CH4/O2同轴射流扩散火焰辐射发光特性

利用分光成像系统,实时收集O2/CH4同轴射流扩散火焰辐射发光信息,重点讨论火焰中3种激发态自由基组分(OH*、CH*和C2*)的发光特征.探讨3种激发态自由基的产生机理;分析OH*、CH*和C2*的辐射强度随氧气/燃料当量比的变化,及其沿扩散火焰轴向分布特征等.结果表明:激发态自由基均由化学激发途径产生,其辐射发光为化学发光;OH*辐射发光强度随氧气/燃料当量比变化显著,可采用[OH*]/[CH*]表征当量比变化;OH*辐射强度可以作为扩散火焰化学反应区及高度的表征.     

Hydrogen addition effects in a confined swirl-stabilized methane-air flame

The effect of hydrogen addition in methane-air premixed flames has been examined from a swirl-stabilized combustor under confined conditions. The effect of hydrogen addition in methane-air flame has been examined over a range of conditions using a laboratory-scale premixed combustor operated at 5.81 kW. Different swirlers have been investigated to identify the role of swirl strength to the incoming mixture. The flame stability was examined for the effect of amount of hydrogen addition, combustion air flow rates and swirl strengths. This was carried out by comparing adiabatic flame temperatures at the lean flame limit. The combustion characteristics of hydrogen-enriched methane flames at constant heat load but different swirl strengths have been examined using particle image velocimetry (PIV), micro-thermocouples and OH chemiluminescence diagnostics that provided information on velocity, thermal field, and combustion generated OH species concentration in the flame, respectively. Gas analyzer was used to obtain NO_x and CO concentration at the combustor exit. The results show that the lean stability limit is extended by hydrogen addition. The stability limit can reduce at higher swirl intensity to the fuel-air mixture operating at lower adiabatic flame temperatures. The addition of hydrogen increases the NO_x emission; however, this effect can be reduced by increasing either the excess air or swirl intensity. The emissions of NO_x and CO from the premixed flame were also compared with a diffusion flame type combustor. The NO_x emissions of hydrogen-enriched methane premixed flame were found to be lower than the corresponding diffusion flame under same operating conditions for the fuel-lean case.     

Syngas combustion radiation profiling through spectrometry and DFCD processing techniques

Experimental investigations of H₂ and H₂-enriched syngas flame radiation properties have been conducted through spectroscopic and DFCD (Digital Flame Colour Discrimination) techniques. A spectrograph was employed to quantify the emission profile of H₂-based flames in the UV–visible spectral domain. The OH* emission was found to be the strongest in reactants with highest amount of H₂. Further addition of CO and/or CO₂ resulted in the reduction of OH* intensity with the addition of CO₂ causing greater radical intensity-loss than that of CO. The decrease in OH* intensity is accompanied by a corresponding increase in the CO–O* broadband continuum in the short-wavelength domain of the visible spectrum. Such reduction of OH* along with increase in CO–O* intensity can be related to the endothermic reaction mechanism of CO + OH => CO₂ + H, which describes the role of CO/CO₂ addition in H₂-enriched syngas flames. Comparison with direct imaging results provided additional credence to the effect of temperature reduction as flames with CO and/or CO₂ additions resembled colourations closer to typical bluish premixed hydrocarbon flame. The employment of DFCD processing effectively characterised different syngas combustion conditions by combining aspects of digital flame colour intensities with spatial combustion distributions. This colour signal quantification method was shown to yield useful characterisation of H₂-based flames, similar to the use of OH* chemiluminescence intensity variation from spectrometry. Also, DCFD analysis was able to depict the variances between the burning of different syngas gaseous constituents. Thus, useful image-based parameters related to the H₂-based combustion can be derived and potentially applied as a practical monitoring and characterisation mean for syngas combustion.     

Study on the characteristics of evaporation–atomization–combustion of biodiesel

A great deal of research is being carried out on renewable diesel fuels. The number of raw materials (especially waste, animal, and vegetable oils), production technologies, and additives of biodiesel is increasing. In our work, a evaporation–atomization–combustion system consisting of a biomass liquid fuel was designed to produce a laminar premixed flame for studying the combustion–emission characteristics of biodiesel. The combustion characteristics of biodiesel including flame height, flame front area, flame speed, and OH total signal intensity were studied by planar laser-induced fluorescence of OH (OH-PLIF). The emission characteristics of biodiesel (CO, CO₂, and NO) were studied with a flue gas analyzer. The experimental results showed that the flame height, flame front area, flame speed, and the OH total signal intensity changed with the equivalence ratio (Φ). The relationship between the OH radical intensity and the emission of CO/CO₂ was obtained from the OH-PLIF average signal intensity. The [CO]/[CO₂] ratio decreased with the OH-PLIF average signal intensity. Finally, we obtained the relationship between the OH-PLIF average signal intensity and the NO emissions.     

Correlation of optical emission and turbulent length scale in a coaxial jet diffusion flame

This article investigates the correlation between optical emission and turbulent length scale in a coaxial jet diffusion flame. To simulate the H₂O emission from an H₂/O₂ diffusion flame, radiative transfer is calculated on flame data obtained by numerical simulation. H₂O emission characteristics are examined for a one-dimensional opposed-flow diffusion flame. The results indicate that H₂O emission intensity is linearly dependent on flame thickness. The simulation of H₂O emission is then extended to an H₂/O₂ turbulent coaxial jet diffusion flame. Time series data for a turbulent diffusion flame are obtained by Large Eddy Simulation, and radiative transfer calculations are conducted on the LES results to simulate H₂O emission optical images. The length scales of visible structures in the simulated emission images are determined by the procedure proposed by Ivancic and Mayer (2002) [8]. The length scales of emission intensity are compared with the integral length scales of velocity and temperature evaluated from LES flowfield data. The results clearly indicate that the length scale of emission intensity agrees well with the integral length scale of temperature, and is also close to that of the radial velocity component. Finally, the explanation as to why the integral length scale of temperature can be extracted from emission intensity distributions is stated.     

Oxy-fuel combustion of heavy oil based on OH - PLIF technique

Oxy-fuel combustion of heavy oil can be applied to oil field steam injection boilers, allowing the utilization of both heavy oil and CO₂ resources. This paper studied the local distribution characteristics of OH on oxy-fuel combustion of heavy oil during the ignition and stable combustion processes. During the ignition process, we observed the generation and evolution of fire kernel, and got the flame propagation velocity. During the stable combustion process, the results showed that the OH distribution and its relative signal intensity were influenced by the oxygen concentration, excess air coefficient, gas flow, reaction atmosphere, oil mist scattering, incident laser energy and laser sheet position. In the same reaction atmosphere, the ranges of OH dense distribution and the high temperature area increased as O₂ concentration increased. In the same O₂ concentration, both the ranges of OH dense distribution and the high temperature area in O₂/N₂ were larger than that in O₂/CO₂. In 29% O₂/71% CO₂, the flame shape was similar to combust in air, while the OH relative signal intensity and its volatility were much larger than that in air. In the same combustion condition, the location of high concentration of OH relative concentration field lagged behind the high temperature area. The results further reveal the differences between the conventional and oxy-fuel combustion.     

烷烃燃料微小平板催化燃烧的火焰特性

对微平板燃烧器内4种烷类燃料（C₁ ~ C₄）进行铂催化燃烧实验,获得其点火过程和静态火焰的特征,并进行对比分析。当量比相同时,点火过程火焰传播速度大小顺序为甲烷 > 乙烷 > 丁烷 > 丙烷。随着当量比增大,火焰传播速度加快,稳态火焰根部位置向气流上游移动。观察可见光、430 nm（OH*光谱）、516 nm（C₂*光谱）成像火焰发现,当量比越大,火焰亮度越大,OH*和C₂*浓度越高。当量比相同时,乙烷的OH*、CH*和C₂*浓度最高,而甲烷和丙烷的则较低。     

Relation between hydroxyl and formaldehyde in a direct-injection heavy-duty diesel engine

Formaldehyde and the hydroxyl radical have been recorded simultaneously in a diesel spray in an optically-accessible heavy-duty diesel engine. Formaldehyde (CH₂O) was excited by 355 nm radiation, while hydroxyl (OH) was excited around 284 nm. Both laser beams were focussed into a sheet, of which the overlap while traversing the spray was monitored. The experimental results have been compared to simulations. The ignition phase was modeled by a set of homogeneous reactors, while the diffusion combustion phase was simulated by a flamelet approach, viz. a rich partially premixed counterflow diffusion flame. Two situations were tested, one in which the combustion started only after the end of injection (short and early injection) and one with a longer injection duration where injection and combustion partly overlapped in time. In the former, OH initially shows up far from the injector and then quickly proceeds towards the injector. The position of OH varied significantly from cycle to cycle, suggesting that the combustion is partly governed by turbulence. When combustion and injection do partly overlap, the flame structure is much more stable. In both cases, OH and CH₂O generally form complementing spatial distributions, indicating that CH₂O is consumed locally during the hot combustion.     

Effect of preferential diffusion and flame stretch on flame structure and laminar burning velocity of syngas Bunsen flame using OH-PLIF

By using OH-PLIF technique, experiments were conducted for laminar Bunsen flame of premixed CO/H₂/air mixtures with equivalence ratio ranging from 0.5 to 1.8. Reynolds number was varied from 800 to 2200, X_H2 = H₂/(H₂+CO) in the mixture was varied from 20% to 100% to study the effects of both preferential diffusion and flame curvature on flame structures and laminar flame burning velocities. Results showed that the combined effects of preferential diffusion and curvature gave an interesting phenomenon of the flame OH radical distributions on high hydrogen content flames. Furthermore, with the increase of H₂ fraction in fuel mixture, the effects of both preferential diffusion and flame curvature were increased. Interpretation of flame stretch effect on laminar burning velocity is also provided in this paper.     

Effects of acoustic excitation on pinch-off flame structure and NOx emissions in H2/CH4 flame

The flame structure and exhaust emissions of pinch-off flames, in which flames are separated under acoustic excitation conditions, were studied. Flame structure analysis was performed using OH1 chemiluminescence measurements, and exhaust emissions analysis was performed with a gas analyzer. Structure analysis of the pinch-off flame for acoustic excitation was performed basis the OH1 images, and various structures were confirmed according to the forcing frequency and velocity perturbation intensity. To analyze the correlation between the flame structure and NO_x emission, the flame residence time and emission index of NO_x (EINO_x) were analyzed according to the Strouhal number. The flame residence time and EINO_x decreased as the velocity perturbation intensity increased; analyzing the NO_x emission characteristics is limited when based only on the flame residence time. 1/2-power scaling was followed by EINO_x analysis and the Strouhal number was normalized to the flame residence time.     

Effect of hydrogen on H2/CH4 flame structure of MILD combustion using the LES method

Large eddy simulation (LES) method is employed to investigate the effect of the hydrogen content of fuel on the H₂/CH₄ flame structure under the moderate or intense low-oxygen dilution (MILD) condition. The turbulence–chemistry interaction of the numerically unresolved scales is modelled using the PaSR method, where the full mechanism of GRI-2.11 represents the chemical reactions. The influence of hydrogen concentration on the flame structure is studied using the profiles of temperature, CH₂O and OH mass fractions and the diffusion profiles of un-burnt fuel through the flame front. Furthermore, more details are investigated by contours of OH, HCO and CH₂O radicals in an area near the nozzle exit zone. Results show that increasing the hydrogen content of fuel reinforces the MILD combustion zone and increases the peak value of the flame temperature and OH mass fraction. This increment also increases the flame thickness and reduces the OH oscillations and diffusion of the un-burnt fuel through the flame front.     

Turbulence and combustion interaction: High resolution local flame front structure visualization using simultaneous single-shot PLIF imaging of CH, OH, and CH2O in a piloted premixed jet flame

High resolution planar laser-induced fluorescence (PLIF) was applied to investigate the local flame front structures of turbulent premixed methane/air jet flames in order to reveal details about turbulence and flame interaction. The targeted turbulent flames were generated on a specially designed coaxial jet burner, in which low speed stoichiometric gas mixture was fed through the outer large tube to provide a laminar pilot flame for stabilization of the high speed jet flame issued through the small inner tube. By varying the inner tube flow speed and keeping the mixture composition as that of the outer tube, different flames were obtained covering both the laminar and turbulent flame regimes with different turbulent intensities. Simultaneous CH/CH₂O, and also OH PLIF images were recorded to characterize the influence of turbulence eddies on the reaction zone structure, with a spatial resolution of about 40 μm and temporal resolution of around 10 ns. Under all experimental conditions, the CH radicals were found to exist only in a thin layer; the CH₂O were found in the inner flame whereas the OH radicals were seen in the outer flame with the thin CH layer separating the OH and CH₂O layers. The outer OH layer is thick and it corresponds to the oxidation zone and post-flame zone; the CH₂O layer is thin in laminar flows; it becomes broad at high speed turbulent flow conditions. This phenomenon was analyzed using chemical kinetic calculations and eddy/flame interaction theory. It appears that under high turbulence intensity conditions, the small eddies in the preheat zone can transport species such as CH₂O from the reaction zones to the preheat zone. The CH₂O species are not consumed in the preheat zone due to the absence of H, O, and OH radicals by which CH₂O is to be oxidized. The CH radicals cannot exist in the preheat zone due to the rapid reactions of this species with O₂ and CO₂ in the inner-layer of the reaction zones. The local PLIF intensities were evaluated using an area integrated PLIF signal. Substantial increase of the CH₂O signal and decrease of CH signal was observed as the jet velocity increases. These observations raise new challenges to the current flamelet type models.     

LES flamelet modeling of hydrogen combustion considering preferential diffusion effect

A flamelet-generated manifold (FGM) method that explicitly considers the preferential diffusion effect, referred to as FGM-PD method, is employed for large-eddy simulations (LESs) of a lean-premixed H₂/air low-swirl lifted flame, and the validity is examined by comparing with the experiment. First, the applicability of the FGM-PD method is investigated by one-dimensional numerical simulations of planar laminar premixed H₂/air flames. Next, LESs of a lean-premixed H₂/air low-swirl lifted flame are performed employing the FGM-PD and conventional FGM methods. Results of the one-dimensional numerical simulations show the importance of considering preferential diffusion to accurately predict species concentrations near the flame front. The FGM-PD method accurately predicts this, and therefore, reproduces the laminar burning velocity and spatial distributions of temperature and mixture fraction. Three-dimensional LES results confirm that the prediction accuracy of the velocities near the flame front is improved by employing this FGM-PD method. Additionally, the OH mass fraction distribution predicted by the FGM-PD method exhibits the inhomogeneous finger-like structure, which has been observed in previous experiments. This inhomogeneity of OH mass fraction distribution, which corresponds to that of the reaction rate, predicted by the FGM-PD method, strongly affects the flame front structure.     

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

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

Flame characteristics of hydrogen-enriched methane–air premixed swirling flames

The effect of hydrogen addition in methane–air premixed flames has been examined from a swirl-stabilized combustor under unconfined flame conditions. Different swirlers have been examined to investigate the effect of swirl intensity on enriching methane–air flame with hydrogen in a laboratory-scale premixed combustor operated at 5.81 kW. The hydrogen-enriched methane fuel and air were mixed in a pre-mixer and introduced into the burner having swirlers of different swirl vane angles that provided different swirl strengths. The combustion characteristics of hydrogen-enriched methane–air flames at fixed thermal load but different swirl strengths were examined using particle image velocimetry (PIV), OH chemiluminescence, gas analyzers, and micro-thermocouple diagnostics to provide information on flow field, combustion generated OH radical and gas species concentration, and temperature distribution, respectively. The results show that higher combustibility of hydrogen assists to promote faster chemical reaction, raises temperature in the reaction zone and reduces the recirculation flow in the reaction zone. The upstream of flame region is more dependent on the swirl strength than the effect of hydrogen addition to methane fuel. At lower swirl strength condition the NO concentration in the reaction zone reduces with increase in hydrogen content in the fuel mixture. Higher combustibility of hydrogen accelerates the flow to reduce the residence time of hot product gases in the high temperature reaction zone. At higher swirl strength the NO concentration increases with increase in hydrogen content in the fuel mixture. The effect of dynamic expansion of the gases with hydrogen addition appears to be more dominant to reduce the recirculation of relatively cooler gases into the reaction zone. NO concentration also increases with decrease in the swirl strength.     

Effect of steam addition on flame structure and NO formation in H2–O2–N2 diffusion flame

Numerical analysis is conducted to clarify chemical effects of added steam to either fuel‐ or oxidizer‐side on flame structure and NO emission behaviour with detailed chemistry in hydrogen–oxygen–nitrogen diffusion flames. An artificial species, which has the same thermodynamic, transport, and radiation properties to added H₂O, is introduced to feasibly isolate chemical effects of added H₂O. It is found that the reaction step (‐R23) is the starting point to induce chemical effects of added steam. Special concern is, thus, focused on the impact of OH radical on flame structure and NO emission behaviour. A strong dependency of the amount of steam addition on OH radical behaviour is clearly displayed, and this modifies flame structure sufficiently to produce higher flame temperature at more than a certain mole fraction of added steam in comparison to that diluted with artificial species. It is also shown that the reaction step (‐R23) is closely related to flame temperature and thereby the location of maximum flame temperature. The behaviour of NO emission index is shown to be greatly influenced by the competition between the reaction steps of (R63) and (R65) in addition to Zeldovich NO. It is, consequently, seen that the intermediate active species, HNO, affects NO emission behaviour remarkably. These results may be helpful to understand the role of recirculated steam in the combustion systems with flue gas recirculation to either fuel‐ or oxidizer‐side. Copyright © 2004 John Wiley & Sons, Ltd.     

Soot suppression by nonthermal plasma in coflow jet diffusion flames using a dielectric barrier discharge

The effect of nonthermal plasma on diffusion flames in coflow jets has been studied experimentally by adopting a dielectric barrier discharge (DBD) technique. The plasma reactor had wire-cylinder-type electrodes with AC power supply operated at 400 Hz. The effect of flame on the behavior of electrical discharge was first investigated to identify the regime of plasma generation, discharge onset voltage, and delivered power to the plasma reactor. The generation of streamers was enhanced with a flame by the increase in the reduced electric field intensity due to high-temperature burnt gas and by the abundance of ions in the flame region. The effect of streamers on flame behavior reveals that the flame length was significantly decreased as the applied voltage increased. The yellow luminosity by the radiation of soot particles was also significantly diminished. The temperature of burnt gases, the concentrations of major species, and the spatial distribution of OH radical, PAH, and soot have been measured. The formation of PAH and soot was influenced appreciably by the nonthermal plasma, while the flame temperature and the concentration of major species were not influenced much with the plasma generation. The results demonstrated that the application of nonthermal plasmas can effectively suppress PAH and soot formation in the flames with low power consumption even in the order of 1 W.     

A numerical study on NOx formation in laminar counterflow CH4/air triple flames

Formation of NO_x in counterflow methane/air triple flames at atmospheric pressure was investigated by numerical simulation. Detailed chemistry and complex thermal and transport properties were employed. Results indicate that in a triple flame, the appearance of the diffusion flame branch and the interaction between the diffusion flame branch and the premixed flame branches can significantly affect the formation of NO_x, compared to the corresponding premixed flames. A triple flame produces more NO and NO₂ than the corresponding premixed flames due to the appearance of the diffusion flame branch where NO is mainly produced by the thermal mechanism. The contribution of the N₂O intermediate route to the total NO production in a triple flame is much smaller than those of the thermal and prompt routes. The variation in the equivalence ratio of the lean or rich premixed mixture affects the amount of NO formation in a triple flame. The interaction between the diffusion and the premixed flame branches causes the NO and NO₂ formation in a triple flame to be higher than in the corresponding premixed flames, not only in the diffusion flame branch region but also in the premixed flame branch regions. However, this interaction reduces the N₂O formation in a triple flame to a certain extent. The interaction is caused by the heat transfer and the radical diffusion from the diffusion flame branch to the premixed flame branches. With the decrease in the distance between the diffusion flame branch and the premixed flame branches, the interaction is intensified.     

Extinction limit extension of unsteady counterflow diffusion flames affected by velocity change

The unsteady extinction limit of (CH₄ + N₂)/air diffusion flames was investigated in terms of the time history of the strain rate and initial strain rates. A spatially locked flame in an opposed-jet counterflow burner was perturbed using linear velocity variation, and time-dependent flame luminosity and unsteady extinction limits were measured with a high-speed intensified CCD (ICCD) camera. In addition, the transient maximum flame temperature and hydroxyl (OH) radical were measured as a function of time using Rayleigh scattering and OH laser-induced fluorescence, respectively. In this experiment, unsteady flames survive at strain rates that are much higher than the extinction limit of steady flames and unsteady extinction limits increase as the slope of the strain rate increases or as the initial strain rate decreases. We found that the equivalent strain rate represents well the unsteady behavior in the outer convective-diffusive layer of the flame. By using the equivalent strain rate, we were able to accurately estimate the contribution of the unsteady effect in the outer convective-diffusive layer to the extinction limit extension, and we also identified the unsteady effect in the inner diffusive-reactive layer of the flame. Consequently, the extension of unsteady extinction limits results from the unsteady effects of both the convective-diffusive layer and the diffusive-reactive layer. The former effect is dominant at the beginning of the velocity change, and the latter effect is dominant near the extinction limit.     

Characteristics of Lean Premixed Flame in Various Oxygen Concentrations Measured by Laser Imaging Techniques

For heavy-duty gas-turbine engines, one of the promising approaches to reducing NO_X emissions is the adoption of lean premixed combustion. This technique could be combined with the conventional technique of exhaust gas recirculation (EGR). However, the reduction in the oxygen concentration will influence the burning velocity and reaction zone characteristics of the lean premixed flame. To elucidate this effect, in this study, we measured the lean premixed flame temperature and OH concentration distributions for various oxygen concentrations instantaneously and simultaneously using laser imaging techniques. Based on the results, we investigated the characteristics of a lean premixed flame under various oxygen concentrations and found that the OH laser-induced fluorescence (LIF) intensity in the reaction zone decreased with the oxygen concentration, as did the flame temperature at a given axial distance from the exit nozzle. The characteristics of the premixed flame changed from a small-scale convexo-concave surface to a smoother one, leading to a decrease in the ratio of turbulent to laminar burning velocity. In addition, local extinction of the premixed flame was observed under conditions with a high air ratio and low oxygen concentration.