Design and optimization of a cavitating device for Congo red decolorization: Experimental investigation and CFD simulation

The aim of this work is to perform design and optimization of a cavitating device based on CFD simulation. A set of operational and geometrical parameters such as convergence angle, divergence angle, length of throat, and inlet pressure that can affect the hydrodynamic cavitation phenomenon generating in a Venturi are evaluated through CFD simulation and experimental approaches. Response surface methodology (RSM) was employed to achieve the optimum geometrical configuration. The CFD results show that the maximum cavitation zone in the Venturi can be obtained when half angle of the convergence section, throat length and half angle of the divergence section are 22.7°, 4 mm, and 6.5°, respectively. A maximum decolorization of 38.8% has been obtained using the designed Venturi at cavitation number (Cv) of 0.12. Additionally, the results were compared to that of various orifice plates. A decolorization of 26.2% using 33 holes orifice plate and 11.55% in one hole orifice plate approved the superiority of the designed Venturi.     

Versatile anisotropic mesh adaptation methodology applied to pure quantity of interest error estimator. Steady,laminar incompressible flow

We introduce a new flexible mesh adaptation approach to efficiently compute a quantity of interest by the finite element method. Efficiently, we mean that the method provides an evaluation of that quantity up to a predetermined accuracy at a lower computational cost than other classical methods. The central pillar of the method is our scalar error estimator based on sensitivities of the quantity of interest to the residuals. These sensitivities result from the computation of a continuous adjoint problem. The mesh adaptation strategy can drive anisotropic mesh adaptation from a general scalar error contribution of each element. The full potential of our error estimator is then reached. The proposed method is validated by evaluating the lift, the drag, and the hydraulic losses on a 2D benchmark case: the flow around a cylinder at a Reynolds number of 20.     

Exploring the undrained cyclic behavior of methane-hydrate-bearing sediments using CFD–DEM

Based on the mechanical experimental results of methane hydrate (MH), a bond contact model considering the rate-dependency of MH is proposed. A CFD–DEM scheme considering fluid compressibility is used to simulate a series of undrained cyclic shear tests of numerical methane-hydrate-bearing sediment (MHBS) samples. The dynamic behavior, including stress–strain relationship, dynamic shear modulus, and damping ratio, is investigated. In addition, the force chains, contact fabric and averaged pure rotation rate (APR) are examined to investigate the relationships between micromechanical variables and macromechanical responses in the DEM MH samples. The effects of temperature, confining pressure and MH saturation are also analyzed. Due to the micro-structural strengthening by the MH bonds, no obvious change in microscopic quantities is observed, and the samples remain at the elastic stage under the applied low-shear stress level. When confining pressure and MH saturation increase, the dynamic elastic modulus increases, while the damping ratio decreases. An increasing temperature (leading to weakening of MH bonds) can lower the dynamic elastic modulus, but has almost no impact on the damping ratio. On the contrary, an increasing cyclic shear stress level lowers the damping ratio, but has almost no effect on the dynamic elastic modulus.     

Assessment of a high‐order MUSCL method for rotor flows

This work presents the implementation of a high‐order, finite‐volume scheme suitable for rotor flows. The formulation is based on the variable extrapolation MUSCL‐scheme, where high‐order spatial accuracy (up to fourth‐order) is achieved using correction terms obtained through successive differentiation. A variety of results are presented, including 2‐ and 3‐dimensional test cases. Results with the proposed scheme, showed better wake and higher resolution of vortical structures compared with the standard MUSCL, even when coarse meshes were employed. The method was also demonstrated for 3‐dimensional unsteady flows using overset and moving grids for the UH‐60A rotor in forward flight and the Enhanced Rotorcraft Innovative Concept Achievement tiltrotor in aeroplane mode. For medium grids, the present method adds reasonable CPU and memory overheads and offers good accuracy on relatively coarse grids.     

Prediction of particle sticking efficiency for fly ash deposition at high temperatures

The tendency of ash particles to stick under high temperatures is dictated by the ash chemistry, particle physical properties, deposit surface properties and furnace operation conditions. A model has been developed in order to predict the particle sticking efficiency for fly ash deposition at high temperatures. The model incorporates the particle properties relevant to the ash chemistry, particle kinetic energy and furnace operation conditions and takes into consideration the partial sticking behaviour and the deposit layer. To test the model, the sticking behaviours of synthetic ash in a drop tube furnace are evaluated and the slagging formation from coal combustion in a down-fired furnace is modelled. Compared with the measurements, the proposed model presents reasonable prediction performance on the particle sticking behaviour and the ash deposition formation. Through a sensitivity analysis, furnace operation conditions (velocity and temperature), contact angle and particle size have been found to be the significant factors in controlling the sticking behaviours for the synthetic ash particles. The ash chemistry and furnace temperature dictate the wetting potential of the ash particles and the melting ability of the deposit surface; particle size and density not only control the particle kinetic energy, but also affect the particle temperature. The furnace velocity condition has been identified as being able to influence the selective deposition behaviour, where the maximum deposition efficiency moves to smaller particles when increasing the gas velocity. In addition, the thermophoresis effect on the arrival rate of the particles reduces with increasing the gas velocity. Further, increasing the melting degree of the deposit layer could greatly enhance the predicted deposition formation, in particular for the high furnace velocity condition.     

Numerical simulation of droplet ejection of thermal inkjet printheads

In this study, we present a method to predict the droplet ejection in thermal inkjet printheads including the growth and collapse of a vapor bubble and refill of the firing chamber. The three‐dimensional Navier–Stokes equations are solved using a finite‐volume approach with a fixed Cartesian mesh. The piecewise‐linear interface calculation‐based volume‐of‐fluid method is employed to track and reconstruct the ink–air interface. A geometrical computation based on Lagrangian advection is used to compute the mass flux and advance the interface. A simple and efficient model for the bubble dynamics is employed to model the effect of ink vapor on the adjacent ink liquid. To solve the surface tension‐dominated flow accurately, a hierarchical curvature‐estimation method is proposed to adapt to the local grid resolution. The numerical methods mentioned earlier have been implemented in an internal simulation code, CFD3. The numerical examples presented in the study show good performance of CFD3 in prediction of surface tension‐dominated free‐surface flows, for example, droplet ejection in thermal inkjet printing. Currently, CFD3 is used extensively for printhead development within Hewlett‐Packard. Copyright © 2015 John Wiley & Sons, Ltd.     

Coupling of CFD Simulations and Population Balance Modeling to Predict Brownian Coagulation in an Emulsion Polymerization Reactor

In this study a framework consisting of a computational fluid dynamics simulation coupled to a population balance model for the modeling of emulsion polymerizations is proposed. The combined approach is used to understand the impact of changing length and time scales, as well as mixing conditions on the particle size distribution (PSD) of a polymer latex under different conditions. It is shown that the effect of agitation rate can have a profound impact on the distribution of ionic species in the reactor, and thus on the evolution of the PSD.

     

建筑风压场数值模拟中压力波动的平抑

在以同位网格为基础的简单流场压力计算中,通常采用动量插值方法来平抑流场中的压力波动现象;但是对于建筑风场等复杂的钝体绕流问题,由该平抑方法得到的收敛风压场仍可能存在小幅波动。为彻底解决同位网格格式下的压力波动,除采用动量插值方法外,本文提出了在压力校正方程的界面流速中添加压力梯度差值项的方法。算例分析表明,该方法计算得到的建筑风压场完全避免了压力波动现象,风压解与试验结果吻合良好。     

CFD SIMULATION OF FLUID CATALYTIC CRACKING IN DOWNER REACTORS

A mathematical model has been developed for the simulation of gas-particle flow and fluid catalytic cracking in downer reactors. The model takes into account both cracking reaction and flow behavior through a four-lump reaction kinetics coupled with two-phase turbulent flow. The prediction results show that the relatively large change of gas velocity affects directly the axial distribution of solids velocity and void fraction, which significantly interact with the chemical reaction. Furthermore, model simulations are carried out to determine the effects of such parameters on product yields, as bed diameter, reaction temperature and the ratio of catalyst to oil, which are helpful for optimizing the yields of desired products. The model equations are coded and solved on CFX4.4.     

增殖剂球床载气体流动特性

增殖剂球床是聚变堆或混合堆产氚包层可选结构之一,准确把握增殖剂球床中载带气体的流动特性有助于提高对球床载氚过程的认识并优化包层设计。采用离散元程序PFC3D模拟增殖剂小球的填充行为,在球床内不同位置随机截取不同尺寸的控制体,利用布尔运算中的"差集"得到孔隙范围,建立孔隙分布的三维几何模型,进一步划分网格并用计算流体力学(CFD)方法求解,得出控制体上单位长度的压降以及单元体内的速度分布特征,计算结果发现载带气体速度分布与γ分布很类似,且只要选取恰当的控制体,通过计算流体力学方法可以较好地分析整个球床孔隙内流体的流动,有利于进一步研究载氚及相关过程。