Scalar energy fluctuations in Large-Eddy Simulation of turbulent flames: Statistical budgets and mesh quality criterion

Large-Eddy Simulation (LES) provides space-filtered quantities to compare with measurements, which usually have been obtained using a different filtering operation; hence, numerical and experimental results can be examined side-by-side in a statistical sense only. Instantaneous, space-filtered and statistically time-averaged signals feature different characteristic length-scales, which can be combined in dimensionless ratios. From two canonical manufactured turbulent solutions, a turbulent flame and a passive scalar turbulent mixing layer, the critical values of these ratios under which measured and computed variances (resolved plus sub-grid scale) can be compared without resorting to additional residual terms are first determined. It is shown that actual Direct Numerical Simulation can hardly accommodate a sufficiently large range of length-scales to perform statistical studies of LES filtered reactive scalar-fields energy budget based on sub-grid scale variances; an estimation of the minimum Reynolds number allowing for such DNS studies is given. From these developments, a reliability mesh criterion emerges for scalar LES and scaling for scalar sub-grid scale energy is discussed.     

Hybrid presumed pdf and flame surface density approaches for Large-Eddy Simulation of premixed turbulent combustion: Part 1: Formalism and simulation of a quasi-steady burner

Extended Coherent Flame Model for Large-Eddy Simulation (ECFM-LES) and Presumed Conditional Moments-Flame Prolongation of Intrisic Low Dimensional Manifolds (PCM-FPI) are some of the combustion models exploited for Large-Eddy Simulations (LES) of turbulent premixed flames. Combustion is then either modeled by tracking the flame surface density or by combining computations of flamelets with presumed probability density functions (pdf). The first approach enables to control the turbulent flame speed but models chemical kinetics in a simplified manner. The second directly accounts for detailed chemistry via the flamelet structure but the turbulent propagation speed cannot be easily estimated a priori. Simple one-dimensional tests are then performed in this study to evaluate flame velocities of PCM-FPI. A restricted operating range of this model, that enables to retrieve an evolution of the propagating speed similar to ECFM-LES predictions, is highlighted. This zone is limited in terms of filter width and sub-grid scale turbulent viscosity. An attractive alternative to both ECFM-LES and PCM-FPI approaches thus appears to be a model integrating their respective main strengths. For this purpose, two hybrid models are proposed in this paper and tested through LES of a lean-premixed turbulent swirling flame. Results in terms of statistics for temperature and mass fractions of chemical species are compared to experimental data and to previous results already obtained with PCM-FPI. The coupled models enable to properly locate the reaction zone via the flame surface density closures along with a correct prediction of the chemistry evolution in the flame front.     

Parametric study of upstream flame propagation in hydrogen-enriched premixed combustion: Effects of swirl,geometry and premixedness

The effect of swirl, premixedness and geometry has been investigated for hydrogen enriched premixed flame using Large Eddy Simulation (LES) with a Thickened Flame (TF) model. Swirl strength has been varied to study the effects of swirl on flame behavior in a laboratory-scale premixed combustor operated under atmospheric conditions. In addition, the levels of premixedness and geometry have also been changed to study the role of these quantities on flame behavior. The turbulent flow field and the chemistry are coupled through TF model. In the LES-TF approach, the flame front is resolved on the computational grid through artificial thickening and the individual species transport equations are directly solved with the reaction rates specified using Arrhenius chemistry. Good agreement is found when comparing predictions with the published experimental data including the predicted RMS fluctuations. Also, the results show that higher swirl strength and increase in level of premixedness make the system more susceptible to upstream flame movement due to higher combustibility of hydrogen, which increases the reaction along the flame front, thereby raises temperature in the reaction zone and leads to combustion induced vortex breakdown (CIVB). Moreover, upstream flame movement is always observed at higher swirl strength irrespective of level of premixedness and burner geometry, whereas the premixed systems exhibit stable behavior while operating at low swirl.     

Large Eddy Simulation of a premixed jet flame stabilized by a vitiated co-flow: Evaluation of auto-ignition tabulated chemistry

A tabulation technique assuming an auto-ignition dominated reaction pathway for highly turbulent premixed combustion is presented, implemented in an LES framework and evaluated. The tabulation method enables the reduction of the chemical system dimension to two scalars, allowing a computationally efficient model implementation, yet still retaining a sufficiently accurate representation of the chemical kinetics. The sensitivity of the LES model to the grid, inflow conditions, subgrid model, tabulation method assumptions and the chemical mechanism used in the tabulation process is evaluated with reference to detailed experimental measurements. The particular chemical mechanism utilized for the tabulation is shown to have a significant effect on the CO and OH concentrations, whilst only a small influence on the temperature and mixing fields. Comparisons with laminar flame based tabulation explain the misprediction of CO concentration. However, both the auto-ignition and laminar flame based tabulations fail to capture the OH concentration. The ability of the two tabulation techniques to capture the non-flamelet structure is discussed and the predictive capability of the two approaches is established. The general utility of a global Karlovitz number for describing the combustion regime and hence the selection of an applicable combustion model is brought into question considering that the variation of the local Karlovitz number in the simulations varies by up to 2 orders of magnitude, indicating a broad range of accessed flame structures.     

Effect of turbulent motions at different length scales on turbulent premixed swirl-stabilised flame topology

Three-dimensional direct numerical simulation data of H₂-air turbulent swirling premixed combustion at two different swirl numbers are analysed to investigate the local reaction zone morphology and its relation with local turbulent motions at different length scales. The effect of small scale turbulent mixing on local flames is investigated, and the results have shown that the contribution of microscale turbulent diffusivity on the local flamelet is insignificant, although there is some evidence of flame thinning for the higher swirl number case. The flame morphology such as high-level convolution and interacting flames, on the other hand, shows greater influence on local flamelets, suggesting the importance of local reaction zone topology on overall combustion processes. The local reaction zones are analysed by using the shapefinders to quantify their topology. Although the shapefinders showed various local reaction zone shapes consisting of “pancakes” and “tubes” and intermissive intense reaction zone distributions, the smallest characteristic length scale shows that the local reaction zones are thin. Finally, the relationship between these local reaction zone topology and turbulent motions at different sizes were discussed. The local reaction zone topology has a direct relation with Taylor microscale, integral length scale and their associated velocity scale, whereas almost no correlation is observed with Kolmogorov length scale, in the presence of inhomogeneous turbulence and strong mean shears. The present results suggest the importance of Taylor microscale on flame surface topology, which is often understated in turbulent combustion modelling frameworks.     

圆湍射流的轴对称大涡模拟

对空间发展的不可压缩圆湍射流进行了大涡模拟研究。在流动轴对称假定下,对Re数等于11300的圆浩射流流动进行数值模拟。大涡模拟很好地再现了圆湍射流中拟序结构非定常演化的前期过程,成功地捕获到了射流中Kelvin～Helmholtz不稳定性的触发与初级涡环的卷起及其第一次和第二次配对合并现象。但在流动轴对称假设下,大涡模拟不能模拟出湍流拟序涡环结构的破碎过程。对圆湍射流的轴对称大涡模拟结果进行长时间统计平均,能够预报出圆湍射流的核心区特征,但与圆湍射流的理论分析解和经典的实验数据对比发现,核心区后大涡模拟预报的流向速度降低缓慢。从控制方程的数学本质和拟序结构的物理机制上对圆湍射流在轴对称假设下产生上述大涡模拟结果的原因进行了分析与探讨。     

Effects of Lewis number on turbulent scalar transport and its modelling in turbulent premixed flames

The behaviour of the turbulent scalar flux in premixed flames has been studied using Direct Numerical Simulation (DNS) with emphasis on the effects of Lewis number in the context of Reynolds-averaged closure modelling. A database was obtained from DNS of three-dimensional freely propagating statistically planar turbulent premixed flames with simplified chemistry and a range of global Lewis numbers from 0.34 to 1.2. Under the same initial conditions of turbulence, flames with low Lewis numbers are found to exhibit counter-gradient transport, whereas flames with higher Lewis numbers tend to exhibit gradient transport. The Reynolds-averaged transport equation for the turbulent scalar flux is analysed in detail and the performance of existing models for the unclosed terms is assessed with respect to corresponding quantities extracted from DNS data. Based on this assessment, existing models which are able to address the effects of non-unity Lewis number on turbulent scalar flux transport are identified, and new or modified models are suggested wherever necessary. In this way, a complete set of closure models for the scalar flux transport equation is prescribed for use in Reynolds-Averaged Navier-Stokes simulations.     

分级进风旋流燃烧室内湍流燃烧的数值模拟

应用考虑湍流-化学反应相互作用的代数二阶矩-概率密度函数(PDF)湍流燃烧模型,对分级进风旋流燃烧室内两组工况下的甲烷湍流燃烧进行了数值模拟,得到的二氧化碳浓度和气体轴向脉动速度均方根值分布与实验数据相符合,得到的气体轴向和切向速度、轴向一切向脉动速度关联量、温度和氧气浓度分布与实验数据基本相符合.研究结果表明,选取适当的二次风率可以起到优化燃烧过程的作用.     

The valorization of plastic solid waste (PSW) by primary to quaternary routes: From re-use to energy and chemicals

Polymers are the most versatile material in our modern day and age. With certain chemicals and additives (pigments, concentrates, anti-blockers, light transformers (LTs), UV-stabilizers, etc.), they become what we know as plastics. The aim of this review is to provide the reader with an in depth analysis regarding the recovery, treatment and recycling routes of plastic solid waste (PSW), as well as the main advantages and disadvantages associated with every route. Recovery and recycling of PSW can be categorized by four main routes, i.e. re-extrusion, mechanical, chemical and energy recovery. Re-extrusion (primary recycling) utilizes scrap plastics by re-introducing the reminder of certain extruded thermoplastics (mainly poly-α-olefins) into heat cycles within a processing line. When plastic articles are discarded after a number of life cycles, mechanical recycling techniques present themselves as a candidate for utilizing a percentage of the waste as recyclate and/or fillers. Collectively, all technologies that convert polymers to either monomers (monomer recycling) or petrochemicals (feedstock recycling) are referred to as chemical recycling. The technology behind its success is the depolymerization processes (e.g. thermolysis) that can result in a very profitable and sustainable industrial scheme, providing a high product yield and a minimal waste. Nevertheless, due to their high calorific value and embodied energy, plastics are being incinerated solely or in combination with municipal solid waste (MSW) in many developed countries. This review also presents a number of application and technologies currently being used to incinerate plastics. Cement kilns and fluidized beds are the two most common units used to recover energy from PSW or MSW with high PSW content. It is concluded that, tertiary (chemical methods) and quaternary (energy recovery) are robust enough to be investigated and researched in the near future, for they provide a very sustainable solution to the PSW cycle.