DNS Analysis of Wall Heat Transfer and Combustion Regimes in a Turbulent Non-premixed Wall-jet Flame

Understanding the heat-release effects on the wall heat transfer in turbulent reacting flows, i.e. heat transfer with or without significant density variation, is essential for a wide variety of industrial flows, especially combustion problems. The present study focuses on the wall heat transfer and the near-wall reaction characteristics. The heat-release effects on the wall heat transfer and skin-friction coefficients are investigated using three-dimensional direct numerical simulations of a turbulent reacting wall-jet flow with and without heat release. Reductions in the skin-friction coefficient are observed in the exothermic case, compared to the isothermal one, and the underlying mechanism is explained. The absolute wall heat flux also increases, while the corresponding Nusselt number decreases with increasing heat release. Furthermore, the wall effects on the near-wall average burning rate are assessed. It is found that the isothermal cold wall results in an appreciable decrease of the burning rate in the exothermic cases. We observed indications that the wall increases the chances for the development of the premixed mode and its occurrence is very fast in the wall-normal direction.     

液滴二次雾化破碎模式数值模拟

在许多现代工程领域中,二次雾化对于增强液体雾化以及提高混合率都起到了关键的作用．基于SIMPLE方法对这个问题进行了数值模拟,VOF方法与Level-Set方法耦合捕捉界面,自适应网格的运用平衡了对提高计算精度以及降低计算成本两方面的要求．在Oh数较低(Oh < 0.1)的情况下,通过数值模拟得到了四种典型的液滴分裂模式,并且对其中相对复杂的第二第三种分裂模式进行了详细的分析与比较．得出了该模式形成的条件与相关特性．最后,将以上结果与前人的实验结果作了对比,在大多数重要特征方面,两者有非常好的一致性．     

Numerical Simulation of Pulverized Coal MILD Combustion Using a New Heterogeneous Combustion Submodel

The scope of this investigation is the application and analysis of a recently developed submodel (Schulze et al., Oil Gas Science Technol, 2013, doi:10.2516/ogst/2012069) for char particle combustion and gasification. The distinguishing feature of this model is a detailed representation of the diffusion and convection processes as well as the homogeneous reactions in the boundary layer around the char particle. These processes are fully coupled to the heterogeneous particle kinetics. The model was implemented into the CFD code ANSYS-Fluent. The coupled solver is used for simulating the IFRF full scale pulverized coal combustion MILD furnace, for which detailed experimental data are available for model evaluation (Orsino et al., IFRF Doc. No F46/y/3, 2000) The new model yields improved agreement with measured data as compared to the standard modeling approach. This can be directly related to the prediction of the char burnout rate. For further analysis, the mixing field in the IFRF furnace is investigated in detail by introducing four mixture fractions for pyrolysis products, char burn-off gases, primary and secondary air, respectively. The solutions of the respective transport equations are used to define the local stoichiometry both in the gas phase and on the particle surface in such a multi-stream system. The conditions in the particle surrounding gas phase as well as on the particle surface are used to define the regime of particle-gas interaction based on the simulations with the new submodel. It can be shown that for certain conditions the homogeneous reactions in the particle boundary must be accounted for.     

Statistics and Modelling of Turbulent Kinetic Energy Transport in Different Regimes of Premixed Combustion

The statistical behaviour of turbulent kinetic energy transport in turbulent premixed flames is analysed using data from three-dimensional Direct Numerical Simulation (DNS) of freely propagating turbulent premixed flames under decaying turbulence. For flames within the corrugated flamelets regime, it is observed that turbulent kinetic energy is generated within the flame brush. By contrast, for flames within the thin reaction zones regime it has been found that the turbulent kinetic energy decays monotonically through the flame brush. Similar trends are observed also for the dissipation rate of turbulent kinetic energy. Within the corrugated flamelets regime, it is demonstrated that the effects of the mean pressure gradient and pressure dilatation within the flame are sufficient to overcome the effects of viscous dissipation and are responsible for the observed augmentation of turbulent kinetic energy in the flame brush. In the thin reaction zones regime, the effects of the mean pressure gradient and pressure dilatation terms are relatively much weaker than those of viscous dissipation, resulting in a monotonic decay of turbulent kinetic energy across the flame brush. The modelling of the various unclosed terms of the turbulent kinetic energy transport equation has been analysed in detail. The predictions of existing models are compared with corresponding quantities extracted from DNS data. Based on this a-priori DNS assessment, either appropriate models are identified or new models are proposed where necessary. It is shown that the turbulent flux of turbulent kinetic energy exhibits counter-gradient (gradient) transport wherever the turbulent scalar flux is counter-gradient (gradient) in nature. A new model has been proposed for the turbulent flux of turbulent kinetic energy, and is found to capture the qualitative and quantitative behaviour obtained from DNS data for both the corrugated flamelets and thin reaction zones regimes without the need to adjust any of the model constants.     

微型开缝钝体燃烧器燃烧特性数值分析

本文将开缝钝体稳燃技术应用于微型燃烧器中,采用详细化学反应机理模拟了不同速度下微型开缝钝体燃烧器与微型常规钝体燃烧器的燃烧情况．结果表明：开缝钝体燃烧器火焰宽度一致性较好,火焰中心温度沿轴向分布更加均匀,尤其在速度较大时,开缝钝体燃烧器优势更加明显;开缝钝体燃烧器燃烧效率高于常规钝体燃烧器,速度大于25m/s时,开缝钝体燃烧器效率高出常规钝体燃烧器5%左右;由于开缝钝体中钝体缝隙过大,濒临吹熄极限时,钝体后值班火焰被吹熄,开缝钝体燃烧器吹熄极限略有降低．     

On the Validation of Large Eddy Simulation Applied to Internal Combustion Engine Flows Part II: Numerical Analysis

In internal combustion engines, the characteristic in-cylinder flow field is essential and significantly contributes to engine efficiency and performance. This paper describes the numerical investigation of the flow field in a motored 4-stroke, single-cylinder research engine. Quantitative and qualitative comparisons between experimental and numerical data have been performed at selected crank angle and results obtained in this work are discussed. Statistical flow properties are examined to analyze the averaged and instantaneous flow field. In order to investigate higher order statistical velocity moments and gain insight in the physical processes describing the engine flow structure, multi-cycle Large Eddy Simulation (LES) was carried out on two meshes with different spatial resolution. The three-dimensional structure of the flow has been also visualized by means of iso-surfaces of vortical structures, based on the Q criterion for individual cycles during intake. In order to assess the analysis and to verify that the computational mesh is applicable for the performance of LES simulations, the turbulence resolution M and the ratio of sgs-viscosity to the laminar viscosity were evaluated along the planes of interest. A direct comparison of the statistics of the flow field extracted from the numerical predictions shows a very good agreement with measurements conducted in the same configuration. Discrepancies have been however observed, in particular in the higher moments of the velocity components. Whilst this can be attributed mostly to the limited number of statistical sample (50 LES cycles) collected during the simulation, further investigation is certainly necessary to assess the relevance of modeling and spatial resolution issues.     

温度对内燃机排气凸轮/挺柱润滑的影响研究

对内燃机排气凸轮/挺柱摩擦副进行了瞬态热弹性流体润滑仿真和瞬态等温弹性流体动力润滑仿真,并比较了2种仿真的结果.结果表明,尽管2种仿真得到摩擦副的接触压力、润滑油膜厚度以及摩擦系数在凸轮1个旋转周期内的变化趋势非常相近,但其参数的大小却差别较大,尤其是油膜厚度在2种数学模型下的计算结果相差5倍左右.在对此类摩擦副进行润滑研究时,不能忽略温度因素的影响.     

几种内燃机磨合油的摩擦学性能评价及在缸套和活塞环摩擦副上的应用

在分析内燃机缸套和活塞环摩擦副的运行环境和运行机制的基础上,针对内燃机频繁出现的缸套和活塞环摩擦副擦伤现象,研制出了2种内燃机磨合油,在四球摩擦磨损试验机上评价了其磨合效果,采用扫描电子显微镜对磨合表面进行了形貌观察,并通过热分析考察了其热稳定性和热氧化安定性,结果表明,在物理化学指标符合内燃机油的前提条件下,所研制的2种磨合油的承载和抗磨能力较高,经其磨合后的缸套-活塞环表面光滑,磨合效果良好,两种磨合油的热稳定性和热氧化安定性亦较坑,是潜在的优良的内燃机磨合油。     

固体火箭发动机燃烧室光弹性分析

本文运用光弹性法研究了某型固体火箭发动机燃烧室内端燃型药柱和星柱型药柱两种形状的固体复合推进剂的残余应力场,测定了光弹材料的弹性模量、泊松比、线胀系数、材料条纹值和冻结温度,获得了光弹材料在压应力和温度应力下的等差线条纹图,分析了应力分布情况.     

Numerical Modeling of Combustion of Fuel-Droplet-Vapour Releases in the Atmosphere

The paper is concerned with a numerical simulation of fuel cloud behaviour which follows releases of a liquid fuel. The main aim of the work is to develop further a mathematical model to simulate such releases into the atmosphere. The model is validated by a comparison with experimental results. The influence of boundary conditions for turbulent kinetic energy k and its dissipation rate ε on the solution is investigated. It is concluded that the solution depends mainly on the combination of k and ε in the form k ^3/2/ε rather than each of these values separately. A way to define the boundary conditions for k and ε is suggested. The KIVA-II code has been used as the base of the code used. The original code has been modified to simulate low Mach number atmospheric flows, radiation, soot formation and turbulent combustion. This revised version was published online in July 2006 with corrections to the Cover Date.     

内燃机摩擦学综合设计决策研究

以内燃机低摩擦功耗设计为目的，以满足各项摩擦学性能指标为约束，提出并建立了基于基因模型的混合进化设计方法，构建了适合领域特点的专家修正算子和屏蔽算子，并进一步拓展出分层基因模型及相应的协同进化算法，建立了参数协调机制．结果表明，利用该模型和方法可在设计空间中获得优选方案的种群，从而为设计决策提供依据，相关研究结果在内燃机活塞子系统以及内燃机整体设计的设计决策中得到了成功应用．     

Flow Regimes of Interaction of a Turbulent Plane Jet into a Rectangular Cavity: Experimental Approach and Numerical Modelling

The present work is devoted to the experimental and numerical study ofthe interaction of a turbulent plane jet with a rectangular cavity.Several flow regimes have been found to occur: the non-oscillationregime, the stable oscillation regime and an unstable oscillationregime. The first two regimes have been particularly considered. Theexperimental study has been carried out using hot wire anemometry andsome visualisations. The numerical predictions based on statisticalmodelling have been made using on the one hand the standard k– model and on the other hand a two-scales split spectrum model. The structuralproperties of the flow have been described for the different situations.For the oscillatory regime, a parametrical study allowed to determinethe influence of the jet exit location and the Reynolds number on thefrequency of the jet flapping. The one point closures have been able topredict the oscillatory regime, and in particular the two-scales modelled to improved results because better account is taken of lag effectsin unsteady non-equilibrium situations.     

内燃机磨合与表面改性实验研究

针对EQ6100型汽油机,利用内燃机磨合台架试验装置对所研制的内燃机专用磨合油与普通15W／40内燃机油进行对比磨合试验,对磨合油液进行直读铁谱分析,对缸套－活塞环磨合表面形貌及元素组成进行测试分析。结果表明：专用磨合油的磨合效果明显优于普通内燃机油;超细金刚石微粉可以加速磨合进程,分化并减小磨屑尺寸,从而避免缸壁拉伤和改善缸套－活塞环摩擦副的摩擦学特性。     

汽油机燃烧室内温度场测量的激光剪切干涉法

采用激光剪切干涉法测量了汽油机燃烧室内的温度场,并分析了剪切干涉法测温的基本原理,推导出利用干涉条纹图求解温度分布的关系式。在一台二冲程火花点火发动机上设置石英窗和信号同步系统,建立了适合高速摄影的激光剪切干涉测量装置。通过改变剪切干涉量,测取发动机燃烧室内干涉条纹图,从而获得缸内燃烧的二维温度场。结果表明,激光剪切干涉法抗振性强,光路简单可靠,可以进行高速摄像,是研究内燃机燃烧过程的有效方法。从温度场可以看出,燃烧过程中缸内大致可分为三个区,即已燃区、未燃区和燃烧区,具有不同的温度分布和温度梯度。燃烧区温度最高,温度梯度大;已燃区温度次之,梯度较小;未燃区温度最低,但梯度较大。     

Bulk Flow Regimes and Fractional Flow in 2D Porous Media by Numerical Simulations

We investigate a two-dimensional network simulator that models the dynamics of two-phase immiscible bulk flow where film flow can be neglected. We present a method for simulating the detailed dynamical process where the two phases are allowed to break up into bubbles, and bubbles are allowed to merge together. The notions of drainage and imbibition are not adequate to describe this process since there is no clear front between the fluids. In fact, the simulator is constructed so that one can study the behaviour of the system far from inlets and outlets, where the two fluids have been mixed together so much that all initial fronts have broken up. The simulator gives the fractional flow as a function of the saturation of each of the fluids. For the case of two fluids with equal viscosity, we classify flow regimes that are parametrized by the capillary number.     

典型磨损情况下的内燃机性能退化预测

通过内燃机热力学建模,考察了两类典型的磨损情况对整机性能的影响:一类是由于磨损和变形而引起的气门间隙变化(进而导致气门升程变化),另一类是由于缸套-活塞组件之间的磨损而引起的漏气间隙变化.通过分析可知,发动机在运行中的磨损等结构退化现象使发动机的性能大幅下降.因此,应当在设计阶段通过抗磨损的摩擦学设计等手段,延长关键部件的正常工作寿命.     

The Numerical Simulation of Diesel Spray Combustion with LES-CMC

A Large Eddy Simulation (LES) approach together with the Conditional Moment Closure (CMC) method have been used for the simulation of spray combustion in engine-like conditions. The strategy consists of coupling an academic CMC code with the commercial CFD software Star-CD?(CD-adapco). Two issues have been investigated: firstly, the applicability of conventional spray models to LES and secondly, LES-CMC for spray combustion. Conventional spray models that were originally developed for use in Reynolds-averaged equations have been assessed for their applicability within the LES framework by conducting non-reacting spray computations. Liquid core penetration, spray spreading angle and vapour phase penetration have been compared to the available experimental data and the agreement between LES and experiments is satisfactory. Several reacting spray calculations have been performed with a range of initial mixture and temperature conditions, which mimic Diesel engine configurations. The computed auto-ignition time and flame lift-off length are in good agreement with the experimental data. Despite the uncertainties associated with the spray models and the chemistry, the results illustrate that the LES-CMC methodology can reproduce well the experimental results.     

Imaging and PDA analysis of a GDI spray in the near-nozzle region

This paper describes a combined LDA, PDA and imaging analysis of the pressure swirl spray in the near-nozzle region of a GDI injector. This innovative approach in the use of multiple, complementary diagnostics facilitates the interpretation of a complex spray flow field.The LDA and PDA data were ensemble-averaged into time bins to produce comprehensive time-history and spatial profiles of liquid velocity, droplet velocity and size and the sample number. They indicated times at which the spray exhibited seven different characteristics. These were identified as: (a) pre-swirl spray, (b) spray cone develops, (c) spray cone relaxes, (d) maximum velocity in spray cone, (e) fully developed steady state, (f) spray cone collapses and (g) the spray detaches from the nozzle. The most effective method to present the spatial and temporal development of the spray was to superimpose the velocity vector and drop size field plots onto the spray images.This article is part of the special issue 11^th International Symposium on Applications of Laser Techniques to Fluid Mechanics, Lisboa, Portugal, July 2002, January 2004, Vol. 36, Issue no. 1