微通道气液同向流动中气泡射流特性

利用高速CCD成像技术,本文研究了梯形横截面微通道内的气泡喷射流动.通过调节同向流动时空气和水流量,研究了气泡形成的大小和频率等变化特性;以及微通道内的流型变化.实验结果表明,气泡喷射频率随空气流量的增加而增大,同时随水流量的增加而上升,且上升幅度渐趋平缓.     

有限导流压裂定向井耦合流动模型

考虑人工裂缝、斜井筒和地层中的耦合流动,将压裂定向井的流动分为地层向裂缝渗流、裂缝内流动和斜井筒变质量管流.采用镜像反映和势叠加原理,建立地层中势分布和渗流数学模型.应用边值理论,将人工裂缝面离散为裂缝网格,并与斜井筒变质量管流模型进行耦合,建立有限导流压裂定向井耦合流动数学模型,并形成相应的迭代算法.应用分析表明:该模型与Prats图版法相比,对于垂直缝吻合度较高;裂缝导流能力和井斜角对产量及裂缝面压力分布影响较大.     

Surface instability and pattern formation by ion-induced erosion and mass redistribution

The contribution of curvature dependent sputtering and mass redistribution to ion-induced self-organized formation of periodic surface nanopatterns is revisited. Ion incidence angle-dependent curvature coefficients and ripple wavelengths are calculated from 3-dimensional collision cascade data obtained from binary collision Monte Carlo simulations. Significant modifications concerning mass redistribution compared to the model of Carter and Vishnyakov and also models based on crater functions are introduced. Furthermore, I find that curvature-dependent erosion is the dominating contribution to pattern formation, except for very low-energy irradiation of a light matrix with heavy ions. The major modifications regarding mass redistribution and ion-induced viscous flow are related to the ion incidence angle-dependent thickness of the irradiated layer. A smaller modification concerns the relaxation of inward-directed mass redistribution. Ion-induced viscous flow in the surface layer also depends on the layer thickness and is thus strongly angle dependent. Simulation results are presented and compared to a variety of published experimental results. The simulations show that in most cases curvature-dependent erosion is the dominant contribution to surface instability and ripple pattern formation and also determines the pattern orientation transition. The simulations predict the occurrence of perpendicular ripple patterns at larger ion incidence angles, in agreement with experimental observations. Mass redistribution causes stabilization of the surface at near-normal ion incidence angles and dominates pattern formation only at very low ion energies.     

Magnetohydrodynamic mechanism for pedestal formation

Time-dependent two-dimensional magnetohydrodynamic simulations are carried out for tokamak plasmas with edge poloidal flow. Differently from conventional equilibrium theory, a density pedestal all around the edge is obtained when the poloidal velocity exceeds the poloidal sound speed. The outboard pedestal is induced by the transonic discontinuity, the inboard one by mass redistribution. The density pedestal follows the formation of a highly sheared flow at the transonic surface. These results may be relevant to the L-H transition and pedestal formation in high performance tokamak plasmas.     

THERMOSOLUTAL CONVECTION WITH COUPLED HEAT AND MASS TRANSFER PROCESS IN A SQUARE ENCLOSURE

Thermosolutal convection flow and its effect on the heat and the mass transfer in a square enclosure is studied experimentally. Both thermal and solute diffusion are induced from the sides, and natural convection is initiated by the combined thermal and solutal buoyancies, which either augment or oppose to each other. The solute diffusion is initiated in an electrochemical system that uses copper sulfate-sulfuric acid solution as an electrolyte. Depending on the magnitude of buoyancy ratio, three different kinds of flow regimes and structures can occur, which lead to different distributions of concentration in the enclosure. The formation and growth of layered flow structure is attributed to the solutal boundary-layer flow that can intrude and accumulate along the horizontal wall. The nearly stagnant layer that occurs can reduce the heat transfer rate. The Nusselt numbers at different flow regimes are measured and correlated in terms of relevant nondimensional parameters. This suggests the correlation of Sherwood number in different ranges of buoyancy ratio. The visualization of flow structures and measurements of both heat and mass transfer allow better understanding of the complicated system.     

Dynamics and applications of a microscale liquid surface interacting with an electric field

We analyze the dynamics of a microscale liquid surface interacting with a time-dependent electrostatic field. The analysis is based on Hamilton’s theory, which can deal with electrodynamics as well as mechanics. The analysis predicts that the mass of a liquid interacting with an electrostatic field is reduced as a result of its motion. The mass reduction occurs through the formation of a high-aspect liquid meniscus, which is eventually transformed to a molecular flow of the liquid. The high-aspect liquid meniscus is used to erect a CNT on the glass surface, and the molecular flow is used to initiate a plasma-induced reaction that produces a hydrogen-storing polymer.     

Influence of pressure on near nozzle flow field and soot formation in laminar co-flow diffusion flames

Soot formation from combustion devices, which tend to operate at high pressure, is a health and environmental concern, thus investigating the effect of pressure on soot formation is important. While most fundamental studies have utilised the co-flow laminar diffusion flame configuration to study the effect of pressure on soot, there is a lack of investigations into the effect of pressure on the flow field of diffusion flames and the resultant influence on soot formation. A recent work has displayed that recirculation zones can form along the centreline of atmospheric pressure diffusion flames. This present work seeks to investigate whether these zones can form due to higher pressure as well, which has never been explored experimentally or numerically. The CoFlame code, which models co-flow laminar, sooting, diffusion flames, is validated for the prediction of recirculation zones using experimental flow field data for a set of atmospheric pressure flames. The code is subsequently utilised to model ethane-air diffusion flames from 2 to 33 atm. Above 10 atm, recirculation zones are predicted to form. The reason for the formation of the zones is determined to be due to increasing shear between the air and fuel steams, with the air stream having higher velocities in the vicinity of the fuel tube tip than the fuel stream. This increase in shear is shown to be the cause of the recirculation zones formed in previously investigated atmospheric flames as well. Finally, the recirculation zone is determined as a probable cause of the experimentally observed formation of a large mass of soot covering the entire fuel tube exit for an ethane diffusion flame at 36.5 atm. Previously, no adequate explanation for the formation of the large mass of soot existed.     

基于MP-PIC模型的流化床煤气化过程三维数值模拟

本文以多相流质点网格模型(MP-PIC)为基础,采用欧拉-拉格朗日方法,建立了流化床煤气化过程的三维可压缩数理模型,该模型同时考虑了稠密气固流动,传热传质和相内、相间的化学反应。通过不同操作参数下模拟计算,获得反应器内的流型、气体组分分布、化学反应速率变化等规律。模型计算所得的出口气体组分与实验结果进行了比较,数值模拟与实验吻合。     

不相溶系统内酸碱中和反应驱动动力学的不稳定性

采用含Mach-Zehnder干涉光路和Hele-Shaw反应器的实验系统,研究了重力场作用下,在Hele-Shaw系统内沿水平界面发生的由酸碱中和反应驱动的动力学不稳定性.反应器内包含上下两层反应物,即下层密度较大的四甲基氢氧化铵水溶液和上层密度较小的溶解于有机相的丙酸溶液. 研究了在伴随有界面传质的中和反应过程中,化学组分对于动力学不稳定性的影响. 观察发现了由于反应物初始浓度不均引起的多种形式的Marangoni对流结构,包含有胞状结构和各种震动波形式的结构.测量了不稳定性发生过程中碱溶液的浓度. 结果表明不稳定性对流的产生可以显著提高系统内的传质效率,并造成传质结 构的剧烈变形.     

CFD在燃煤细粒子凝聚过程中的应用

燃煤产生的细粒子富集了大量的有毒痕量元素,对大气环境和人类健康造成严重危害。本文针对燃煤细粒子的形成过程,在计算流体力学(CFD)的基础上,结合气溶胶动力学理论,模拟了烟气中细粒子在圆柱形流场中由于碰撞而凝聚的过程,并通过计算结果分析细粒子颗粒特征参量(速度、质量、直径和颗粒数目)随流场的变化趋势,为深入研究燃煤细粒子的形成演化机理提供理论基础。     

An aerosol model to predict size and structure of soot particles

An aerosol model to simulate soot formation and growth was developed using moving- and fixed-sectional methods. The new model is composed of a set of subroutines that can be easily combined with the Chemkin package. Using the model, we have simulated soot formation and growth in plug flow reactors.

Our model was compared with a previously published method of moments model for a simulation of the plasma pyrolysis of methane in a plug flow reactor. Inclusion of the transition correction factor for the condensation coefficient led to the prediction of a smaller condensation rate compared with the method of moments model. The average coagulation rate calculated by the sectional model was much higher than that by the method of moments model for a broad particle size distribution. The two models predicted significantly different soot precursor concentration and rates of aerosol processes, but substantially similar particle mass and number for the pyrolysis process.

We have also simulated soot formation and growth in a jet-stirred/plug flow reactor (JSR/PFR) system for which soot size distribution measurements are available in the literature. It is shown that the adjusted-point fixed-sectional method can provide comparable accuracy to the moving-sectional model in a simulation of soot formation and growth. It is also shown that the measured surface growth rate could be much higher than the value used in this study. Soot mass concentrations and size distributions for particles larger than 10 nm were well predicted with a surface reaction enhancement. The primary particle size was underpredicted by only about 30% compared with the measurements, without any model adjustments. As the new model can predict both the particle size distribution and structure, and is suitable for application in complex flows, its application to diverse soot formation conditions will enhance our knowledge on the evolution of soot structures.     

Local nonequilibrium mass transfer in a two-component system under external pulsed irradiation with energy fluxes

The nonisothermal mass transfer in metal materials under irradiation with concentrated energy fluxes is studied in the one-dimensional approximation. Local nonequilibrium equations of extended irreversible thermodynamics are used to describe the transfer phenomena. It is established that, for short times (on the order of the time required for relaxation of the diffusion flow to its local-equilibrium value), the wave mechanism for mass transfer is dominant over the diffusion one, ensuring that the impurity-concentration profiles have a nonmonotonous form. The degree of influence of the space-time nonlocality of the transfer processes on the formation of concentration profiles is estimated, and the model results are compared with the experimental data.     

Physical aspects of polymer combustion and the inhibition mechanism

The characteristics of polymer combustion were studied. The contributions from the conductive, convective, and radiation components of the total heat flow from the flame to the polymer surface were determined. The influence of inhibitors on the rate of chemical reactions in the preignition zone and on the rate of heat and mass exchange between the flame and the burning surface was estimated. It was shown that a change in the heat balance and heat and mass exchange accompanied by changing the optical properties of the flame and burning surface has a pronounced effect on the combustion rate at the flame edge. The formation of a protective coke layer reduces heat flow to the surface of a nonreacted polymer, leading to a decrease in the rate of evolution of the volatile combustible polymer-destruction products into the gas phase. As a result, the flame temperature decreases and it is extinguished.     

Formation mechanism of a circumferential roller in a drying biofluid drop

Using the methods of theoretical thermohydromechanics, a model of protein roller formation at the periphery of a drying biofluid drop is constructed. As basic mechanisms of mass transfer-, temperature-, concentration-, and gravity-induced convections are considered, which produce a global toroidal flow with an ascending fluid filament at the center, dipping fluid along the free surface, and compensatory centripetal flow at the bottom. The shape of the protein roller is analyzed under the assumptions that the material deposition rate is proportional to the (i) material concentration and (ii) material flux.     

BOILING HEAT TRANSFER MECHANISMS IN A HORIZONTAL TUBE BUNDLE

The effect of thermal environment on boiling heat transfer performance in a section of a horizontal tube bundle was investigated using R-113 as the working fluid. The in-line tube bundle has five columns and 27 rows with a pitch-to-diameter ratio of 1.3. Heal transfer coefficients obtained from the instrumented tube in the tube bundle with only one tube heated while the other tubes remained unhealed and with all the lubes in the bundle heated are reported for a range of heat flux, pressure, mass flow rate, and quality. The results showed that heat transfer coefficient of a tube in a heated bundle is slightly higher than that in an unhealed bundle, with the variation of heat transfer coefficient decreasing as heat flux, mass flow rate, or pressure increased. It was also found that higher quality would tend to improve the heat transfer. However, the effect of quality disappeared as heat flux, mass flow rate, and pressure increased. Based on the experimental data, the mechanism of the heat transfer augment due to thermal environment was analyzed. It was proposed that fluid agitation and thin liquid film formation are two main factors for a heated bundle to have better transfer performance than an unhealed bundle,     

Low- and high-temperature study of n-heptane combustion chemistry

n-Heptane has been used extensively in various fundamental combustion experiments as a prototypical hydrocarbon fuel. While the formation of polycyclic aromatic hydrocarbon (PAH) in n-heptane combustion has been studied preferably in premixed flames, this study aims to investigate the combustion chemistry of n-heptane in less-studied diffusion flame and highly rich high-temperature homogeneous oxidation configurations by using a counterflow burner and a flow reactor, respectively. This work addresses the formation of higher-molecular species in the mass range up to about 160 u in both configurations. Samples are analyzed by time-of-flight (TOF) molecular beam mass spectrometry (MBMS) using electron-impact (EI) and single-photon ionization (PI). Highly resolved speciation data are reported. Laminar flow reactor experiments cover a wide temperature range. Especially the measurements at low temperatures provide speciation data of large oxygenates produced in the low-temperature oxidation of n-heptane, which are scarce in the literature. Important precursor molecules for PAH and soot formation, such as C₉H₈, C₁₀H₈, C₁₁H_10, and C₁₂H₈, are formed during the high-temperature combustion process in the counterflow flame, while oxygenated growth species are observed under low-temperature conditions, even at the fuel-rich equivalence ratio of ?=4.00.Numerical modeling for both conditions is performed by using a newly developed kinetic model of n-heptane, which includes the n-heptane and PAH formation chemistry with state-of-the-art kinetic knowledge. Good agreement between model predictions and experimental data of counterflow flame and flow reactor is observed for the major species and some intermediates of n-heptane oxidation. While the concentrations of benzene and toluene measured in the counterflow burner are well-reproduced, the numerical results for flow reactor data are not satisfactory. Differences are found between the formation pathways of fulvene, from whose isomerization benzene is produced in diffusion flame and flow reactor.     

密度锁反向启动特性实验研究

实验研究了密度锁反向启动特性,结果表明：启动流量是影响密度锁反向启动过程的关键因素,启动流量偏离平衡流量较小时,余热排出回路反向流量较小,依靠系统本身具有的自稳定性,将很快建立密度锁内水力平衡关系;而启动流量偏离平衡流量较大时,由于余热排出回路流量较大,较多冷却水进入主回路,导致加热水箱入口温度降低,入口温度的降低对密...     

Jets

This is a discussion of concentrated large-scale flows in planetary atmospheres and oceans, argued from the viewpoint of basic geophysical fluid dynamics. We give several elementary examples in which these flows form jets on rotating spheres. Jet formation occurs under a variety of circumstances: when flows driven by external stress have a rigid boundary which can balance the Coriolis force, and at which further concentration can be caused by the beta effect; when there are singular lines like the line of vanishing windstress or windstress-curl, or the Equator; when compact sources of momentum, heat or mass radiate jet-like beta plumes along latitude circles; when random external stirring of the fluid becomes organized by the beta effect into jets; when internal instability of the mass field generates zonal flow which then is concentrated into jets; when bottom topographic obstacles radiate jets, and when frontogenesis leads to shallow jet formation. Essential to the process of jet formation in stratified fluids is the baroclinic life cycle described in geostrophic turbulence studies; there, conversion from potential to kinetic energy generates eddy motions, and these convert to quasibarotropic motions which then radiate and induce jet-like large-scale circulation. Ideas of potential vorticity stirring by eddies generalize the notion of Rossby-wave radiation, showing how jets embedded in an ambient potential vorticity gradient (typically due to the spherical geometry of the rotating planet) gain eastward momentum while promoting broader, weaker westward circulation. Homogenization of potential vorticity is an important limit point, which many geophysical circulations achieve. This well-mixed state is found in subdomains of the terrestrial midlatitude oceans, the high-latitude circumpolar ocean, and episodically in the middle atmosphere. Homogenization expels potential vorticity gradients vertically to the top and bottom of the fluid, and sideways to the edges of flow domains or gyres; in both these ways is jet formation enhanced.     

Incorporation of film theory in single droplet combustion model for prediction of precursor release in flame spray pyrolysis

The precursor release rate during flame synthesis has been shown to influence the uniformity of synthesized particles. However, its quantification through single droplet combustion modeling was based on its immediate release from the droplet without considering the effect of the mass boundary layer surrounding the droplet. Here, the film theory is applied with the single droplet combustion model to understand the precursor release in the droplet. The resulting mass boundary layer thickness is coupled with droplet temperature to qualitatively investigate precursor release in flame spray pyrolysis. It is shown that small droplets can enhance the precursor release rate due to their small mass boundary layer thickness and higher heating rate. Increasing the EHA content in the EHA/toluene solvent mixture reduces the mass boundary layer thickness and increases the droplet temperature due to EHA's low specific heat capacity. Using droplet sizes estimated by the phase Doppler interferometry, the model shows that the temporal droplet temperature profile remains relatively constant for six synthesis conditions. Concurrently, the mass boundary layer thickness is increased when the liquid and oxygen flow rates are reduced, and atomizing pressure drop is enhanced, resulting in the overall suppression of precursor release from the droplet and consequently increased formation of smaller-size primary particles. Insight into the relative tendencies of the pure gas-to-particle formation route during the synthesis was also derived as a function of the synthesis conditions. This new methodology for the characterization of precursor release is essential for a more accurate understanding and design of homogeneous nanomaterial using flame spray pyrolysis.     

强脉冲电子束辐照材料表面形貌演化的模拟

在回顾和总结强脉冲电子束表面改性实验的基础上, 利用有限元数值计算方法对强脉冲电子束辐照铝和304不锈钢产生的温度场进行模拟, 给出了靶的近表面区域流体状态存在的特征尺度和特征时间, 并对不同材料特性下熔坑的产生原因进行了讨论. 采用两相流模型, 通过水平集方法和有限元方法结合的计算流体力学模拟了熔坑和表面突起形貌在表面处于熔融状态下的运动特征, 通过和实验数据相对比, 验证了对于高黏度, 高表面张力的高熔点金属, 表面处于流体状态下的张力驱动效应是熔坑等表面形貌演化的重要原因.