气固两相自由射流颗粒弥散实验研究综述

本文为水平浓淡煤粉燃烧技术研发基础工作之一。以双矩形气固两相平行射流为研究背景,作者考察了气固两相平面和圆自由射流的实验研究。综述表明,大涡拟序结构的存在和初始条件对自由射流颗粒弥散的影响是关键的,利用St数可定性估计这种影响,颗粒尺寸和负荷对气固两相流场的湍流结构具有显著影响,大颗粒尾涡脱落显著地影响湍流结构的事实使数值模拟气固两相流动的努力面临着模拟方法选择与指导工程设计的矛盾要求。因此,数值模拟技术需要评估,这是继续的工作。     

入流滑移条件对两相射流特性影响的大涡模拟研究

研究了入流滑移条件对空间发展的气粒两相平面湍射流的非定常流动特性的影响。以Re数13000的平面不可压缩湍射流流动为例,气相场用Euler方法求解,通过大涡模拟(large-eddy　simulation,LES),直接求解大尺度涡运动的Navier-Stokes方程,小尺度涡用标准Smagorinsky亚格子模式模拟。颗粒相的运动用Lagrangian方法直接求解。在不同入流滑移条件下(U     

三维混合层中湍流拟序结构对颗粒扩散的影响

对时间发展模式的三维气固两相混合层湍流拟序结构及其对不同尺寸颗粒扩散的影响进行直接数值模拟，对于气相场，应用拟谱方法（Pseudospectral method）直接求解Navier-Stokes方程组；对于颗粒场，对于Lagrangian方法跟踪半场颗粒，针对不同Stokes数的颗粒，分别模拟颗粒场在展向和流向的分布变化，进而分析流场三维大涡结构对不同颗粒扩散的影响，并引入一系列参数，着重定量描述流场展向大涡结构和流向大涡结构对不同尺寸颗粒分布的不同影响。     

气粒两相平面湍射流拟序结构的大涡模拟

采用Eulerian/Lagrangian方法,对空间发展的气粒两相平面湍射流的非定常流动过程进行了数值模拟。以Re数为13000的平面不可压缩湍射流流动为例,气相场采用大涡模拟（large-eddy simulaiton,LES）技术,直接求解大尺度涡运动的Navier-Stokes方程,小尺度涡采用标准Smagorinsky亚格子模式模拟。为了示踪两相射流中气相的运动,同时球 解了标志物的浓度输运方程。颗粒相的运动用Lagrangian方法直接求解。大涡模拟结果表明,在平面射流的过渡区及充分发展区存在丰富的拟序结构及其相互作用。对于稀疏两相射流,不同Stokes数的颗粒运动规律和浓度分布取决于颗粒惯性和气相拟序结构的共同作用。对于Stokes数小于10的两相射流,颗粒相的瞬时浓度场分布与拟序结构密切相关,研究颗粒相的扩散应当考虑拟序结构的影响。     

重力对水平两相湍射流中颗粒行为的影响

研究了重力对空间发展的水平两相平面湍射流中颗粒运动的影响。气相场用 Euler方法求解 ,通过大涡模拟直接求解大尺度涡运动的 Navier- Stokes方程 ,小尺度涡采用 Smagorinsky亚格子模式模拟。颗粒相的运动采用L agrangian方法直接求解。射流 Re数为 1130 0。模拟发现 ,对于 St 1及 St～ 0 (1)的颗粒 ,其在平面射流下游的瞬时分布对重力的影响不敏感。随着颗粒 St数的增大 ,重力对平面射流场中颗粒行为的影响逐渐明显 ,但其作用效果还明显地与两相入射流滑移系数的大小有着直接联系。在小两相入流滑移系数情况下 ,对于 St～ 0 (10 )的颗粒 ,在重力作用下的沉降过程还受到了湍流拟序结构的作用 ,而重力作用导致的更大 St数颗粒的沉降 ,将引起固相粒子在射流下游的非对称分布 ,但它既不是均匀各向同性湍流中颗粒的梯度扩散结果 ,也未呈现出受到湍流拟序结构影响的特征     

两相圆孔射流颗粒喷入方法关键问题的直接数字模拟研究

对两相圆孔射流颗粒喷入方法的关键问题进行了直接数值模拟研究.气相采用可压缩的N-S方程直接求解,颗粒相采用拉格朗日方法( Lagrangian)跟踪实际颗粒的运动.主要探讨了空间发展的两相圆孔射流在采用直接数值模拟算法的前提下,颗粒喷入的方法以及喷入方法对两相圆孔射流中不同直径颗粒的扩散特点.在拉格朗日坐标系下实际跟踪了每一个颗粒运动的轨迹,通过反复的数值试验研究发现,颗粒喷入方法是可信的,得到数值模拟结果成功地再现了不同直径颗粒扩散特性.     

流化床内非等密度双组分颗粒流动特性的研究

基于颗粒动力学和气固两相流体动力学，建立流化床稠密气一固两相非等密度双组分颗粒运动碰撞解耦模型，采用硬球模拟方法研究颗粒间碰撞，大涡模拟方法研究气相湍流流动。基于牛顿第二定律建立单颗粒运动方程，应用牛顿第三定律确定颗粒相和气相相间相互作用的双向耦合。数值模拟二维鼓泡流化床内非等密度双组分颗粒气一固两相流动，计算结果表明颗粒弹性恢复系数影响分层流动特性。     

气固流化床的离散颗粒运动-碰撞解耦模型与模拟

基于分子动力学和气固两相流体动力学，建立流化床稠密气-固两相离散颗粒运动-碰撞解耦模型，采用硬球模拟方法处理颗粒与颗粒之间的碰撞，及大涡模拟方法处理气相湍流流动．单颗粒运动满足牛顿第二定律，颗粒相和气相相间相互作用的双向耦合由牛顿第三定律确定，数值模拟二维鼓泡流化床内稠密气-固两相流动，得到了气泡的形成、发展及颗粒的流化过程，计算结果表明颗粒弹性恢复系数影响气-固两相流动特性。     

单相平面射流实验研究进展

本文为水平浓淡煤粉燃烧技术研发基础工作之一。以双矩形气固两相平行射流为研究背景,作者首先考察了主要应用于航空航天的单相平面射流的实验研究。综述表明,描述平面（单一或多平行）射流的主要概念包括：势流核长度、速度半宽、最大速度衰减、湍流场的相似性、湍流特性;影响因素包括：喷口宽度、喷口结构特性,初始条件;测量方法包括：皮托管、热线风速仪、LDV等。调查发现,近场和远场的射流混合特性有本质区别,喷口宽度、喷口间距、动量比对混合影响显著。对于双平行射流,三区结构和湍流特性受初始条件和喷口结构影响明显,主要测试方法的可靠性较高,均能满足工程要求。重要的是,尚没有关于喷口间距较小时双平行射流的研究。     

大涡模拟二维气固两相平面射流

首先用大涡模拟对二维平面射流进行了数值模拟，并就断面流向速度分布、速度脉动量分布以及雷诺切应力分布与实验值进行对比分析，得出大涡模拟方法可以较好地模拟二雏平面射流。在此基础上对二维气固两相平面射流中直径为30μm(St=2．5)的颗粒扩散特性进行数值模拟。发现由于大尺度涡团的作用，颗粒分布于大尺度涡团的外缘而不是集中于中线附近。这与过去基于时均雷诺方程的数值模拟结果得到的颗粒主要集中于中线的结论有所不同，但与直接模拟二维平面混合层中颗粒运动扩散情况相吻合。     

Simulation of a high Reynolds number reactive transverse jet and the formation of a triple flame

This paper briefly describes a hybrid Eulerian–Lagrangian approach for the numerical simulation of turbulent combustion and its application to the study of transverse reactive jets. Because of their interesting mixing properties, transverse jets are important to a variety of industrial applications such as film cooling, primary or dilution jets in gas turbines, and flame stabilization in high speed combustion. To capture the jet complex structure and the associated reaction dynamics, we developed a fast, multiscale and parallel 3D code using a Lagrangian particle method to solve the vorticity transport equation and an Eulerian adaptive grid-based method to solve the reactive transport equations.     

CFD simulation of jet behaviour and voidage profile in a gas–solid fluidized bed

Based on an Eulerian–Eulerian approach, a computational fluid dynamic (CFD) model for gas–solid system has been developed to investigate the hydrodynamics in fluidized beds. With this model, jet penetration height, jet frequency, time‐averaged axial gas velocity profile, and time‐averaged voidage profile have been simulated in a two‐dimensional bed. The computational results indicate that the jet penetration height increases with increasing the jet gas velocity. The jet frequency decreases with increasing the jet gas velocity and decreasing particle diameter. The time‐averaged axial gas velocity profile becomes ‘lower’ and ‘wider’ and the time‐averaged voidage decreases with increasing distance from the jet nozzle. These conclusions appear in good agreement with the experimental and simulated data in the literature. Copyright © 2004 John Wiley & Sons, Ltd.     

Numerical study on mixture formation characteristics in a direct-injection hydrogen engine

Numerical modeling of direct hydrogen injection and in-cylinder mixture formation is performed in this paper. Numerical studies on direct-injection hydrogen engines are very limited due mainly to the complexity in modeling the physical phenomena associated with the high-velocity gas jet. The high injection pressure will result in a choked flow and develop an underexpanded jet at the nozzle exit, which consists of oblique and normal shock waves. A robust numerical model and a very fine computational mesh are required to model these phenomena. However, a very fine mesh may not be feasible in the practical engine application. Therefore, in this study a gas jet injection model is implemented into a multidimensional engine simulation code to simulate the hydrogen injection process, starting from the downstream of the nozzle. The fuel jet is modeled on a coarse mesh using an adaptive mesh refinement algorithm in order to accurately capture the gas jet structure. The model is validated using experimental and theoretical results on the penetrations of single and multiple jets. The model is able to successfully predict the gas jet penetration and structure using a coarse mesh with reasonable computer time. The model is further applied to simulate a direct-injection hydrogen engine to study the effects of injection parameters on the in-cylinder mixture characteristics. The effects of the start of fuel injection, orientation of the jets, and the injector location on the mixture quality are determined. Results show that the hydrogen jets impinge on the walls soon after injection due to the high velocity of the gas jet. The mixing of hydrogen and air takes place mainly after wall impingement. The optimal injection parameters are selected based on the homogeneity of the in-cylinder mixture. It is found that early injection can result in more homogeneous mixture at the time of ignition. Results also indicate that it is more favorable to position the injector near the intake valve to take advantage of the interaction of hydrogen jets and the intake flow to create a more homogeneous mixture.     

Numerical analysis of discrete phase induced effects on a gas flow in a turbulent two-phase free jet

The paper addresses numerical simulation of turbulent two-phase flow in a long vertical tube and turbulent two-phase free jet formed at the tube outlet, analyzing agreement between the numerical results and the results of corresponding experimental investigation carried out earlier.In the numerical analyses conducted, gas phase was modeled as an air flow (having a mass flow-rate in the range of 1.25–4.00 g/s), while the sand particles of two different sizes (0.25–0.30 and 0.8–1.0 mm) represented a discrete phase (particle to gas mass flow ratio of 0.72–4.08) in the two-phase flow considered. Gas-particle interaction was analyzed based on the gas velocities in the particle-laden two-phase flow and the particle-free gas flow, calculated and measured at various locations along the longitudinal axis and radius of the jet.Mathematical model of continuous phase flow was developed based on the single phase flow models, with certain corrections introduced to account for the effects of particles in the flow. In the simulation model developed, the flow analyzed was modeled as a two-phase mixture, with Eulerian simulation used to account for the gas phase behavior and the Lagrangian simulation modeling the particle movement in the two-phase flow considered. In order to appropriately close the system of time-averaged equations, k–ε turbulent model, deemed the most reliable, was used. Phase coupling i.e. fluid-particle interaction was modeled using the PSI-CELL concept. The results obtained via numerical simulation have shown a good agreement with the experimental data acquired.     

The behaviour of lifted oxy-fuel flames in burners with separated jets

Oxy-fuel combustion in separated-jet burners has been proven to increase thermal efficiency and to have a potential for NOx emission reduction. This paper presents an investigation into confined, turbulent, oxy-flames generated by a burner consisting of a central natural gas jet surrounded by two oxygen jets. The study is focused on the identifying the influence of burner parameters on the flame characteristics and topology, namely stability, lift-off height and flame length. The effects of the natural gas and oxygen jet exit velocities, the distance separating the jets and the deflection of oxygen jets towards the natural gas jet are examined. The OH chemiluminescence. Results show that the lift-off heights increase when jet exit velocities and the distance separating the jets are increased. The deflection of oxygen jets decreases the lift-off height and increases the volume of flame in the transversal plane. The flame length increases principally with the oxygen exit velocity and the separation distance, and decreases considerably when the angle of oxygen jets is increased.     

Modeling of nonreacting and reacting turbulent spray jets using a fully stochastic separated flow approach

Numerical modeling of several turbulent nonreacting and reacting spray jets is carried out using a fully stochastic separated flow (FSSF) approach. As is widely used, the carrier-phase is considered in an Eulerian framework, while the dispersed phase is tracked in a Lagrangian framework following the stochastic separated flow (SSF) model. Various interactions between the two phases are taken into account by means of two-way coupling. Spray evaporation is described using a thermal model with an infinite conductivity in the liquid phase. The gas-phase turbulence terms are closed using the k–ε model. A novel mixture fraction based approach is used to stochastically model the fluctuating temperature and composition in the gas phase and these are then used to refine the estimates of the heat and mass transfer rates between the droplets and the surrounding gas-phase. In classical SSF (CSSF) methods, stochastic fluctuations of only the gas-phase velocity are modeled.Successful implementation of the FSSF approach to turbulent nonreacting and reacting spray jets is demonstrated. Results are compared against experimental measurements as well as with predictions using the CSSF approach for both nonreacting and reacting spray jets. The FSSF approach shows little difference from the CSSF predictions for nonreacting spray jets but differences are significant for reacting spray jets. In general, the FSSF approach gives good predictions of the flame length and structure but further improvements in modeling may be needed to improve the accuracy of some details of the predictions.     

Mixing enhancement mechanism of combined H2–Water jets in supersonic crossflows in a combustor with an expanded section

To obtain the mixing enhancement mechanism of H₂–Water combined jets in supersonic crossflows in a combustor with expanded section for rotating detonation ramjet, the flow field shape and spray structure were studied by experimental and numerical methods. The Eulerian–Lagrangian method was used to investigate the diffusion mechanism and H₂–Water interaction law of combined jets with different sequences. At the same time, high-speed photography and the schlieren technique were used to capture the flow field. The effects of jet pressure drop, orifice diameter, orifice spacing, incoming Mach number, and other parameters on the penetration depth of water jets were studied. The results of experiment and simulation show that using H₂–Water combined jets, the penetration depth of the jet spray can be greatly increased and the jet mixing effect can be significantly improved, which will contribute to the engine's ignition and stable combustion. In the case of pre-water/post-H₂, the penetration depth of the hydrogen jet is greater. In the case of pre-H₂/post-water, the hydrogen jet raises the water spray mainly by protecting the integrity of the water column.     

High pressure direct injection of gaseous fuels using a discrete phase methodology for engine simulations

Direct gaseous fuel injection in internal combustion engines is a potential strategy for improving in-cylinder combustion processes, performance and emissions outputs and, in the case of hydrogen, could facilitate a transition away from fossil fuel usage. Computational fluid dynamic studies are required to fully understand and optimise the combustion process, however, the fine grids required to adequately model the underexpanded gas jets which tend to result from direct injection make this a difficult and cumbersome task. In this paper the gaseous sphere injection (GSI) model, which utilises the Lagrangian discrete phase model to represent the injected gas jet, is further improved to account for the variation in the jet core length with better estimation due to total pressure ratio change. The improved GSI model is then validated against experimental hydrogen and methane underexpanded freestream jet studies, mixing in a direct injection hydrogen spark ignition engine and combustion in a pilot ignited direct injection methane compression ignition engine. The improved GSI model performs reasonably well across all cases examined which cover various pressure ratios, injector diameters, injection conditions and disparate gases (hydrogen and methane) while also allowing for relatively coarse meshes (cheaper computational cost) to be used when compared to those needed for fully resolved modelling of the gaseous injection process. The improved GSI model should allow for efficient and accurate investigation of direct injection gaseous fuelled engines.     

计算循环流化床锅炉旋风分离器分级效率的数值模拟方法

利用数值模拟方法对某210MW循环流化床锅炉旋风分离器进行了数值模拟,其中气相采用Euler坐标系下的雷诺应力模（Reynolds Stress Model,RSM）型,固相采用Lagrangian坐标系下的离散相模型（Discrete Phase Model,DPM）,气固两相之间进行耦合处理,最终计算得到了分离器的分级效率,对比实炉获取的灰样后发现数值模拟计算结果较传统的分级效率计算方法更为准确,数值模拟可以用于旋风分离器的分级效率计算及性能预测。