多孔介质燃烧室内湍流气液两相流的数值研究

以多孔介质发动机为背景,用数值模拟方法考察气缸内加入多孔介质蓄热体后对燃烧室内湍流流场及混合气形成的影响.建立了多孔介质随机结构的简化模型,把多孔介质视为随机分布的大量固相单元的集合.在多孔介质区域引入了Antohe和Lage提出的双方程κ-ε湍流模型.为研究喷雾液滴与多孔介质的高温壁面碰撞,引入新的喷雾模型.在模型中考虑了液滴的破碎、碰撞和合并,并且描述了喷雾和多孔介质壁面之间的相互作用.针对燃油喷雾在不同结构多孔介质内流场进行了数值计算.计算表明,喷雾与多孔介质相互作用对燃烧室内燃油液滴的蒸发、混合气的形成过程和均匀化具有重要的影响.     

多孔介质结构对燃烧室内流场和喷雾特性影响的数值研究

以多孔介质发动机为背景,用数值模拟方法考察气缸内加入多孔介质蓄热体后对燃烧室内湍流流场及混合气形成的影响.计算基于Antohe和Lage的适用于多孔介质的κ-ε模型,其中引入了Darcy项和Forchheimer项.建立了一种可描述多孔介质随机结构的简化模型.针对燃油喷雾在不同结构多孔介质内的流场进行了数值计算.计算表明,多孔介质结构对燃烧室内燃油液滴的蒸发和混合气的形成过程具有重要的影响.     

气体和气溶胶在多孔介质中迁移的数值模拟

为了研究多孔介质对气体和气溶胶的封闭能力,用数值方法模拟了气体和气溶胶在多孔介质中的迁移.考虑了渗透、吸附过滤、扩散、热传导等因素,在柱对称坐标下用双孔双渗模型进行了数值模拟,并与实验结果进行了比较.计算结果显示,多孔介质对气溶胶有较强的阻挡作用,模拟实验结果也证明了这一点.     

多孔介质模型在管壳式换热器数值模拟中的应用

本文详细讨论了多孔介质模型在管壳式换热器数值模拟中的应用,开发了一套能自动生成多孔介质特性参数的通用程序。该程序主要基于三维交错网格及SIMPLE算法,然后运用该模型,采用改进的κ-ε模型和壁面函数法,对换热器壳侧的湍流流动进行了数值模拟。计算结果与换热器冷态实验结果符合良好,表明该模型和计算方法是切实可行的。     

双层多孔介质内预混燃烧的实验和二维数值模拟

对双层多孔介质燃烧器内丙烷/空气预混燃烧进行实验和数值研究.实验对多孔介质燃烧器内固体温度场分布进行测量分析;数值计算利用商业软件FLUENT6.2,通过添加用户自定义标量方程和用户自定义函数,对有壁面散热的双层多孔介质内预混燃烧进行了二维模拟,并与实验结果进行了比较.结果表明,双层多孔介质燃烧器具有良好的稳定燃烧范围和较低的污染物排放;壁面散热对多孔介质燃烧的影响不可忽视.     

多孔介质强化池内沸腾换热特性

为探讨多孔介质对强化池内沸腾传热的机理,在38×3mm的容器内上,应用多相流VOF模型,耦合多孔介质流动与传热模型,通过添加用户自定义程序实现了对光滑和多孔加热壁面的池内沸腾传热过程进行数值模拟。数值模拟结果与实验结果相符合,所建立的数学模型为多孔介质的沸腾传热研究提供了依据。     

湍流对多孔介质内预混火焰影响的初步研究

当气流速度较大时,多孔介质内预混燃烧的模拟需要考虑湍流的影响,本文利用简化的k-ε双方程湍流反应流模型对多孔介质内的预混火焰进行了数值模拟.结果表明,湍流大大加强了气流的组分和能世扩散,计算得到的火焰传播速度、CO及NO的排放量都与实验值符合得比较好,与层流模型相比,湍流模型能够改善计算结果.     

考虑弥散效应的多孔介质中超绝热燃烧的数值模拟

研究多孔介质内往复流动下的超绝热燃烧。一维模型包括气体输运、多孔介质固体的辐射、导热和气固两相间的对流换热。通过数值计算研究超绝热燃烧的形成、以及弥散效应、当量比和多孔介质材料本身对超绝热燃烧特性的影响。计算结果的有效性通过实验进行了验证并取得了相同的趋势。结果表明,组分弥散效应对气体温度分布和反应热影响很小;同一工况下,不考虑气体混合物的热弥散效应,会导致过高的气体温度计算值。同时,计算结果表明小孔径的多孔介质更有利于贫可燃极限的扩展,对30 ppi的多孔介质燃烧器,得到了当量比为0．092的可燃极限。     

粘性耗散及变物性对多孔介质中对流换热的影响研究

本文通过理论分析、数值模拟与实验研究的综合,研究了粘性耗散及变物性对多孔介质中对流换热的影响。发现：用理想的等热流边界条件可以较好地数值模拟等热流边界条件下多孔介质中的对流换热;在实际可实现的参数条件下,粘性耗散对多孔介质中对流换热的影响不大;油的变物性对换热的影响很大—热流密度越高,对流换热越强。     

Lattice Boltzmann方法模拟多孔介质Klinkenberg效应

格子Boltzmann数值模拟方法是研究复杂的多孔介质结构特别是Klinkenberg效应的有效方法之一，对处理复杂边值问题尤其有效，用格子Boltzmann方法研究了气流穿越多孔介质问题，并将数值计算结果与实验结果进行了比较，结果表明格子Boltzmann方法是数值模拟气流穿越多孔介质问题的有效方法之一。     

油束撞击形成混合气及其燃烧过程的机理研究

本文介绍了用多次喷雾叠加摄影与激光粒子分析技术对燃油喷雾撞击前后的粒径、贯穿度以及喷雾锥角等因素变化所进行的观察和测量以及介绍了利用双像高速摄影技术对燃油束撞击雾化形成的混合气以及燃烧过程特点的研究。结果表明,燃油经撞击后可显著地增大油束扩散角、不同程度地影响了燃油束的贯穿度,但对燃油束撞击前后滴径变化的影响不大。混合气形成及其燃烧过程的高速摄影研究结果表明,燃油束撞击雾化对加快燃油与空气的混合并促进其火焰扩展起到重要作用。另外撞击反弹方向和喷油压力等也是影响混合气形成和燃烧的重要因素。     

柴油混合闪蒸喷雾的探索研究

基于同时降低柴油机中的NOx和微粒的想法，作者提出混合闪蒸喷雾的设想，井建立了混合闪蒸试验台，以证实这种设想在喷雾阶段是否能实现．试验证实了混合闪蒸雾化改善柴油雾化的机理．文中分析了水油比、水的温度、压力对雾束形状、油滴平均直径的影响，对混合闪蒸用于实际柴油机的潜力做了估计．     

一种新型喷嘴的提出及流量特性的研究

本文在对各种气动喷嘴及其雾化机理分析基础上提出了一种新型的气动雾化喷嘴-"旋转型气-液雾化喷嘴"。在此喷嘴中,油与气分别从不同的槽道切向进入混合室,且油与气一一对应,油与气互相混合、旋转后从喷口喷出。其气液比在热态实验时为4%-6%(用压缩空气雾化),雾化状态良好。本文中对其流量系数及雾化角进行了系统的研究。主要考虑了喷嘴的结构参数,气液比(ALR),液体粘度等因素对流量系数的影响。通过实验测量与拟合,最后得到了喷嘴的流量系数和雾化角的表达式,可以用来指导喷嘴的设计。     

On radiative transfer in water spray curtains using the discrete ordinates method

Radiative transfer through water spray curtains has been presently addressed in conditions similar to devices used in fire protection systems. The radiation propagation from the heat source through the medium is simulated using a 2D Discrete Ordinates Method. The curtain is treated as an absorbing and anisotropically scattering medium, made of droplets injected in a mixing of air, water vapor and carbon dioxide. Such a participating medium requires a careful treatment of its spectral response in order to model the radiative transfer accurately. This particular problem is dealt with using a correlated-K method. Radiative properties for the droplets are calculated applying the Mie theory. Transmissivities under realistic conditions are then simulated after a validation thanks to comparisons with some experimental data available in the literature. Owing to promising results which are already observed in this case of uncoupled radiative problem, next step will be to combine the present study with a companion work dedicated to the careful treatment of the spray dynamics and of the induced heat transfer phenomena.     

Stagnation-point polydisperse spray flame ignition

A theory of stagnation-point flow polydisperse spray flame ignition by an isothermal hot surface is presented for the first time. The configuration investigated consists of a mixture of fuel droplets and air flowing against an isothermal hot surface (such as a hot ignition probe). The polydisperse spray of droplets is modelled using the sectional approach. A single global chemical reaction is assumed for the case when ignition occurs. The mathematical analysis makes use of a small parameter that is exploited for an asymptotic approach. An analytical criterion for ignition is derived which includes effects of the flow field, the reactants and all the fuel spray-related parameters, including the initial size distribution of the spray's droplets. Numerical calculations disclose how the actual droplet size distribution impacts on the critical stagnation point temperature necessary to promote ignition. Additionally, the analytical estimates are compared with predictions of a numerical finite difference code with very satisfactory agreement.     

含甲缩醛柴油喷雾和燃烧排放特性的试验研究

应用激光相位多普勒技术测量了含甲缩醛柴油喷雾的速度场和粒径场,在直喷式柴油机上研究了该含氧混合燃油的燃烧排放特性。结果表明,添加甲缩醛可改善柴油的雾化,增加喷雾轴线上的粒子速度,但减小喷雾锥角;同时以远大于其加入的比例降低柴油机排气烟度,但对氮氧化物的排放影响不大。柴油机采用甲缩醛作燃油添加剂时,需改造燃油系统。     

Numerical simulation and laser-based imaging of mixture formation, ignition, and soot formation in a diesel spray

Laser-based imaging of fuel vapor distribution, ignition, and soot formation in diesel sprays was carried out in a high-pressure, high-temperature spray chamber under conditions that correspond to temperature and pressure in a diesel engine. Rayleigh scattering and laser-induced incandescence are used to image fuel density and soot volume fraction. The experimental results provide data for comparison with numerical simulations. An interactive cross-sectionally averaged spray model based on Eulerian transport equations was used for the simulation of the spray, and the turbulence-chemistry interaction was modeled with the representative interactive flamelet (RIF) concept. The flamelet calculation is coupled to the Kiva3V computational fluid dynamics (CFD) code using the scalar dissipation rate and pressure as an input to the RIF-code. The flamelet code computes the instationary flamelet profiles for every time step. These profiles were integrated over mixture fraction space using a prescribed β-PDF to obtain mean values, which are passed back to the CFD-code. Thereby, the temperature and the relevant species in each CFD-cell were obtained. The fuel distribution, the average ignition delay as well as the location of ignition are well predicted by the simulation. Furthermore, simulations show that the experimentally observed injection-to-injection variations in ignition delay are due to temperature inhomogeneities. Experimental and simulated spatial soot and fuel vapor density distributions are compared during and after second stage ignition.     

材料性质和燃料特性对浸没在多孔介质中的液体燃料燃烧特性影响

本文研究了浸没在沙、钢珠和氟石等不同材料的多孔介质床中的液体燃料的燃烧特性.探讨了燃料性质对火焰特性、床层温度分布、离表面最近测点处蒸气区的下边界到达时间以及燃料消耗的影响.实验结果揭示了浸没在多孔床中的液体燃料燃烧的特殊规律.     

On the behaviour of organic gel multi-size spray diffusion flames

The recently reported, experimentally observed, unusual behaviour of organic gellant-based fuel droplets which, under appropriate ambient thermal conditions, evaporate and burn in an oscillatory fashion is incorporated in a phenomenological manner in a model of a two-dimensional arbitrary multi-size spray diffusion flame. Non-unity Lewis numbers are permitted for the fuel vapour and oxidant. A combined analytical/numerical solution of the governing equations is presented and used to investigate how a spray's initial polydispersity and the frequency of oscillatory evaporation influence the combustion field. It is demonstrated that the initial droplet size distribution and the frequency of evaporation of the burning gel droplets can have an acute impact both on the homogeneous diffusion flame shape, height and width and on the thermal field downstream of the flame front. Hot spots of individual (or clusters of) burning droplets can be created and under certain operating conditions can lead to hotter temperatures than experienced in the main homogeneous flame. The intensity of these hotspots, their number and location are sensitive to spray related parameters. In realistic combustion chambers there is a danger inherent in the existence of hotspots in undesirable regions as they can damage the structural integrity. Other computed results demonstrate that, in relation to the spray diffusion flames obtained using an equivalent purely liquid fuel spray, the use of a gel fuel spray can lead, under certain operating conditions, to a reduction in flame height and temperature. The latter effect is critical when considering flame extinction.