptane > isooctane > toluene may due to the NTC action. The OH intensity and flame lift-off height show a monotonous increasing trend with the toluene ratio. At the same toluene ratio, the peak OH intensity and flame lift-off height of n-heptane/toluene is lower than that of isooctane/toluene. The CH intensity in n-heptane, isooctane, and toluene flames shows completely different distribution characteristics. As the toluene ratio increases, the 320 nm-LIF in n-heptane and isooctane flames shows a weakening monotonous increasing trend with different inflection points. The 360/400/450 nm-LIF in n-heptane and isooctane flames show a nonmonotonic trend with different peak points. The peak point is 50% toluene ratio in n-heptane/toluene flames and 40% in isooctane/toluene flames, and it corresponds exactly to the smoke point of the flame. The kinetic analysis showed that the benzyl has a significant effect on the formation of A2–A4 through the generation of C10H9, C9H7, and C14H12. A1 trend is dominated by C6H5CH3 + H = A1 + CH3 and A1– + CH4 → A1 + CH3. The trends of A2, A3 and A4 are mainly dominated by the reactions between their own dehydrogenation groups with H2.

Comparison on the Absolute Concentrations of Hydroxyl and Atomic Oxygen Generated by Five Different Nonequilibrium Atmospheric-Pressure Plasma Jets

Nonequilibrium atmospheric pressure plasma jets (N-APPJs) generate near or at room temperature plasmas in open space, which is attractive for various applications such as plasma medicine. Because of this advantage, different types of N-APPJs have been reported. Reactive species, such as hydroxyl (OH) and atomic oxygen (O), generated by N-APPJs, are believed to play important roles for many applications. However, there is no report on the comparison of the reactive species concentrations generated by different types of N-APPJs. In this paper, the absolute concentrations of OH and O generated by five different configurations of widely used N-APPJs are compared. For this initial study, all five N-APPJS are driven by the same pulsed dc power supply. Besides, the effects of different operating parameters, including the amplitude of the applied voltage, frequency, the content of the working gas, and the gas flow rate, on their concentrations are investigated. It is found that the OH and O concentrations of the five N-APPJs could have up to one order and five-folder difference, respectively. On the other hand, for all five devices, their peak O concentrations are about two orders higher than their peak OH concentrations. Finally, from the energy point of view, the reactive species generation efficiency of these devices are discussed and future work is proposed.

The TOMCAT global chemical transport model v1.6: description of chemical mechanism and model evaluation

Abstract. This paper documents the tropospheric chemical mechanism scheme used in the TOMCAT 3-D chemical transport model. The current scheme includes a more detailed representation of hydrocarbon chemistry than previously included in the model, with the inclusion of the emission and oxidation of ethene, propene, butane, toluene and monoterpenes. The model is evaluated against a range of surface, balloon, aircraft and satellite measurements. The model is generally able to capture the main spatial and seasonal features of high and low concentrations of carbon monoxide (CO), ozone (O3), volatile organic compounds (VOCs) and reactive nitrogen. However, model biases are found in some species, some of which are common to chemistry models and some that are specific to TOMCAT and warrant further investigation. The most notable of these biases are (1) a negative bias in Northern Hemisphere (NH) winter and spring CO and a positive bias in Southern Hemisphere (SH) CO throughout the year, (2) a positive bias in NH O3 in summer and a negative bias at high latitudes during SH winter and (3) a negative bias in NH winter C2 and C3 alkanes and alkenes. TOMCAT global mean tropospheric hydroxyl radical (OH) concentrations are higher than estimates inferred from observations of methyl chloroform but similar to, or lower than, multi-model mean concentrations reported in recent model intercomparison studies. TOMCAT shows peak OH concentrations in the tropical lower troposphere, unlike other models which show peak concentrations in the tropical upper troposphere. This is likely to affect the lifetime and transport of important trace gases and warrants further investigation.

From elementary kinetics in perfectly stirred reactors to reduced kinetics utilizable in turbulent reactive flow simulations for combustion devices

This study addresses the protocol of tests for evaluating the performance of skeletal, reduced and highly-reduced chemical kinetics mechanisms for one-dimensional (1D) laminar flame simulations and the validity of these tests for turbulent flame applications. Taking n-heptane as an example fuel, it is first shown that there are apparent disagreements regarding the elementary-reaction chemical kinetics models which should be emulated by reduced kinetic mechanisms. Further it is also shown, expectably, that the functional relationship between mass fractions and temperature is different between 0D and 1D models, and it is different among 1D models having different molecular species-transport formulations. Having identified the retention of this functional relationship as pivotal in the success of highly-reduced reaction mechanisms to accurately simulate spatial configurations, it is demonstrated that the unity Lewis-number assumption biases this functional relationship such that the highly-reduced chemical kinetic mechanism LS2T 20-species mechanism (LS2T 20) used for 1D flame simulations fails to agree with the template but provides accurate predictions in conjunction with, for example, the mixture-average diffusion model at otherwise the same initial conditions as the unity-Lewis-number simulation. The LS2T 20 success extends to the cold-ignition regime. For rich conditions the mixture-average diffusion approximation is less acceptable, and if the transport model does not include the direct effect of the heavy species (which are all modeled in LS2T) on the light species (which are all computed in LS2T), moderate departures of the 1D predictions from the template are obtained around the peak OH mass fraction, whereas CO2 is still excellently reproduced by LS2T 20. It is then conjectured that, independent of the reduction method, the more reduced is a kinetic mechanism, the more accurate the transport formulation should be to ensure the preservation of the species/temperature functional relationship; due to the extensive investigation necessary to evaluate this conjecture, this activity is relegated to future studies.

Synthesis of Catechin-Gelatin Nanofiber by Electrospinning

Catechin and gelatin are important natural products for food, medical, pharmaceutical and cosmetic industry. We have successfully synthesized catechin-gelatin nanofiber by electrospinning process. Catechin-gelatin nanofiber was synthesized by using gelatin from yellow fin skin tuna fish as biopolymer, polyethylene oxide (PEO) as spinnability improver polymer, acetic acid as solvent and catechin as bioactive component, respectively. Morphology and structure of bioactive catechin-gelatin nanofiber were characterized by scanning electron microscopy (SEM) and fourier transform infrared spectroscopy (FTIR), respectively. SEM analysis showed that morphology of nanofiber was very smooth without bead on nanofiber string. The average of catechin-gelatin nanofiber diameter was 389 nm. FTIR analysis results were used to confirm structure of catechin-gelatin nanofiber. Catechin-gelatin nanofiber has vibration band peak of amide A (N-H) at 3289,043 cm-1 and amide B (N-H) 3062,310 cm-1, amide I (C=O) at 1643,812 cm-1, amide II (N-H and CN) at 1538,949 cm-1, amide III (C-N) at 1237,11 cm-1 from gelatin, C-O-C from PEO at 1143,583 cm-1, and vibration band peak OH at 3200-3600 cm-1, and at C-O ether around 1300-1100 from catechin, respectively. FTIR spectra showed us that there is no change in chemical structure of gelatin and catechin in nanofiber which was produced by electrospinning process. Catechin-gelatin nanofiber can inhibit S. Aureus bacteria around 43.38%

OH density optimization in atmospheric-pressure plasma jet by using multiple ring electrodes

OH radical is one of the important reactive species generated by non-equilibrium atmospheric-pressure plasma jets, which is believed to play an important role in plasma medicine applications such as cancer therapy, wound healing and sterilization. In this study, a method to increase OH density is proposed. By using multiple pairs of ring electrodes, we generate 3–5 times more OH radicals than in the common device which uses only one high-voltage ring electrode. Discharge imaging shows that the plasma plume with only one ring electrode is longer and its emission intensity is higher than those with multiple pairs of ring electrodes. Further studies indicate that the distribution of OH radicals is significantly influenced by the gas flow rate. At higher gas flow rates, the OH peak concentration is detected further away from the nozzle, and the position of the peak OH concentration correlates with the product of the gas flow velocity and the pulse duration. As observed from the emission spectra, multiple electrodes only enhance the plasma inside the tube rather than the plasma plume in the surrounding air. These results suggest that the OH radicals are mainly generated inside the tube and then delivered to the outer plasma plume region by the gas flow.

Numerical study of the effect of water content on OH production in a pulsed-dc atmospheric pressure helium–air plasma jet**Project supported by the National Natural Science Foundation of China (Grant No. 11465013), the Natural Science Foundation of Jiangxi Province, China (Grant No. 20151BAB212012), and the International Science and Technology Cooperation Program of China (Grant No. 2015DFA61800).

A numerical study of the effect of water content on OH production in a pulsed-dc atmospheric pressure helium–air plasma jet is presented. The generation and loss mechanisms of the OH radicals in a positive half-cycle of the applied voltage are studied and discussed. It is found that the peak OH density increases with water content in air (varying from 0% to 1%) and reaches 6.3×1018 m−3 when the water content is 1%. Besides, as the water content increases from 0.01% to 1%, the space-averaged reaction rate of three-body recombination increases dramatically and is comparable to those of main OH generation reactions.

Combustion Processes as a Source of High Levels of Indoor Hydroxyl Radicals through the Photolysis of Nitrous Acid.

Hydroxyl radicals (OH) are known to control the oxidative capacity of the atmosphere but their influence on reactivity within indoor environments is believed to be of little importance. Atmospheric direct sources of OH include the photolysis of ozone and nitrous acid (HONO) and the ozonolysis of alkenes. It has been argued that the ultraviolet light fraction of the solar spectrum is largely attenuated within indoor environments, thus, limiting the extent of photolytic OH sources. Conversely, the ozonolysis of alkenes has been suggested as the main pathway of OH formation within indoor settings. According to this hypothesis the indoor OH radical concentrations span in the range of only 10(4) to 10(5) cm(-3). However, recent direct OH radical measurements within a school classroom yielded OH radical peak values at moderate light intensity measured at evenings of 1.8 × 10(6) cm(-3) that were attributed to the photolysis of HONO. In this work, we report results from chamber experiments irradiated with varying light intensities in order to mimic realistic indoor lighting conditions. The exhaust of a burning candle was introduced in the chamber as a typical indoor source causing a sharp peak of HONO, but also of nitrogen oxides (NOx). The photolysis of HONO yields peak OH concentration values, that for the range of indoors lightning conditions were estimated in the range 5.7 ×· 10(6) to 1.6 × 10(7) cm(-3). Excellent agreement exists between OH levels determined by a chemical clock and those calculated by a simple PSS model. These findings suggest that significant OH reactivity takes place at our dwellings and the consequences of this reactivity-that is, formation of secondary oxidants-ought to be studied hereafter.

Minor-species structure of premixed cellular tubular flames

Quantitative spatially resolved measurements of atomic hydrogen and hydroxyl radicals are reported of cellular phenomena in lean laminar premixed hydrogen–air tubular flames. Femtosecond two-photon laser-induced fluorescence (fs-TPLIF) enables photolytic-interference-free measurement of H atoms. Quenching corrections are calculated from previously measured temperature and major-species concentrations. In the sub-unity Lewis number, highly stretched and curved premixed flame structure, the strong effect of thermal-diffusive imbalance is observed through clear local extinction (dearth of H atoms) and highly curved reaction cells. Peak H-atom number densities of 5*1015cm–3 and peak OH number densities of 9*1015cm−3 are found but remain approximately the same value despite doubling the flame stretch from 200 to 400s–1. Comparisons to non-cellular tubular flame numerical predictions show that the cellular transition increases temperature and local equivalence ratio but lowers peak number densities of H and OH. For cellular instabilities in the tubular flame, the thermal-diffusive imbalance maintains a similar local flame structure (i.e., same curvature and peak values of H, OH, T, and local equivalence ratio) that is insensitive to stretch rate.

Soot formation characteristics in a lab-scale turbulent pulverized coal flame with simultaneous planar measurements of laser induced incandescence of soot and Mie scattering of pulverized coal

Soot formation characteristics of a lab-scale pulverized coal flame were investigated by performing carefully controlled laser diagnostics. The spatial distributions of soot volume fraction and the pulverized coal particles were measured simultaneously by laser induced incandescence (LII) and Mie scattering imaging, respectively. In addition, the radial distributions of the soot volume fraction were compared with the OH radical fluorescence, gas temperature and oxygen concentration obtained in our previous studies [1,2]. The results indicated that the laser pulse fluence used for LII measurement should be carefully controlled to measure the soot volume fraction in pulverized coal flames. To precisely measure the soot volume fraction in pulverized coal flames using LII, it is necessary to adjust the laser pulse fluence so that it is sufficiently high to heat up all the soot particles to the sublimation temperature but also sufficiently low to avoid including a too large of a change in the morphology of the soot particles and the superposition of the LII signal from the pulverized coal particles on that from the soot particles. It was also found that the radial position of the peak LII signal intensity was located between the positions of the peak Mie scattering signal intensity and peak OH radical signal intensity. The region, in which LII signal, OH radical fluorescence and Mie scattering coexisted, expanded with increasing height above the burner port. It was also found that the soot formation in pulverized coal flames was enhanced at locations where the conditions of high temperature, low oxygen concentration and the existence of pulverized coal particles were satisfied simultaneously.

Local flame structure and turbulent burning velocity by joint PLIF imaging

In this study, we have examined local flame structure by joint PLIF imaging, using simultaneous HCHO/OH-PLIF and orthogonal OH-PLIF measurements. By considering the degree of flame wrinkling, three-dimensional flame surface area is obtained from 2D images. We have used a cyclone–jet combustor to establish turbulent premixed flames for propane/air mixtures in a wide range of turbulence, covering the flamelet regime and thin-reaction-zones regime on combustion diagram. Results show that as the mean velocity at the combustor exit is increased, more complex flame structure is observed with multiple flamelets. Near the global extinction, peak OH concentration in reaction zone becomes almost zero whereas HCHO concentration in preheat zone still remains. As a result, the large reduction of heat release rate occurs, resulting in local extinction. The turbulent burning velocity determined by the mean flame shape is increased with turbulence, and the bending effect is observed in thin-reaction-zones regime. On the other hand, different from the flame perimeter, the flame surface area is linearly increased. Since the turbulent burning velocity is well predicted by the classical formula in wrinkled flames, the bending is explained by the reduction of local burning velocity. Thus, within the frame work in our measurement, the flamelet approach could be still valid even in the thin-reaction-zones regime.

Study of the H + O + M reaction forming OH ∗: Kinetics of OH ∗ chemiluminescence in hydrogen combustion systems

The temporal variation of OH ∗ (A 2Σ +) chemiluminescence in hydrogen oxidation chemistry has been studied in a shock tube behind reflected shock waves at temperatures of 1400–3300 K and at a pressure of 1 bar. The aim of the present work is to obtain a validated reaction scheme to describe OH ∗ formation in the H 2/O 2 system. Temporal OH ∗ emission profiles and ignition delay times for lean and stoichiometric H 2/O 2 mixtures diluted in 97–98% argon were obtained from the shock-tube experiments. Based on a literature review for the hydrogen combustion system, the key reaction considered was H + O + M = OH ∗ + M (R1). The temperature dependence of the measured peak OH ∗ emission from the shock tube and the peak OH ∗ concentration from a homogeneous closed reactor model are compared. Based on these results a reaction rate coefficient of k 1 = (1.5 ± 0.4) × 10 13 exp(−25 kJ mol −1/ RT) cm 6 mol −2 s −1 was found for the forward reaction (R1) which is slightly higher than the rate coefficient suggested by Hidaka et al. (1982). The comparison of measured and simulated absolute concentrations shows good agreement. Additionally, a one-dimensional laminar premixed low-pressure flame calculation was performed for where absolute OH ∗ concentration measurements have been reported by Smith et al. (2005). The absolute peak OH ∗ concentration is fairly well reproduced if the above mentioned rate coefficient is used in the simulation.

Kinetic effects of toluene blending on the extinction limit of n-decane diffusion flames

The impact of toluene addition in n-decane on OH concentrations, maximum heat release rates, and extinction limits were studied experimentally and computationally by using counterflow diffusion flames with laser induced fluorescence imaging. Sensitivity analyses of kinetic path ways and species transport on flame extinction were also conducted. The results showed that the extinction strain rate of n-decane/toluene/nitrogen flames decreased significantly with an increase of toluene addition and depended linearly on the maximum OH concentration. It was revealed that the maximum OH concentration, which depends on the fuel H/C ratio, can be used as an index of the radical pool and chemical heat release rate, since it plays a significant role on the heat production via the reaction with other species, such as CO, H 2, and HCO. Experimental results further demonstrated that toluene addition in n-decane dramatically reduced the peak OH concentration via H abstraction reactions and accelerated flame extinction via kinetic coupling between toluene and n-decane mechanisms. Comparisons between experiments and simulations revealed that the current toluene mechanism significantly over-predicts the radical destruction rate, leading to under-prediction of extinction limits and OH concentrations, especially caused by the uncertainty of the H abstraction reaction from toluene, which rate coefficient has a difference by a factor of 5 in the tested toluene models. In addition, sensitivity analysis of diffusive transport showed that in addition to n-decane and toluene, the transport of OH and H also considerably affects the extinction limit. A reduced linear correlation between the extinction limits of n-decane/toluene blended fuels and the H/C ratio as well as the mean fuel molecular weight was obtained. The results suggest that an explicit prediction of the extinction limits of aromatic and alkane blended fuels can be established by using H/C ratio (or radical index) and the mean fuel molecular weight which represent the rates of radical production and the fuel transport, respectively.

Burning velocity and OH concentration in premixed combustion

In the present paper, we have simulated laminar premixed flames in several types of flow configuration. The counter-flow flame, 1D flame in uniform flow, and 2D flame on a slot burner have been considered. The fuel is propane, and detailed chemistry has been used. We have obtained burning velocity in these flames to examine the response of peak OH concentration in flame zone to burning velocity (SL). In case of counter-flow flame, peak OH concentration is decreased at the larger stretch rate. At the same time, the burning velocity is decreased. The peak OH concentration of 1D flame in uniform flow is decreased at lower equivalence ratio, with smaller burning velocity. It is interesting to note that, for both flames as well as 2D flame, the same linear relationship is observed between two parameters of peak OH and SL. Thus, consistent with earlier findings of turbulent flames, peak OH concentration can be a good measure of burning velocity. In the experiments, we have estimated local burning velocity in turbulent combustion with a cyclone-jet combustor. The slot burner has been used for calibration. It is found that the peak OH concentration in the flame becomes smaller with an increase of turbulence. The local burning velocity is monotonically decreased at higher turbulence. In some cases, its value is almost zero, which could be local extinction. When the fuel is propane, Markstein number is positive for lean mixture. In that case, the burning velocity is decreased due to the stretch effect, which is well in accordance with the fact that the turbulence always suppresses the burning velocity in our measurements.

Hydroxyl space-time correlation measurements in partially premixed turbulent opposed-jet flames

A high repetition-rate, two-point, time-resolved, laser-induced fluorescence technique is used to perform simultaneous two-point OH time-series measurements in a series of turbulent opposed-jet partially premixed flames with varying fuel-side equivalence ratio and bulk Reynolds number. Time scales of OH in these flames have previously been reported; however, the extension to two-point detection permits measurements of new spatial and temporal statistics previously unavailable in such flames. In particular, the simultaneous OH time series are used here to compute spatial and temporal autocorrelation functions. Filtered OH length scales (lr,OH), corresponding to radial OH fluctuations in turbulent stagnation flames, are obtained from the spatial autocorrelation function, including their variation across the stagnation plane. In general, maximum OH fluctuations occur at the stagnation plane, thus minimizing the OH integral length scale at the axial location of peak OH. For all flames of this study, trends in OH length scale follow those of axial time scale (τI,OH). For flames with constant Re, lr,OH decreases with less partial premixing. However, this change in integral length scale appears to be more significant for flames at lower Re in comparison to those at higher Re. Similar to OH integral time scales, for flames with the same fuel composition, lr,OH decreases with increasing Re. Moreover, fuel-lean mixtures appear to be more sensitive to Re variations as compared to fuel-rich mixtures.

Structure of laminar sooting inverse diffusion flames

The flame structure of laminar inverse diffusion flames (IDFs) was studied to gain insight into soot formation and growth in underventilated combustion. Both ethylene–air and methane–air IDFs were examined, fuel flow rates were kept constant for all flames of each fuel type, and airflow rates were varied to observe the effect on flame structure and soot formation. Planar laser-induced fluorescence of hydroxyl radicals (OH PLIF) and polycyclic aromatic hydrocarbons (PAH PLIF), planar laser-induced incandescence of soot (soot PLII), and thermocouple-determined gas temperatures were used to draw conclusions about flame structure and soot formation. Flickering, caused by buoyancy-induced vortices, was evident above and outside the flames. The distances between the OH, PAH, and soot zones were similar in IDFs and normal diffusion flames (NDFs), but the locations of those zones were inverted in IDFs relative to NDFs. Peak OH PLIF coincided with peak temperature and marked the flame front. Soot appeared outside the flame front, corresponding to temperatures around the minimum soot formation temperature of 1300 K. PAHs appeared outside the soot layer, with characteristic temperature depending on the wavelength detection band. PAHs and soot began to appear at a constant axial position for each fuel, independent of the rate of air flow. PAH formation either preceded or coincided with soot formation, indicating that PAHs are important components in soot formation. Soot growth continued for some time downstream of the flame, at temperatures below the inception temperature, probably through reaction with PAHs.

Transient response of premixed methane flames

The response of premixed methane–air flames to transient strain and local variations in equivalence ratio is studied during isolated interactions between a line–vortex pair and a V-flame. The temporal evolution of OH and CH is measured with planar laser-induced fluorescence for N 2-diluted flames with equivalence ratios ranging from 0.8 to 1.2. One-dimensional laminar flame calculations are used to simulate the flame response to unsteady strain and variations in reactant composition. When the reactant composition of the vortex pair and the V-flame are identical, the measurements and predictions show that the peak mole fractions of OH and CH decay monotonically in lean, stoichiometric, and rich flames. We also investigate the effects of a vortex pair with a leaner composition than the V-flame. In a stoichiometric flame, the leaner vortex enhances the decay of both OH and CH. In a rich flame, we observe an abrupt increase in OH-LIF signal and a disappearance of CH-LIF signal that are consistent with a previous experimental investigation. Our results indicate that the previously observed OH burst and CH breakage were caused by a difference in the equivalence ratios of the vortex pair and the main reactant flow. A numerical study shows that N 2 dilution enhances the response of premixed flames to unsteady strain and variations in stoichiometry. Reaction-path and sensitivity analyses indicate that the peak OH and CH mole fractions exhibit significant sensitivity to the main branching reaction, H + O 2 ↔ OH + O. The sensitivity of OH and CH to this and other reactions is enhanced by N 2 dilution. As a result, N 2-diluted flames provide a good test case for studying the reliability of chemical kinetic and transport models.