Pore-scale simulation of water/oil displacement in a water-wet channel

Water/oil flow characteristics in a water-wet capillary were simulated at the pore scale to increase our understanding on immiscible flow and enhanced oil recovery. Volume of fluid method was used to capture the interface between oil and water and a pore-throat connecting structure was established to investigate the effects of viscosity, interfacial tension (IFT) and capillary number (Ca). The results show that during a water displacement process, an initial continuous oil phase can be snapped off in the water-wet pore due to the capillary effect. By altering the viscosity of the displacing fluid and the IFT between the wetting and non-wetting phases, the snapped-off phenomenon can be eliminated or reduced during the displacement. A stable displacement can be obtained under high Ca number conditions. Different displacement effects can be obtained at the same Ca number due to its significant influence on the flow state, i.e., snapped-off flow, transient flow and stable flow, and ultralow IFT alone would not ensure a very high recovery rate due to the fingering flow occurrence. A flow chart relating flow states and the corresponding oil recovery factor is established.     

The effect of wall roughness on the liquid removal in micro-channels related to a proton exchange membrane fuel cell(PEMFC)

In this paper, the effect of the wall roughness on the water behavior related to the PEMFCs gas channel is investigated by the two-phase flow simulation. And, the different wetting conditions of the wall surface are considered, i.e. hydrophilic surface and hydrophobic surface. The relative roughness height and the roughness element density as well as the roughness element type are also considered in the study. And the results show: (1) for hydrophilic surface, water behavior for smooth case is different from the roughness cases, due to the effect of roughness on the water slug morphology even for r/H = 0.2% roughness. (2) r/H = 0.2% is positive for water removal and will not lead to the high pressure drop for hydrophilic surface, (3) r/H = 5% is advantageous for water removal for hydrophilic surface but disadvantageous for hydrophobic case, and the pressure drop greatly increases for both cases, (4) for hydrophobic surface, roughness of r/H = 1% and r/H = 2% slow down the water removal speed, but will not affect the amount of the removable water, (5) there is nearly no effect for r/H = 0.2% for hydrophobic case, (6) for both conditions, the average pressure drop obviously increases when r/H ≥ 2%. (7) Increase of the roughness element can help water removal for hydrophilic case but no obvious function for hydrophobic surface. (8) The triangle roughness element is better than rectangle element with the same height.     

A coupled volume-of-fluid and level set (VOSET) method for computing incompressible two-phase flows

A coupled volume-of-fluid and level set (VOSET) method, which combines the advantages and overcomes the disadvantages of VOF and LS methods, is presented for computing incompressible two-phase flows. In this method VOF method is used to capture interfaces, which can conserve the mass and overcome the disadvantage of nonconservation of mass in LS method. An iterative geometric operation proposed by author is used to calculate the level set function ? near interfaces, which can be applied to compute the accurate curvature κ and smooth the discontinuous physical quantities near interfaces. By using the level set function ? the disadvantages of VOF method, inaccuracy of curvature and bad smoothness of discontinuous physical quantities near interfaces, can be overcome. Finally the computing results made with VOSET method are compared with those made with VOF and LS methods.     

背压对敞口型旋流喷嘴液膜特性影响的数值研究

背压对空气物性影响较大,使喷嘴液膜受到的气动力发生大幅变化。采用计算流体力学（Computational Fluid Dynamics,CFD）方法对比3种常见液体工质和3种喷嘴尺寸,研究背压对雾化锥角、液膜厚度、旋流强度、液膜表面自激励不稳定性以及液膜形态的影响。结果表明：雾化锥角随背压增加而减小,其中部分喷嘴当背压达到某定值后,雾化锥角小幅波动;选用正庚烷或正十二烷为工质时,液膜厚度随背压增加而增加,达到一定背压后,3种液体液膜厚度呈小幅波动;旋流强度随背压变化无明显规律性;液膜自激励不稳定性随背压增加而降低;因水的表面张力较大,各雾化参数较为稳定;液膜形态与韦伯数呈明显规律性,背压越小韦伯数越大,液膜形态也更趋近于完美圆锥形;以正十二烷为工质时,其液膜形态在大背压下明显收缩,呈洋葱形,其余工质的液膜形态随背压增加,均逐渐从空心圆锥转变为郁金香形。     

带横向自压管道的开敞式调节池体型优化

调节池与泵站进水池相似,具有调节流态的功能等,但传统泵站进水池的出水管多为竖向布置且靠外力吸水,而调节池出水管横向布置且依靠水流自身重力出流.为此,基于FLUENT平台,采用雷诺时均N-S方程、VOF模型及RNG κ-ε湍流模型,通过单因素控制法研究了调节池的几何参数变化(调节池长宽比、调节池宽度、出水口边壁形状)对调...     

Two-dimensional numerical pore-scale investigation of oxygen evolution in proton exchange membrane electrolysis cells

Oxygen blocking the porous transport layer (PTL) increases the mass transport loss, and then limits the high current density condition of proton exchange membrane electrolysis cells (PEMEC). In this paper, a two-dimensional transient mathematical model of anode two-phase flow in PEMEC is established by the fluid volume method (VOF) method. The transport mechanism of oxygen in porous layer is analyzed in details. The effects of liquid water flow velocity, porosity, fiber diameter and contact angle on oxygen pressure and saturation are studied. The results show that the oxygen bubble transport in the porous layer is mainly affected by capillary pressure and follows the transport mechanism of ‘pressurization breakthrough depressurization’. The oxygen bubble goes through three stages of growth, migration and separation in the channel, and then be carried out of the electrolysis cell by liquid water. When oxygen breaks through the porous layer and enters the flow channel, there is a phenomenon that the branch flow is merged into the main stream, and the last limiting throat affects the maximum pressure and oxygen saturation during stable condition. In addition, increasing the liquid water velocity is helpful to bubble separation; changing the porosity and fiber diameter directly affects the width of pore throat and the correlative capillary pressure; increasing porosity, reducing fiber diameter and contact angle can promote oxygen breakthrough and reduce the stable saturation of oxygen.     

Numerical investigation of liquid water dynamics in wave‐like gas channels of PEMFCs

For the air feed in proton exchange membrane fuel cells (PEMFCs), the wave‐like gas channel (GC) shows obvious advantages over the straight GC because the former enhances collision of secondary flow and diffusion in the gas diffusion layer (GDL). However, it is prone to water flooding, which brings greater pressure drop, larger pressure oscillation, and blocking of reaction area. In the present study, numerical models of the water dynamic processes, including water droplets emerging from micropores on the GDL surface and removing through the GC, are established based on the volume of fluid (VOF) method. Water coverage ratio and pressure drop are calculated to evaluate the water flooding. The effects of the dimensional parameters of wave‐like GC and contact angle of channel walls on the water accumulation are studied. The emergence and removal of liquid water is a quasiperiodic and oscillating process. Multicycle simulations show that channel pressure drop increases linearly with greater growth rate than channel length. The equilibrium position of water droplet is strongly dependent on the relative wettability of the GDL and bipolar plate (BPP) surfaces. And the geometric parameters of GC have a significant impact on the pressure, water removal behavior and detachment time. Smaller bent angle brings bigger pressure drop, and larger cycle length is helpful for relieving the oscillation of pressure.     

Validation of a 3D multiphase-multicomponent CFD model for accidental liquid and gaseous hydrogen releases

As hydrogen-air mixtures are flammable in a wide range of concentrations and the minimum ignition energy is low compared to hydrocarbon fuels, the safe handling of hydrogen is of utmost importance. Additional hazards may arise with the accidental spill of liquid hydrogen. Such a release of LH2 leads to a formation of a cryogenic pool, a dynamic vaporization process, and consequently a dispersion of gaseous hydrogen into the environment. Several LH2 release experiments as well as modeling approaches address this phenomenology. In contrast to existing approaches a new CFD model capable of simulating liquid and gaseous distribution was developed at Forschungszentrum Jülich. It is validated against existing experiments and yields no substantial lacks in the physical model and reveals a qualitatively consistent prediction. Nevertheless, the deviation between experiment and simulation raises questions on the completeness of the database, in particular with regard to the boundary conditions and available measurements.     

抽水蓄能电站同发同抽水力特性VOF数值模拟

为了有效缓解电网突然出现的电力不平衡现象,同一个抽水蓄能电站不同水力单元机组同时分别发电及抽水运行,是对抽水蓄能运行及调度方式的一个重大创新措施。以某抽水蓄能电站上库为例,通过数学模型研究上库进/出水口在同发同抽工况下的库区水流流态、流道的水力特性、库区漩涡特性等水力参数的变化规律。结果表明,VOF方法可很好地模拟抽水蓄能同发同抽过程;同发同抽工况下,库区内进出水口附近局部水流流态与单向运行工况基本一致,但是两进/出水口之间形成连通的水流通道,库区内形成大范围横向流动,致使抽水单元大部分水流再次进入发电单元。     

Effect of hydrophilic pipe structure of proton exchange membrane fuel cell on water removal from the gas diffusion layer surface

In a proton exchange membrane fuel cell (PEMFC), effective GDL surface water elimination is significant to water management. This paper used the volume-of-fluid method (VOF) method to carry out simulation research on transferring liquid water in the flow channel with a hydrophilic pipe. The findings indicated that compared with a straight channel, a hydrophilic pipe structure could effectively remove water from the gas diffusion surface (GDL) and reduce the surface water coverage of the GDL. With the increase in the diameter and height of the pipe structure, the GDL surface's water coverage first increased and then decreased, and it was less with the pipe structure than with the direct flow channel. The removal rate of water on the GDL surface was accelerated. The spacing of hydrophilic pipes has a significant impact on the transportation of water. As the spacing increases, the removal rate of water on the GDL surface slowed. A hydrophilic pipe structure with a diameter of 75 μm, a height of 400 μm, and spacing of 300 μm has good water removal performance on the GDL surface. This research work proposes a new internal structure design of the flow channel, which has specific implications for removing water on the GDL surface.