CFD simulation of oil–water separation characteristics in a compact flotation unit by population balance modeling

Huge amounts of produced water are generated in offshore oil production. The Compact Flotation Unit (CFU) is an excellent pretreatment technology of produced water with high separation efficiency, low residence, and small split ratio. The Computational Fluid Dynamics-population balance model (CFD-PBM) method is used in the present work to study the oil–water separation characteristics in the self-developed Beijing Institute of Petrochemical Technology Compact Flotation Unit (BIPTCFU) at both micro-scale and macro-scale, which would help us gain more insights into the mechanism and the influence of flow field on the oil–water separation process such as the oil droplets’ diameter distribution and separation efficiency. The effects of the inlet diameter, the height of the preliminary separation zone, and the width of the annular space on the oil–water separation characteristics of CFU were discussed systematically. It is illustrated that the appropriate increase of inlet velocity, decrease of annular gap width, and increase of the height in the preliminary separation zone can effectively promote the collision and coalescence process of oil droplets. However, the overlarge height of the preliminary separation zone and the too narrow width of the annular space will both have a significant negative effect on the migration and separation of oil and water and lead to the decrease of separation efficiency.     

Dye-dependent studies on droplet pattern and electro-optic behaviour of polymer dispersed liquid crystal

In order to study the droplet pattern and electro-optic (EO) behaviour of polymer dispersed liquid crystal (PDLC) with the addition of dye, dichroic polymer dispersed liquid crystal (DPDLC) films were prepared using a nematic liquid crystal (NLC), photo-curable polymer (NOA 65) and anthraquinone blue dichroic dye (B2), in equal ratio (1:1) of polymer and liquid crystal (LC) by polymerisation induced phase separation (PIPS) technique. Dichroic dye was taken in different concentration (wt./wt. ratio) as 0.0625%, 0.125%, 0.25%, 0.5% and 1% of the LC mixture in DPDLC films. Initially, in an open circuit when there is no proviso for external electric field (0 V), LC droplets in polymer matrix exhibited bipolar pattern, though on closing the circuit with the increase of electric field pattern of droplets starts changing, LC molecules align along the direction of applied electric field and aligned completely relatively at higher field (30 V), which illustrate vertical radial pattern. Further, results show that the DPDLC film containing 0.0625% dye concentration with consistent average droplet size ~4.30 μm, exhibits the best transmission at lower operating voltage.     

Recent progress in experiments for sessile droplet wetting on structured surfaces

Wetting of a sessile droplet on structured or patterned surface can be found in a broad range of applications. The researchers have been promoted to keep working on the topic. The review is on the basis of the recent experimental advances on the sessile droplet wetting on the hydrophobic, hydrophilic, or combined hydrophobic and hydrophilic surfaces under isothermal conditions, and on heating or cooling substrates having nonisothermal conditions. More attention has been paid on the wetting configuration between the sessile droplet and the structured substrate; the research gap has been discussed on identifying the three-phase line shape. Further, the three-dimensional measurement for the sessile droplets on the patterned surfaces with focusing more on the contact line of sessile droplets might reveal new physical insights. This review targets at building a holistic overview on the sessile droplet wetting behaviors on the structured substrate in the past 2 years.     

Electrokinetic actuation of an uncharged polarizable dielectric droplet in charged hydrogel medium

Electrokinetic transport of an uncharged nonconducting microsized liquid droplet in a charged hydrogel medium is studied. Dielectric polarization of the liquid drop under the action of an externally imposed electric field induces a non-homogeneous charge density at the droplet surface. The interactions of the induced surface charge of the droplet with the immobile charges of the hydrogel medium generates an electric force to the droplet, which actuates the drop through the charged hydrogel medium. A numerical study based on the first principle of electrokinetics is adopted. Dependence of the droplet velocity on its dielectric permittivity, bulk ionic concentration, and immobile charge density of the gel is analyzed. The surface conduction is significant in presence of charged gel, which creates a concentration polarization. The impact of the counterion saturation in the Debye layer due to the dielectric decrement of the medium is addressed. The modified Nernst–Planck equation for ion transport and the Poisson equation for the electric field is considered to take into account the dielectric polarization. A quadrupolar vortex around the uncharged droplet is observed when the gel medium is considered to be uncharged, which is similar to the induced charge electroosmosis around an uncharged dielectric colloid in free-solution. We find that the induced charge electrokinetic mechanism creates a strong recirculation of liquid within the droplet and the translational velocity of the droplet strongly depends on its size for the dielectric droplet embedded in a charged gel medium.     

Static droplet array for the synthesis of nonspherical microparticles

We reported a manually operated static droplet array (SDA)-based device for the synthesis of nonspherical microparticles with different shapes. The improved SDA structure and reversible bonding between poly(dimethylsiloxane) (PDMS) were used in the device for the large-scale synthesis and rapid extraction of nonspherical microparticles. To understand the device physics, the effects of flow rate, SDA well size, and shape on droplet generation performances were explored. The results indicated that droplet generation in SDA structures was insensitive to the flow rate, and monodisperse droplets were generated by the SDA-based device through manually pushing the syringe. Finally, we integrated four kinds of SDA structures in one device and successfully realized the synthesis and extraction of nonspherical microparticles with different shapes and materials. Our SDA-based device offers numerous advantages, such as simple manual operation, low equipment cost, controllable microparticle shapes and sizes, and large-scale production. Thus, it holds the potential to be used as a flexible tool for the production of nonspherical microparticles.     

液滴平壁铺展过程中的滞后效应及力学机制研究

研究了液滴平壁铺展过程中的接触角滞后效应,从接触线附近流体的压力、速度以及能量分布等角度考虑滞后效应的成因和变化规律.在此基础之上分析了固体表面粗糙度对滞后效应的影响,并借助部分三维形貌参数(ISO 25178)建立了固体表面形貌与接触角滞后效应之间的量化关系.为了研究以上内容,应用数值仿真软件建立了液滴铺展动力学模型,并结合固体表面上的滞后性实验进行了相关验证.研究结果表明:由于表面粗糙度的存在,液滴铺展至平衡位置时,位于铺展前沿的液体分子被钉扎在固体表面的凹坑或低谷中,使得前沿接触角逐渐增大,后沿接触角逐渐减小,接触角发生滞后;驱动液滴铺展的Laplace压力和自身重力与阻碍液滴铺展的黏性阻力之间的平衡关系,是接触角发生滞后的主要力学机制.另外,实验结果表明接触角滞后效应与固体表面形貌密切相关,具有相同表面粗糙度(Sa)的固体表面,由于表面形貌不同接触角滞后效应可能存在明显的差异.     

Numerical prediction of droplet dynamics in turbulent flow,using the level set method

In the present article, the droplet dynamics in turbulent flow is numerically predicted. The modelling is based on an interfacial marker-level set (IMLS) method, coupled with the Reynolds-averaged Navier–Stokes (RANS) equations to predict the dynamics of turbulent two-phase flow. The governing equations for time-dependent, two-dimensional and incompressible two-phase flow are described in both phases and solved separately using a control volume approach on structured cell-centred collocated grids. The topological changes of the interface are predicted by applying the level set approach. The kinematic and dynamic conditions on the interface separating the two phases are satisfied. The numerical method proposed is validated against a well-known computational fluid dynamics problem. Further, the deformation and breakup of a single droplet either suddenly moved in air or exposed to turbulent stream are numerically investigated. In general, the developed numerical method demonstrates remarkable capability in predicting the characteristics of complex turbulent two-phase flows.     

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.     

双流体雾化液滴荷电特性分析

本文通过理论分析,提出了双流体荷电雾化过程中,增大电场强度或减小液滴粒径均能提高荷电效果的论点。针对不同的电极布置情况建立了数值模拟模型,计算结果表明,双流体喷嘴接地时,喷嘴与环状电极之间的电场强度比喷嘴不接地时大很多。通过实验,验证了电场强度、液滴粒径等参数对荷电效果的影响规律,在环状电极荷电情况下,双流体喷嘴接地时,液体荷质比高于双流体喷嘴不接地时的荷质比;电场强度增大,荷质比增加;液滴粒径越小,荷质比越大;在一定的电导率范围内,电导率越大,荷质比越小。实际喷雾中,运动液滴发生二次破碎的临界荷电量小于Rayleigh极限。