Rich Combustion Conditions Research Articles

Two-Photon Planar Laser-Induced Fluorescence Of Near-Wall CO During The Interaction Between A Flame And A Cooling Air Film

Reducing the anthropogenic pollutant emissions is a major concern in the aeronautical community. Implementing high-power density core engines made of lighter materials would undoubtedly increase the thermal efficiency and reduce CO 2 emissions. Nevertheless, near-wall combustion processes will become significant, raising concerns regarding thermal management. As a result, walls in aero-engine combustors are commonly cooled, but it also introduces more complex physical phenomena associated to pollutant formation during flame-cooling air interaction (FCAI). This experimental study aims at elucidating the role of the cooling air film on CO emissions during FCAI. The experiments are conducted in an atmospheric pressure and optically-accessible lab-scale combustion test rig dedicated to near-wall combustion. It generates a lean premixed turbulent methane/air V-shaped flame, with one branch of the flame that interacts with an oil-cooled stainless-steel wall. A splash-cooling plate system enables to generate a momentum-controlled parietal cooling air film. The novelty of this study is to develop and implement planar laser-induced fluorescence of the CO molecule (CO-PLIF), complemented by planar laser-induced fluorescence of the OH radical (OH-PLIF) as well as global CO emissions in the exhaust gases. Results show that the excitation of the Hopfield-Birge system enables to detect the AN ngström bands and the third positive system. However, significant interferences with C 2 and CN emissions are highlighted, being more pronounced in rich combustion conditions, and originating from the incomplete fuel oxidation as well as the high energy density of the two-photon excitation process. A broadband collection strategy of the AN ngström bands is selected, since these interferences are limited in lean combustion regimes. The analysis of the flame dynamics indicates that the increase of the cooling air film momentum shifts the reactive flow away from the wall, but also controls the flame wrinkling through modified aerodynamics. As a result, the flame is not influenced anymore by thermal quenching processes, and can effectively burn further away from the wall. Global emission measurements indicate an increase in CO when the cooling air film momentum increases. While thermal quenching favors a faster oxidation downstream of the flame, the establishment of the cooling air film leads to a longer flame with more production areas. CO-PLIF imaging concurs this phenomenon, with a CO distribution all along the wall when the cooling air film is well established.

Study of CH4-H2-CO-Air premixed combustion characteristics based on fractal dimension and flame crack method

The laminar combustion characteristics of CH4-H2-CO-Air are studied in a constant volume combustion chamber with different hydrogen content ratios and the initial conditions of 300 K and 0.2 MPa by schlieren technology. The influence laws of flame instability are analyzed through the influence laws of hydrogen content ratio and equivalence ratio (Φ) on the fractal dimensions of the flame contour and the ratio of the total flame face cracks' lengths to the flame radius (L/R), as well as the influence laws of hydrodynamic instability and thermal-diffusive instabilities on the flames. The results show that L/R is more sensitive to flame instabilities than flame front fractal dimensions due to a wider sampling range. After R exceeds a certain value, for lean and rich combustion conditions with the same distance from the stoichiometric ratio, the L/R is greater during lean combustion, and as the equivalence ratio gradually increases from 0.6 to 1.4, the effect of increasing hydrogen content (Hydrogen mole fraction ranges from 60% to 66.67%) on flame instability gradually shifts from promotion to suppression. This study has certain reference value for the combustion application of biomass synthesis gas.

Acoustic and Optical Investigations of the Flame Dynamics of Rich and Lean Kerosene Flames in the Primary Zone of an Air-Staged Combustion Test-Rig

Abstract In this study, the thermoacoustic behavior of nonpremixed kerosene flames from rich to lean combustion conditions is investigated. Flame-transfer-functions (FTF) measured purely acoustically are compared with results based on flame chemiluminescence. OH*, CH*, C2*, and CO2* were selected as potential measures for representing steady and fluctuating heat release when burning nonpremixed kerosene. In addition, their ability for the quantification of equivalence ratio fluctuations will be highlighted. The measurements were performed in the primary zone of an atmospheric rich-quench-lean (RQL) combustion test-rig. The new experimental approach allows a characterization of the primary zone independent of the secondary zone. Rich and lean operating points were analyzed by fueling an aero-engine prototype injector with and without acoustic excitation. To improve the quality of the acoustic wavefield reconstruction a thermocouple correction method was implemented. The flame dynamics determined with the multimicrophone method (MMM) exhibit a frequency and equivalence ratio depending effect of rich combustion conditions. The results for the steady behavior of the chosen radicals by altering equivalence ratio and thermal power indicate proportionality of the chemiluminescence to thermal power. Furthermore, the CH*/C2* ratio is found to be a promising indicator for the global equivalence ratio in the combustion chamber. The flame-transfer-functions based on chemiluminescence show a good qualitative agreement with the multimicrophone method. Based on the experimental findings a calibration curve for the different radicals to obtain quantitatively correct flame-transfer-functions from chemiluminescence is presented.

Characteristics of Explosion Hazards in Methane–Air Mixtures Diluted by Hydrogen

The combustion efficiency of methane can be effectively enhanced with the occurrence of hydrogen. However, the combustion characteristic of premixed methane/hydrogen/air is not fully understood. In this study, the effect of the amount of hydrogen addition on the explosion risk of premixed CH4/air combustion was fully investigated through experiments and simulations. The explosion overpressure of premixed CH4/air combustion with various hydrogen additions was measured in a standard 20 L spherical closed vessel. Meanwhile, the microscopic flame structures for the same cases were simulated using 2022 Chemkin-Pro software. The results showed that hydrogen could increase the explosion risks of premixed CH4/air combustion. The rate of key elementary reactions R38: H + O2 <=> O + OH and R84: OH + H2 <=> H + H2O in the system could be accelerated by hydrogen. The peak explosion overpressure in the closed chamber is boosted and the arrival time of peak overpressure rise rate is shortened, which raises the danger. Especially under lean and rich combustion conditions, hydrogen could potentially lead to more dangerous situations. With the increase in hydrogen concentration, the reaction rate of key elementary reactions accelerates faster, the peak explosion overpressure increases more, and the peak overpressure rise rate arrives earlier.

Transient kinetic analysis of passive SCR systems for NH3 abatement from natural gas fueled heavy duty engines over dual-layer ASC catalysts: An experimental and modelling study

Air-to-fuel ratios fluctuating around the stoichiometric value enable the best CH4, NOX and CO abatement performance of the three-way catalytic converters implemented in NGVs after-treatment systems; however, this leads to undesired NH3 production and potential slip under rich combustion conditions. An additional Passive SCR catalytic device placed downstream of the TWC, based on a dual-layer SCR+PGM configuration, could represent the solution.Through both transient experimental tests and modelling analysis, we demonstrate that effective NH3 abatement is achieved at low temperatures by its adsorption onto the SCR catalyst during the rich phases, and its further consumption by NO+O2 under lean conditions. Differently, at high temperatures NH3 is efficiently and selectively converted to nitrogen by direct oxidation through a redox reaction mechanism. The developed model can predict the performances of both fresh and aged catalysts under the transient rich-lean cycling experimental conditions investigated.

RANS simulations on combustion and emission characteristics of a premixed NH3/H2 swirling flame with reduced chemical kinetic model

Ammonia (NH3) is considered as a potential alternative carbon free fuel to reduce greenhouse gas emission to meet the increasingly stringent emission requirements. Co-burning NH3 and H2 is an effective way to overcome ammonia’s relative low burning velocity. In this work, 3D Reynolds Averaged Navier-Stokes (RANS) numerical simulations are conducted on a premixed NH3/H2 swirling flame with reduced chemical kinetic mechanism. The effects of (A) overall equivalence ratio Φ and (B) hydrogen blended molar fraction XH2 on combustion and emission characteristics are examined. The present results show that when 100%NH3-0%H2-air are burnt, the NO emission and unburned NH3 of at the swirling combustor outlet has the opposite varying trends. With the increase of Φ, NO emission is found to be decreased, while the unburnt ammonia emission is increased. NH2 → HNO, NH → HNO and HNO → NO sub-paths are found to play a critical role in NO formation. Normalized reaction rate of all these three sub-paths is shown to be decreased with increased Φ. Hydrogen addition is shown to significantly increase the laminar burning velocity of the mixed fuel. However, adding H2 does not affect the critical equivalence ratio corresponding to the maximum burning velocity. The emission trend of NO and unburnt NH3 with increased Φ is unchanged by blending H2. NO emission with increased XH2 is increased slightly less at a larger Φ than that at a smaller Φ. In addition, reaction rates of NH2 → HNO and HNO → NO sub-paths are decreased with increased XH2, when Φ is larger. Under all tested cases, blending H2 with NH3 reduces the unburned NH3 emission, especially for rich combustion conditions. In summary, the present work provides research finding on supporting applying ammonia with hydrogen blended in low-emission gas turbine engines.

Combustion system optimization for dimethyl ether using a genetic algorithm

Dimethyl ether is a gaseous fuel which can easily be liquefied under moderate pressures. Its high reactivity makes it suitable for combustion in a compression ignition engine, and due to the high oxygen content, its combustion is virtually free of soot. The high oxygen content and low density of dimethyl ether lead to a lower volumetric heating value compared to Diesel fuel. Therefore, the hydraulic flow rates of the injectors have to be increased with larger nozzle holes. The influence of larger nozzle holes on the dimethyl ether spray formation and ignition are presented in this article. Experimental investigations were conducted at a constant-pressure vessel with optical access and with a single-cylinder research engine. Subsequently, a numerical optimization of the piston bowl and injector nozzle has been carried out. A very fast air/fuel mixture formation with dimethyl ether was observed, which leads to a lean combustion with small nozzle diameters. With increasing nozzle diameters, the combustion moves toward stoichiometric conditions and with very large diameters to rich combustion conditions. The ignition delay for small diameters is mostly dominated by the lean mixture, and for large diameters, the ignition delay is strongly influenced by cooling effects. For the optimization, the oxidation potential number was maximized, which proved suitable to simultaneously increase efficiency and reduce emissions. A conventional ω-shaped bowl and a step bowl have been optimized, and large bowl diameters were found to be beneficial for dimethyl ether combustion. Furthermore, nozzle diameters around 150 µm showed the most promising results. Compared to the dimethyl ether reference, the simulations with the optimized ω-shaped bowl showed a power increase of 2.7%. Experimentally, the optimized ω-shaped bowl in combination with the reference injector showed an efficiency increase by more than 1% at 2000 r/min full load compared to the dimethyl ether reference.

Combustion and pyrolysis of iso-butanol: Experimental and chemical kinetic modeling study

The first reaction mechanism for iso-butanol (372 species and 8723 reversible elementary reactions) pyrolysis and combustion that includes pressure dependent kinetics and proposes reaction pathways to soot precursors has been automatically generated using the open-source software package RMG. High-pressure reaction rate coefficients for important hydrogen abstraction reactions from iso-butanol by hydrogen, methyl and HO2 were calculated using quantum chemistry at the CBS-QB3 level. The mechanism was validated with recently published iso-butanol combustion experiments as well as new pyrolysis speciation data under diluted and undiluted conditions from 900 to 1100K at 1.72atm representative of fuel rich combustion conditions. Sensitivity and rate of production analysis revealed that the overall good agreement for the pyrolysis species, and in particular for the soot precursors like benzene, toluene and 1,3-cyclopentadiene, depends strongly on pressure dependent reactions involving the resonantly stabilized iso-butenyl radical. Laminar flame speed, opposed flow diffusion flame speciation profiles, and autoignition are also well-captured by the model. The agreement with speciation profiles for the jet-stirred reactor could be improved, in particular for temperatures lower than 850K. Flux and sensitivity analysis for iso-butanol consumption revealed that this is primarily caused by uncertainty in iso-butanol+OH, iso-butanol+HO2 and the low temperature peroxy chemistry rates. Further theoretical and quantum chemical studies are needed in understanding these rates to completely predict the combustion behavior of iso-butanol using detailed chemistry.

Combustion of an Illinois No. 6 coal char simulated using an atomistic char representation and the ReaxFF reactive force field

Coal or biomass chars are complex carbonaceous materials that are important energy sources for electricity production. Reactive molecular dynamics simulations are a useful tool to examine the chemical reactions occurring in complex processes, providing that a realistic structural representation and an applicable reactive force field can be utilized. Combustion of coal (or biomass) char is one such area were additional insight would be helpful for utilization enhancements and pollution control. In this investigation a devolatilized Illinois No. 6 coal char atomistic representation was generated, using Fringe3D and additional Perl scripts, coupled with the ReaxFF reactive force field for hydrocarbon combustion. Fringe3D facilitates the char structure generation process by producing a distribution of aromatic structures based on HRTEM lattice fringe image analyses. Perl scripts were used for incorporating heteroatoms and aliphatic components to aid elimination of investigator bias, and facilitate a more rapid construction process. The char structure was constrained by a combination of elemental and NMR literature data. Chemical and physical parameters were found to be broadly consistent with the experimental data. The ReaxFF force field for hydrocarbon combustion was used to perform simulations to examine the structural transformations and chemical processes associated with char combustion. In this initial work, very high temperatures (3000–4000 K) were selected for ReaxFF simulation under stoichiometric, fuel lean and rich combustion conditions. These elevated temperatures were chosen to observe chemical reactions proceed to completion within a computationally practical simulation time. Analyses indicated that char oxidation process was primarily initialized by either thermal degradation of char structure to form small fragments, that were subsequently oxidized, or by hydrogen abstraction reactions by oxygen molecules and O and OH radicals. A more rapid oxidation and combustion of the polyaromatic structures occurred at fuel lean (oxygen rich) conditions compared with fuel rich combustion. Char transitions included 6-membered ring conversion into 5- and 7-membered rings that further decomposed or reacted with mostly O and OH radicals. This work further demonstrates the utility of ReaxFF force field integration with representative char structural models to investigate physical and chemical transformations of char structure during combustion at high-temperature conditions.

Observation of local heat release rate with changing combustor pressure in the CH 4/air flame (wrinkled laminar regime)

The influence of combustor pressure on the local heat release rate of methane/air flames (wrinkled laminar regime) was investigated by measurements of local chemiluminescence intensity. Induced flow is often applied to the industrial boiler systems to improve heat transfer and prevent leakage of combustor. To investigate the combustion characteristics in the induced flow pattern, the pressure index ( P *) was controlled in the range of 0.7–1.3 for various equivalence ratio conditions for the present combustion system, where P * is defined as the ratio of absolute pressure to atmospheric pressure. Relationships between local heat release rate and pressure index have been investigated by simultaneous measurements of C 2 *, CH * and OH * chemiluminescence intensities. Flame length became longer with decreasing P *. The mean value of C 2 * and CH *, which indicated the reaction intensity in the hydrocarbon flames, decreased with decreasing pressure index for Φ ≤ 1, but increased with decreasing pressure index for Φ > 1. The C 2/CH value, which has a strong relationship with the local equivalence ratio, was almost the same for Φ ≤ 1, regardless of the change of the pressure index, but showed a large level for lower pressure index and Φ > 1 conditions. For rich combustion conditions, a combustor pressure lower than the atmospheric pressure enhanced the combustion reaction by increasing the diffusion velocity of unburned fuel.

Controlling carbon nanofibre morphology for improved composite reinforcement

Carbon nanofibres were grown at low temperatures on a novel fibre catalyst, palladium particles, using a novel method, growth in fuel rich combustion conditions. The growth of carbon filaments by this process was found to be remarkably rapid. Preliminary analysis of how changes in operating parameters, for example gas flow rate and reactant ratio, showed that fibre diameter and other factors such as helicity are very sensitive to the precise conditions employed, suggesting simple means to impact overall properties of the fibres. The ability to control diameter, helicity, and surface roughness are discussed with possible importance to advanced composites.

Characteristic Response of a Production Diesel Oxidation Catalyst Exposed to Lean and Rich PCI Exhaust

Although low-temperature premixed compression ignition (PCI) combustion in a light-duty diesel engine offers dramatic and simultaneous reductions in nitric oxides (NOx) and soot, associated increases in unburned hydrocarbons (HC) and carbon monoxide (CO) become unacceptable. Production diesel oxidation catalysts (DOCs) are effective in oxidizing the increased levels of HC and CO under lean combustion conditions. However, the low-temperature∕high CO combination under rich PCI conditions, designed as a lean NOx trap (LNT) regeneration mode, generally renders the DOC ineffective. The objectives of this study are to characterize the oxidizing efficiency of a production DOC under lean and rich PCI conditions, and attempt to identify probable causes for the observed ineffectiveness under rich PCI. The study uses several tests to characterize the behavior of the DOC under lean PCI and rich PCI combustion conditions, including (1) steady-state feed gas characterization, (2) transient feed gas characterization, (3) air injection (4) insulated air-fuel sweep, and (5) combustion mode switching. The DOC never becomes effective under rich PCI for any of the tests, suggesting that the platinum-based catalyst may be incorrect for use with rich PCI. Furthermore, combustion mode switching between lean PCI and rich PCI (mimicking LNT loading and regeneration) demonstrates diminishing effectiveness of the DOC during and after continuous mode transitioning.

Evidence of PAH production under lean combustion conditions

Abstract While it is common knowledge that PAH formation can be expected at rich combustion conditions, this paper presents experimental and computational evidence that PAH formation can also be expected under lean, or cool, combustion conditions. Experimental evidence for diesel engine operation at lean conditions, as well as the literature references related to waste tire and coal combustion, are cited as supportive evidence of conclusions drawn here. A computational model based on reaction mechanisms for three surrogate fuels: C 2 H 2 , C 7 H 16 and C 14 H 28 , was validated against the available diesel engine data, and projected to both lean and rich combustion conditions. The finding was that for the adiabatic case, minimum production of PAHs occurs at equivalence of unity and increases as equivalence becomes either lean or rich; the latter observation being consistent with the common knowledge.

A Computational Study of the Reaction of Methyl Formate with ·H and ·CH3 Radicals

Ab initio molecular orbital theory has been used to examine the kinetics and mechanism for the reaction of hydrogen and methyl radicals with methyl formate. From the ab initio results the rate of reaction of hydrogen atoms with methyl formate is found to be significantly faster than the analogous reactions initiated by methyl radicals. Under oxygen rich combustion conditions reactions with H and OH are likely to dominate. Abstraction of the carbonyl hydrogen of methyl formate by H atom is significantly (85%) favored at 298 K. As the temperature increases, however, methyl and carbonyl hydrogen abstraction pathways become more equally probable.

Mathematical modelling of catalytic monolithic reactors with storage of reaction components on the catalyst surface

Effects of periodic switching between lean and rich combustion conditions on CO, HC and NO x conversion on a monolithic catalyst with NO x storage were simulated by mathematical model. The model includes description of oxygen and NO x storage on the washcoat. Parametric study showed possibility to reach much improved time-averaged NO x conversion on a single monolith in comparison with steady-state operation, but lower CO and HCs conversions under reducing (rich) conditions. Sequence of monoliths, the first one with NO x storage catalyst and the second one with oxidizing catalyst, then enables to obtain satisfactory conversions of all pollutants.

SiC Recession Caused by SiO2 Scale Volatility under Combustion Conditions: I, Experimental Results and Empirical Model

A high‐pressure burner rig was developed to evaluate the response of chemical‐vapor‐deposited SiC material during exposure to simulated gas turbine combustor conditions. Linear weight loss and surface recession rates of SiC were observed in both fuel‐lean and fuel‐rich gas mixtures. This response was shown to result from SiO2 scale volatility. Arrhenius‐type temperature dependence was demonstrated. In addition, the effects of pressure and gas velocity were defined in terms of a gaseous‐diffusion‐controlled process for volatile reaction products (such as SiO, Si(OH)4, and iO(OH)x). Accordingly, multiple linear regression was used to develop empirical recession relationships of the form exp(‐DeltaQ/RT)Pxvyfor both lean and rich combustion conditions. Part II of this paper discusses the thermodynamics and gaseous‐diffusion model of this recession. The empirical models discussed here enable prediction of SiC recession for any combination of T, P, and vin turbine environments. For typical combustion conditions, recession of 0.2‐2 µm/h was predicted at 1200°‐1400°C. Thus, long‐term, high‐temperature, high‐velocity exposure may degrade silicon‐based or SiO2‐forming material by recession in combustion gas environments.

The Kinetics of Limestone/Dolomite with H2S Under Rich Combustion Conditions

Abstract The kinetics of the reaction of limestone/dolomite with H2S have been examined under fuel rich combustion conditions. The experiments were done in a tubular laminar flow reactor with co-current flow of gas and solids. In the temperature region, 1065-1310°C, the results were consistent with the absorption reaction of H2S first order with respect to calcium and half order with respect to H2S. Reactivity of the stones fell off substantially between 1065-1310°C because of severe desurfacing occurring at these temperatures to the limestone and dolomite. A diffusion controlled reaction is believed to be the dominant mechanism.

Conversion of fixed nitrogen to N2 in rich combustion

Equilibrium considerations indicate that the concentration of total fixed nitrogen species (other than molecular N2) should reach a minimum under fuel rich combustion conditions. Increasing the combustion temperature should alleviate the kinetic constraints that prevent the fixed nitrogen concentration from approaching the low values predicted by equilibrium. In this study the concept of fuel rich combustion at high temperatures was explored, and the dependence of fixed nitrogen conversion to N2 on the equivalence ratio, temperature and reaction time was determined in oxygen-enriched methane/air combustion doped with ammonia or nitric oxide. To maintain constant, high temperature conditions required for the fixed nitrogen to N2 conversion, a tubular flow reactor with its walls heated to the adiabatic temperature of the premixed flames supplied by a flat flame burner was used for these reaction studies. The results indicate the validity of the concept of minimizing fixed nitrogen species (HCN+NH3+NO) concentration by increasing the temperature in the range of 1850 K to 2050 K at the optimum equivalence ratio of 1.7–1.9. The predominant initial nitrogen species is HCN which is converted to N2 presumably through amine and NO intermediates. A comprehensive treatment of the mechanism of nitrogen chemistry in rich combustion led to a kinetic analysis of the results, which indicates that near the flame front fixed nitrogen conversion to N2 follows a second order rate law because of the rate controlling nature of bimolecular reactions involving fixed nitrogen intermediates (NH4, N, NO). At the same temperatures, but longer reaction times HCN reaction with chain carrier radicals becomes rate controlling, as evidenced by the first order decay of HCN.

Soot nucleation kinetics in premixed methane combustion

The reflected shock region of a conventional shock tube was used to study soot formation from rich methane combustion. The combustion environment, reactant partial pressures of approximately one atmosphere, and temperature exceeding 1800 °K, simulated conditions in practical systems. The simultaneous time-resolved observation of ion precursors, ions and neutral species along with the nascent soot particles in the homogeneous reflected core region provided unambiguous kinetic data about the earliest stages of soot formation. Emission spectra of CH0 (4315 ) and C2 0(5165 ) and Langmuir probe current histories of chemi-ions paralleled each other and changed radically in appearance from fuel-lean combustion to rich combustion conditions. Electric fields of moderate intensity were applied across the reaction zone to control ion-electron recombination. At equivalence ratios less than two, the soot production was extinguished while at equivalence ratios approaching three, an enhancement of soot formation was achieved. These observations confirm the presence of a previously proposed dual nucleation path. An analytical study of the gas-phase reaction mechanism prior to the nucleation of soot was undertaken using existing rate data. The effects of electric fields were modeled by eliminating the ion-electron recombination step since the electron concentration will be drastically reduced in an electric field of appreciable strength. The analytical results suggest that the dual nucleation path is a tenable concept in the combustion system studied.