Three Dimensional Numerical Analysis of Two Phase Flow Separation Using Swirling Fluidics

Vapor-water two phase flow separation in pressure vessel of nuclear power plants is accomplished with swirl motion using vanes. In order to reduce separation pressure loss and to make it economic, a new type of low cost simplified innovative separator using lattice core configuration is proposed where swirling is caused by the orthogonal driving flow. The performance of the separator has been assessed numerically with the commercial CFD code FLUENT 14.0. The numerical analysis is compared with the experiment. The geometry and flow conditions are chosen according to the experiment. In the analysis, standard k – e and realizable k – e turbulence models are implemented. The prediction of maximum air void fraction with realizable k – e model was almost the same as input air void fraction but the void fraction computed by standard k – e model was compared better with the experimental results than the realizable k – e model. Some discrepancies in flow pattern between the experimental and simulation results are observed which might be due to the difference of nozzle shape. However, a more detailed model is necessary to arrive at the final conclusion.

Piezoelectric Power Harvesting Process via Phase Changes of Low-Boiling-Point Medium Together with Water for Recovering Low-Temperature Heats

Low-temperature thermal energy conversions down to exergy zero to electric power must contribute energy sustainability. That is to say, reinforcements of power harvesting technologies from extremely low temperatures less than 373 K might be at least one of minimum roles for the current generations. Then, piezoelectric power harvesting process for recovering low-temperature heats was invented by using a unique biphasic operating medium of an underlying water-insoluble/low-boiling-point medium (i.e. NOVEC manufactured by 3M Japan Ltd.) in small quantity and upper-layered water in large quantity. The higher piezoelectric power harvesting densities were naturally revealed with an increase in heating temperatures. Excessive cooling of the operating medium deteriorated the power harvesting efficiency. The denser operating medium was surpassingly helpful to the higher piezoelectric power harvesting density. Concretely, only about 5% density increase of main operating medium (i.e. water with dissolving alum at 0.10 mol/dm3) came to the champion piezoelectric power harvesting density of 92.6 pW/dm2 in this study, which was about 1.4 times compared to that with the original biphasic medium of pure water together with a small quantity of NOVEC.

Mixture flow of particles and power-law fluid in round peristaltic tube

The erythrocyte and blood flowing in the blood vessel can be treated as the two-phase flow of the mixture of particles and a power-law fluid in a peristaltic tube.In the present work, the peristaltic transport of a power-law fluid and the suspension of particles in a tube is investigated by a perturbation method using the long wavelength approximation. The influence of different parameters on the velocity profile and streamlines is explored. Results show that there is a deflection of the flow field when the power-law index n = 0.5 or 1.5 compared with the Newtonian fluid where the trapping zone is symmetric to a certain cross section. The flux rate and reflux of the material are identified,and the conditions under which the reflux appears are determined. Moreover, a reflux phenomenon occurs near the wall. The trapping zone is related to not only the tube geometry and the flow flux but also the fluid properties. Both the length and width of the trapping zone increase with an increase in θ or φ. The trapping zone is more difficult to produce in the shear-thinning fluid than the shear-thickening fluid.

基于全纯嵌入法的三相潮流通用算法

随着高比例新能源接入,电网三相不平衡问题日益突出,迫切需要工程适用的三相潮流算法进行分析。传统三相潮流算法存在初值依赖性强、模型复杂、计算效率较低和适用场景受限等问题。全纯嵌入法是一种解析求解非线性潮流方程的新方法,具有不依赖初值、求解过程中矩阵定常的优势。因此,该文首先基于电网三相元件模型特性,运用相序变换矩阵,构建三相潮流的全纯嵌入模型;其次,基于幂级数系数相等原则,推导模型系数的递推关系,并运用矩阵范型技术简化计算模型;然后,对零序约束方程进行解析重构,运用矩阵行列替换技术,实现不同接地方式下电网三相潮流的高效求解;最后,在IEEE 33、改造的IEEE 123及我国某区域电网系统上,验证所提方法效率高、收敛可靠且不依赖于计算初值的优势。

双芯移相变压器在主动配电网电压和潮流调控中的应用分析

双芯移相变压器可以有效控制线路电压和潮流,优化潮流分布,提高电网电能质量。针对分布式电源的高比例接入造成的主动配电网电压波动和偏差问题,以及对潮流大小、方向和均衡度的影响,文章研究了双芯移相变压器的拓扑结构和工作原理;然后分析了串联变压器和励磁变压器的具体结构和作用,梳理了移相变压器分别应用于电压调节和潮流控制时的档位控制策略;最后基于MATLAB/Simulink建立了详细的仿真模型,研究了双芯移相变压器的电压调节效果,并对双线路并联、两路电源并联供电、多路电源并联供电三种应用场景下的潮流控制效果作了比较。仿真结果表明,双芯移相变压器可以在有限的档位内将线路电压控制在允许范围;与双线路并联、多路电源并联供电应用场景相比,两路电源并联供电时的潮流均衡效果最好,潮流偏差最小,本研究为双芯移相变压器的广泛应用提供了参考。

声共振混合器高黏度流体混合的功耗特性研究

声共振混合利用机械共振产生高加速度振动,从而促进流体流动,其功耗特性对于其设计及应用具有重要作用。为研究声共振混合器的功耗特性,基于CFD建立了声共振混合过程仿真模型,分析了高黏度流体声共振混合过程壁面对物料的作用力和做功功率,探究了黏度和振动参数改变对混合器功耗特性的影响,并建立了声共振混合器混合功率的预测函数。研究结果表明,在混合过程中,壁面对液相做功的瞬时功率呈现先减少后稳定波动的趋势,而有效功率呈现先增加后稳定波动的趋势,这种不同的变化趋势是由于两者的相位差发生变化所导致的。增加振幅、频率或等加速度下低频大振幅都能够增加瞬时功率和有效功率,并减少液相进入稳定流动阶段所需吸收的外界能量。

贯通同相供电系统建模及运行特性分析

贯通同相供电系统可以取消全线电分相,实现牵引网不间断供电,对贯通同相供电系统进行建模和运行特性分析能够为系统的设计和优化提供理论依据。构建同相牵引变电所模型;建立能够灵活模拟不同拓扑结构和不同运行状态的贯通同相供电系统的链式电路模型;针对模型高效求解和导线载流量校验需求,提出一种改进的导体电流间接求解方法;基于构建的模型对贯通同相供电系统在空载和负载工况下的运行特性进行分析。研究结果表明,三相电压不平衡度的计算值与理论值相等,验证了模型的正确性;与既有导体电流计算方法相比,改进的导体电流间接求解方法的计算精度无差异,计算时间缩短为原来的1/3;潮流计算能够充分反映贯通同相供电系统的穿越功率以及牵引网电压、电流等特性。

闪沸喷雾孔几何形状对孔内流动特性影响的数值模拟

研究了汽车直喷汽油机喷嘴的几何形状对于闪沸喷雾孔内流动以及相变过程的影响机理。基于热不平衡假设建立了一维的两相相变流动模型,对于具有不同喷孔流道长度、直径、入口处圆角以及流道锥角的结构,分析了闪沸喷雾孔内流动的特性。将该模型结果与作者团队先前的实验结果进行了精度比对,验证了模型的准确性。结果表明:喷孔流道长度越长,孔径越小,从而孔内相变越剧烈。流道入口处圆角会减少喷孔内部蒸汽的生成。收缩型喷孔压降较小,孔内蒸汽相的生成也相应减少,从而扩张型喷嘴增大蒸汽产生速率。不同的喷孔结构,改变了流道内的压力以及速度分布;压力分布影响了气泡的生长速率,速度分布影响了气泡的生长时间,而两者共同作用影响了孔内的相变特性。

计及分布式光伏的三相不平衡低压配电网快速潮流计算

随着国家“碳达峰碳中和”战略的持续推进,分布式光伏接入配电网项目的渗透率逐步提高,为保证分布式光伏大量并网安全可靠运行的要求,提出计及分布式光伏的三相不平衡低压配电网快速潮流计算算法,在建立配电网的含分布式光伏的元件模型的前提下,提出含潮流方向和拓扑参数的上三角阻抗矩阵,充分利用该稀疏矩阵,提高潮流计算速度;同时,为分析三相拓扑或负荷(分布式光伏作为负的负荷)不平衡引起的低压配电网中性点位移,设计了计算中性线远端节点电压的方法,使该节点等效为三相相电流集中回路节点;最后,详细给出该低压配电网快速潮流计算算法流程,通过改进的IEEE 33节点验证了该算法的超实时特性和准确性,为大规模分布式光伏接入提供强有力的支撑。

新型单心式独立型移相变压器及其调节特性研究

为提升高比例新能源配电网的潮流控制能力和精度,提出了一种新型单心式独立型移相变压器(SCIPST),对调压绕组匝数配置进行了优化,通过独立控制2个调压绕组,SCIPST能够独立调节线路电压的相位和幅值。与传统移相变压器相比,增加了独立调节线路电压幅值的功能和可调移相角的数量,提高了调节精度。通过独立调节线路电压相位和幅值,能够在一定范围内实现独立调节线路有功、无功。介绍了SCIPST的拓扑结构,详细分析了工作原理并给出了控制策略。建立了SCIPST的仿真模型,对SCIPST调节电网潮流和电压的稳态和暂态性能进行仿真验证。研制出小容量SCIPST实验模型,搭建了潮流调节特性测试平台,开展了调节潮流模拟实验。仿真和实验结果证明了所提出的SCIPST的可行性和有效性。

基于数据潮流模型的高比例光伏配电网三相不平衡优化

随着分布式光伏的大规模接入,配电网固有的三相不平衡问题日益严重,给系统的电能质量、经济运行等带来不利影响。此外,高比例光伏的接入使得配电网的物理结构和运行方式更加复杂多变,导致当前依赖精确拓扑结构和线路参数的三相不平衡优化方法难以应用。因此,提出一种基于数据潮流模型的高比例光伏配电网三相不平衡优化方法。首先,采用基于双阶段注意力机制的循环神经网络方法建立数据潮流模型,拟合三相潮流约束中变量之间的函数关系。同时,提出图特征嵌入的方法将部分已知的拓扑信息嵌入到数据潮流模型中以提高拟合精度。其次,以训练后的数据潮流模型为基础重建配电网三相不平衡优化模型。最后,通过条件梯度下降方法求解该模型,以修改的IEEE33节点配电网络为例,验证了所提方法的有效性。

计及频率耦合的单相光伏变流器谐波潮流分析模型

单相光伏逆变器被广泛应用于配电系统,研究其谐波模型对于谐波潮流分析、谐波交互影响与电力系统稳定具有重要意义。首先,通过分析电网与逆变器的交互机理与逆变器交直流侧的谐波传导,推导出单相光伏逆变器在不同谐波下的阻抗公式;其次,分析谐波阻抗的频率耦合机制并结合控制环节,建立不同谐波下单相逆变器的谐波耦合阻抗矩阵模型与等效戴维南电路;再次,基于所提模型对系统进行谐波分析,极大地简化单相逆变器的谐波潮流分析,提高系统谐波潮流的计算精度,并能有效评估单相逆变器的接入对电能质量的影响;最后,通过时域仿真验证了所建立模型的正确性和有效性。

混合潮流控制器阻抗特性分析及控制策略研究

随着新能源并网的占比不断提高,新能源输送通道受到电网结构及线路热稳定极限的限制。为此,提出一种混合型潮流控制器(HPFC)用于灵活、精准调控潮流,平衡潮流分布,提高断面输送能力。HPFC由无源移相器(PST)部分与有源级联H桥换流器(CHBC)部分组合而成,结合了PST调节潮流容量大与CHBC灵敏性高的优点,实现了对潮流的快速精准调节。首先,利用PST阻抗特性分析了潮流与PST挡位之间的非线性耦合关系,并提出一种基于sinh函数的变步长高效求解的迭代算法;其次,对PST有载调压过程进行仿真建模与暂态分析,推导出切换过程中PST输出电压解析式,为实现暂态应力协同控制(TSCC)提供理论基础;再次,针对有源CHBC与无源PST调节区域及速度之间的差异问题,提出一种潮流柔性协同控制策略,实现了暂态应力协同控制,减小了HPFC调控过程中对电网的潮流冲击及换流阀承受的应力集中;最后,基于PSCAD/EMTDC的仿真结果验证了理论分析的正确性及协调控制策略的有效性。

低载波比调制下的MMC-UPFC电容电压减频效应

低载波比(载波频率与基波频率之比)的载波移相调制策略可有效降低MMC-UPFC的开关损耗,但会导致“减频效应”造成电容电压谐波频率明显降低,严重削弱电容电压均衡策略的控制效果。根据电容电压谐波频率随载波比降低的规律,定量分析了低载波比调制下MMC-UPFC电容电压的减频效应,明确了低载波比与电容电压低频波动、子模块电容不稳定充放电状态和子模块间电容电压不均衡间的内在联系及其发生机理;在此基础上,提出一种基于RBF神经网络的减频效应抑制策略,通过检测电容电压的谐波分量进行最优载波比选择和电容电压控制。最后以苏州南部500kV UPFC示范工程为参考进行仿真,证明了MMC-UPFC电容电压的减频效应的存在性以及抑制方法的有效性。

高功率质子交换膜燃料电池两相流数值模拟

随着氢能产业的进步与发展,质子交换膜燃料电池(PEMFC)的运行压力和功率密度不断提高。基于FLUENT程序包,采用非饱和流动理论(UFT)建立了梯形流道的PEMFC三维稳态非等温两相流数值模型,针对PEMFC运行压力特别是高压高功率密度工况下的两相流行为进行了模拟分析。研究表明,高运行压力会明显提高电堆功率密度、液态水冷凝速率、液态水饱和度以及膜水含量。随着压力和电流密度的提高,水的冷凝、电拖拽和反渗透会产生强烈的非线性变化,导致液态水的积聚位置向入口侧偏移。

搅拌槽内两种桨型组合固液悬浮的数值模拟

本文对工业搅拌罐内原搅拌桨型组合和改造的搅拌桨型组合产生的固液两相流动进行了数值模拟,通过对原搅拌方案和改造后的搅拌方案搅拌罐内所形成的流场、固含率分布以及功率消耗等方面的分析,发现改造方案底层桨叶排出流体的倾斜角更大,形成的搅拌循环更高。原方案搅拌罐内底部固含率很高,从下往上固含率逐渐降低,而搅拌桨型组合改造方案能够形成比较均匀的固含率。改造方案的固液悬浮效果更好,并且改造后的搅拌功率没有增加。

打浆釜内流场和功率特性数值模拟研究

针对打浆釜内颗粒悬浮和搅拌功耗问题,本文采用计算流体力学(Computational Fluid Dynamics,CFD)方法,应用欧拉-欧拉双流体模型,对打浆釜内的流场和搅拌功率特性进行了数值模拟分析。得到了打浆釜内单相速度场、4种不同颗粒的悬浮特性和搅拌功率特性,操作条件对釜内流场和功率的影响规律。研究可为相关搅拌设备的设计和结构优化等提供参考和依据。

Numerical simulation of mud-flows impacting structures

The study of the interaction of mud-flows with obstacles is important to define inundation zones in urban areas and to design the possible structural countermeasures. The paper numerically investigates the impact of a mud-flow on rigid obstacles to evaluate the force acting on them using two different depth-integrated theoretical models, Single-Phase Model(SPM) and Two-Phase Model(TPM), to compare their performance and limits. In the first one the water-sediment mixture is represented as a homogeneous continuum described by a shearthinning power-law rheology. Alternatively, the twophase model proposed by Di Cristo et al in 2016 is used, which separately accounts for the liquid and solid phases. The considered test cases are represented by a 1D landslide flowing on a steep slope impacting on a rigid wall and a 2D mud dam-break flowing on a horizontal bottom in presence of single and multiple rigid obstacles. In the 1D test case, characterized by a very steep slope, the Two-Phase Model predicts the separation between the two phases with a significant longitudinal variation of the solid concentration. In this case the results indicate appreciable differences between the two models in the estimation of both the wave celerity and the magnitude of the impact, with an overestimation of the peak force when using the Single-Phase Model. In the 2D test-cases, where the liquid and solid phases remain mixed, even if the flow fields predicted by the two models present some differences, the essential features of the interaction with the obstacles, along with the maximum impact force, are comparable.

Numerical Simulation of the Churning Power Losses in the Automotive Hypoid Gear Reducer

Improving vehicle transmission efficiency and reducing vehicle fuel consumption is currently one of the main objectives in the automotive field. Reducing gear churning power losses has significant influence on the decreasing vehicle fuel consumption. Based on the two phase flow theory, a 2D two-phase model of the simplified hypoid gear is established to predict the churning losses in different conditions, the VOF method is introduced to track the volume fraction of the free surface, a standard k-ε model is also built to calculate complex turbulence. The oil distributions at the different rotational speed, the different immersion depth and the different viscosity as well as the churning losses of the hypoid gear are obtained and discussed in detail. In general, the churning power losses increase with the increase of the speed, the immersion depth and the viscosity, while the rotational speed shows the greatest influence on the churning losses. It is hoped that this investigation will be helpful in automotive industry applications.

Further Stabilization and Power Density Improvement of Stack-Type Thermoelectric Power Generating Module with Biphasic Medium by Using Various Flexible Metals as Electrodes

In order to realize further stability of a stack-type thermoelectric power generating module (i.e. no electrical connections inside), flexible materials of metal springs and/or rods having restoring forces were installed between lower-temperature-sides of thermoelectric elements. These flexible materials were expected to play three important roles of interpolating different thermal expansions of the module components, enlarging heat removal area and penetration of any media through themselves. Then, a low-boiling-point medium (i.e. NOVEC manufactured by 3M Japan Ltd.) was also applied for a high-speed direct heat removal via its phase change from the lower-temperature-sides of the thermoelectric elements in the proposing stack-type thermoelectric power generating module. No electrical disconnections inside the module were confirmed for more than 9 years of use, indicating further module stability. The power generating density was improved to about 120 mW·m-2 with SUS304 springs having 0.7 mm diameter. Increasing power generating density can be expected in terms of suitable selection of flexible metal with high Vickers hardness, cavities control on the spring surface, more vigorous multiphase flow with adding powders to the medium and optimization of the module configurations according to numerical simulations.