Experimental and numerical modeling of the gas atomization nozzle for gas flow behavior

Gas atomization is a widely used process for manufacturing of fine metal- and alloy-powder. To ensure a stable process with high yields of metal powder, the negative pressure at the melt delivery tube tip base and no flow separation conditions are necessary for a good atomization process. An important feature of these jets is that flow separation may occur over the outer surface of the liquid delivery tube for some conditions. Flow separation cause solidification and accumulation of metal, leading to a shape alteration of the liquid delivery tube in gas atomization process. Using computational fluid dynamics (CFD) software, a parametric study was conducted to determine the effects of atomizing gas pressure on the melt delivery tube tip base pressure and flow separation. Atomization gas pressures of 1.0, 1.3, 1.7, 2.2, and 2.7 MPa were used in the CFD model to initialize the pressure in gas inlet. CFD simulations were performed and the modeling results were compared with experimental data. These results showed that the CFD modeling can be used for the estimation of the melt tip base pressure of the nozzle. It is found that the flow separation formation is strongly dependent on the atomizing gas pressure.     

Pressure distributions of gaseous slip flow in straight and uniform rectangular microchannels

This paper presents analytical derivations of the pressure distribution in straight and uniform rectangular microchannels in the slip flow regime and new experimental data in those channels. The flow is to be steady state, two-dimensional, isothermal, and to have negligible transverse velocities with a first order slip boundary condition. The measured pressure distributions of airflows are compared with newly derived analytical results. There is close agreement between the measurements and calculation by the slip flow formula. The dimensionless location of the maximum deviation from the linear pressure distribution is found analytically and compared with the measurements. This dimensionless location of the maximum deviation increases with the increasing pressure ratios in the slip flow regime. The effect of several parameters such as the channel aspect ratio and the Knudsen number on the locations of maximum deviation from linearity are investigated. The nonlinearity of the pressure distribution is also discussed.     

A combined boundary integral and vortex method for the numerical study of three-dimensional fluid flow systems

Vortex methods using vorticity–velocity formulations have become an increasingly powerful and popular means of studying complex fluid flow systems. The problem of combining an integral equation method and grid-free discrete vortex method (DVM) when studying three-dimensional wall-bounded flows is considered. While the normal boundary condition is satisfied by means of a boundary integral equation (BIE), we also consider the problem of recovering pressure from given vorticity and velocity fields when using Lagrangian DVMs in terms of a BIE. For validation purposes, vortical flow past a sphere and past a flat plate are considered, for which the commonly used method of images is available. Results of near-wall boundary-layer flow simulations are then presented as an illustration of the numerical scheme. The importance of hairpin vortices is highlighted. Finally, results on wall compliance fluid flow are displayed emphasizing the versatility of the numerical method.     

Performance assessment of OpenFOAM and FLOW-3D in the numerical modeling of a low Reynolds number hydraulic jump

A comparative performance analysis of the CFD platforms OpenFOAM and FLOW-3D is presented, focusing on a 3D swirling turbulent flow: a steady hydraulic jump at low Reynolds number. Turbulence is treated using RANS approach RNG k-ε. A Volume Of Fluid (VOF) method is used to track the air–water interface, consequently aeration is modeled using an Eulerian–Eulerian approach. Structured meshes of cubic elements are used to discretize the channel geometry. The numerical model accuracy is assessed comparing representative hydraulic jump variables (sequent depth ratio, roller length, mean velocity profiles, velocity decay or free surface profile) to experimental data. The model results are also compared to previous studies to broaden the result validation. Both codes reproduced the phenomenon under study concurring with experimental data, although special care must be taken when swirling flows occur. Both models can be used to reproduce the hydraulic performance of energy dissipation structures at low Reynolds numbers.     

Instability of the flow in the vicinity of trailing edge of a class of thin aerofoils

A numerical study has been undertaken to investigate the notion of absolute/convective instability in laminar incompressible trailing edge flows past wedge-like shapes with curved boundaries of the form y=α(−x)^m. The effects of various trailing edge shapes m and relative thickness α on the flow separation and the development of instabilities in the vicinity of trailing edge are investigated. The nonlinear viscous-inviscid interaction equations, which have been derived by means of the asymptotic theory of flow separation, are solved first numerically to construct genuine mean velocity profiles representing the correct flow in the vicinity of the trailing edge. The absolute/convective nature of the asymptotically formed velocity profiles via a composite expansion is then ascertained by using a spatio-temporal analysis based on the Briggs-Bers pinching criterion. Although no absolute growth is encountered upstream of the trailing edge of the airfoil shapes considered, in particular the wake region behind the trailing edge of Joukowski type profiles is found to be persistently susceptible to absolute instability. It is found that separation is enhanced as the relative thickness of the airfoil gets bigger. This, in turn, is shown to lead to an additional enhancement of the absolute instability character by both increasing the absolute growth rate as well as the extent of the unstable region. Shedding frequency of the Karman vortex street is also determined behind the trailing edge shapes considered.     

Investigation of viscosity effect on droplet formation in T-shaped microchannels by numerical and analytical methods

Both numerical and analytical models have been developed to explore the viscosity effect of the continuous phase on drop formation at a T-shaped junction in immiscible liquids. The effects of the generalized power law coefficient, the power law exponent and the yield stress on the mechanism of drop breakup, final drop size and frequency of drop formation are studied by using the numerical three-dimensional volume of fluid model. Droplets coalescence in Bingham fluids is observed in the beginning transient period. The effect of yield stress on drop extension is also discussed. Predictions of drop size by using an analytical force balance show satisfactory agreement with simulation results for Newtonian and power law fluids with different viscosity ratios. The approximation error associated with the analytical model for Bingham fluids is also acceptable. This analytical model can greatly shorten the prediction time as compared with the numerical model, which is helpful for on-line control.     

Comfort design and optimization of direct expansion multi-connected radiant air conditioning based on 3D flow field simulation

The air conditioning method based on radiation heat exchange has the characteristics of small vertical temperature gradient, high thermal comfort and energy saving, and has become a hot spot of attention. The Fluent numerical simulation, the experiment in this paper studies the direct expansion multi-line radiant air conditioner under the artificially simulated climate environment in winter heating, summer cooling and dehumidification. The temperature difference of the radiation + fresh air mode at the same time indoors under heating conditions is less than 2.5 °C, and the time to reach the indoor set temperature of 24 °C is about 2–3 h. Under cooling conditions, the temperature difference of the radiation + fresh air mode at the same time in the room is about 0.5–2 °C, and the time to reach the indoor set temperature of 26 °C is about 1–3 h. In the fresh air mode, the indoor temperature difference and response time at the same time are slightly larger than the radiation + fresh air mode. The freezing and dehumidification effect of fresh air is obvious, the moisture content of dehumidifying fresh air is between 6.3 and 10.5 g/kg, and the dehumidification efficiency can reach 50%. Under the same artificial simulated climate environment, the consumption of the three modes is not much different. When the outdoor temperature in heating conditions is higher than 9 °C, the fresh air mode can get better, and the radiation + fresh air mode can achieve better comfort when running indoors under various conditions.     

Finding the focus of expansion and estimating range using optical flow images and a matched filter

The focus of expansion plays an important role in many vision applications such as three-dimensional reconstruction, range estimation, time-to-impact computation, and obstacle avoidance. Most current techniques are based on correspondence or on accurate flow estimation and are therefore considered computationally heavy. This paper presents an efficient technique to find the focus of expansion from optical flow. The technique utilizes a specially designed matched filter that does not require an exact estimation of the optical flow but rather can use a low-quality estimation of it. In addition, based on the location of the focus of expansion and its immediate neighborhood, the paper suggests a way to estimate the range to the focus of expansion. Based on the experimental results, the technique has proved to be both accurate and efficient.Received: 26 January 2003, Accepted: 18 March 2004, Published online: 14 September 2004 Correspondence to: Didi Sazbon     

Peristaltic flow of a Sisko fluid in an endoscope: analytical and numerical solutions

This article deals with the peristaltic flow of a Sisko fluid in an endoscope. The inner tube of the endoscope is fixed, while outer tube is flexible. Continuity and momentum equations are utilized in the mathematical analysis and obtained both the analytical and numerical solutions. The analytical solution has been found by the homotopy analysis method and the numerical solutions are carried out by the shooting technique. The comparison of both the solutions are presented. And the quantitative behaviours of the solutions are discussed.     

Analytical and numerical solutions of the unsteady 2D flow of MHD fractional Maxwell fluid induced by variable pressure gradient

The nonlocal property of the fractional derivative can supply more precise mathematical models for depicting flow dynamics of complex fluid which cannot be modelled appropriately by normal integer order differential equations. This paper studies the analytical and numerical methods of unsteady 2D flow of Magnetohydrodynamic (MHD) fractional Maxwell fluid in a rectangular pipe driven by variable pressure gradient. The governing equation is formulated with Caputo time dependent fractional derivatives whose orders are distributed in interval (0, 2). A challenge is to firstly obtain the exact solution by combining modified separation of variables method with Mikusiński-type operational calculus. Meanwhile, the numerical solution is also obtained by the implicit finite difference method whose validity has been confirmed by the comparison with the exact solution constructed. Different to the most classical works, both the stability and convergence analysis of two-dimensional multi-term time fractional momentum equation are derived. Based on numerical analysis, the results show that the velocity increases with the rise of the fractional parameter and relaxation time. While an increase in the values of Hartmann number leads to a slower velocity in the rectangular pipe.     

Separation of CO2 by single and mixed aqueous amine solvents in membrane contactors: fluid flow and mass transfer modeling

Removal of carbon dioxide from gas mixtures is of vital importance for the control of greenhouse gas emission. This study presents a numerical simulation using computational fluid dynamics of mass and momentum transfer in hollow-fiber membrane contactors. The simulation was conducted for physical and chemical absorption of CO₂. A mass transfer model was developed to study CO₂ transport through hollow-fiber membrane contactors. The model considers axial and radial diffusions in the contactor. It also considers convection in the tube and shell side with chemical reaction. The model equations were solved by numerical method based on finite element method. Moreover, the simulation results were validated with the experimental data obtained from literature for absorption of CO₂ in amine aqueous solutions as solvent. The simulation results were in good agreement with the experimental data for different values of gas and liquid velocities. The simulation results indicated that the removal of CO₂ increased with increasing liquid velocity in the tube side. Simulation results also showed that hollow-fiber membrane contactors have a great potential in the area of gas separation specially CO₂ separation from gas mixtures.     

Simulation of supercritical flow in crossroads: Confrontation of a 2D and 3D numerical approaches to experimental results

This work deals with the modeling of a flood in an urban environment. Among the various types of urban flood events, it was decided to study specifically the severe surface flooding events, which take place in highly urbanized areas. This work concerns particularly the numerical resolution of the two-dimensional Saint Venant equations for the study of the propagation of flood through the crossroads in the city. A discontinuous finite-element space discretization with a second-order Runge-Kutta time discretization is used to solve the two-dimensional Saint Venant equations. The scheme is well suited to handle complicated geometries and requires a simple treatment of boundary conditions and source terms to obtain high-order accuracy. The explicit time integration, together with the use of orthogonal shape functions, makes the method for the investigated flows computationally more efficient than comparable second-order finite volume methods. The scheme is applied to several supercritical flows in crossroads, which are investigated by Mignot. The experimental results obtained by the author are used to verify the accuracy and the robustness of the method. The results obtained are compared to those obtained by a second-order finite volume method (Rubar20 (2D)) and by FLUENT (3D). A very good agreement between the numerical solution obtained by the Runge-Kutta discontinuous Galerkin (RKDG) method and the experimental measured data were found. The method is then able to simulate the flow patterns observed experimentally and able to predict well the water depths, the discharge distribution in the downstream branches of the crossroad and the location of the hydraulic jumps and other flow characteristics more than the other methods.     

Honeycomb Monolith微反应器中NH3还原NOx的一维数学模型

综述了最近燃料气中NOx和SOx的脱除,为了达到最好的设计和操作条件,对Honeycomb Monolith微反应器中进行了模拟,使用数学模型对一种壁面上涂有催化剂的方形HMMR管道进行了脱除NOx和SOx的模拟,并研究了质量和能量的传递问题。气相的一维方程是属于一阶常微分方程,本论文采用了四阶Runge-Kutta方法进行了数值求解。催化相方程属于一维二阶常微分方程,采用数值逼近法对其进行了求解,采用面向对象的C 对其进行了编程求解。在模拟中改变了HMMR反应器的操作条件,研究了气相中的温度和浓度的分布以及催化剂层的温度剃度和浓度的分布。     

Coupled experimental study and thermodynamic modeling of the MnO-Mn2O3-Ti2O3-TiO2 system

A coupled experimental study and thermodynamic modeling of the MnO-Mn₂O₃-Ti₂O₃-TiO₂ system at 1 bar total pressure is presented. Classical equilibration and quenching experiments followed by the phase analysis using electron probe microanalysis (EPMA) and X-ray diffraction (XRD) were employed to obtain equilibrium compositions of the liquid and solid solutions in air. The molten oxide phase was described by using the Modified Quasichemical Model which considers short-range ordering, and the Gibbs energies of the solid solutions (pseudobrookite, ilmenite and spinel) were described using the Compound Energy Formalism based on their crystal structures. A set of optimized model parameters of all phases was obtained, which reproduces all available and reliable thermodynamic data and phase diagrams within experimental error limits from 298 K (25 °C) to above the liquidus temperatures over the entire range of composition under oxygen partial pressures from metallic saturation to 1 bar. The complex phase relationships in the system have been elucidated and discrepancies among the experimental data have been resolved. The database of the model parameters can be accessed by FactSage software with the Gibbs energy minimization to calculate any phase diagrams and thermodynamic properties of the MnO-Mn₂O₃-Ti₂O₃-TiO₂ system.     

Experimental characterization of the temperature dependence of zeta potential and its effect on electroosmotic flow velocity in microchannels

This study attempts to characterize the influence of temperature on zeta potential for a number of commonly used buffers in both poly(dimethylsiloxane) (PDMS):glass and PDMS:PDMS microchannels. The study is motivated by the apparent inability of the Smoluchowski equation for electroosmotic flow (EOF) velocity, U = [ε ₀ ε _r ζ/μ]E _z , to accurately predict EOF velocities at elevated temperatures. Error can result if zeta potential (ζ) is taken to be constant, even if permittivity (ε) and viscosity (μ) are treated as temperature-dependent variables. In some cases, velocity may be underestimated by more than 30%. In this study, the time-interval current-monitoring method was used to measure zeta potential. A hotplate maintained precise channel temperatures and applied electric field strengths were selected so that Joule heating was negligible. Results show that in some solutions (e.g., KCl, TBE), the zeta potential can exhibit a strong dependence on temperature, changing by as much as 50% over a span of 60°C. This influence was found to increase with solution concentration. Other buffers (e.g., TE, Na₂CO₃/NaHCO₃) were stable over all measured temperatures.     

A workflow for handling heterogeneous 3D models with the TOUGH2 family of codes: Applications to numerical modeling of CO2 geological storage

This paper is addressed to the TOUGH2 user community. It presents a new tool for handling simulations run with the TOUGH2 code with specific application to CO₂ geological storage. This tool is composed of separate FORTRAN subroutines (or modules) that can be run independently, using input and output files in ASCII format for TOUGH2. These modules have been developed specifically for modeling of carbon dioxide geological storage and their use with TOUGH2 and the Equation of State module ECO2N, dedicated to CO₂-water-salt mixture systems, with TOUGHREACT, which is an adaptation of TOUGH2 with ECO2N and geochemical fluid-rock interactions, and with TOUGH2 and the EOS7C module dedicated to CO₂-CH₄ gas mixture is described. The objective is to save time for the pre-processing, execution and visualization of complex geometry for geological system representation. The workflow is rapid and user-friendly and future implementation to other TOUGH2 EOS modules for other contexts (e.g. nuclear waste disposal, geothermal production) is straightforward. Three examples are shown for validation: (i) leakage of CO₂ up through an abandoned well; (ii) 3D reactive transport modeling of CO₂ in a sandy aquifer formation in the Sleipner gas Field, (North Sea, Norway); and (iii) an estimation of enhanced gas recovery technology using CO₂ as the injected and stored gas to produce methane in the K12B Gas Field (North Sea, Denmark).     

Flow in context: Development and validation of the flow experience instrument for social networking

Flow theory is a popular theoretical framework for understanding the underpinnings of the prolonged use of information systems. While there has been an increasing interest in examining flow experience during nearly four decades, the concept of flow still suffers from various limitations concerning its use as a measurable construct in empirical research. To address these limitations, the present study developed a 26-item instrument for examining flow experience in the domain of social networking services. A cross-sectional survey was administered to 804 Facebook users. The development and validation process consisted of exploratory and confirmatory factor analyses, second-order factor analysis, and examination of instrument validity and reliability. The developed instrument represents six components of flow experience: skill, machine interaction, social interaction, playfulness, concentration and enjoyment. The developed instrument possesses good model fit and high validity and reliability. This paper discusses the uses and limitations of the instrument in the examination of users' experiences of social networking services, and suggests avenues for future research on the topic with a special focus on research on user-centric innovations in online service.     

Stability and robustness issues in numerical modeling of material failure with the strong discontinuity approach

Robustness and stability of the Continuum Strong Discontinuity Approach (CSDA) to material failure are addressed. After identification of lack of symmetry of the finite element formulation and material softening in the constitutive model as possible causes of loss of robustness, two remedies are proposed: (1) the use of an specific symmetric version of the elementary enriched (E-FEM) finite element with embedded discontinuities and (2) a new implicit-explicit integration of the internal variable, in the constitutive model, which renders the tangent constitutive algorithmic operator positive definite and constant. The combination of both developments leads to finite element formulations with constant, in the time step, and non-singular tangent structural stiffness, allowing dramatic improvements in terms of robustness and computational costs. After assessing the convergence and stability properties of the new strategies, three-dimensional numerical simulations of failure problems illustrate the performance of the proposed procedures.     

煤岩膨胀破裂应变及声发射特征实验研究

针对目标对煤岩膨胀破裂过程中的声发射特征研究较少的问题,开展了煤岩膨胀破裂实验,分析了煤岩膨胀破裂全过程应变与声发射信号的变化规律。结果表明:①煤岩膨胀破裂过程可划分为微破裂阶段(Ⅰ)、宏观裂纹生成及扩展阶段(Ⅱ)和劈裂阶段(Ⅲ)。②煤、岩试样膨胀过程应变演化具有一致性,均呈现“缓变→加速→极值”变化趋势,但两者各阶段声发射信号差异较大。③煤样在第Ⅰ、Ⅱ阶段声发射振铃计数较为丰富,在临近第Ⅲ阶段累计振铃计数呈指数型递增;而砂岩在第Ⅰ、Ⅱ阶段声发射振铃计数较少,在临近第Ⅲ阶段累计振铃计数表现出“突增→平静”趋势。④煤岩膨胀破裂过程均存在变形局部化现象,应变变异系数的突变和声发射峰值频率段增多现象可作为煤岩胀裂失稳破坏的前兆特征。研究结果可为煤岩致裂等工程的监测预警提供依据。