A hybrid EDC/Flamelet approach for modelling biomass combustion of grate-firing furnace

This paper proposes a new combustion model for the simulation of biomass combustion. It is developed based on the framework of the well-known Eddy Dissipation Concept (EDC) approach, which has the ability to incorporate chemical kinetics in turbulent reacting flows and thus makes it suitable for modelling gas-phase combustion. However, its high computational cost when using detailed chemistry has made it impractical for modelling large/industrial setups. To address this handicap, the proposed approach decouples the real-time calculation of chemical and mixing processes by importing a pre-calculated steady laminar flamelet library into EDC. The development of this new model is performed based on a modified version of EDC (called Extended EDC), which is capable of modelling the gas-phase of biomass combustion over a wide range of turbulent flow conditions. The proposed model is validated by simulating the well-documented experiment of the piloted jet flames of Barlow and Frank. The performance of the model is then evaluated by simulating a small-scale grate firing biomass furnace. The results show that, overall, the proposed model can be used to model biomass combustion at substantially low computational cost.     

Characteristic boundary conditions for simulations of compressible reacting flows with multi-dimensional,viscous and reaction effects

A generalized formulation of the characteristic boundary conditions for compressible reacting flows is proposed. The new and improved approach resolves a number of lingering issues of spurious solution behaviour encountered in turbulent reacting flow simulations in the past. This is accomplished (a) by accounting for all the relevant terms in the determination of the characteristic wave amplitudes and (b) by accommodating a relaxation treatment for the transverse gradient terms with the relaxation coefficient properly determined by the low Mach number asymptotic expansion. The new boundary conditions are applied to a comprehensive set of test problems including: vortex-convection; turbulent inflow; ignition front propagation; non-reacting and reacting Poiseuille flows; and counterflow cases. It is demonstrated that the improved boundary conditions perform consistently superior to existing approaches, and result in robust and accurate solutions with minimal acoustic wave interactions at the boundary in hostile turbulent combustion simulation conditions.     

Large eddy simulation of turbulent combustion systems

This paper reviews recent and ongoing work on numerical models for turbulent combustion systems based on a classical LES approach. The work is confined to single-phase reacting flows. First, important physico-chemical features of combustion-LES are discussed along with several aspects of overall LES models. Subsequently, some numerical issues, in particular questions associated with the reliability of LES results, are outlined. The details of chemistry, its reduction, and tabulation are not addressed here. Second, two illustrative applications dealing with non-premixed and premixed flame configurations are presented. The results show that combustion-LES is able to provide predictions very close to measured data for configurations where the flow is governed by large turbulent structures. To meet the future demands, new key challenges in specific modelling areas are suggested, and opportunities for advancements in combustion-LES techniques are highlighted. From a predictive point of view, the main target must be to provide a reliable method to aid combustion safety studies and the design of combustion systems of practical importance.     

Mind the gap: Turbulent combustion model validation and future needs

This overview collects a range of well characterized experiments used in the step-wise validation of turbulent combustion models, from gas phase non-premixed jet flames to spray flames, and from simple symmetric jets to real device geometries, focusing primarily on statistically steady state experiments. We discuss how the experiments and models are constructed, approaches to modelling, and the tradeoffs between the level of detail and computational demands. The review highlights a number of experiments used for benchmarking models, selecting a few examples where models have clearly succeeded, as well as some areas where there are clear needs in the experimental database. In particular, the areas of turbulent spray combustion and soot prediction, as well as combustion under high pressures appear as the least developed and present the clearest gaps for both models and experiments. Based on the successful application of advanced methods of uncertainty quantification to a number of problems in reacting flows, we suggest that these methods might be used to advantage in the design of experiments. This would enable an upfront examination of the extent to which comparisons between measurable scalars and velocities allow clear distinction between model features.     

Combustion modeling using Principal Component Analysis: A posteriori validation on Sandia flames D,E and F

The present work shows the first application of the PC-transport approach in the context of Large Eddy Simulation (LES) of turbulent combustion. Detailed kinetic mechanisms, together with advanced computational tools, are needed to advance our knowledge of turbulent reacting systems. However, the cost related to high-fidelity simulations of turbulent reacting flows is still prohibitive for realistic configurations. Therefore, there is a need to reduce the complexity of the problem by identifying low-dimensional manifolds. To this end, the potential offered by Principal Component Analysis (PCA) in parameterizing the thermo-chemical state-space is very appealing. The present paper extends the PC-transport framework to three-dimensional Large Eddy Simulation (LES), coupling PCA with Gaussian Process Regression (GPR). To demonstrate the potential of the method, LES simulations of Sandia flames D, E and F are shown. Results show the great potential of the PC-GPR model, as indicated by the accuracy of the simulation results when compared with experimental data, using only 2 principal components. The sensitivity to the kinetic mechanism and subgrid closure model is also investigated.     

湍流燃烧的新二阶矩模型

本文针对湍流燃烧中模拟包括污染物生成的详细有限反应动力学的要求，用二阶矩封闭与简化概率密度的概念相结合的方法，提出了湍流燃烧的新二阶矩模型。此模型可兼顾适用于工程问题中的合理性和经济性。     

Numerical simulation of turbulent combustion: Scientific challenges

Predictive simulation of engine combustion is key to understanding the underlying complicated physicochemical processes, improving engine performance, and reducing pollutant emissions. Critical issues as turbulence modeling, turbulence-chemistry interaction, and accommodation of detailed chemical kinetics in complex flows remain challenging and essential for high-fidelity combustion simulation. This paper reviews the current status of the state-of-the-art large eddy simulation (LES)/prob-ability density function (PDF)/detailed chemistry approach that can address the three challenging modelling issues. PDF as a subgrid model for LES is formulated and the hybrid mesh-particle method for LES/PDF simulations is described. Then the development need in micro-mixing models for the PDF simulations of turbulent premixed combustion is identified. Finally the different acceleration methods for detailed chemistry are reviewed and a combined strategy is proposed for further development.     

Filtered density function simulator on unstructured meshes

A new computational filtered density function (FDF) methodology is developed for large eddy simulation (LES) of turbulent reacting flows. This methodology is based on a Lagrangian Monte Carlo (MC) FDF solver constructed on a domain portrayed by an unstructured mesh. The base filtered transport equations on this mesh are solved by a finite-volume (FV) method. The consistency of the hybrid FV-MC solver and the realizability of the simulated results are demonstrated via LES of a temporally developing mixing layer. The overall performance of the model is appraised by comparison with direct numerical simulation (DNS) data. The algorithmic implementation in the commercial software ANSYS-FLUENT facilitates future FDF-LES of turbulent combustion in complex configurations.     

High order finite volume methods on wavelet-adapted grids with local time-stepping on multicore architectures for the simulation of shock-bubble interactions

We present a space–time adaptive solver for single- and multi-phase compressible flows that couples average interpolating wavelets with high-order finite volume schemes. The solver introduces the concept of wavelet blocks, handles large jumps in resolution and employs local time-stepping for efficient time integration. We demonstrate that the inherently sequential wavelet-based adaptivity can be implemented efficiently in multicore computer architectures using task-based parallelism and introducing the concept of wavelet blocks. We validate our computational method on a number of benchmark problems and we present simulations of shock-bubble interaction at different Mach numbers, demonstrating the accuracy and computational performance of the method.     

Simulation of Combustion Field with Lattice Boltzmann Method

Turbulent combustion is ubiquitously used in practical combustion devices. However, even chemically non-reacting turbulent flows are complex phenomena, and chemical reactions make the problem even more complicated. Due to the limitation of the computational costs, conventional numerical methods are impractical in carrying out direct 3D numerical simulations at high Reynolds numbers with detailed chemistry. Recently, the lattice Boltzmann method has emerged as an efficient alternative for numerical simulation of complex flows. Compared with conventional methods, the lattice Boltzmann scheme is simple and easy for parallel computing. In this study, we present a lattice Boltzmann model for simulation of combustion, which includes reaction, diffusion, and convection. We assume the chemical reaction does not affect the flow field. Flow, temperature, and concentration fields are decoupled and solved separately. As a preliminary simulation, we study the so-called counter-flow laminar flame. The particular flow geometry has two opposed uniform combustible jets which form a stagnation flow. The results are compared with those obtained from solving Navier–Stokes equations.     

A direct approach to generalised multiple mapping conditioning for selected turbulent diffusion flame cases

This work presents a direct and transparent interpretation of two concepts for modelling turbulent combustion: generalised Multiple Mapping Conditioning (MMC) and sparse-Lagrangian Large Eddy Simulation (LES). The MMC approach is presented as a hybrid between the Probability Density Function (PDF) method and approaches based on conditioning (e.g. Conditional Moment Closure, flamelet, etc.). The sparse-Lagrangian approach, which allows for a dramatic reduction of computational cost, is viewed as an alternative interpretation of the Filtered Density Function (FDF) methods. This work presents simulations of several turbulent diffusion flame cases and discusses the universality of the localness parameter between these cases and the universality of sparse-Lagrangian FDF methods with MMC.     

贴体坐标系下模型加力室的大涡模拟

本文利用贴体网格对带V形槽稳定器模型加力燃烧室素流化学反应流流动进行大涡模拟的研究。采用区域法生成模型加力燃烧室的二维贴体网格,并采用多区域耦合法进行区域之间的数据传递,求解加力室整体流场。采用k方程亚网格尺度模型和亚网格EBU燃烧模型分别估算其亚网格紊流粘性和化学反应速率,用热通量辐射模型估算辐射通量,并用交错网格下SIMPLE算法和混合差分格式求解离散方程,壁面函数处理固壁边界条件。计算结果显示了稳定器后面的回流区气流结构,所得的热态流场模拟结果与实验比较吻合,表明采用贴体网格对模型加力燃烧室进行大涡模拟能真实反映流体流动及燃烧过程。     

Studies of the flow and turbulence fields in a turbulent pulsed jet flame using LES/PDF

A turbulent piloted jet flame subject to a rapid velocity pulse in its fuel jet inflow is proposed as a new benchmark case for the study of turbulent combustion models. In this work, we perform modelling studies of this turbulent pulsed jet flame and focus on the predictions of its flow and turbulence fields. An advanced modelling strategy combining the large eddy simulation (LES) and the probability density function (PDF) methods is employed to model the turbulent pulsed jet flame. Characteristics of the velocity measurements are analysed to produce a time-dependent inflow condition that can be fed into the simulations. The effect of the uncertainty in the inflow turbulence intensity is investigated and is found to be very small. A method of specifying the inflow turbulence boundary condition for the simulations of the pulsed jet flame is assessed. The strategies for validating LES of statistically transient flames are discussed, and a new framework is developed consisting of different averaging strategies and a bootstrap method for constructing confidence intervals. Parametric studies are performed to examine the sensitivity of the predictions of the flow and turbulence fields to model and numerical parameters. A direct comparison of the predicted and measured time series of the axial velocity demonstrates a satisfactory prediction of the flow and turbulence fields of the pulsed jet flame by the employed modelling methods.     

On the importance of the heat release rate in numerical simulations of fires in mechanically ventilated air‐tight enclosures

A fire in a compartment with limited ventilation can cause a significant pressure rise, up to hundreds of Pascal. This is important in practice, as the pressure rise can cause damage or hinder evacuation, but also from the perspective of fire safety science. From the energy balance, taking into account the interaction between compartment pressure, fire dynamics and mechanical ventilation, the importance of the net heat gained per unit time in the gas phase is well recognized. This leads to the need to accurately quantify the heat release rate inside the compartment as a function of time. It is explained that scaling of the transient phenomena is not straightforward. The paper then focuses on numerical simulations, in particular on CFD in the gas phase. An overview is presented of different existing approaches for turbulent combustion modelling in turbulent buoyancy-driven flames with low values of scalar dissipation rate, typical for fire flames. A dynamic approach for modelling turbulent combustion, and the coupling with radiation modelling, is briefly discussed. Extinction and re-ignition are discussed extensively, in the context of reduced ventilation conditions. Finally, low-frequency oscillatory behaviour in mechanically ventilated air-tight compartments is addressed. It is argued that CFD simulations are a very valuable tool to gain further insight in this phenomenon. Suggestions for future research are formulated.     

Small scales,many species and the manifold challenges of turbulent combustion

A major goal of combustion research is to develop accurate, tractable, predictive models for the phenomena occurring in combustion devices, which predominantly involve turbulent flows. With the focus on gas-phase, non-premixed flames, recent progress is reviewed, and the significant remaining challenges facing models of turbulent combustion are examined. The principal challenges are posed by the small scales, the many chemical species involved in hydrocarbon combustion, and the coupled processes of reaction and molecular diffusion in a turbulent flow field. These challenges, and how different modeling approaches face them, are examined from the viewpoint of low-dimensional manifolds in the high-dimensional space of chemical species. Most current approaches to modeling turbulent combustion can be categorized as flamelet-like or PDF-like. The former assume or imply that the compositions occurring in turbulent combustion lie on very-low-dimensional manifolds, and that the coupling between turbulent mixing and reaction can be parameterized by at most one or two variables. PDF-like models do not restrict compositions in this way, and they have proved successful in describing more challenging combustion regimes in which there is significant local extinction, or in which the turbulence significantly disrupts flamelet structures. Advances in diagnostics, the design of experiments, computational resources, and direct numerical simulations are all contributing to the continuing development of more accurate and general models of turbulent combustion.     

Lattice Boltzmann methods on the ClearSpeed Advance™ accelerator board

Numerical analysts and programmers are currently facing a conceptual change in processor technology. Multicore concepts, coprocessors and accelerators are becoming a vital part in scientific computing. The new hardware techno- logies lead to new paradigms and require adapted methodologies and techniques in numerical simulation. These developments play an important role in computational fluid dynamics (CFD) where many highly CPU-time demanding problems arise. In this paper, we propose a parallel lattice Boltzmann method (LBM) in the context of a coprocessor technology, the ClearSpeed Advance™ accelerator board. Implementations of LBMs on parallel architectures benefit from localities of the necessary interactions and the regular structure of the underlying meshes. The considered board supports high-level parallelism and double precision conforming to the IEEE 754 standard. However, the solution process relies on a huge amount of data which needs to propagate along the mesh. This prototypical fact shows up the bottleneck of internal communication bandwidth and indicates the limits of this type of small-scale parallel systems.     

Desktop supercomputer: what can it do?

The paper addresses the issues of solving complex problems that require using supercomputers or multiprocessor clusters available for most researchers nowadays. Efficient distribution of high performance computing resources according to actual application needs has been a major research topic since high-performance computing (HPC) technologies became widely introduced. At the same time, comfortable and transparent access to these resources was a key user requirement. In this paper we discuss approaches to build a virtual private supercomputer available at user’s desktop: a virtual computing environment tailored specifically for a target user with a particular target application. We describe and evaluate possibilities to create the virtual supercomputer based on light-weight virtualization technologies, and analyze the efficiency of our approach compared to traditional methods of HPC resource management.     

Multi-environment probability density function method for modelling turbulent combustion using realistic chemical kinetics

A computational fluid dynamics (CFD) tool for performing turbulent combustion simulations that require finite-rate chemistry is developed and tested by modelling a series of bluff-body stabilized flames that exhibit different levels of finite-rate chemistry effects ranging from near equilibrium to near global extinction. The new modelling tool is based on the multi-environment probability density function (MEPDF) methodology and combines the following: the direct quadrature method of moments (DQMOM); the interaction-by-exchange-with-the-mean (IEM) mixing model; and realistic combustion chemistry. Using DQMOM, the MEPDF model can be derived from the transport PDF equation by depicting the joint composition PDF as a weighted summation of a finite number of multi-dimensional Dirac delta functions in the composition space. The MEPDF method with multiple reactive scalars retains the unique property of the joint PDF method of treating chemical reactions exactly. However, unlike the joint PDF methods that typically must resort to particle-based Monte-Carlo solution schemes, the MEPDF equations (i.e. the transport equations of the weighted delta-peaks) can be solved by traditional Eulerian grid-based techniques. In the current study, a pseudo time-splitting scheme is adopted to solve the MEPDF equations; the reaction source terms are computed with a highly efficient and accurate in-situ adaptive tabulation (ISAT) algorithm. A 19-species reduced mechanism based on quasi-steady state assumptions is used in the simulations of the bluff-body flames. The modelling results are compared with the experimental data, including mixing, temperature, major species and important minor species such as CO and NO. Compared with simulations using a Monte-Carlo joint PDF method, the new approach shows comparable accuracy.     

模型加力燃烧室燃烧流场的并行计算

本文采用基于MPI的并行算法,采用动态内存分配、分区算法和多点重合交错网格系统,在贴体网格下对带V形槽稳定器模型加力燃烧室素流化学反应流场进行数值模拟,湍流模型采用k方程亚网格尺度模型,燃烧模型采用亚网格EBU模型,采用热通量辐射模型估算辐射通量。计算结果表明并行计算对复杂形状的化学反应流动计算效率很高,是模拟大规模的燃烧问题的有力工具。