共查询到20条相似文献,搜索用时 49 毫秒
1.
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. 相似文献
2.
3.
This is an experimental study of soot formation in precessing jet flames. The Mie diagnostic technique was implemented to
provide qualitative visualisation of the zones of soot formation. A range of conditionally sampled experiments was carried
out. The characteristic Reynolds number based on the nozzle diameter, was varied from 4329 to 11223 and the Strouhal number
based on the nozzle diameter, was varied from 0.0042 to 0.0245. The nozzle diameter was fixed at 5 mm and the jet exit angle
at 45 deg.
Experimental data were collected and used to show the tendencies in the formation of soot at different experimental conditions.
It was found that the relative soot intensity increases with increase in both Re and St numbers.
The instantaneous images reveal that soot is predominantly formed in sheets of varying thickness. Very little soot is observed
in the near nozzle region, which is consistent with the idea that the formation of soot in appreciable quantities is kinetically
limited. Readily observable are very broad regions of low signal spanning much of the flame. These broad regions are more
prevalent in the high St number flames where strain rates are lower and residence times are longer. The experimental results
support the hypothesis that low strain in a diffusion flame promotes soot formation and high emissivity (i.e., soot formation
correlates inversely with flame strain).
An erratum to this article can be found at 相似文献
4.
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. 相似文献
5.
Olivier Gicque Dominique Thévenin Nasser Darabiha 《Flow, Turbulence and Combustion》2005,73(3-4):307-321
In this paper we wish to investigate the occurrence of super-equilibrium temperature values, observed in many experimental
configurations. We would like to understand the origin of this phenomenon. Previous authors have already shown that differential
diffusion can lead to considerable changes in the temperature field and we would like to build on top of this observation.
We investigate numerically super-equilibrium combustion by considering both laminar counter-flow and turbulent diluted hydrogen/air
diffusion flames. These turbulent flames are computed using direct numerical simulations (DNS). A detailed reaction mechanism
is employed and the transport properties are modeled using multicomponent diffusion velocities, including the Soret effect.
Analyzing these results we introduce three complementary parameters (dilution-free mixture fraction, dilution excess and local
enthalpy) to describe the local combustion conditions. Introducing a measure of dilution separately from the mixture fraction
is necessary for a proper analysis. Using this set of parameters it becomes possible to explain super-equilibrium temperature
levels as a consequence of differential diffusion. 相似文献
6.
The influence of differential diffusion of chemical species in the soot initiation process in turbulent flows is investigated through Direct Numerical Simulations coupled to a compact global chemical mechanisms for ethylene (C2H4) flame combustion (Løvås et al., Combust Sci Tech 182(11):1945?C1960, 2010) featuring the important reaction steps for acetylene production. Our focus is on the formation of acetylene (C2H2) which is one of the most important species indicative of soot formation layers, especially in relation to the location of the H and H2 layers. The effect of preferential diffusion is assessed by comparison of results from unity and non-unity Lewis number simulations. The results indicate that under moderate turbulent conditions, where preferential diffusion effects become prominent, and with the global scheme used preferential diffusion greatly enhances the spread of the radical H whose peak value in mass fraction is reduced by a factor of about two; the spread of H2 is also enhanced though to a lesser extent. Importantly, the H and H2 spread into a range of mixture fraction Z between 0.2 and 0.3 which contains the soot formation range, supporting the hypothesis that soot formation is enhanced by preferential diffusion. Nevertheless, the acetylene formation layers themselves show little adjustment in the presence of non-unity Lewis numbers suggesting that the acetylene formation is dominated under the current conditions by the direct thermal decomposition of ethylene to acetylene in the global chemistry used. The specific F i factors that appear in flamelet models are explicitly computed; only F H, F H2 and F CO show appreciable differences on the fuel lean range of mixture fraction due to non-unity Lewis numbers, suggesting that the effects of non-unity Lewis numbers could be incorporated by a selective inclusion of only a few of the F i factors in order to save computational time. 相似文献
7.
In the present study, Reynolds Averaged Navier Stokes (RANS) simulations are applied to a series of turbulent V-shaped flames. Two formulations of Conditional Source-term Estimation (CSE) are developed using singly and doubly conditioned averages for turbulent premixed and partially premixed flames, respectively. Detailed chemistry is included. Conditionally averaged chemical source terms are closed by conditional averaged scalars which are obtained by inverting an integral equation. The objectives are to study a turbulent premixed V-shaped flame using the premixed CSE approach and apply the Doubly Conditional CSE (DCSE) combustion model to a case of stratified combustion. The partially premixed implementation involves double conditioning on two variables, mixture fraction and progress variable. The present study represents the first application of DCSE for a series of turbulent stratified flames. First, CSE is analysed for fully premixed conditions. A sensitivity analysis on the number of CSE ensembles and different scalar dissipation model closures is performed. Good results are obtained in terms of velocity and progress variable profiles. Finally, the partially premixed formulation is applied to the stratified case at three different conditions, corresponding to two different turbulence grids and three different profiles of the equivalence ratio, providing promising results. 相似文献
8.
R. Mercier P. Auzillon V. Moureau N. Darabiha O. Gicquel D. Veynante B. Fiorina 《Flow, Turbulence and Combustion》2014,93(2):349-381
Common combustion chambers often exhibit turbulent flames propagating in partially-premixed mixtures. This propagation is generally governed by aerodynamics, unsteady mixing and chemical processes and may also be affected by conductive heat losses when the reactive zone develops close to the burner lips. The Filtered TAbulated Chemistry for Large Eddy Simulation (F-TACLES) model has been recently developed to include tabulated chemistry in Large Eddy Simulation (LES) of adiabatic stratified flames in flamelet regimes. The present article proposes a modeling approach to account for both differential diffusion and non-adiabatic effects on flame consumption speed following the F-TACLES formalism. The adiabatic F-TACLES model is first detailed using a generalized formalism for diffusive fluxes allowing either to account for differential diffusion or not. The F-TACLES model is then extended to non-adiabatic situations. A correction factor based on the non-adiabatic consumption rate is introduced to recover a realistic filtered flame consumption speed. The objective is here to tackle flame stabilization mechanisms when heat losses affect the reaction zone. The proposed approach is validated through the simulation of the unconfined stratified turbulent jet flame TSF-A for which stabilization process is affected by heat losses. Five simulations are performed for both adiabatic and non-adiabatic flow conditions comparing unity Lewis number and complex diffusion assumptions. The adiabatic F-TACLES model predicts a flame anchored at the burner lip disagreeing with experimental data. The non-adiabatic simulation exhibits local extinction due to heat losses near the burner exit. The flame is then lifted improving the comparison with experiments. Results also show a significant impact of molecular diffusion model on both mean flame consumption rate and angle. 相似文献
9.
A Priori Testing of Flamelet Generated Manifolds for Turbulent Partially Premixed Methane/Air Flames
W. J. S. Ramaekers J. A. van Oijen L. P. H. de Goey 《Flow, Turbulence and Combustion》2010,84(3):439-458
To reduce high computational cost associated with simulations of reacting flows chemistry tabulation methods like the Flamelet
Generated Manifold (FGM) method are commonly used. However, H2, CO and OH predictions in RANS and LES simulations using the FGM (or a similar) method usually show a substantial deviation
from measurements. The goal of this study is to assess the accuracy of low-dimensional FGM databases for the prediction of
these species in turbulent, partially-premixed reacting flows. It will be examined to what extent turbulent, partially-premixed
jet flames can be described by FGM databases based on premixed or counterflow diffusion flamelets and to what extent the chosen
molecular transport model for the flamelet influences the accuracy of species mass fraction predictions in CFD-simulations.
For LES and RANS applications a model that accounts for subgrid fluctuations has to be added introducing additional errors
in numerical results. A priori analysis of FGM databases enables the exclusion of numerical errors (scheme accuracy, convergence) that occur in CFD simulations
as well as the exclusion of errors originating from subgrid modeling assumptions in LES and RANS. Four different FGM databases
are compared for H2O, H2, CO, CO2 and OH predictions in Sandia Flames C to F. Species mass fractions will be compared to measurements directly and conditioned
on mixture fraction. Special attention is paid to the representation of experimentally observed differential diffusion effects
by FGM databases. 相似文献
10.
Joint Scalar versus Joint Velocity-Scalar PDF Simulations of Bluff-Body Stabilized Flames with REDIM
Two transported PDF strategies, joint velocity-scalar PDF (JVSPDF) and joint scalar PDF (JSPDF), are investigated for bluff-body
stabilized jet-type turbulent diffusion flames with a variable degree of turbulence–chemistry interaction. Chemistry is modeled
by means of the novel reaction-diffusion manifold (REDIM) technique. A detailed chemistry mechanism is reduced, including
diffusion effects, with N
2 and CO
2 mass fractions as reduced coordinates. The second-moment closure RANS turbulence model and the modified Curl’s micro-mixing
model are not varied. Radiative heat loss effects are ignored. The results for mean velocity and velocity fluctuations in
physical space are very similar for both PDF methods. They agree well with experimental data up to the neck zone. Each of
the two PDF approaches implies a different closure for the velocity-scalar correlation. This leads to differences in the radial
profiles in physical space of mean scalars and mixture fraction variance, due to different scalar flux modeling. Differences
are visible in mean mixture fraction and mean temperature, as well as in mixture fraction variance. In principle, the JVSPDF
simulations can be closer to physical reality, as a differential model is implied for the scalar fluxes, whereas the gradient
diffusion hypothesis is implied in JSPDF simulations. Yet, in JSPDF simulations, turbulent diffusion can be tuned by means
of the turbulent Schmidt number. In the neck zone, where the turbulent flow field results deteriorate, the joint scalar PDF
results are in somewhat better agreement with experimental data, for the test cases considered. In composition space, where
results are reported as scatter plots, differences between the two PDF strategies are small in the calculations at hand, with
a little more local extinction in the joint scalar PDF results. 相似文献
11.
In the present study a Large Eddy Simulation and Filtered Density Function model is applied to three premixed piloted turbulent methane flames at different Reynolds Numbers using the Eulerian stochastic fields approach. The model is able to reproduce the flame structure and flow characteristics with a low number of fields (between 4 and 16 fields). The results show a good agreement with experimental data with the same closures employed in non-premixed combustion without any adjustment for combustion regime. The effect of heat release on the flow field is captured correctly. A wide range of sensitivity studies is carried out, including the number of fields, the chemical mechanism, differential diffusion effects and micro-mixing closures. The present work shows that premixed combustion (at least in the conditions under study) can be modelled using LES-PDF methods.. Finally, the ability of the model to predict flame quenching is studied. The model can accurate capture the conditions at which combustion is not sustainable and large pockets of extinction appear. 相似文献
12.
A new tabulated chemistry approach for representing turbulent combustion in industrial furnaces is presented. This model is based on the tabulation of two dimensional diffusion flamelets to account for ternary mixtures between fuel, oxidant and burned gases which are integrated over probability density functions. To avoid excessive CPU time for the table generation, the calculation of the two dimensional flamelets is performed using the method proposed in the ADF-PCM (Approximated Diffusion Flame - Presumed Conditional Moment) approach: only the equation for the progress variable is solved, instead of the equations for all species. The progress variable reaction rate is given by a table of homogeneous reactors using the DHR model (Diluted Homogeneous Reactor) proposed by Locci et al. These approximated diffusion flames are first compared to exact diffusion flames computed using the flamelet equations and the chemistry for all species. The resulting model, called A2DF (Approximate 2 Dimensional Flamelet) is then applied to the RANS (Reynolds Averaged Navier-Stokes) simulations of Sandia Flames D and F, showing a good agreement with experimental measurements. Finally, this model is applied to the flameless and conventional combustion cases of the burner of Verissimo et al., showing a correct agreement for temperature and species predictions. 相似文献
13.
The influences of differential diffusion rates of heat and mass on the transport of the variances of Favre fluctuations of reaction progress variable and non-dimensional temperature have been studied using three-dimensional simplified chemistry based Direct Numerical Simulation (DNS) data of statistically planar turbulent premixed flames with global Lewis number ranging from Le?= 0.34 to 1.2. The Lewis number effects on the statistical behaviours of the various terms of the transport equations of variances of Favre fluctuations of reaction progress variable and non-dimensional temperature have been analysed in the context of Reynolds Averaged Navier Stokes (RANS) simulations. It has been found that the turbulent fluxes of the progress variable and temperature variances exhibit counter-gradient transport for the flames with Lewis number significantly smaller than unity whereas the extent of this counter-gradient transport is found to decrease with increasing Lewis number. The Lewis number is also shown to have significant influences on the magnitudes of the chemical reaction and scalar dissipation rate contributions to the scalar variance transport. The modelling of the unclosed terms in the scalar variance equations for the non-unity Lewis number flames have been discussed in detail. The performances of the existing models for the unclosed terms are assessed based on a-priori analysis of DNS data. Based on the present analysis, new models for the unclosed terms of the active scalar variance transport equations are proposed, whenever necessary, which are shown to satisfactorily capture the behaviours of unclosed terms for all the flames considered in this study. 相似文献
14.
Computation of Conditional Average Scalar Dissipation in Turbulent Jet Diffusion Flames 总被引:2,自引:0,他引:2
The modelling of conditional scalar dissipation in locally self-similar turbulent reacting jets is considered. The streamwise
dependence in the transport equation of the conserved scalar pdf is represented by a function solely dependent on centreline
mixture fraction. This procedure provides a simple model suitable for non-homogeneous flows and ensures positive values for
conditional scalar dissipation. It has been tested in pure hydrogen-air jet diffusion flames using a Conditional Moment Closure
method with detailed 12species, 23 reactions chemistry. The calculations show good agreement of the averaged scalar dissipation
with reference values and the model proves to be superior to previous models based on homogeneous flows if the distribution
of the conditional scalar dissipation in mixture fraction space is compared with experimental results. A dependence of NO
predictions on the model of conditional scalar dissipation can be observed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
15.
Numerical simulation results are presented for three turbulent jet diffusion flames, stabilized behind a bluff body (Sydney
Flames HM1-3). Interaction between turbulence and combustion is modeled with the transported joint-scalar PDF approach. The
focus of the study is on the impact of the quality of simulation results in physical space on the behavior of two micro-mixing
models in composition space: the Euclidean Minimum Spanning Tree (‘EMST’) model and the modified Curl coalescence dispersion
(‘CD’) model. Profiles of conditional means and variances of thermo-chemical quantities, conditioned on the mixture fraction,
are discussed in the recirculation region and in the neck zone behind. The impact of the flow and mixing fields in physical
space on the mixing model behavior in composition space is strong for the CD model and increases as the turbulence – chemistry
interaction becomes stronger. The EMST conditional profiles, on the contrary, are hardly affected. 相似文献
16.
Traditional turbulence models using constant turbulent Prandtl number fail to predict the experimentally observed anisotropies
in the thermal eddy diffusivity and thermal turbulent intensity fields. Accurate predictions depend strongly on the turbulence
model employed. Consequently, the objective of this paper is to assess the performance of turbulence model with variable turbulent
Prandtl number in predicting of thermal and scalar fields quantities. The model is applied to axisymmetric turbulent round
jet with variable density and in turbulent hydrogen diffusion flames using the flamelet concept. The k − ɛ turbulence model is used in conjunction with thermal field; the model involves solving supplemental scalar equations
for the temperature variance and its dissipation rate. The model predictions are compared with available experimental data
for the purpose of validating model. In reacting cases, velocity and scalar (including temperature and mass fractions) predictions
agree relatively well in the near field of the investigated diluted hydrogen flames. 相似文献
17.
Laser induced incandescence (LII) is used in this study to measure soot volume fractions in steady and pulsed laminar diffusion
flame. The main objective of this study is to investigate the effect of flame pulsing on soot formation inside the flame region.
Phase-locked soot images were obtained for flame pulsing frequency between 10 and 200 Hz. The phase-locked soot images revealed
the entire motion process of the soot field during one pulsation period. The results showed that the total soot volume fraction
in the flame region increased by 45% when the pulsing frequency was increased from 10 to 200 Hz. 相似文献
18.
Quantitative three-dimensional imaging of soot volume fraction in turbulent non-premixed flames 总被引:1,自引:0,他引:1
J. Hult A. Omrane J. Nygren C. Kaminski B. Axelsson R. Collin P.-E. Bengtsson M. Aldén 《Experiments in fluids》2002,33(2):265-269
A three-dimensional (3-D) imaging system for studies of reactive and non-reactive flows is described. It can be used to reveal the topology of turbulent structures and to extract 3-D quantities, such as concentration gradients. Measurements are performed using a high repetition rate laser and detector system in combination with a scanning mirror. In this study, the system is used for laser-induced incandescence measurements to obtain quantitative 3-D soot volume fraction distributions in both laminar and turbulent non-premixed flames. From the acquired data, iso-concentration surfaces are visualised and concentration gradients calculated. 相似文献
19.
Franziska Hunger Meor F. Zulkifli Benjamin A. O. Williams Frank Beyrau Christian Hasse 《Flow, Turbulence and Combustion》2016,97(1):231-262
In the present study laminar and turbulent oxy-fuel jet flames are investigated both experimentally and numerically with emphasis on the direct comparison of the Rayleigh signal. The Rayleigh signal was measured for both flame setups, correcting for background light appropriately. Two downstream regions were recorded for the laminar flame and three for the turbulent flame. Equivalently, the signal was processed numerically based on the numerical species data and temperature. The laminar flame was used for validating the procedure of processing the Rayleigh signal. Both the numerical species data and the temperature are known from detailed simulations, so a predicted Rayleigh signal can easily be obtained. Further, the influence of the choice of the kinetic mechanism, radiation and diffusion model was investigated. In contrast, in the turbulent Large Eddy Simulation, the Rayleigh signal has to be computed using an appropriate turbulence-chemistry interaction model in order to obtain the Reynolds-filtered Rayleigh signal which is of non-linear nature. In the present investigation, the Rayleigh signal was incorporated in the flamelet/progress variable approach. The statistics of the experimental and numerical Rayleigh signal were then compared. The proposed procedure of directly comparing the experimental and predicted Rayleigh signal was shown to be advantageous in model validation especially in turbulent flame configurations. The procedure enables accurate model validation across an entire 2D field of view whilst using a realistic fuel-oxidizer combination and reducing experimental complexity. 相似文献
20.
This paper follows the evolution in understanding of the multiple mapping conditioning (MMC) approach for turbulent combustion
and reviews different implementations of MMC models. As the MMC name suggests, the original version represents a consistent
combination of CMC-type conditional equations (conditional moment closure) and generalised mapping closure. It seems that
the strength of the MMC model, and especially that of its stochastic version, lies in a more general (and much more transparent)
interpretation. In this new generalised interpretation, we can replace complicated derivations by physical reasoning and the
model appears to be a natural extension of modelling approaches developed in recent decades. MMC can be seen as a methodology
for enforcing certain known characteristics of turbulence on a conventional mixing model. This is achieved by localising the
mixing operation in a reference space. The reference space variables are selected to emulate the properties of a turbulent
flow which have a strong effect on reactive quantities. The best and simplest example is an MMC model which has a single reference
variable emulating the mixture fraction. In diffusion flames turbulent fluctuations of reacting quantities are strongly correlated
with fluctuations of the mixture fraction. By making mixing local in the reference mixture fraction space a CMC-type mixing
closure is enforced. In the original interpretation of MMC the reference variables are modelled as Markov processes. Since
the reference variables should emulate properties of turbulent flows as realistically as possible the next step, and the basis
of generalised MMC, is to remove the Markovian restriction and set reference variables equal to traced Lagrangian quantities
within DNS or LES flow fields. Indeed, no Markov value can emulate the mixture fraction better than the mixture fraction itself.
(Using a Markov vector process of dimension higher than the number of conditioning variables represents a more economical
alternative for producing reference variables in generalised MMC.) The generalised MMC approach effectively incorporates the
mixture fraction-based models, the PDF methods and LES/DNS techniques into a single methodology with possibility of blending
useful features developed previously for conventional models. The generalised approach to MMC stimulates a more flexible understanding
of simulations using sparsely placed Lagrangian particles as tools that may provide accurate joint distributions of reactive
scalars at relatively low computational cost. The physical reasoning behind the new interpretation of MMC is supported by
example computations for a partially premixed methane/air diffusion flame (Sandia Flame D). The scheme utilises LES for the
dynamic field and a sparse-Lagrangian filtered density function method with MMC mixing for the scalar field. Two different
particle mixing schemes are tested. Simulations are performed using only 35,000 Lagrangian particles (of these only 10,000
are chemically active) on a single workstation. The relatively low computational cost allows the use of realistic chemical
kinetics containing 34 reactive species and 219 reactions.
Intended for publication in the special issue of Flow, Turbulence and Combustion arising from the 2nd ECCOMAS Thematic Conference
on Computational Combustion held at Delft in mid-2007. 相似文献