共查询到20条相似文献,搜索用时 31 毫秒
1.
Low swirl flame characteristics under external flow excitations are numerically investigated using large eddy simulations with a dynamically thickened flame combustion model. A finite volume scheme on a Cartesian grid with a dynamic one equation eddy viscosity subgrid scale model is used for large eddy simulations. The excitations are imposed on inlet velocity profiles by a sinusoidal forcing function over a wide range of amplitudes and frequencies. Present investigation shows that although, the swirling motion of the low swirl flame is not intense enough to induce a recirculation zone in ensemble averaged results, external flow excitations increase the local swirl number upstream of the flame front. Such increase in the local swirl number is at maximum value when the low swirl flame is excited at the dominant frequency of the flow field, which in turn induces a vortex breakdown and hence a central recirculation zone. The strength and size of the time averaged recirculation zone depend on both the amplitude and frequency of the excitations. Moreover, phase-locked results indicate that external flow excitations induce local swirl fluctuations ahead of the flame front which alter the strength of the recirculation zone at different phase angles of the excitations. 相似文献
2.
In this study, large eddy simulation (LES) has been used to examine supersonic flow, mixing, self-ignition and combustion in a model scramjet combustor and has been compared against the experimental data. The LES model is based on an unstructured finite-volume discretization, using monotonicity-preserving flux reconstruction of the filtered mass, momentum, species and energy equations. Both a two-step and a seven-step hydrogen–air mechanism are used to describe the chemical reactions. Additional comparisons are made with results from a previously presented flamelet model. The subgrid flow terms are modeled using a mixed model, whereas the subgrid turbulence–chemistry interaction terms are modeled using the partially stirred reactor model. Simulations are carried out on a scramjet model experimentally studied at Deutsches Zentrum für Luft- und Raumfahrt consisting of a one-sided divergent channel with a wedge-shaped flame holder at the base of which hydrogen is injected. The LES predictions are compared with experimental data for velocity, temperature, wall pressure at different cross sections as well as schlieren images, showing good agreement for both first- and second-order statistics. In addition, the LES results are used to illustrate and explain the intrinsic flow, and mixing and combustion features of this combustor. 相似文献
3.
Large Eddy Simulations of Unconfined Non-reacting and Reacting Turbulent Low Swirl Jets 总被引:1,自引:0,他引:1
Mohammad Shahsavari Mohammad Farshchi Mohammad Hossien Arabnejad 《Flow, Turbulence and Combustion》2017,98(3):817-840
The low swirl flow is a novel method for stabilizing lean premixed combustion to achieve low emissions of nitrogen oxides. Understanding the characteristics of low swirl flows is of both practical and fundamental interest. In this paper, in order to gain better insight into low swirl stabilized combustion, large eddy simulation and dynamically thickened flame combustion modeling are used to characterize various features of non-reacting and reacting low swirl flows including vortex breakdown, shear layers’ instability, and coherent structures. Furthermore, four test cases with different equivalence ratios are studied to evaluate the effects of equivalence ratio on the flame and flow characteristics. A finite volume scheme on a Cartesian grid with a dynamic one equation eddy viscosity subgrid model is used for large eddy simulations. The obtained results show that the combustion heat release and increase in equivalence ratio toward the stoichiometric value decrease the local swirl number of the flow field, while increasing the flow spreading at the burner outlet. Results show that the flame becomes W shaped as the equivalence ratio increases. Moreover, the combination of the swirling motion and combustion heat release temporally imposes a vortex breakdown in the post-flame region, which leads to occurrence of a transient recirculation zone. The temporal recirculation zone disappears downstream of the burner outlet due to merging of the inner shear layer from all sides at the centerline. Also, various analyses of shear layers’ wavy and vortical structures show that combustion heat release has the effect of decreasing the instability amplitude and vortex shedding frequency. 相似文献
4.
5.
M.P. Kirkpatrick S.W. Armfield A.R. Masri S.S. Ibrahim 《Flow, Turbulence and Combustion》2003,70(1-4):1-19
A large eddy simulation of a turbulent premixed flame propagatingthrough a chamber containing a square obstruction is presented anddiscussed. The governing equations for compressible, reacting flowsare Favre filtered and turbulence closure is achieved using thedynamic Smagorinsky subgrid model. A simple flame surface densitymodel based on the flamelet concept is employed for the subgrid scalereaction rate. The simulation gives very good agreement with experimentalresults for the speed and the shape of the flame as it propagates throughthe chamber. The peak pressures, however, are underpredicted by20–30%. Reasons for this are discussed and it is concluded that amore sophisticated combustion model is required for complex flowssuch as this one, if a more accurate prediction of the pressureis to be achieved. 相似文献
6.
本文采用三种不同亚网格尺度模型对带有V型稳定器的模型燃烧室二维瞬态紊流流动进行了大涡模拟。并在交错网格系下用SIMPLE算法和混合差分格式求解离散方程。数值研究拟不同型式入口速度分布和不同亚网格尺度模型下模型燃烧室二维瞬态紊流流场。计算结果表明不同入口速度分布和不同亚网格尺度模型对瞬态流场和出口速度分布有一定的影响。本文通过数值模拟,揭示了V型稳定器后旋涡的产生和脱落过程。通过计算结果及实验数据的比较可知,本文采用的亚网格尺度模型可以用来模拟模型燃烧室紊流流场及稳定器后面回流区的流动情况。 相似文献
7.
Christophe Duwig 《Flow, Turbulence and Combustion》2007,79(4):433-454
Despite significant advances in the understanding and modelling of turbulent combustion, no general model has been proposed
for simulating flames in industrial combustion devices. Recently, the increase in computational possibilities has raised the
hope of directly solving the large turbulent scales using large eddy simulation (LES) and capturing the important time-dependant
phenomena. However, the chemical reactions involved in combustion occur at very small scales and the modelling of turbulent
combustion processes is still required within the LES framework. In the present paper, a recently presented model for the
LES of turbulent premixed flames is presented, analysed and discussed. The flamelet hypothesis is used to derive a filtered
source term for the filtered progress variable equation. The model ensures proper flame propagation. The effect of subgrid
scale (SGS) turbulence on the flame is modelled through the flame-wrinkling factor. The present modelling of the source term
is successfully tested against filtered direct numerical simulation (DNS) data of a V-shape flame. Further, a premixed turbulent
flame, stabilised behind an expansion, is simulated. The predictions agree well with the available experimental data, showing
the capabilities of the model for performing accurate simulations of unsteady premixed flames. 相似文献
8.
We propose a new flame index for the transported probability density function(PDF) method. The flame index uses mixing flux projections of Lagrangian particles on mixture fraction and progress variable directions as the metrics to identify the combustion mode, with the Burke-Schumann solution as a reference. A priori validation of the flame index is conducted with a series of constructed turbulent partially premixed reactors. It indicates that the proposed flame index is able to identify the combustion mode based on the subgrid mixing information. The flame index is then applied the large eddy simulation/PDF datasets of turbulent partially premixed jet flames. Results show that the flame index separate different combustion modes and extinction correctly. The proposed flame index provides a promising tool to analyze and model the partially premixed flames adaptively. 相似文献
9.
Large eddy simulation (LES) models for flamelet combustion are analyzed by simulating premixed flames in turbulent stagnation
zones. ALES approach based on subgrid implementation of the linear eddy model(LEM) is compared with a more conventional approach
based on the estimation of the turbulent burning rate. The effects of subgrid turbulence are modeled within the subgrid domain
in the LEM-LES approach and the advection (transport between LES cells) of scalars is modeled using a volume-of-fluid (VOF)
Lagrangian front tracking scheme. The ability of the VOF scheme to track the flame as a thin front on the LES grid is demonstrated.
The combined LEM-LES methodology is shown to be well suited for modeling premixed flamelet combustion. The geometric characteristics
of the flame surfaces, their effects on resolved fluid motion and flame-turbulence interactions are well predicted by the
LEM-LES approach. It is established here that local laminar propagation of the flamelets needs to be resolved in addition
to the accurate estimation of the turbulent reaction rate. Some key differences between LEM-LES and the conventional approach(es)
are also discussed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
10.
V. Di Sarli A. Di Benedetto G. Russo S. Jarvis E. J. Long G. K. Hargrave 《Flow, Turbulence and Combustion》2009,83(2):227-250
In gas explosions, the unsteady coupling of the propagating flame and the flow field induced by the presence of blockages
along the flame path produces vortices of different scales ahead of the flame front. The resulting flame–vortex interaction
intensifies the rate of flame propagation and the pressure rise. In this paper, a joint numerical and experimental study of
unsteady premixed flame propagation around three sequential obstacles in a small-scale vented explosion chamber is presented.
The modeling work is carried out utilizing large eddy simulation (LES). In the experimental work, previous results (Patel
et al., Proc Combust Inst 29:1849–1854, 2002) are extended to include simultaneous flame and particle image velocimetry (PIV) measurements of the flow field within the
wake of each obstacle. Comparisons between LES predictions and experimental data show a satisfactory agreement in terms of
shape of the propagating flame, flame arrival times, spatial profile of the flame speed, pressure time history, and velocity
vector fields. Computations through the validated model are also performed to evaluate the effects of both large-scale and
sub-grid scale (SGS) vortices on the flame propagation. The results obtained demonstrate that the large vortical structures
dictate the evolution of the flame in qualitative terms (shape and structure of the flame, succession of the combustion regimes
along the path, acceleration-deceleration step around each obstacle, and pressure time trend). Conversely, the SGS vortices
do not affect the qualitative trends. However, it is essential to model their effects on the combustion rate to achieve quantitative
predictions for the flame speed and the pressure peak. 相似文献
11.
A large eddy simulation (LES) is performed for turbulent flow around a bluff body inside a sudden expansion cylinder chamber,
a configuration which resembles a premixed gas turbine combustor. To promote turbulent mixing and to accommodate flame stability,
a flame holder is installed inside the combustion chamber. The Smagorinsky model and the Lagrangian dynamic subgrid-scale
model are employed and tested. The calculated Reynolds number is 5,000 based on the bulk velocity and the diameter of inlet
pipe. The simulation code is constructed by using a general coordinate system based on the physical contravariant velocity
components. The predicted turbulent statistics are evaluated by comparing with the laser-doppler velocimetry (LDV) measurement
data. The agreement of LES with the experimental data is shown to be satisfactory. Emphasis is placed on the time-dependent
evolutions of turbulent vortical structures behind the flame holder. The numerical flow visualizations depict the behavior
of large-scale vortices. The turbulent behavior behind the flame holder is analyzed by visualizing the sectional views of
vortical structure.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
12.
A. W. Vreman J. A. van Oijen L. P. H. de Goey R. J. M. Bastiaans 《Flow, Turbulence and Combustion》2009,82(4):511-535
Large-eddy simulation (LES) of turbulent combustion with premixed flamelets is investigated in this paper. The approach solves
the filtered Navier–Stokes equations supplemented with two transport equations, one for the mixture fraction and another for
a progress variable. The LES premixed flamelet approach is tested for two flows: a premixed preheated Bunsen flame and a partially
premixed diffusion flame (Sandia Flame D). In the first case, we compare the LES with a direct numerical simulation (DNS).
Four non-trivial models for the chemical source term are considered for the Bunsen flame: the standard presumed beta-pdf model,
and three new propositions (simpler than the beta-pdf model): the filtered flamelet model, the shift-filter model and the
shift-inversion model. A priori and a posteriori tests are performed for these subgrid reaction models. In the present preheated
Bunsen flame, the filtered flamelet model gives the best results in a priori tests. The LES tests for the Bunsen flame are
limited to a case in which the filter width is only slightly larger than the flame thickness. According to the a posteriori
tests the three models (beta-pdf, filtered flamelet and shift-inversion) show more or less the same results as the trivial
model, in which subgrid reaction effects are ignored, while the shift-filter model leads to worse results. Since LES needs
to resolve the large turbulent eddies, the LES filter width is bounded by a maximum. For the present Bunsen flame this means
that the filter width should be of the order of the flame thickness or smaller. In this regime, the effects of subgrid reaction
and subgrid flame wrinkling turn out to be quite modest. The LES-results of the second case (Sandia Flame D) are compared
to experimental data. Satisfactory agreement is obtained for the main species. Comparison is made between different eddy-viscosity
models for the subgrid turbulence, and the Smagorinsky eddy-viscosity is found to give worse results than eddy-viscosities
that are not dominated by the mean shear.
Paper presented on the Eccomas Thematic Conference Computational Combustion 2007, submitted for a special issue of Flow, Turbulence
and Combustion. 相似文献
13.
This large eddy simulation (LES) study is applied to three different premixed turbulent flames under lean conditions at atmospheric
pressure. The hierarchy of complexity of these flames in ascending order are a simple Bunsen-like burner, a sudden-expansion
dump combustor, and a typical swirl-stabilized gas turbine burner–combustor. The purpose of this paper is to examine numerically
whether the chosen combination of the Smagorinsky turbulence model for sgs fluxes and a novel turbulent premixed reaction
closure is applicable over all the three combustion configurations with varied degree of flow and turbulence. A quality assessment
method for the LES calculations is applied. The cold flow data obtained with the Smagorinsky closure on the dump combustor
are in close proximity with the experiments. It moderately predicts the vortex breakdown and bubble shape, which control the
flame position on the double-cone burner. Here, the jet break-up at the root of the burner is premature and differs with the
experiments by as much as half the burner exit diameter, attributing the discrepancy to poor grid resolution. With the first
two combustion configurations, the applied subgrid reaction model is in good correspondence with the experiments. For the
third case, a complex swirl-stabilized burner–combustor configuration, although the flow field inside the burner is only modestly
numerically explored, the level of flame stabilization at the junction of the burner–combustor has been rather well captured.
Furthermore, the critical flame drift from the combustor into the burner was possible to capture in the LES context (which
was not possible with the RANS plus k–ɛ model), however, requiring tuning of a prefactor in the reaction closure. 相似文献
14.
Fuel efficiency improvement and harmful emission reduction are the paramount driving forces for development of gas turbine combustors. Lean-burn combustors can accomplish these goals, but require specific flow topologies to overcome their sensitivity to combustion instabilities. Large Eddy Simulations (LES) can accurately capture these complex and intrinsically unsteady flow fields, but estimating the appropriate numerical resolution and subgrid model(s) still remain challenges. This paper discusses the prediction of non-reacting flow fields in the DLR gas turbine model combustor using LES. Several important features of modern gas turbine combustors are present in this model combustor: multiple air swirlers and recirculation zones for flame stabilisation. Good overall agreement is obtained between LES outcomes and experimental results, both in terms of time-averaged and temporal RMS values. Findings of this study include a strong dependence of the opening angle of the swirling jet inside the combustion chamber on the subgrid viscosity, which acts mainly through the air mass flow split between the two swirlers in the DLR model combustor. This paper illustrates the ability of LES to obtain accurate flow field predictions in complex gas turbine combustors making use of open-source software and computational resources available to industry. 相似文献
15.
Ronan Vicquelin Benoît Fiorina Nasser Darabiha Olivier Gicquel Denis Veynante 《Comptes Rendus Mecanique》2009,337(6-7):329-339
A new modeling strategy is developed to introduce tabulated chemistry methods in the LES of turbulent premixed combustion. The objective is to recover the correct laminar flame propagation speed of the filtered flame front when the subgrid scale turbulence vanishes. The filtered flame structure is mapped by 1D filtered laminar premixed flames. Closure of the filtered progress variable and the energy balance equations are carefully addressed. The methodology is applied to 1D and 2D filtered laminar flames. These computations show the capability of the model to recover the laminar flame speed and the correct chemical structure when the flame wrinkling is completely resolved. The model is then extended to turbulent combustion regimes by introducing subgrid scale wrinkling effects on the flame front propagation. Finally, the LES of a 3D turbulent premixed flame is performed. To cite this article: R. Vicquelin et al., C. R. Mecanique 337 (2009). 相似文献
16.
在可压缩多介质粘性流体动力学高精度计算方法MVPPM(multi-viscous-fluid piecewise parabolicmethod)基础上,引入Smagorinsky和Vreman亚格子湍流模型,采用大涡数值模拟方法求解可压缩粘性流体NS(Navier-Stokes)方程,给出适用于可压缩多介质流体界面不稳定性发展演化至湍流阶段的计算方法和二维计算程序MVFT(multi-viscosity-fluid and turbulence)。在2种亚格子湍流模型下计算了LANL(Los Ala-mos National Laboratory)激波管单气柱RM不稳定性实验,分析了气柱的形状、流场速度以及涡的特征,通过与LANL实验和计算结果的比较可知,Vreman模型略优于Smagorinsky模型,MVFT方法和计算程序可用于对界面不稳定性发展演化至湍流阶段的数值模拟。 相似文献
17.
K. N. Volkov 《Journal of Applied Mechanics and Technical Physics》2011,52(1):48-56
Results of large eddy simulations in a subsonic isothermal turbulent jet exhausting from a circular nozzle into a submerged
space or a cocurrent flow are presented. The flow is described by space-averaged Navier-Stokes equations and by the RNG model
of subgrid scale viscosity. Results computed for different values of the cocurrency parameter are compared with available
results of numerical simulations and experimental data. The results obtained are found to agree well with measured data and
to confirm the basic laws of variation of gas-dynamic and fluctuating parameters of submerged and cocurrent jets. 相似文献
18.
We present an original timesaving joint RANS/LES approach to simulate turbulent premixed combustion. It is intended mainly for industrial applications where LES may not be practical. It is based on successive RANS/LES numerical modelling, where turbulent characteristics determined from RANS simulations are used in LES equations for estimation of the subgrid chemical source and viscosity. This approach has been developed using our TFC premixed combustion model, which is based on a generalization of the Kolmogorov’s ideas. We assume existence of small-scale statistically equilibrium structures not only of turbulence but also of the reaction zones. At the same time, non-equilibrium large-scale structures of reaction sheets and turbulent eddies are described statistically by model combustion and turbulence equations in RANS simulations or follow directly without modelling in LES. Assumption of small-scale equilibrium gives an opportunity to express the mean combustion rate (controlled by small-scale coupling of turbulence and chemistry) in the RANS and LES sub-problems in terms of integral or subgrid parameters of turbulence and the chemical time, i.e. the definition of the reaction rate is similar to that of the mean dissipation rate in turbulence models where it is expressed in terms of integral or subgrid turbulent parameters. Our approach therefore renders compatible the combustion and turbulent parts of the RANS and LES sub-problems and yields reasonable agreement between the RANS and averaged LES results. Combining RANS simulations of averaged fields with LES method (and especially coupled and acoustic codes) for simulation of corresponding nonstationary process (and unsteady combustion regimes) is a promising strategy for industrial applications. In this work we present results of simulations carried out employing the joint RANS/LES approach for three examples: High velocity premixed combustion in a channel, combustion in the shear flow behind an obstacle and the impinging flame (a premixed flame attached to an obstacle). 相似文献
19.
20.
The paper aims to assess the performance of large eddy simulation (LES) in predicting the unsteady reacting flows in internal combustion engines. The incompatibility due to the turbulence dissipation was avoided in the k-equation LES formulation. Two versions of the LES have been tested with different filtering. The cell-specific filtering was found to give realistic prediction of the instantaneous temperature and pressure field during the combustion process. The coupling of combustion heat release, temperature field and turbulent flow field was found to be strong in the LES predicted flow and combustion fields which showed wrinkled flame structures. The formulation gives improved agreement with available experimental data. 相似文献