A numerical study of stir mixing of liquids with particle method

The process of stir mixing of two viscous liquids is simulated using the moving particle semi-implicit (MPS) method. A mixing rate is defined within the particle method to characterize the level of mixing, as the number, position, period, and rotating speed of the stirring stick(s) and liquid viscosity are changed. The motions of liquid particles are tracked to reveal the flow field and mixing mechanisms. The variation of the mixing rate shows that the mixing rate is higher when the sticks are rotating monotonically at high speed, and an optimum position of the stick can be identified. The mixing rate does not enhance significantly when three or more sticks are employed, and the liquid viscosity has minor influences on the mixing rate. These results give useful qualitative suggestions on controlling the mixing rate during chemical reactions.     

井筒中起下钻过程的两相波动压力研究

为避免井下井喷、井漏、井塌等复杂情况和事故的发生,在钻井设计和施工时应考虑起下钻作业中产生的波动压力的影响。实际工况中,钻井流体多以气液两相流的形式出现,而以往的波动压力计算模型将钻井流体作为单相液流,存在一定的误差。本文以气液均相流为研究对象,通过理论推导,建立了井筒起下钻或下套管过程中,以气液两相形式存在的钻井液的粘滞性产生的波动压力预测模型,编制了气液两相波动压力预测软件,对不同工况下的起下钻波动压力进行了预测,绘制了不同情况下波动压力系数变化规律图,并与油田钻井实例进行对比,结果表明,该预测模型对钻井现场的起下钻速度控制有一定的指导作用。     

Characterisation of flow and heat transfer patterns in low aspect ratio packed beds by a 3D network-of-voids model

Using an illustrative sphere packing assembly, it is demonstrated that flow structure and wall heat transfer patterns in low aspect ratio fixed bed reactors are more realistically modelled by properly accounting for the discrete void fraction variations. A 3D network-of-voids (NoV) model has been devised to characterise and examine the discrete flow and heat transfer phenomena in a low aspect ratio packed bed with d_t/d_p = 1.93. The model as formulated is deliberately designed to be not too complicated so as not to place severe demands on computational resources. Hence, the model can potentially easily be applied to simulate the typically large sets of tubes (often comprising more than 10,000) in the case of industrial multi-tubular reactors, where every tube is different due to the random insertion of the packing particles. Because of its simplicity, the model offers an opportunity of coupling the individual catalyst pellet level transport with the complex interstitial flows at the reactor scale. Illustrative studies of this NoV model on a random packed bed of spheres predict large variations of discrete in-void angular velocities and consequently wall heat transfer coefficients within a single tube. The wide variations of wall heat transfer coefficients imply that the different angular sections of the tube will transfer heat at radically different rates resulting in potentially large temperature differences in different segments of the tube. This may possibly result in local temperature runaway and/or hot spot development leading to several potentially unanticipated consequences for safety and integrity of the tube and hence the reactor. The NoV model predictions of the overall pressure drop behaviour are shown to be consistent with the quantitative and qualitative features of correlations available in the literature.     

Scale-up of fluidized-bed combustion - A review

Methods for scaling of fluidized-bed combustors are reviewed. It is found that a general scaling methodology, including simultaneously fluid-dynamic and combustion scaling, cannot be applied in practical scaling tests. Simplifications are needed. The approach followed here is to differentiate between fluid-dynamic scaling, combustion scaling, both related to the basic equations describing the phenomena, and boiler scaling that means scale-up from one boiler size to another, where established design elements can be utilized in the scaling procedure.     

工程力学与化工设备机械基础课程整合

阐述了武汉科技大学化学工程与技术学院对化学工程与工艺专业的工程力学与化工设备机械基础课程进行整合的缘由,介绍了各部分内容的选取与整合。指出了教学过程中发现的问题,并给出了解决这些问题的对策。     

Ideal micromixing performance in packed microchannels

In this work, the hydrodynamics and the micromixing characteristics in the packed and non-packed microchannels were studied experimentally at low Reynolds numbers (8–300). The mixing performances in microchannels were observed with high-speed CCD camera, and were evaluated in terms of a segregation index by the Villermaux/Dushman method. The fluid elements were drastically stretched, folded, and sheared with the effects of micro-particles in packed microchannels, resulting in extremely shorter diffusion distance and larger effective interfacial area, and much higher micromixing efficiency compared with those of non-packed microchannels. Under enough packing length and appropriate packing position of micro-particles, the ideal micromixing performance could be obtained in packed microchannels. Furthermore, the micromixing time in packed microchannels was determined based on the incorporation model, and its value was in the range of 0–0.1 ms.     

An analytical method for selecting the optimal nozzle external geometry for fluid dynamic gauging

Fluid dynamic gauging (FDG) was developed to measure, in situ and in real time, the thickness of a soft deposit layer immersed in a liquid without contacting the surface of the layer. An analysis based on the lubrication assumption for the flow patterns in the space between the nozzle and the surface being gauged yielded analytical expressions for the relationships between the main flow variables and system parameters. Nozzle shapes for particular pressure, pressure gradient and shear stress profiles could then be identified. The effect of flow rate, nozzle geometry and nozzle position on the pressure beneath the nozzle and shear stress on the gauged surface showed very good agreement with computational fluid dynamics (CFD) simulations. Case studies presented include nozzle shapes for uniform pressure and shear stress profiles, which are useful for measuring the strength of soft deposit layers.     

A two-scale model for discrete cell gravure roll coating

A two-dimensional model for predicting the fluid pickout and coated film thickness characteristics of a discrete cell direct gravure roll coater operating in reverse mode is derived. A novel multiscale approach is adopted for this purpose and the resulting equations solved numerically for inertia-less flow conditions. A system of stiff ordinary differential equations is found to be sufficient to capture the major gross flow features, while at the cell level the analysis is based on a finite element solution of the momentum and continuity equations. It represents the first such predictive model of its kind, with particular interest placed on the nature of both the pressure distribution and web-to-roll gap profile spanning the coating bead. The effect of key operating parameters, web-to-roll speed ratio, web-tension, wrap-angle, capillary number and cell-geometry, on the degree of fluid pickout from gravure cells and the coated film thickness is explored. Although an idealised model, the trends observed show qualitative agreement with existing experimental data collected on a small-scale gravure coating rig and point the way forward to the eventual formulation of a full three-dimensional predictive model of the process.     

Simulations of microfluidic droplet formation using the two-phase level set method

Microdroplet formation is an emerging area of research due to its wide-ranging applications within microfluidic based lab-on-a-chip devices. Our goal is to understand the dynamics of droplet formation in a microfluidic T-junction in order to optimize the operation of the microfluidic device. Understanding of this process forms the basis of many potential applications: synthesis of new materials, formulation of products in pharmaceutical, cosmetics and food industries. The two-phase level set method, which is ideally suited for tracking the interfaces between two immiscible fluids, has been used to perform numerical simulations of droplet formation in a T-junction. Numerical predictions compare well with experimental observations. The influence of parameters such as flow rate ratio, capillary number, viscosity ratio and the interfacial tension between the two immiscible fluids is known to affect the physical processes of droplet generation. In this study the effects of surface wettability, which can be controlled by altering the contact angle, are investigated systematically. As competitive wetting between liquids in a two-phase flow can give rise to erratic flow patterns, it is often desirable to minimize this phenomenon as it can lead to a disruption of the regular production of uniform droplets. The numerical simulations predicted that wettability effects on droplet length are more prominent when the viscosity ratio λ (the quotient of the viscosity of the dispersed phase with the viscosity of the continuous phase) is O(1), compared to the situation when λ is O(0.1). The droplet size becomes independent of contact angle in the superhydrophobic regime for all capillary numbers. At a given value of interfacial tension, the droplet length is greater when λ is O(1) compared to the case when λ is O(0.1). The increase in droplet length with interfacial tension, σ, is a function of with the coefficients of the regression curves depending on the viscosity ratio.     

2D modeling of moving particles with phase-change effect

The main aim of this work is to study the behavior of particulate flows taking into account the interfacial heat transfer on the particle surface including the phase change phenomena. Two flow configurations of increasing complexity are studied with the aim of unraveling fundamental properties of heat transfer in two-phase flows. The first configuration corresponds to the fluid flow past a single cylindrical ice particle melted in the water. The focus is on the influence of the Reynolds number on the melting time of a single particle and comparison of numerical data against Nu-based models. The second configuration refers to ice particles moving up in the water due to the gravity force in a two-dimensional channel. Here, the focus is on the study of particle dynamics in the presence of neighboring particles, including the influence of the viscous torque on the particle trajectories and finally on the melting time compared to the case of a single particle. An implicit fictitious boundary method (FBM) over a fixed Cartesian grid is extended to model the heat transfer and the phase change in particulate flows in two dimensions. The hydrodynamic forces acting on the particles are calculated directly through the surface integrals without the use of any semi-empirical correlations. The particle collisions are modeled directly using the hard sphere approach, taking into account the inelastic collisions. The interface velocity of the melting (solid–liquid) was calculated by means of the Stefan condition for each particle. To illustrate the impact of particle rotation caused by the viscous torques on the particle trajectory, a set of simulations was performed with and without viscous torque effect. A comparative analysis of the results showed that when the viscous moment is taken into account, the particle melting time is reduced significantly. Additionally, based on an analysis of the time history of the volume-averaged velocity in the entire domain, three regimes were found. In particular, the first regime is characterized by the acceleration of particles due to the gravity force, the second is the transitional regime and the last is the passive regime, where the melted particles follow the flow induced by particles in the past. During the last regime the particles do not have any influence on the fluid flow. The influence of the initial configuration of particles on the flow pattern and regimes is discussed.