Differential Diffusion Modelling in LES with RCCE-Reduced Chemistry

The present work shows results obtained from the incorporation of a soot model into a combined Large Eddy Simulation and Conditional Moment Closure approach to modelling turbulent non-premixed flames. Soot formation is determined via the solution of two transport equations for soot mass fraction and particle number density, where acetylene is employed as the incipient species responsible for soot nucleation. The concentrations of the gaseous species are calculated using a Rate-Controlled Constrain Equilibrium approach to reduce the number of species to solve from a detailed gas-phase kinetic scheme involving 63 species. The study focuses on the influence of differential diffusion of soot particles on soot volume fraction predictions. The results of calculations are compared with experimental data for atmospheric methane flames, Overall, the study demonstrates that the model, when used in conjunction with a representation of differential diffusion effects, is capable of predicting soot formation at a fundamental level in the turbulent non- premixed flames considered.     

Radiative heat transfer calculations in real gases

A general formulation for radiative heat transfer calculations is presented, based on integrated quantities such as total emissivities and absorptivities. The procedure is intended particularly for combustion chamber applications of varying degree of complexity, the radiative active medium consisting of gases such as H₂O and CO₂ and of soot. First, some preliminary calculations are given for the often treated radiative equilibrium cases of plane parallel plates and infinite concentric cylinders. Then an example of a combustion chamber calculation is studied where the radiative heat transfer calculation is included in a system of partial differential equations describing momentum, heat and mass transfer with combustion.     

Large eddy simulation of turbulent premixed piloted flame using artificial thickened flame model coupled with tabulated chemistry

A sub-grid scale (SGS) combustion model, which combines the artificial thickened flame (ATF) model with the flamelet generated manifold (FGM) tabulation method, is proposed. Based on the analysis of laminar flame structures, two self-contained flame sensors are used to track the diffusion and reaction processes with different spatial scales in the flame front, respectively. The dynamic formulation for the proposed SGS combustion model is also performed. Large eddy simulations (LESs) of Bunsen flame F3 are used to evaluate the different SGS combustion models. The results show that the proposed SGS model has the ability in predicting the distributions of temperature and velocity reasonably, while the predictions for the distributions of some species need further improvement. The snapshots of instantaneous normalized progress variables reveal that the flame is more remarkably and severely wrinkled at the flame tip for flame F3. More satisfactory results obtained by the dynamic model indicate that it can preserve the premixed flame propagation characteristics better.     

Behavior of Moderately Oscillating Sooting Methane-Air Diffusion Flames

The properties of oscillating sooting methane air diffusion flames have been investigated by different methods in order to examine instationary effects in these flames. The pulsation has been induced by modulation of the methane gas flow with an amplitude of 30% of the mean gas flow. The focus of the investigations is on the flame oscillated at 10 Hz, which is close to the frequency of self-induced flickering. Additionally, further measurements at varying frequencies have been performed to determine the transition towards steady-state behavior. Different measurement techniques allowed the determination of soot volume fractions, particle number densities, mean particle radii, particle temperatures, and OH*-chemiluminescence. The oscillating flame shows strong instationary effects and increased soot concentrations compared to the steady-state flame of equivalent mean fuel flow. Accompanying calculations are based on a kinematic analysis of diffusion flames. The model can sufficiently well reproduce the flame height and the contour of the flame. Furthermore, the model describes the asymmetric course of the OH*-emission signal. A simple numerical approach is deduced that explains qualitatively the strong variations of the soot volume fraction in an oscillating flame. This paper is based on work presented at the 2nd ECCOMAS Thematic Conference on Computational Combustion, Delft, 2007.     

Laser-induced incandescence calibration in a three-dimensional laminar diffusion flame

A non-buoyant laminar diffusion flame has been studied using laser-induced incandescence (LII) and light extinction measurements. The present flame is established within a laminar boundary layer, producing a complex three-dimensional flow field. This produces a three-dimensional soot concentration field. LII can provide spatially resolved three-dimensional concentration measurements of the soot field, nevertheless it requires calibration. Calibration needs to be conducted under identical conditions to the actual measurements, given the complex interaction between the flow field and soot production. This study reports a calibration procedure that allows the determination of a calibration constant correlating LII signal to soot volume fraction. The potential sources of error are identified and quantified.     

Radiation effect on temperature distribution and NO formation in a diffusion flame under reduced gravity conditions

Combustion of hydrocarbon fuel is accompanied with the formation of nitric oxide (NO) amongst other harmful emissions. In this work, a numerical investigation has been made for understanding the effect of radiative heat transfer on temperature distribution and formation of thermal NO in a methane–air diffusion flame under different reduced gravity environments. Conservation equations of overall mass, species concentration, momentum and energy for the reactive flows have been numerically solved with the use of finite difference scheme. In addition to that a semi-empirical soot model and an optically thin radiation model have been incorporated in the simulation. Gravity level is varied by the changed values of acceleration due to gravity. A thermal NO model incorporated accounts for the NO formation process which is decoupled from the hydrocarbon combustion. The relevant conservation equations have been solved as a post combustion reaction process. The flame height drops marginally with the reduction of gravity. Temperature becomes more uniformly distributed at lower gravity. NO formation boosts up with the fall of gravity below normal level when no radiation effect is considered. However, when radiation is considered, NO formation declines marginally with the reduction of gravity levels. Also in this case, concentration values of NO compare substantially lower with those without radiation. The upsurge of NO formation due to decline in gravity; and on the other hand, a shrinkage in concentration values of NO due to radiation effect can be attributed mainly to the rise and fall of temperature respectively in the computational zone.     

Soot distribution in turbulent nonpremixed chloromethane — Air flames

The results of an experimental study on soot and temperature distribution in turbulent, nonpremixed chloromethane-air jet flames are presented. Transient measurements of soot volume fraction and temperature are made using a three-line optical pyrometry method. This method enables measurement of the “total” (i.e., absorption-related), and “hot” (i.e., emission-related) soot. Significant amounts of cold soot with hot soot is observed to coexist for all of the measurements made in these flames. Images of soot presence using a gated camera provide information about the mixing phenomena. The effects due to differential probe lengths are also discussed.     

Construction of a collisional radiative model of complex multiple charged ions for mid- to high-Z elements

A method of constructing a compact and complete collisional radiative model of multiple charged ions of mid- to high-Z elements is proposed, for studying radiative properties of the plasmas. The proposed tungsten model, which is based on atomic data calculated by the HULLAC code, incorporates a computer algorithm to identify well-populated atomic states and dielectronic recombination channels that have a significant effect on the ionization balance. The model is validated by investigating the convergence of the mean charge and radiative power loss with respect to the size of the model, and by comparing results with other calculations presented at the nLTE kinetics workshop.     

Numerical Simulation of Diesel Particulate Filter Regeneration Considering Ash Deposit

Non-combustible ash can be deposited on channel walls through the full length of a diesel particulate filter (DPF). This type of ash can affect the exhaust condition and heat transfer process during the periodical soot regeneration of DPF. A model of soot regeneration is established in this paper to describe the effects of ash deposits on exhaust condition and heat transfer. Mass, momentum, and energy balances are considered for the multiphase system of gas, soot, and wall. The good agreement with experimental data confirms that the model is able to describe the processes. The axial patterns of wall temperature and soot regeneration rate during DPF regeneration are investigated by numerical simulation. Results indicate that the deposited ash layer reduces the exhaust flow rate and increases the heat conduction resistance during DPF regeneration, thus leading to high wall temperature and soot oxidation rate. When 5 g/L of ash is deposited, the complete oxidation of soot can be achieved 90 s faster with a 60 ^°C increase in wall temperature. The gain on soot oxidation rate increases with increasing amounts of deposited ash and reaches a plateau when the deposited ash approaches 15 g/L. Owing to the sintering and melting of ash when the temperature reaches 900 ^°C and the consequent uncontrolled regeneration, the soot carrying capacity should be identified based on the amount of deposited ash within DPF.     

Multi-Technique Analysis of Soot Reactivity from Conventional and Paraffinic Diesel Fuels

A 2.0 L, 4-cylinder, turbocharged, common rail diesel engine was used for generating soot samples. Three fuels were tested: a “first fill” diesel fuel, a gas-to-liquid fuel (GTL) and a hydrotreated fuel derived from vegetable oils (HVO). A stationary low-load operating mode (1667 rpm and 78 Nm) was selected for testing, and some modifications in the injection process (strategy, timing and pressure) were evaluated experimentally to assess their influence in the soot reactivity. The collected soot samples were characterized using a thermogravimetric analyzer (TGA), a differential scanning calorimeter (DSC), a diffuse reflectance infrared Fourier transform spectrometer (DRIFTS) and a surface area analyzer. All techniques anticipated that HVO and GTL soot samples are more reactive (i.e. show higher potential to be oxidized at lower temperatures leading to more efficient regeneration processes in a Diesel Particle Filter – DPF) compared to diesel soot. Additionally, the four characterization techniques showed the same tendencies when analyzing the effect of the engine operating parameters. In view of the results, the paraffinic fuels – HVO and GTL – here tested confirm their promising perspective for future use in automotive diesel engines, while some guides are proposed to enhance the soot reactivity via calibration of engine operating parameters.     

One-dimensional numerical model for degradation and combustion of polymethyl methacrylate

A transient one-dimensional model is applied to study degradation and combustion of a poly methyl methacrylate (PMMA) sample. Ignition of PMMA is a complex interaction among different mechanisms, including solid fuel degradation, heat transfer, in-depth absorption of radiation, surface regression, gas-phase advective heat/mass transfer, and combustion. The present task has the significant feature of coupling the solid and gas phases. Besides the mathematical model has been solved numerically by using a fast iterative method and has yielded realistic results.     

Multiscale modeling of the dynamics of solids at finite temperature

We develop a general multiscale method for coupling atomistic and continuum simulations using the framework of the heterogeneous multiscale method (HMM). Both the atomistic and the continuum models are formulated in the form of conservation laws of mass, momentum and energy. A macroscale solver, here the finite volume scheme, is used everywhere on a macrogrid; whenever necessary the macroscale fluxes are computed using the microscale model, which is in turn constrained by the local macrostate of the system, e.g. the deformation gradient tensor, the mean velocity and the local temperature. We discuss how these constraints can be imposed in the form of boundary conditions. When isolated defects are present, we develop an additional strategy for defect tracking. This method naturally decouples the atomistic time scales from the continuum time scale. Applications to shock propagation, thermal expansion, phase boundary and twin boundary dynamics are presented.     

POLAR project: Numerical modeling of the accretion column

In this paper we study the formation and evolution of radiative accretion shocks in the context of accretion columns in magnetic cataclysmic variables. The phenomenon induces extreme regimes of matter and the radiation emitted by those region dominates the observed emission from polars. A 2D hydrodynamical code, HYDRO-COOL, has been developed in order to deal with high-Mach number flows and is currently improved to produce a numerical model of the accretion column. This code is the hydrodynamical part of the HADES code which aims at modeling radiative hydrodynamical systems. The final goal of this model is to improve our understanding of observational results. First numerical results are reported here, as well as a discussion about the time-dependent evolution of the column.     

A weakly compressible MPS method for modeling of open‐boundary free‐surface flow

A mesh‐free particle method, based on the moving particle semi‐implicit (MPS) interaction model, has been developed for the simulation of two‐dimensional open‐boundary free‐surface flows. The incompressibility model in the original MPS has been replaced with a weakly incompressible model. The effect of this replacement on the efficiency and accuracy of the model has been investigated. The new inflow–outflow boundary conditions along with the particle recycling strategy proposed in this study extend the application of the model to open‐boundary problems. The final model is able to simulate open‐boundary free surface flow in cases of large deformation and fragmentation of free surface. The models and proposed algorithms have been validated and applied to sample problems. The results confirm the model's efficiency and accuracy. Copyright © 2009 John Wiley & Sons, Ltd.     

Coupled conductive radiative transfer in CO2 with narrow band statistical model and Monte-Carlo simulation

This paper describes an approach in order to determine the radiative source term and the temperature fields in a coupled conductive and radiative transfer. We consider an emitting and absorbing gas (carbon dioxide) enclosed between two horizontal and parallel reflecting plates. The medium is in local thermodynamic equilibrium (LTE). Only radiative and conductive exchanges are considered. An implicit finite difference technique is used to solve the energy conservation equation, and the narrow band statistical model (NBSM) with the Monte Carlo method (MCM) are used for simulation of radiative transfer equation in a coupled manner. This study of coupled conductive-radiative shows many phenomenons: The interaction radiation conduction in an emitting and absorbing gas is sensitive to the molar fraction of gas and the emissivities of walls. Received on 6 July 1999     

A simple heat transfer model for furnaces based on the zoning method

A simplified method for calculation of the radiative heat transfer in furnaces with rectangular crosssection based on the zoning method has been developed. The furnace is divided longitudinally into chambers. The net radiative intensity between any two chambers is considered to be independent of the angle. The chambers are also coupled by the gas mass flow.     

Acoustic filtration and sedimentation of soot particles

 Removal of soot particles from a static chamber by an intense acoustic field is investigated. Combustion of a solid fuel fills a rectangular chamber with small soot particles, which sediment very slowly. The chamber is then irradiated by an intense acoustic source to produce a three dimensional standing wave field in the chamber. The acoustic excitation causes the soot particles to agglomerate, forming larger particles which sediment faster from the system. The soot also forms 1–2 cm disks, with axes parallel to the axis of the acoustic source, which are levitated by the sound field at half-wavelength spacing within the chamber. Laser extinction measurements are made to determine soot volume fractions as a function of exposure time within the chamber. The volume fraction is reduced over time by sedimentation and by particle migration to the disks. The soot disks are considered to be a novel mechanism for particle removal from the air stream, and this mechanism has been dubbed acoustic filtration. An experimental method is developed for comparing the rate of soot removal by sedimentation alone with the rate of soot removal by sedimentation and acoustic filtration. Results show that acoustic filtration increases the rate of soot removal by a factor of two over acoustically-induced sedimentation alone. Received: 26 August 1996/Accepted: 31 March 1997     

煤油简化动力学和ISAT在超燃计算中的应用

利用基于``准稳态'方法建立的模型简化软件包SPARCK, 从建立的详细模型出发得到了一个包含22组分18步总包反应的煤油简化动力学模型. 简化模型计算得到的点火延迟时间与文献计算结果和实验结果相一致, 验证了模型的有效性. 采用简化模型和当地自适应建表(ISAT)方法, 对超燃冲压发动机进行了二维并行数值模拟, 计算得到的壁面压力分布与试验结果吻合较好, 表明简化模型能够很好地用来模拟煤油燃料超燃发动机内部的复杂燃烧过程. 在并行计算环境下, 和直接积分方法相比, 该方法将化学反应项的计算速度提高了3.73倍, 大大提高了计算效率.