利用简化的化学反应动力学模型耦合CFD代码模拟气体燃烧器中的烟黑生成(英文)

A computational study of soot formation in ethylene/air coflow jet diffusion flame at atmospheric pressure was conducted using a reduced mechanism and soot formation model. A 20-step mechanism was derived from the full mechanism using sensitivity analysis, reaction path analysis and quasi steady state (QSS) approximation. The model in premixed flame was validated and with computing savings in diffusion flame was applied by incorporating into a CFD code. Simulations were performed to explore the effect of coflow air on flame structure and soot formation. Thermal radiation was calculated by a discrete-ordinates method, and soot formation was predicted by a simple two-equation soot model. Model results are in good agreement with those from experiment data and detailed mechanism at atmospheric conditions. The soot nucleation, growth, and oxidation by OH are all enhanced by decrease in coflow air velocity. The peak soot volume fraction region appears in the lower annular region between the peak flame temperature and peak acetylene concentration locations, and the high soot oxidation rate due to the OH attack occurs in the middle annular region because of high temperature.     

Sooting and non-sooting propylene diffusion flames with irradiation of laser light

The structural change in co-flow propylene diffusion flames by irradiation of laser light is experimentally investigated to examine the effect of soot radiation at a certain position. The soot volume fraction and the flame/soot temperatures are measured by laser extinction and two-color-ratio pyrometry techniques. Transitions from a non-sooting to a sooting flame and vice versa are observed to depend on the irradiated positions of laser light. The structural change could be attributed to the following processes. When laser light is irradiated at the soot-formation region (lower part in the flame), the temperature of soot particles is increased by absorption of laser energy; the raised temperature enhances soot formation and increases the local volume fraction of soot. The increased amount of soot enhances the radiation loss and eventually lowers the flame temperature at the downstream direction. As a result, the soot-oxidation process is suppressed, and finally a non-sooting flame transforms to a sooting flame. The laser light irradiated at the competing section of soot formation and oxidation or at the oxidation section (upper part of the flame) also increases the temperature of soot particles, however, the increased soot temperature mostly enhances soot oxidation in these regions. Therefore, a decreased amount of soot lowers the radiation loss from the flame and maintains the flame temperature relatively high, compared with that without irradiation of laser light. As a result, the soot-oxidation process is more enhanced, and finally the transition from a sooting flame to a non-sooting flame occurs. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 6, pp. 74–81, November–December, 2006.     

Validation of Submodels for CFD Simulation of n‐Hexane Pool Flames including Interferometry

Computational fluid dynamics simulation is used to predict transient and time‐averaged flame temperatures and species concentrations of an n‐hexane pool flame. Employing a combination of an assumed probability density function approach with laminar flamelets using detailed kinetic data and large eddy simulation with Smagorinsky submodel is shown to be a promising way in modeling pool and tank fires. The measured species concentration and flame temperature profiles from gas chromatography, thermocouple measurements and holographic interferometry are used to validate the submodels for CFD simulation of pool flames.     

Quantitative photocatalyzed soot oxidation on titanium dioxide

We report here the titanium dioxide (TiO₂) photocatalyzed oxidation of deposited hurricane lamp soot. Sol–gel derived TiO₂ was coated on quartz crystal microbalance (QCM) elements. Characterization by spectroscopic ellipsometry (SE) and atomic force microscopy (AFM) revealed low surface roughness of 0–17%, and SE showed a linear variation of the TiO₂ thickness versus the number of sol–gel spin coats.Soot was deposited on the calcined TiO₂ film using an analytical rotor passing through a hurricane lamp flame, and subsequently irradiated with near-UV light. Varying the soot mass on the TiO₂-coated QCM crystals revealed behaviors over 20,000 min ranging from total soot destruction of a single pass soot layer to minimal oxidation of an eight pass soot layer, the latter caused by soot screening of the incident UV light. A series/parallel reaction mechanism [P. Chin, G.W. Roberts, D.F. Ollis, Industrial & Engineering Chemistry Research 46 (2007) 7598] developed to describe previous literature data on TiO₂-catalyzed soot photooxidation was successfully employed to capture the longer time changes in presumably graphitic soot mass as a function of UV illumination time from 1000 to 20,000 min and of soot layer thickness. Short time soot mass loss is attributed to oxidation of organic carbons deposited on the graphitic soot components. This kinetic model can be used to predict the rate of TiO₂-catalyzed soot destruction as a function of near-UV illumination time and initial soot layer thickness.     

Investigation of the size of the incandescent incipient soot particles in premixed sooting and nucleation flames of n-butane using LII,HIM, and 1 nm-SMPS

Laser-induced incandescence (LII) measurements were conducted to explore the ability of LII to detect small soot particles of less than 10 nm in two sooting flat premixed flames of n-butane: a so-called nucleation flame obtained at a threshold equivalence ratio Φ = 1.75, in which the incipient soot particles undergo only minor soot surface growth along the flame, and a more sooting flame at Φ = 1.95. Size measurements were obtained by modeling the time-resolved LII signals detected using 1064 nm laser excitation. Spectrally-resolved LII signals collected in the nucleation flame display a similar blackbody-like behavior as mature soot. Soot particle temperature was determined from spectrally-resolved detection. LII modeling was conducted using parameters either relevant to those of mature soot or derived from fitting the modeled results to the experimental LII data. Particle size measurements were also carried out using (1) ex situ analysis by helium-ion microscopy (HIM) of particles sampled thermophoretically and (2) online size distribution analysis of microprobe-sampled particles using a 1 nm-SMPS. The size distributions of the incipient soot particles, found in the nucleation flame and in the early soot region of the Φ = 1.95 flame, derived from time-resolved LII signals are in good agreement with HIM and 1 nm-SMPS measurements and are in the range of 2–4 nm. The thermal and optical properties of incipient soot were found to be not radically different from those of mature soot commonly used in LII modeling. This explains the ability of incipient soot particles to produce continuous thermal emissions in the visible spectrum. This study demonstrates that LII is a promising in situ optical particle sizing technique that is capable of detecting incipient soot as small as about 2.5 nm and potentially 2 nm and resolving small changes in soot sizes below 10 nm.

© 2017 American Association for Aerosol Research     

Implementation of the population balance equation in CFD codes for modelling soot formation in turbulent flames

The simulation of soot formation in turbulent diffusion flames is carried out within a CFD code, by coupling kinetics and fluid dynamics computations with the solution of the population balance equation via the Direct Quadrature Method of Moments, a novel and efficient approach based on a quadrature approximation of the size distribution of soot particles. A turbulent non-premixed ethylene-air flame is used as the test case for validation of the model. Simplified kinetic expressions are employed for modelling nucleation, molecular growth and oxidation of particles, along with a Brownian aggregation kernel. A recently proposed approach for modelling the evolution of fractal dimension is used with a monovariate population balance to predict the morphological properties of aggregates.     

Kinetics of catalyzed and non‐catalyzed soot oxidation with nitrogen dioxide under regeneration particle trap conditions

BACKGROUND: For compliance with the regulations on diesel particulate matter, car manufacturers have developed diesel particulate filters (DPF). These technologies require a regeneration method which oxidizes soot deposits in the filter. In diesel exhaust emissions there are two suitable oxidizing gases: oxygen and nitrogen dioxide. Nitrogen dioxide is much more active than O₂ and can directly attack the carbon surface. This work describes the kinetics of the oxidation of soot by NO₂ over a wide range of conditions relevant for DPF. RESULTS: The catalyzed and the non‐catalyzed oxidation of soot have been performed in a fixed‐bed reactor. The experimental results show that the overall oxidation process can be described by two additive parallel reactions: a direct C ? NO₂ reaction catalyzed by H₂O and a cooperative C ? NO₂ ? O₂ reaction catalyzed by the Pt/Al₂O₃ catalyst. The results obtained allow to propose the following kinetic law for the specific rates of the catalyzed and the non‐catalyzed oxidation of soot in the regeneration filter conditions: CONCLUSION: The kinetic parameters describing the oxidation rate of soot by NO₂ over a range of temperature and gas composition have been obtained. The extracted kinetics data are relevant for modeling the removal of trapping soot in automotive gas exhaust technology. Copyright © 2009 Society of Chemical Industry     

Simulation of NOx and soot abatement with Cu‐Cha and Fe‐ZSM5 catalysts

The embodiment of the NO_x selective catalytic reduction (SCR) functionality in a diesel particulate filter (DPF), so‐called SCR‐on‐Filter (SCRoF), is investigated through numerical modeling with SCR kinetics corresponding to Cu‐Chabazite and Fe‐ZSM5 catalysts. The results of the simulations of the SCR activity, performed in the absence and presence of soot, indicate that the presence of soot negligibly affects the NO_x conversion efficiency, given the slow dynamics of passive regeneration. Conversely, the reduction in cake thickness by soot passive oxidation is significantly different in the absence of SCR activity (uncatalyzed DPF) compared to that in its presence (SCRoF). In fact, in the SCRoF only 60–80% of the original soot consumption obtained in the absence of SCR reaction over 1 h can be achieved. Individual Cu‐Chabazite and Fe‐ZSM5 catalysts, as well as in‐series layers of the two catalysts, are investigated to devise the widest temperature window for SCRoF. © 2016 American Institute of Chemical Engineers AIChE J, 63: 238–248, 2017     

Modeling Oxidation of Soot Particles Within a Laminar Aerosol Flow Reactor Using Computational Fluid Dynamics

This work examines the measurement of surface specific soot oxidation rates with the High Temperature Oxidation-Tandem Differential Mobility Analyzer (HTO-TDMA) method. The Computational Fluid Dynamics package CFD-ACE⁺ is used to understand particle flow, oxidation and size dependent particle losses in the laminar aerosol flow reactor using an Eulerian-Lagrangian framework. Decrease of DMA selected mono-disperse particle size distribution due to oxidation within the aerosol tube is modeled using fitted kinetic soot oxidation parameters. The effects of Brownian diffusion and thermophoresis on particle flow and loss to the reactor walls are evaluated. The position of peak particle diameter, which is used as an indicator to determine oxidation rate, is found to be independent of diffusion, thermophoresis and secondary flow effects, thus validating its use in deriving kinetic soot oxidation parameters. Diffusion does not affect the evolution of particle size distribution within the reactor. However, thermophoresis is found to be the dominant mechanism influencing both shape of particle size distribution and particle loss to the walls of the aerosol reactor. Simulations show reduced effects of secondary recirculating flows on the particle flow trajectories in a vertical furnace as compared to horizontal furnace orientation. This work highlights the importance of making accurate measurements of temperature within the modeling domain. Since gas temperature within the flow tube could not be measured with high radial resolution using radiation shielded thermocouple, the derived soot oxidation rate may be uncertain by a factor of 2. Importantly, CFD simulations suggest that a distribution of temperature and size-dependent particle reactivities may be present in the reactor, requiring further theoretical and experimental investigation.     

Impact of Fe Content in Laboratory-Produced Soot Aerosol on its Composition,Structure, and Thermo-Chemical Properties

Soot aerosol, which is a major pollutant in the atmosphere of urban areas, often contains not only carbonaceous matter but also inorganic material. These species, for example, iron compounds, originated from impurities in fuel or lubricating oil, additives or engine wear may change the physico-chemical characteristics of soot and hence its environmental impact. We studied the change of composition, structure, and oxidation reactivity of laboratory-produced soot aerosol with varying iron content. Soot types of various iron contents were generated in a propane/air diffusion flame by adjusting the doping amount of iron pentacarbonyl Fe(CO)₅ to the flame. Scanning electron microscopy (SEM)/energy-dispersive X-ray spectroscopy (EDX) was combined with cluster analysis (CA) to separate individual particles into definable groups of similar chemical composition representing the particle types in dependence of the iron content in soot. Raman microspectroscopy (RM) and infrared spectroscopy were applied for the characterization of the graphitic soot structure, hydrocarbons, and iron species. For the analysis of soot reactivity, temperature-programmed oxidation (TPO) was used. It is demonstrated that iron is most dominantly present in the form of amorphous Fe (III) oxide crystallizing to hematite α-Fe₂O₃ upon thermal treatment. Iron contaminations do not change the soot microstructure crucially, but Fe(CO)₅ doping of the flame impacts hydrocarbon composition. Soot oxidation reactivity strongly depends on the iron content, as the temperature of maximum carbon (di)oxide emission T _max follows an exponential decay with increasing iron content in soot. Based on the results of the thermo-chemical characterization of laboratory-produced internally mixed iron-containing soot, we can conclude that iron-containing combustion aerosol samples cannot be characterized unambiguously by current thermo-optical analysis protocols.

Copyright 2012 American Association for Aerosol Research     

Investigation of soot formation in turbulent flames with a pseudo-bivariate population balance model

Soot inception and subsequent aggregation, surface growth and oxidation are described through a new pseudo-bivariate population balance model solved with the direct quadrature method of moments (DQMOM) and implemented in a commercial computational fluid dynamics (CFD) code. This modelling strategy, that in this work is presented in its Reynolds-averaged Navier-Stokes (RANS) equation formulation, has the advantage, over conventional approaches based on the solution of a single transport equation for the soot volume fraction, to overcome the assumption of mono-dispersed soot particle size distribution. On the contrary the pseudo-bivariate approach presented in this work is able to represent the evolution of the soot particle size distribution with good accuracy and affordable computational costs, especially when compared with other multi-variate formulations previously developed. This new pseudo-bivariate model is firstly formulated and presented, then predictions obtained with different soot inception models are compared with some recent experimental data from the literature and the role played by the different phenomena involved (e.g., turbulence, oxidation and radiation) is investigated.     

Effects of air/fuel combustion ratio on the polycyclic aromatic hydrocarbon content of carbonaceous soots from selected fuels

Patterns of polycyclic aromatic hydrocarbon (PAH) content were observed from GC/MS analysis of the extracts of soots at various air/fuel combustion ratios of three commonly used fuels: n-hexane, JP-8 (Jet fuel), and diesel. With increasing air/fuel ratio, from a simple diffusion flame up to an air/fuel ratio of 3.94, there is a significant loss of high molecular weight PAHs and an increasing abundance of oxidized lower molecular weight aromatics. The formation of high molecular weight PAHs is favored for JP-8 and diesel fuels at higher air/fuel combustion ratios than is the case with n-hexane, probably due to the aromatic content in JP-8 and diesel fuels acting as centers for large aromatic and soot nucleation. The efficiency and reproducibility of two techniques, Soxhlet and supercritical fluid extraction (SFE), used for extraction of PAHs from soot were compared. Electron paramagnetic resonance (EPR) measurements were performed on the soot both before and after supercritical fluid and Soxhlet extraction, and a substantial decrease in the spin density of soot following extraction indicates that extractable molecules are associated with 40–50% of the unpaired electrons in soot. This analysis generally supports trends observed in our earlier work for surface oxidation, surface area, unpaired electron spin density, hydration, and ozone oxidation.     

Evolution of maturity levels of the particle surface and bulk during soot growth and oxidation in a flame

We performed a study of the evolution of soot composition and fine structure, i.e., maturity level, in an atmospheric ethylene-air diffusion flame. We used laser-induced incandescence (LII) to provide information about maturity level of the bulk primary particle and X-ray photoelectron spectroscopy (XPS) to provide complementary information about particle-surface-maturity level. The results demonstrate that the bulk material and the particle surface evolve separately in the flame. Increased soot-maturity level is associated with increased long-range order of the particle fine structure. This increased order leads to an increase in the absorption cross-section in the visible and near-infrared and a shift of the absorption to longer wavelengths with increasing maturity level of the bulk particle. These trends result in a decrease in the dispersion exponent (?) and increase in the absorption cross-section scaling factor (?), as inferred from LII measurements. LII measurements demonstrate that bulk-maturity level increases with height-above-the-burner (HAB) until it reaches a plateau in the center of the flame at the maximum in the soot volume fraction. Bulk-maturity level only slightly decreases as soot is oxidized at larger HABs. Increased maturity level also leads to an increase in long-range sp² hybridization. XPS measurements of the sp²/defect ratio demonstrate an increase in soot surface-maturity level with increasing HAB, but the surface-maturity level increases more gradually with HAB than the bulk-maturity level. Whereas the bulk-fine-structure order decreases slightly in the oxidation region, the surface order decreases dramatically, indicating that oxidation occurs preferentially at the surface under these conditions.

Copyright © The Authors. Published with license by American Association for Aerosol Research     

Multiplicity and stability of premixed laminar flames: an application of bifurcation theory

Numerical bifurcation techniques are used to describe multiple steady states for a premixed, laminar flame stabilized on a flat flame burner. The flame is assumed to be adiabatic, and the kinetic mechanism is approximated by a single reaction. The numerical methods make it possible to determine all steady states and eliminate computational difficulties near singular points. The possibility of defining solutions near singularities is particularly important in flame modeling for it is near such points that ignition and burn-out may occur. Three steady states are identified: a stable upper state corresponding to a flame burning at or near the adiabatic flame temperature, a lower solution representing an extinguished stable state, and an unstable intermediate state. Sensitivity of the solutions to changes in kinetic parameters is enhanced near burn-out. It is expected that the ability to predict flame behavior near such singular points will be particularly useful in the determination of flame kinetics.     

A new reduced reaction mechanism of a surrogate fuel for kerosene

The introduction of detailed chemical reaction mechanisms for aviation fuels into complex multidimensional fluid dynamics problems is not practical at the present time. Simplified reaction mechanisms that have been thoroughly evaluated must be developed to address specific issues arising in realistic combustor configurations. A reduced chemical kinetic mechanism features 210 elemental reactions (including 92 reversible reactions and 26 irreversible reactions) and 50 species for the ignition and combustion of n‐decane was compiled and validated for a wide range of combustion regimes. Validations were performed using experimental measurements on a premixed flame of Jet‐A1, O₂ and N₂, stabilised at 1 atm on a flat‐flame burner, as well as from n‐decane shock‐tube ignition experiments. Numerical calculations were performed using this reduced mechanism and the detailed mechanism respectively for n‐decane surrogate fuel. The calculated values of ignition delay times at pressures of 12, 50 bar and equivalence ratio is 1.0, 2.0, respectively and the main reactants and main products mole fractions agree well with experimental data. The present study shows that this reduced mechanism for the n‐decane surrogate can be employed to predict premixed combustion of kerosene. © 2012 Canadian Society for Chemical Engineering     

Low-temperature oxidation of soot

The oxidation rate of propane soot over the temperature range (770–1250 K) has been measured using two methods. These were direct measurements of the burn-out of soot produced by a laminar diffusion flame, and a thermal gravimetric technique using collected soot. The reaction order with respect to oxygen (over the range 1–20 kPa) was determined at a variety of particle temperatures and found to lie between 0 and 0.65. The intrinsic reaction rate at an oxygen partial pressure of 101 kPa was found to be given by ρ_i = 1.05 exp(− 143.5/RT) units where the activation energy is given in kJ/mol. Some values for the oxidation rate of methane soot are also given.     

A two‐zone model for fluid catalytic cracking riser with multiple feed injectors

Developments in modeling of the fluid catalytic cracking (FCC) process have progressed along two lines. One emphasizes composition‐based kinetic models based on molecular characterization of feedstocks and reaction products. The other relies on computational fluid dynamics. The aim is to develop an FCC model that strikes a balance between the two approaches. Specifically, we present an FCC riser model consisting of an entrance‐zone and a fully developed zone. The former has four overlapping, fan‐shaped oil sprays. The model predicts the plant data of Derouin et al. and reveals an inherent two‐zone character of the FCC riser. Inside the entrance zone, cracking intensity is highest and changes rapidly, resulting in a steep rise in oil conversion. Outside the entrance zone, cracking intensity is low and varies slowly, leading to a sluggish increase in conversion. The two‐zone model provides a computationally efficient modeling approach for FCC online control, optimization, and molecular management. © 2014 American Institute of Chemical Engineers AIChE J, 61: 610–619, 2015     

Global kinetic modelling of the reaction of soot with O2 and NOx on Fe2O3 catalyst

This study deals with the catalytic reaction of NO_x and soot on Fe₂O₃ to yield N₂ and CO₂ in excess of oxygen. Based on the three types of kinetic experiments, i.e. temperature programmed oxidation (TPO), transient examinations and gradient-free loop reactor experiments, as well as mechanistic studies presented recently a global kinetic model is established. The model includes catalytic effect of the iron oxide on soot/O₂ reaction, whereas it is assumed that NO_x reduction occurs on the soot without direct participation of Fe₂O₃. Furthermore, the model implies global kinetic expressions for the CO_x formation and NO_x reduction. These equations include the evolution of the surface area of soot and the correlation of reactive carbon sites (C_f) with those specifically involved in NO_x reduction (C*). The kinetic model is sequentially developed by accounting for the catalytic and non-catalytic soot/O₂ as well as soot/NO_x/O₂ conversion. Kinetic parameters are taken from the literature and are also determined from a fit to experimental data. Comparison of measured and calculated data shows accurate reproduction of the experiments and the model. Finally, the kinetic model is validated by some simulations.     

Ceria‐based nanomaterials as catalysts for CO oxidation and soot combustion: Effect of Zr‐Pr doping and structural properties on the catalytic activity

In this work, we investigated a set of ceria‐based catalysts prepared by the hydrothermal and solution combustion synthesis. For the first time to our knowledge, we synthesized nanocubes of ceria doped with zirconium and praseodymium. The catalysts were tested for the CO and soot oxidation reactions. These materials exhibited different surface reducibility, as measured by H₂‐TPR, CO‐TPR and Soot‐TPR, despite their comparable chemical compositions. As a whole, Soot‐TPR appears a suitable characterization technique for the soot oxidation catalysts, whereas CO‐TPR technique allows to better discriminate among the CO oxidation activities. Praseodymium contributes positively toward the soot oxidation. On the other hand, it has an adverse effect on the CO oxidation over the same catalysts, as compared to pure ceria. The incorporation of zirconium into the ceria lattice does not have a direct beneficial effect on the soot oxidation activity, although it increases the catalyst performances for CO oxidation. © 2016 American Institute of Chemical Engineers AIChE J, 63: 216–225, 2017