水平轴风力机尾流特性的数值研究

对600kW单风轮和7倍风轮直径(7D)间距的两风轮水平轴风力机在不同来流风速条件进行三维流场数值模拟.结果显示:风力机下游流场存在三维速度流动,轴向速度在尾迹区存在明显亏损,且随尾迹向下游的发展,轴向速度亏损逐渐减少.不同来流风速条件下风轮尾迹流的发展有一定区别,在来流风速较低的条件下尾流风速恢复较快.风场在布置风力机时应考虑当地的风速条件,若多数情况在额定风速或超过额定风速工况下运行,则前后风轮间距应大于7倍风轮直径;否则可考虑缩短前后风轮间距.     

厌氧发酵液中平行气泡间相互作用的数值模拟

在Fluent软件中采用SIMPLE算法,对具有剪切变稀性的厌氧发酵液中平行双气泡间的相互作用进行了数值模拟。探讨了气泡间距、尺寸、流变性质对气泡间相互作用的影响。结果发现:当两气泡间距较大时,气泡相互排斥,直到互相不影响;当两气泡间距较小时,气泡间液相的惯性力起作用,气泡互相吸引、接触进而聚并。临界聚并的距离随着两气泡的体积增大而减小。发酵液的剪切变稀性越高,两气泡之间稳定上升过程中气泡最终达到稳定时速度越低,相互作用的临界距离越小。研究结果可以为生物质及市政垃圾厌氧发酵过程的优化设计提供理论依据。     

基于FLUENT的单个透空四面体三维数值模拟

针对透空四面体按高度分层后形成等边三角形布置的三方柱绕流问题,基于FLUENT进行了三维数值模拟,分析了不同高度下流场分布和不同流速下阻力特性,进一步揭示了透空四面体减速落淤机理。结果表明,三维数值模拟结果与分层后的二维数值模拟结果表现出相似性;不同水位下透空四面体后面两根杆件尾流区流场分布区别较大,在水位h=600 mm时,减速区域大且减速效果最明显;不同流速下的阻力系数的变化规律不同,最后均稳定在常数值1.55左右;投放透空四面体后整个流场挟沙能力的衰减区主要体现在尾流区。     

基于动网格对风力机尾流的数值模拟

为了更真实地分析风机尾流流场,尽量减小尾流影响,提高风电场输出功率及效益,提出一种利用动网格技术进行风场优化的新方法。针对1.5 MW风力机叶片旋转区域进行动态网格化求整个流道内的尾流场,同时利用CFD方法与致动盘理论相结合的方法对单个风力机远尾迹区的流动状况进行数值模拟。利用两种计算方法对单台风力机的尾流区域进行计算,获得单台风力机尾流中风轮中心的轴向速度分布及风力机下游不同位置处的流场分布,并与改进Jenson模型结果进行对比分析,表明该动网格方法能较好地捕捉尾流的流场特性,由于新的尾流计算模型考虑叶片真实旋转对尾流影响,准确捕捉风机尾流中产生大小不同尾漩涡,合理捕捉尾流恢复率及尾流半径的变化规律,因此可作为工程开发有效工具,为分析风机尾流流场及风电场风机布置提供一定参考。     

两种新修正工程模型对多台风力机尾流数值模拟分析

基于Park模型尾流区线性膨胀假设、径向风速呈高斯分布和多项式分布假设,提出两种新的修正工程尾流模型Park-Gauss模型和Park-polynomial模型,并对两台风力机全尾流和偏尾流效应进行数值模拟研究。分别对Park模型、2D Jensen模型、Park-Gauss模型以及Park-polynomial模型进行对比研究。经过与LES数值结果比较,结果表明,新修正的Park-Gauss模型可很好模拟全尾流效应,其计算精度要优于Park模型、2D Jensen模型以及Park-polynomial模型;Park-Gauss和Park-polynomial模型均能比较好地模拟偏尾流效应,但Park-polynomial模型更优于前者;两种新的修正工程尾流模型在精度上不仅与LES结果接近一致,而且在径向分布上也更符合真实流场。     

两种典型来流条件下风力机尾迹特性的数值研究

采用CFD方法,对一台两叶片的水平轴风力机在均匀来流和风切变来流两种来流条件的尾迹特性进行定常数值模拟,研究其尾迹区内轴向速度亏损以及湍流强度变化.结果表明:两种来流条件下,轴向速度在尾迹区内不同位置沿径向的分布都近似由"M"形分布逐渐变为"V"形分布,但在风切变来流下垂直平面的上下部轴向速度并不对称;在水平平面上,风...     

气液两相流泵内气泡大小及分布规律研究

为了研究气液两相流离心泵内的气泡分布规律以及气泡大小对数值模拟的影响,采用高速摄像技术对泵内气液两相流动进行可视化试验,采用Fluent中的Eulerian模型和SST k-ω湍流模型对泵内的气液两相流动进行数值模拟。在转速为400 r/min、液相流量为10 m3/h和气相流量为0.5 L/min时,泵内流型为孤立泡状流,经统计测得叶轮内的气泡平均直径约为0.94 mm,且叶轮内气泡直径分布在0.1～2.0 mm之间,较蜗壳内的0.1～1.4 mm分布更广,蜗壳内的气泡平均直径沿着流道方向由0.55 mm逐渐增大到0.82 mm呈增大趋势,整体而言叶轮内的气泡平均直径大于蜗壳区域。通过设置不同直径的气泡对气液两相流泵进行模拟发现,气泡直径对泵内气相的集聚和分布范围有一定的影响,气泡直径越大,气相越容易聚集成高浓度分布,合理设置气泡直径能够提高数值模拟结果的准确性。     

气泡粘度对水平槽道气泡减阻率影响的数值研究

采用多相流混合模型和雷诺应力湍流模型,对水平槽道气泡减阻进行了数值模拟;同时考虑了液相湍流诱导气泡聚合、气泡尾流诱导气泡聚合、湍流涡与气泡碰撞产生破碎作用对气泡减阻的影响。在保证雷诺数以及气相体积分数不变的情况下,调查了气相粘度变化以及气泡聚合、破碎物理因素对气泡减阻率和液相湍流的影响。研究表明:减阻率和液相湍流变化与气相粘度的大小具有直接关系;气泡减阻和液相湍流抑制是因气相粘度小于液相粘度而引起的,如气相的粘度大于液相的粘度时,液相湍流则被强化、气泡减阻现象消失;当气泡体积分数较低时,气泡聚合和破碎物理现象对液相湍流和减阻率的影响很小。     

NREL 5 MW风力机气动特性的数值模拟研究

以NREL 5 MW风力机为研究对象,忽略风轮的仰角和锥角,采用CFD数值模拟方法并选用SST湍流模型研究均匀来流条件下不同风速时风力机的输出功率,并与FAST软件的计算结果进行比较。分析叶片展向不同截面的压力分布和径向速度流场,讨论风力机尾流场速度和湍动能的变化规律。研究结果表明,沿着叶片展向自叶根至叶尖,吸力面压力逐渐降低,低压区覆盖面积逐渐增大;压力面压力逐渐升高,前缘与尾缘附近压力增幅较大。风穿过风轮能量被大量吸收,风轮对来流的阻塞作用主要集中在近尾流区。风轮后随着流体从近尾流区运动到远尾流区湍动能逐渐减小。     

流化床内气固两相绕流单沉浸管的流体动力计算

应用贴体坐标系，基于颗粒相采用颗粒动力学的气固两相双流体模型，数值模拟单沉浸管流化床内颗粒及气泡的行为，计算得到的瞬时颗粒浓度和速度揭示了气泡绕流沉浸管时出现的合并和破裂过程．瞬时颗粒浓度的功率谱密度表明，颗粒脉动的主频率为0．4-1．0Hz，大于床内无沉浸管颗粒的频率值，数值模拟得到的气泡频率与文献中实测值相吻合。     

TRANSIENT CONDUCTION AND RADIATION HEAT TRANSFER WITH HEAT GENERATION IN A PARTICIPATING MEDIUM USING THE COLLAPSED DIMENSION METHOD

In order to study the mechanisms of heat and mass transfer at the gas–liquid interface, flows inside and around a rising inert bubble are considered and calculated using the numerical algorithm developed in a companion article. Studies of heat and mass transfer are carried out while special attention is paid to the effects of wake vortices. Recoveries of the Sherwood and Nusselt numbers are observed in the wake zone behind bubbles, and a physical explanation is proposed.     

Condensation of bubbles in miscible liquids

We previously developed a theoretical envelope model for bubbles condensing in immiscible liquids. The envelope model defines two zones while condensing. In the first zone the bubble accelerates after detachment from the nozzle and the heat is transferred through a viscous boundary layer at the front of the bubble and through the wake at the rear. In the second zone the bubble decelerates, settles into the wake and the heat is transferred through the wake all around the bubble. At a third zone, the bubble reaches the terminal velocity while the condensation process is terminated. In this paper both models (viscous boundary layer model--VBLM; and envelope model--EM) are modified to suit also bubbles condensing in miscible liquids. According to our visualization study of bubbles condensing in miscible liquids, partly envelopment of bubbles takes place at the deceleration zone. Visualization studies also revealed that the condensate mixes immediately with the surroundings. The experimental results for freon-113 bubbles condensing in subcooled freon-113 and presented in this paper confirm these observations and therefore they are bounded by two theoretical models: the envelope model and the viscous boundary layer model.     

Condensation of a bubble train in immiscible liquids

We previously developed a theoretical envelope model for single bubbles condensing in immiscible liquids, in which the convection outside the bubble is conducted through boundary layers at the front of the bubble and through the wake at the rear while the bubble accelerates, and the convection is dominated by heat transfer through the wake all over the bubble while the bubble is enveloped by its own wake at decelerating. In this paper the envelop model is extended for bubble train condensing in immiscible liquids by assuming that the envelopment occurs from start, i.e., the bubble is enveloped by the previous bubble’s wake right after detachment from the nozzle. The experimental results for freon-113 and hexane bubbles condensing in water confirm the assumption for injection frequencies higher than 12 bubbles per second.     

Computational Fluid Dynamics–Population Balance Modeling of Gas–Liquid Two-Phase Flow in Bubble Column Reactors With an Improved Breakup Kernel Accounting for Bubble Shape Variations

Abstract

Traditionally, bubble shapes have been assumed to be spherical in breakup models such as the one developed by Luo and Svendsen in 1996. This particular breakup model has been widely accepted and implemented into computational fluid dynamics (CFD) modeling of gas–liquid two-phase flows. However, simulation results from this model usually provide unreliable predictions about the breakage of very small bubbles. The incorporation of bubble shape variation into breakup models has rarely been documented in literature but the bubble shape plays an important role in the interactions with the surrounding eddies, especially when the effects of bubble deformation, distortion, and bubble internal pressure change are considered during the events of eddy-bubble collision. Thus, the assumption of spherical bubbles seems to be no longer appropriate in reflecting this phenomenon. This study proposes and implements a modified bubble breakup model, which accounts for the variation of bubble shapes when solving the population balance equations for CFD simulation of gas–liquid two-phase flows in bubble columns. The key parameters predicted by the modified breakup model have been compared with the ones predicted by the original model. The simulation results of interfacial area and mass transfer coefficient for larger bubbles have been greatly enhanced by the modified breakup model.     

Numerical study on bubble behavior in magnetic nanofluid used for waste heat recovery power generation concept

Electrical energy can be generated by the bubble motion inside the magnetic nanofluid under the influence of an external magnetic field. The relative movement of the magnetic particles dispersed in the magnetic fluid is induced through the movement of the bubbles rising by buoyancy force. This disturbs the external magnetic field associated with the generator coil, and electrical energy can be generated. The bubble movement in this complex physical environment was studied through 2D numerical analysis. Commercial magnetic fluids EFH1 and EFH3, manufactured by Ferrotec, were selected as the working fluid for the investigation. A level set method was used to analyze the 2‐phase flow of bubbles motion in the magnetic fluid. The effect of magnetic particle concentration on the behavior of bubbles and the change of bubble flow patterns through interaction between bubbles were observed by analysis. In addition, the influence of the magnetic force caused by the external magnetic field on the behavior of the bubble was also investigated. The following results can be obtained through the analysis of this study. The high concentration of magnetic particles increases the viscosity and attenuates the rising velocity and the lateral oscillation of the bubbles. The interaction of the 2 bubbles depends on the initial relative distance. Merging occurs only between 2 bubbles within a certain initial distance, which maximizes disturbance of the surrounding magnetic fluid. The magnetic force exerted by the permanent magnets externally applied is relatively small in comparison with gravity. Therefore, the effect on the rise behavior of the bubble is not significant. In consideration of the overall external force and flow conditions, the pattern of the bubble flow that maximizes the efficiency in the present electric energy generation concept was found.     

舰船尾流横截面上的激光散射特性

根据Mie散射理论，在认为气泡不相干的情况下，计算了单个气泡在不同散射方向的散射强度，得出舰船尾流某个横截面上气泡群的光散射特性。计算结果表明，气泡群的后向散射增强非常明显；舰船尾流横截面上尾流中心气泡的散射强度远大于尾流边缘处的散射强度，说明利用舰船尾流中气泡与背景海洋中的气泡对光散射的差异来作为鱼雷自导的一种手段是可行的。     

An Improved Bubble Packing Method for Unstructured Grid Generation with Application to Computational Fluid Dynamics

An improved bubble packing method (BPM) is proposed to generate high-quality unstructured grids for prediction of the flow field in a domain with complex geometry. For a curved-boundary domain, bubble departure from the curved boundaries during the dynamic movement of bubbles can be avoided by using the mapping and the arc-length parameterization methods. Furthermore, the grid density of the whole region can be controlled effectively. Local mesh refinement is achieved by adding bubbles with different sizes to the real and artificial vertices of the domain, and vertex information is transferred to the inner nodes of the domain using the Shepard interpolation method. In order to validate the proposed BPM, a finite-volume solver on an unstructured collocated grid is developed to simulate both square and polar lid-driven cavity flows. The numerical simulation results agree well with the experimental data under different Reynolds numbers.     

Numerical investigation of bubble coalescence characteristics under nucleate boiling condition by a lattice-Boltzmann model

A numerical study was performed to investigate the characteristics of bubble growth, detachment and coalescence on vertical, horizontal, and inclined downward-facing surfaces. The FlowLab code, which is based on a lattice-Boltzmann model of two-phase flows, was employed. Macroscopic properties, such as surface tension (σ) and contact angle (β), were implemented through the fluid–fluid (G_σ) and fluid–solid (G_t) interaction potentials. The model predicted a linear relationship between the macroscopic properties (σ,β) and microscopic parameters (G_σ,G_t). The simulation results on bubble departure diameter appear to have the same parametric dependence as the empirical correlation. Hydrodynamic aspects of bubble coalescence are investigated by simulating the growth and detachment behavior of multiple bubbles generated on horizontal, vertical, and inclined downward-facing surfaces. For the case of horizontal surface, three distinct regimes of bubble coalescence were represented in the lattice-Boltzmann simulation: lateral coalescence of bubbles situated on the surface; vertical coalescence of bubbles detached in a sequence from a site; and lateral coalescence of bubbles, detached from the surface. Multiple coalescence was predicted on the vertical surface as the bubble detached from a lower elevation merges with the bubble forming on a higher site. The bubble behavior on the inclined downward-facing surface was represented quite similar to that in the nucleate boiling regime on a downward-facing surface.     

CFD simulations of hydrodynamic characteristics in a gas–liquid vertical upward slug flow

Computational fluid dynamics (CFD) simulations are conducted using the volume-of-fluid (VOF) method to investigate the hydrodynamic characteristics of slug flow and the mechanism of slug flow induced CO₂ corrosion. The hydrodynamic characteristics are significantly affected by the viscous, interfacial, and inertial forces. In inertia dominated flows, the velocity of fully developed falling liquid film is increased with increased Taylor bubble rising velocity. The developing falling liquid film is formed at about the length of 0.5 diameter from the Taylor bubble nose, the fully developed falling liquid film is reached at about the length of 1.5–2.1 diameter from the Taylor bubble nose. The average mass transfer coefficient in the falling liquid film is always higher than that in the Taylor bubble wake zone. The iron ion near wall mass transfer coefficient is higher than that of hydrogen ion. The wall shear stress is increased with increased Taylor bubble rising velocity in fully developed falling liquid film zone, and the wall shear stress has a large fluctuation due to the chaotic and turbulent vortexes in Taylor bubble wake zone. The formation and the damage mechanism of the corrosion product scale are proposed for the gas–liquid two-phase vertical upward slug flow induced CO₂ corrosion. It is found that the wall shear stress of upward gas–liquid slug flow is alternate with high frequency, which is the key factor resulting in the corrosion product scale fatigue cracking. The CFD simulation results are in satisfactory agreement with previous experimental data and models available in literature.     

Numerical Simulation of Bubbles Deformation,Flow, and Coalescence in a Microchannel Under Pseudo-Nucleation Conditions

This paper reports the results of numerical study on bubbles deformation, flow, and coalescence under pseudo-nucleate boiling conditions in horizontal mini-/microchannels. The numerical simulation, which is based on the multiphase model of volume of fluid method, aims to study the corresponding flow behaviors of nucleate bubbles generated from the tube walls in mini-/microchannels so as to understand the effect of confined surfaces/walls on nucleate bubbles and heat transfer. Under the pseudo- or quasi-nucleate boiling condition, superheated small vapor bubbles are injected at the wall to ensure that the bubbles generation is under a similar condition of real nucleation. The numerical study examined the fluid mechanics of bubble motion with heat transfer, but the mass transfer across the bubble–liquid interface is not simulated in the present work.