| 1. |
- Bai, Xue-Song, et al.
(författare)
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Laminar flamelet structure at low and vanishing scalar dissipation rate
- 2000
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Ingår i: Combustion and Flame. - 0010-2180 .- 1556-2921. ; 120:3, s. 285-300
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Tidskriftsartikel (refereegranskat)abstract
- The laminar flamelet structures of methane/air, propane/air, and hydrogen/air nonpremixed combustion at low and vanishing scalar dissipation rates are investigated, by numerical calculations of a system of conservation equations in a counterflow diffusion flame configuration, together with a transport equation defining the mixture fraction and scalar dissipation rate. The chemical reaction mechanisms consist of 82 elementary reactions up to C-3 species. In the limit of vanishing scalar dissipation rate, two types of structures are shown to appear. In one structure fuel and oxygen are consumed in a thin layer located near the stoichiometric mixture fraction, Z(st), where the temperature and the major products reach their peaks. This is similar to the so-called Burke-Schumann single layer flame sheet structure. One example is the hydrogen/air diffusion flame. The second structure consists of multilayers. Fuel and oxygen are consumed at different locations. Oxygen is consumed at Z(l) (near Z(st)), where the temperature and the major products reach their peaks. Fuel is consumed at Z(r) (> Z(st)). Between Z(l) and Z(r) some intermediate and radical species are found in high concentrations. Hydrocarbon/air nonpremixed flames are of this type. It is shown that for methane/air diffusion flames, some chemical reactions which are negligible at large scalar dissipation rate near flame quenching conditions, play essential roles for the existence of the multilayer structure. Examples of such reactions are, CH4 --> CH3 + H, H2O + O-2 --> HO2 + OH, H2O + M --> H + OH + M and CHO + H-2 --> O + H. The sensitivity of the species distributions in the flamelet to the scalar dissipation rate varies for different species. The most sensitive species are the intermediates and radicals at the fuel-rich side. At low scalar dissipation rate the radiative heat transfer can significantly move the fuel consumption layer to the oxygen consumption layer, increase the oxygen leakage to fuel side, and even quench the flame. Differential diffusion modifies the species and temperature profiles in the flamelet, but does not affect the multilayer nature of the flamelet. This result is used to successfully explain the high CO emissions in a turbulent methane/air diffusion flame.
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| 2. |
- Balthasar, Michael, et al.
(författare)
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A computational study of the thermal ionization of soot particles and its effect on their growth in laminar premixed flames
- 2002
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Ingår i: Combustion and Flame. - 0010-2180. ; 129:1-2, s. 204-216
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Tidskriftsartikel (refereegranskat)abstract
- The effect of thermal ionization on the growth of soot particles has been analyzed by detailed kinetic modeling of a low-pressure premixed acetylene flame. The detailed kinetic model considers the oxidation of fuel, the formation and growth of polycyclic aromatic hydrocarbons, and particle inception, coagulation, as well as mass growth via surface reactions. A numerical method has been developed, which considers the production of charged particles by thermal ionization as well as coagulation and surface reactions of these particles. The enhancement of coagulation by collisions between charged-charged and charged-neutral particles is rigorously accounted for in the numerical model. The particle size distribution functions for both neutral and charged particles were solved using the method of moments. The computed relative soot volume fractions for neutral and charged soot particles were compared to measurements and found to be in good agreement with them. The results show also that omitting of thermal ionization of soot particles does not lead to significant errors in the simulation of soot formation in the acetylene flame, as long as the nature of the surface reactions between charged particles and gaseous molecules remains the same as that for neutral particles. This result can be generalized to most laboratory laminar premixed and counterflow diffusion flames with flame temperatures not exceeding 2100 K.
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| 3. |
- Balthasar, Michael, et al.
(författare)
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Detailed Modeling of soot formation in a partially stirred plug flow reactor
- 2002
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Ingår i: Combustion and Flame. - 0010-2180. ; 128:4, s. 395-409
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Tidskriftsartikel (refereegranskat)abstract
- The purpose of this work is to propose a detailed model for the formation of soot in turbulent reacting flow and to use this model to study a carbon black furnace. The model is based on a combination of a detailed reaction mechanism to calculate the gas phase chemistry, a detailed kinetic soot model based on the method of moments, and the joint composition probability density function (PDF) of these scalar quantities. Two problems, which arise when modeling the formation of soot in turbulent flows using a PDF approach, are studied. A consistency study of the combined scalar-soot moment approach reveals that the molecular diffusion term in the PDF-equation can be closed by the IEM and Curl-type mixing models. An investigation of different kernels for the collision frequency of soot particles shows that the influence of turbulence on particle coagulation is negligible for typical flame conditions and the particle size range considered. The model is used as a simple toot to simulate a furnace black process, which is the most important industrial process for the production of carbon blacks. Despite the simplifications in the modeling of the turbulent flow reasonable agreement between the calculated soot yield and data measured in an industrial furnace black reactor is achieved although no adjustments were made to the kinetic parameters of the soot model. The effect of the mixing intensity on soot yield and different soot formation rates is investigated. In addition the influence of different operating conditions such as temperature and equivalence ratio in the primary zone of the reactor is studied.
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| 4. |
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| 5. |
- Bengtsson, Per-Erik, et al.
(författare)
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Vibrational CARS thermometry in sooty flames: Quantitative evaluation of C2 absorption interference
- 1990
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Ingår i: Combustion and Flame. - 0010-2180. ; 82:2
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Tidskriftsartikel (refereegranskat)abstract
- The application of nitrogen vibrational CARS thermometry to sooty, premixed, atmospheric pressure flames has been investigated using a Nd:YAG laser based system. It was found that laser-produced C2 radicals strongly absorb part of the fundamental band peak in the CARS spectrum. This was the most severe interference to the CARS signal. A quantitative investigation of temperature errors caused by the C2 absorption effect is presented. The correlation between the absorption interference and the soot volume fraction was examined for different flame conditions. Also, the increase of the nonresonant susceptibility in sooty flame regions is clearly illustrated and its effect on thev evaluated temperature is quantitatively determined. The single-shot temperature standard deviation has also been investigated in flames with different soot loadings. Finally, other interference effects to the CARS signals in sooty flames are described and discussed.
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| 6. |
- Holmstedt, Göran
(författare)
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The upper limit of flammability of hydrogen in air, oxygen, and oxygen-inert mixtures at elevated pressures
- 1971
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Ingår i: Combustion and Flame. - 0010-2180. ; 17:3, s. 295-301
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Tidskriftsartikel (refereegranskat)abstract
- The upper limit of flammability of hydrogen in air, oxygen, oxygen-helium, oxygen-neon, oxygen-argon and oxygen-carbon dioxide mixtures was measured at room temperature and pressures between 0.97 and 29 atm in two cylindrical bombs with volumes of 1.5 and 5.2 liters. The limit in ternary mixtures was determined in 20%, 40%, 60% and 80% helium, 20% neon and argon and 10% carbon dioxide concentrations. The maximum safe percentage of oxygen in a hydrogen-oxygen-helium mixture was calculated for pressures between 0.97 and 29 atm.
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| 7. |
- Neij, Hans, et al.
(författare)
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Development and demonstration of 2D-LIF for studies of mixture preparation in SI engines
- 1994
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Ingår i: Combustion and Flame. - 0010-2180. ; 99:2, s. 449-457
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Tidskriftsartikel (refereegranskat)abstract
- Laser-induced fluorescence (LIF) has been developed for visualization of fuel distribution fields in an operating spark-ignition (SI) engine. Since the standard research fuel iso-octane, does not yield a useful LIF signal a fluorescent additive was used. None of the commonly used seeds were found adequate. A seed not commonly used in this context, 3-pentanone, C2H5COC2H5, was chosen due to favorable vaporization characteristics and fluorescent properties. Results from preparatory investigations in the actual engine environment are presented and related laboratory data are discussed. The two-dimensional LIF technique was applied to a spark-ignition engine and the fuel distribution at the ignition time was recorded. The resulting images were processed and converted into fuel/air equivalence ratio using an in situ calibration technique. The processed fuel distribution maps presented a noise level of 10% and a systematic error not exceeding 0.03 fuel/air equivalence units. An increased combustion variability was observed when changing from a homogeneous to an inhomogeneous fuel/air mixture. Correlations of image data to the combustion development indicated that the increased cyclic variability could be largely explained by variations in the mean fuel concentration around the spark gap. The initial flame development therefore seems to be controlled by the average amount of fuel near the spark gap, whereas the actual distribution of the fuel within this volume is of less importance.
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| 8. |
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| 9. |
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| 10. |
- Yan, Zhenghua, et al.
(författare)
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Three-dimensional computation of heat transfer from flames between vertical parallel walls
- 1999
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Ingår i: Combustion and Flame. - 0010-2180. ; 117:3, s. 574-588
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Tidskriftsartikel (refereegranskat)abstract
- The heat transfer from turbulent diffusion flames between vertical walls has been computed for different wall and burner configurations. The buoyancy-modified k- model was used to study the turbulent characteristics of the flow. The flamelet concept, coupled to a prescribed probability density function, was employed to model the nonpremixed combustion process. With the nucleation, surface growth, coagulation, and oxidation considered, sooting was modeled by solving the balance equations for mass fraction and number density. The radiation from the main radiating species - carbon dioxide, water vapor and soot - was calculated using the discrete transfer method. A recently developed fast, narrow-band model was adopted to provide the radiation properties of the radiating species. Computations were performed for different cases by varying the wall separation and burner output. The results were analyzed and compared with experimental measurements, with which they showed good agreement. The effects of wall separation and burner output on heat transfer were faithfully reproduced.
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