Soot Number Density Research Articles

On the formation and early evolution of soot in turbulent nonpremixed flames

A Direct Numerical Simulation (DNS) of soot formation in an n-heptane/air turbulent nonpremixed flame has been performed to investigate unsteady strain effects on soot growth and transport. For the first time in a DNS of turbulent combustion, Polycyclic Aromatic Hydrocarbons (PAH) are included via a validated, reduced chemical mechanism. A novel statistical representation of soot aggregates based on the Hybrid Method of Moments is used [M.E. Mueller, G. Blanquart, H. Pitsch, Combust. Flame 156 (2009) 1143–1155], which allows for an accurate state-of-the-art description of soot number density, volume fraction, and morphology of the aggregates. In agreement with previous experimental studies in laminar flames, Damköhler number effects are found to be significant for PAH. Soot nucleation and growth from PAH are locally inhibited by high scalar dissipation rate, thus providing a possible explanation for the experimentally observed reduction of soot yields at increasing levels of mixing in turbulent sooting flames. Furthermore, our data indicate that soot growth models that rely on smaller hydrocarbon species such as acetylene as a proxy for large PAH molecules ignore or misrepresent the effects of turbulent mixing and hydrodynamic strain on soot formation due to differences in the species Damköhler number. Upon formation on the rich side of the flame, soot is displaced relative to curved mixture fraction iso-surfaces due to differential diffusion effects between soot and the gas-phase. Soot traveling towards the flame is oxidized, and aggregates displaced away from the flame grow primarily by condensation of PAH on the particle surface. In contrast to previous DNS studies based on simplified soot and chemistry models, surface reactions are found to contribute barely to the growth of soot, for nucleation and condensation processes occurring in the fuel stream are responsible for the most of soot mass generation. Furthermore, the morphology of the soot aggregates is found to depend on the location of soot in mixture fraction space. Aggregates having the largest primary particles populate the region closest to the location of peak soot growth. On the contrary, the aggregates with the largest number of primary particles are located much further into the fuel stream.

In-situ observations of young contrails – overview and selected results from the CONCERT campaign

Abstract. Lineshaped contrails were detected with the research aircraft Falcon during the CONCERT – CONtrail and Cirrus ExpeRimenT – campaign in October/November 2008. The Falcon was equipped with a set of instruments to measure the particle size distribution, shape, extinction and chemical composition as well as trace gas mixing ratios of sulfur dioxide (SO2), reactive nitrogen and halogen species (NO, NOy, HNO3, HONO, HCl), ozone (O3) and carbon monoxide (CO). During 12 mission flights over Europe, numerous contrails, cirrus clouds and a volcanic aerosol layer were probed at altitudes between 8.5 and 11.6 km and at temperatures above 213 K. 22 contrails from 11 different aircraft were observed near and below ice saturation. The observed NO mixing ratios, ice crystal and soot number densities are compared to a process based contrail model. On 19 November 2008 the contrail from a CRJ-2 aircraft was penetrated in 10.1 km altitude at a temperature of 221 K. The contrail had mean ice crystal number densities of 125 cm−3 with effective radii reff of 2.6 μm. The presence of particles with r>50 μm in the less than 2 min old contrail suggests that natural cirrus crystals were entrained in the contrail. Mean HONO/NO (HONO/NOy) ratios of 0.037 (0.024) and the fuel sulfur conversion efficiency to H2SO4 (εS↓) of 2.9 % observed in the CRJ-2 contrail are in the range of previous measurements in the gaseous aircraft exhaust. On 31 October 2010 aviation NO emissions could have contributed by more than 40% to the regional scale NO levels in the mid-latitude lowest stratosphere. The CONCERT observations help to better quantify the climate impact from contrails and will be used to investigate the chemical processing of trace gases on contrails.

Modelling soot formation in a DISI engine

In this work, the formation of soot in a Direct Injection Spark Ignition (DISI) engine is simulated using the Stochastic Reactor Model (SRM) engine code. Volume change, convective heat transfer, turbulent mixing, direct injection and flame propagation are accounted for. In order to simulate flame propagation, the cylinder is divided into an unburned, entrained and burned zone, with the rate of entrainment being governed by empirical equations but combustion modelled with chemical kinetics. The model contains a detailed chemical mechanism as well as a highly detailed soot formation model, however computation times are relatively short. The soot model provides information on the morphology and chemical composition of soot aggregates along with bulk quantities, including soot mass, number density, volume fraction and surface area. The model is first calibrated by simulating experimental data from a Gasoline Direct Injection (GDI) Spark Ignition (SI) engine. The model is then used to simulate experimental data from the literature, where the numbers, sizes and derived mass particulate emissions from a 1.83 L, 4-cylinder, 4 valve production DISI engine were examined. Experimental results from different injection and spark timings are compared with the model and the qualitative trends in aggregate size distribution and emissions match the exhaust gas measurements well.

Experimental Study on Characteristics of Synergistic Effect of Fuel Mixing on Number Density and Size of Soot in Ethylene-base Counterflow Diffusion Flames by Laser Techniques

The effect of fuel mixing on soot structure with methane, ethane, and propane to ethylene-base counterflow diffusion flames has been investigated by measuring the volume fraction, number density, and particle size of soot by adopting the light extinction/scattering techniques. The experimental result showed that the mixing of ethane and propane in ethylene diffusion flame increased soot volume fraction while the mixing of methane decreased. As compare to the ethylene-base flame, the diameters of soot particles for mixture flames are slightly smaller. While the soot number densities for the mixture flames are much higher. Thus, the increase in the soot volume fraction can be attributed to the appreciably increased soot number density by the fuel mixing.

Numerical investigation of soot reduction potentials with diesel homogeneous charge compression ignition combustion by an improved phenomenological soot model

An improved phenomenological soot model coupled with a reduced n-heptane chemical mechanism was implemented into KIVA-3V code to describe soot formation and oxidation processes in diesel homogeneous charge compression ignition (HCCI) combustion. This model was first validated by the shock tube experiments with a rich n-heptane mixture over wide temperature and pressure ranges. The computational results demonstrate that the phenomenological soot model is capable of predicting the soot yield, particle diameter, and number density with satisfactory accuracy. Then the model was applied to investigate the influence of the orifice diameter and injection pressure on soot emissions in a constant-volume combustion vessel under typical diesel combustion conditions. The predictions showed qualitative agreement with the measurements on the soot volume fraction distribution. The results also indicate that the soot formation can almost be suppressed as the local equivalence ratio is kept lower than 2.0. Finally, the model was used to explore the potentials of soot reduction with HCCI combustion for diesel engines. The overall trend of soot with the variations in the start of injection timing was well reproduced by the model. With the help of an equivalence ratio—temperature map, it was found that nitrogen oxide emissions could be markedly reduced by applying a high exhaust gas recirculation rate and relative low compression ratio for diesel HCCI engines. However, the mixture preparation by using a multi-hole injector with early injection strategy remains a limitation for further reduction in soot emissions.

Hybrid Method of Moments for modeling soot formation and growth

In this work, a new statistical model for soot formation and growth is developed and presented. The Hybrid Method of Moments (HMOM) seeks to combine the advantages of two moment methods, the Method of Moments with Interpolative Closure (MOMIC) and the Direct Quadrature Method of Moments (DQMOM), in an accurate and consistent formulation. MOMIC is numerically simple and easy to implement but is unable to account for bimodal soot Number Density Functions (NDF). DQMOM is accurate but is numerically ill-posed and difficult to implement. HMOM combines the best of both two methods to capture bimodal NDF while retaining ease of implementation and numerical robustness. The new hybrid method is shown to predict mean quantities nearly as accurately as DQMOM and high-fidelity Monte Carlo simulations. In addition, a model for combining particle coalescence with particle aggregation is presented and shown to accurately reproduce experimental measurements in a variety of sooting flames.

Towards a detailed soot model for internal combustion engines

In this work, we present a detailed model for the formation of soot in internal combustion engines describing not only bulk quantities such as soot mass, number density, volume fraction, and surface area but also the morphology and chemical composition of soot aggregates. The new model is based on the Stochastic Reactor Model (SRM) engine code, which uses detailed chemistry and takes into account convective heat transfer and turbulent mixing, and the soot formation is accounted for by SWEEP, a population balance solver based on a Monte Carlo method. In order to couple the gas-phase to the particulate phase, a detailed chemical kinetic mechanism describing the combustion of Primary Reference Fuels (PRFs) is extended to include small Polycyclic Aromatic Hydrocarbons (PAHs) such as pyrene, which function as soot precursor species for particle inception in the soot model. Apart from providing averaged quantities as functions of crank angle like soot mass, volume fraction, aggregate diameter, and the number of primary particles per aggregate for example, the integrated model also gives detailed information such as aggregate and primary particle size distribution functions. In addition, specifics about aggregate structure and composition, including C/H ratio and PAH ring count distributions, and images similar to those produced with Transmission Electron Microscopes (TEMs), can be obtained. The new model is applied to simulate an n-heptane fuelled Homogeneous Charge Compression Ignition (HCCI) engine which is operated at an equivalence ratio of 1.93. In-cylinder pressure and heat release predictions show satisfactory agreement with measurements. Furthermore, simulated aggregate size distributions as well as their time evolution are found to qualitatively agree with those obtained experimentally through snatch sampling. It is also observed both in the experiment as well as in the simulation that aggregates in the trapped residual gases play a vital role in the soot formation process.

Transient Development of Flame and Soot Distribution in Laminar Diffusion Flame With Preheated Air

A numerical investigation of the transient development of flame and soot distributions in a laminar axisymmetric coflowing diffusion flame of methane in air has been carried out considering the air preheating effect. The gas phase conservation equations of mass, momentum, energy, and species concentrations along with the conservation equations of soot mass concentration and number density are solved simultaneously, with appropriate boundary conditions, by an explicit finite difference method. Average soot diameters are then calculated from these results. It is observed that the soot is formed in the flame when the temperature exceeds 1300 K. The contribution of surface growth toward soot formation is more significant compared with that of nucleation. Once the soot particles reach the high temperature oxygen-enriched zone beyond the flame, the soot oxidation becomes important. During the initial period, when soot oxidation is not contributing significantly, some of the soot particles escape into the atmosphere. However, under steady condition the exhaust product gas is nonsooty. Preheating of air increases the soot volume fraction significantly. This is both due to more number of soot particles and the increase in the average diameter. However, preheating of air does not cause a qualitative difference in the development of the soot-laden zone during the flame transient period.

Modeling of Soot and Polycyclic Aromatic Hydrocarbons in Diesel Diffusion Combustion

AbstractBy integrating a reduced n‐heptane oxidation chemical kinetic reaction mechanism in a multidimensional computational fluid dynamic solver, simulation of the turbulent diffusion reactive flow inside the direct injection (DI) diesel combustor through compression and expansion stroke were implemented. Coupled with the soot kinetic model by the method of moments, the soot formation is modeled simultaneously with a gas‐phase flame. Under fuel lean flame, orders of magnitude of the multi‐ring polycyclic aromatic hydrocarbon (PAH) intermediate molar fraction are very small. Though soot surface growth is overwhelmed quickly by oxidation under current modeling conditions, the simulated soot number density and average diameter curves are consistent with experimental results. A detailed soot kinetic mechanism is a prospect for further research in computational fluid dynamics (CFD) modeling.

Aromatic site description of soot particles

A new, advanced soot particle model is developed that describes soot particles by their aromatic structure, including functional site descriptions and a detailed surface chemistry mechanism. A methodology is presented for the description of polyaromatic hydrocarbon (PAH) structures by their functional sites. The model is based on statistics that describe aromatic structural information in the form of easily computed correlations, which were generated using a kinetic Monte Carlo algorithm to study the growth of single PAH molecules. A comprehensive surface reaction mechanism is presented to describe the growth and desorption of aromatic rings on PAHs. The model is capable of simulating whole particle ensembles which allows bulk properties such as soot volume fraction and number density to be found, as well as joint particle size and surface area distributions. The model is compared to the literature-standard soot model [J. Appel, H. Bockhorn, M. Frenklach, Combust. Flame 121 (2000) 122–136] in a plug-flow reactor and is shown to predict well the experimental results of soot mass, average particle size, and particle size distributions at different flow times. Finally, the carbon/hydrogen ratio and the distribution of average PAH sizes in the ensemble, as predicted by the model, are discussed.

Investigation of soot transport and radiative heat transfer in an ethylene jet diffusion flame

Numerical and experimental investigations highlighting the heat and mass transfer phenomena in a laminar co-flowing jet diffusion flame have been carried out. The fuel under consideration is ethylene, with ambient air as the co-flowing oxidizer. The diffusion flame is modeled using a 17-step reduced reaction mechanism with finite rate chemistry and the effects of soot on the radiative heat transfer of the flame have been demonstrated. Soot growth and oxidation processes are studied using a two-equation transport model, while the radiative heat transfer is modeled using the P1 approximation. An in-house finite volume code has been developed to solve the axi-symmetric Navier–Stokes equations in cylindrical coordinates, along with the soot mass fraction, soot number density, energy and species conservation equations. Comparison of predictions with experimental results shows reasonable agreement with regard to the flame height and temperature distribution. A parametric study is also presented, which illustrates the effects of the fuel jet Reynolds number and the flow rate of co-flow air.

Transient soot dynamics in turbulent nonpremixed ethylene–air counterflow flames

The dynamics of soot formation in turbulent ethylene–air nonpremixed counterflow flames is studied using direct numerical simulation (DNS) with a semi-empirical soot model and the discrete ordinate method (DOM) as a radiation solver. Transient characteristics of soot behavior are studies by a model problem of flame interaction with turbulence inflow at various intensities. The interaction between soot and turbulence reveals that the soot volume fraction depends on the combined effects of the local conditions of flow, temperature, and fuel concentration, while the soot number density depends predominantly on the high temperature regions. Depending on the relative strength between mixing and reaction, the effects of turbulence on the soot formation lead to three distinct paths in deviating the data points away from the laminar flame conditions. It is found that turbulence has twofold effects of increasing the overall soot yield by generating additional flame volume and of reducing soot by dissipating soot pockets out of high-temperature regions. The relative importance between the two effects depends on the relative length scales of turbulence and flame, suggesting that a nonmonotonic response of soot yield to turbulence level may be expected in turbulent combustion.

Development of an experimetal test-rig for natrural gas diffusion flame studies

This work presents a part of a continuous research programme designed to provide information and experimental data leading to studies of soot and stable gaseous species from natural gas diffusion flame. This Test – Rig, which operates under atmospheric pressure, has along side with it a small probe (thermocouple attachment) for instantaneous measurement of temperature. The test rig has a co-annular burner consisting of a 16mm diameter steel tube (where the natural gas is injected upwardly) and surrounded by a 109mm diameter air annulus. The Rig and Facilities measures flame temperature, samples particulate matter (soot) and stable gaseous combustion product species within the flame with a well-designed Isokinetic sampling probe insert. After which, the natural gas species compositional change, soot volume fraction, soot number density can be determined with respect to axial and radial position distances from the burner rim. Keywords : diffusion flame, temperature profile and isokinetic Global Journal of Pure and Applied Sciences Vol. 12(2) 2006: 211-219

Numerical simulation and sensitivity analysis of detailed soot particle size distribution in laminar premixed ethylene flames

In this paper, the prediction of a soot model [J. Appel, H. Bockhorn, M. Frenklach, Combust. Flame 121 (2000) 122–136] is compared to a recently published set of highly detailed soot particle size distributions [B. Zhao, Z. Yang, Z. Li, M.V. Johnston, H. Wang, Proc. Combust. Inst. 30 (2005)]. A stochastic approach is used to obtain soot particle size distributions (PSDs). The key features of the measured and simulated particle size distributions are identified and used as a simple way of comparing PSDs. The sensitivity of the soot PSDs to the parameters defining parts of the soot model, such as soot inception, particle and PAH collision efficiency and enhancement, and surface activity is investigated. Incepting soot particle size is found to have a very significant effect on the small-size end of the PSDs, especially the position of the trough for a bimodal soot PSDs. A new model for the decay in the surface activity is proposed in which the activity of the soot particle depends only on the history of that particle and the local temperature in the flame. This is a first attempt to use local flame variables to define the surface aging which has major impact on the prediction of the large-size end of the PSDs. Using these modifications to the soot model it is possible to improve the agreement between some of the points of interest in the simulated and measured PSDs. The paper achieves the task to help advance the soot models to predict soot PSD in addition to soot volume fraction and number density, which has been the focus of the literature.

06/00274 Self-ignition of a lean mixture of n-pentane and airover a wide range of pressures : Zhukov, V. P. et al. Combustion and Flame, 2005, 140, (3), 196–203

Sooting tendencies of fuels are analyzed with respect to the type of experimental configuration used—per-mixed flames, normal and inverse co-annular, Wolfhard-Parker and counter-flowing diffusion flames, or shock tubes. The important effect of temperature in each type of experiment is examined. The relative tendency of various fuels to soot when consumed in pre-mixed and diffusion flames has been determined by the qualitative measurements of critical sooting equivalence ratios and smoke heights at various temperatures and quantitatively confirmed by measurements of chemical species, soot number density and volume fraction.Fuel structure has a significant effect on the sooting tendency of diffusion flames, but little influence in premixed flames. Irrespective of the fuel in diffusion flames soot inception occurs around 1400 K and is dependent somewhat on H atom diffusion. Particle burnout ceases at about 1300 K and is responsible for the smoke height.Incipient particle formation determines the volume fraction of soot and is controlled by the rate of formation of the first aromatic species containing one to two rings. The particle growth process is found to be the same regardless of flame type. A generalized chemical mechanism of soot formation irrespective of flame type or process is presented. The various routes in this mechanism support the relative sooting tendency of fuels found in both flame and shock tube experiments. The effect of oxygen added to various fuel in diffusion controlled experiments is analyzed.

OPTIMIZATION OF SOOT MODELING IN TURBULENT NONPREMIXED ETHYLENE/AIR JET FLAMES

ABSTRACT Two-equation soot models, which solve conservation equations for soot number density and mass concentration, have been extensively used to study soot formation in laboratorial turbulent flame and practical gas-turbine combustors. This study investigates the effects of different inception, growth coagulation, and oxidation source terms in a two-equation semi-empirical soot model that has been implemented to model two turbulent ethylene/air jet flames. The gas-phase chemistry is modeled using the laminar flamelet approach. A new soot inception submodel is proposed that is based on the naphthalene formation rate calculated by the detailed chemical kinetics. The expected value of the formation rate is stored in the flamelet library. Model predictions were compared with the measurements of Young and Moss. The predictions of the soot volume fraction are very sensitive to the soot surface growth rate. The soot predictions agree well with measurements when the surface growth rate is assumed to be proportional to the square root of the surface area. The result also indicate that the naphthalene inception route exhibits better performance. Finally a new soot model with an optimal combination of rates was developed. The model predictions provided good agreement with the experimental temperature, mixture fraction, and soot volume fraction distributions along both the axial and radial directions. The optimal soot model was also successfully validated on another turbulent ethylene/air jet flame.

Quantitative measurements of soot particles in a laminar diffusion flame using a LII/LIS technique

Methods to investigate soot formation are studied experimentally with a co-flow burner. In this study, the soot volume fraction, soot particle diameter and number density in a laminar diffusion flame are measured using LII/LIS (laser-induced incandescence/laser-induced scattering) techniques. For the purpose of quantification, validation tests needed to be conducted before the LII/LIS techniques were applied to measure soot formation, and the development of algorithm was also required to analyse the raw data. In the present study, extinction and scattering tests with a co-flow burner were performed to acquire calibration data. From these results, we decided the optimal laser beam intensity and obtained the LII signal from the diffusion flame. In addition, the algorithm for the LII/LIS simultaneous measurement was developed and applied to this algorithm to measure the soot volume fraction in the diffusion flame.

레이저 및 열전대를 이용한 동축류 확산화염에서의 매연입자 측정에 관한 연구

The temperature and soot particle measurement technique in a laminar diffusion flame has been studied to Investigate the characteristics of soot particle with temperature using a co-flow burner The temperature distribution in the flame were measured by rapid Insertion of a R-type thermocouple and the soot particles by LEM/LIS techniques In these measurement, soot volume fraction, number density and soot diameters were analyzed experimentally As a results, the spacial distributions of particle volume fraction, soot diameter, and number density are mapped throughout the flame using the Rayleigh theory for the scattering of light by particles A laser extinction method was used to measure the soot volume fraction and laser induced scattering method was used to measure the soot particle diameter and number density Also, we measured temperature without the effect of soot particles attached to the thermocouple Junction, which is close to the nozzle In this result, we found that upstream zone has a unstable flowing in co-flow diffusion flame and the y-axis temperature of flame has a uniform temperature distribution in the most soot volume fraction zone<br/>

COMPUTED NO AND SOOT DISTRIBUTION IN TURBULENT TRANSIENT JETS UNDER DIESEL CONDITIONS

In this work, the NO submechanism from GRIMECH 3.0 and three soot models of increasing levels of complexity are employed to predict NO and soot distribution in transient turbulent diesel jets. The soot models are (a) an empirical model in which soot mass formation rate is related to the pressure, temperature, and fuel concentration; (b) a semi-empirical model in which soot nucleation, coagulation, and surface growth are modeled; and (c) a kinetic model. The first two formation models are coupled with soot oxidation models proposed by Nagle and Strickland-Constable and Fenimore and Jones whereas the third model includes soot oxidation in the reaction kinetics. Effects of soot diffusivity and radiant heat transfer are evaluated. Variables studied include the soot volume fraction and number density. NO is found to be a maximum in the high-temperature region of the jet. The empirical soot model predicts maximum soot concentration along the centerline of the jet upstream of the head vortex at all instants during the injection period. The semi-empirical and the kinetic soot models predict maximum soot concentration in the head vortex region of the jet earlier in its development but a maximum along the centerline upstream of the head vortex later in its development. Radiant heat transfer reduces the formation rates of NO and soot and the oxidation rate of soot but its overall effect may not be significant in diesel jets because the residence timescales in the jet are short relative to the timescales associated with heat-loss effects due to radiation.

Modeling soot formation in turbulent kerosene/air jet diffusion flames

Soot volume fraction and number density in a turbulent diffusion flame burning kerosene/air were predicted using two approaches. The first used a conventional soot inception model based on the acetylene concentration and is referred to as the acetylene model. The second used a soot inception model based on the formation rate of three and two ring aromatics [1] and is referred to as the PAH inception model. The soot models account for inception, coagulation, surface growth, and oxidation processes. The Favre-averaged governing equations of mass, momentum, and energy in the turbulent field were solved in conjunction with the k − ε turbulence model. A recently developed detailed reaction mechanism for kerosene/air [2] was coupled to the turbulent flow field by the stretched laminar flamelet approach. A radiation heat transfer model that considered the soot, water and CO 2 levels is included. Models are validated by comparing the numerical results to the experimental results of Young et al. [3] for a turbulent jet-flame burning pre-vaporized aviation kerosene. Significant improvements in the prediction of soot volume fraction are obtained using the PAH inception model for soot inception compared to the conventional acetylene approach.