Large Eddy Simulation of Particle Wake Effect and RANS Simulation of Turbulence Modulation in Gas-Particle Flows

The turbulence enhancement by particle wake effect is studied by large eddy simulation （LES） of turbulent gas flows passing a single particle. The predicted time-averaged and root-mean-square fluctuation velocities behind the particle are in agreement with the Reynolds-averaged Navier-Stokes modeling results and experimental results. A semi-empirical turbulence enhancement model is proposed by the present-authors based on the LES resuits. This model is incorporated into the second-order moment two-phase turbulence model for simulating vertical gas-particle pipe flows and horizontal gas-particle channel flows. The simulation results show that compared with the model not accounting for the particle wake effect, the present model gives simulation results for the gas turbulence modulation in much better agreement with the experimental results.

CFD simulation of flow field and resistance in a 19-core tandem ceramic membrane module

CFD simulation of the permeation process of a 19-core tandem ceramic membrane module was established to investigate flow field and resistance and its change in permeate flux to the membrane element position and the channel of each membrane element.The results show that when the volume flow rate changes from26 m3·h-1 to 89 m3·h-1,the resistance of each part of the membrane module increases gradually.The increase in resistance loss in the membrane element is faster than the plates and the bell mouths.In a single ceramic membrane module,the maximum difference in flow rate of each membrane tube is 7.23%.In a single membrane tube,the outer ring channels 3–5,3–6,3–7,3–8 are relatively slow.The maximum mass flow deviation from the mean is 2.7%.This work helps to clarify the flow mechanism within the modules,optimize the structure of the equipment and provide a reliable basis for the improvement of industrial ceramic membrane modules.

Effect of wall temperature modulation on the heat transfer characteristics of droplet-train flow inside a rectangular microchannel

The numerical studies of water–oil two-phase slug flow inside a two-dimensional vertical microchannel subjected to modulated wall temperature boundary conditions have been discussed in the present paper.Many researchers have contributed their efforts in exploring the characteristics of Taylor flows inside microchannel under constant wall heat flux or isothermal wall conditions.However,there is no study available in the literature which discusses the impact of modulated thermal wall boundary conditions on the heat transfer behavior of slug flows inside microchannels.Hence,to bridge this gap,an effort has been made to understand the heat transfer characteristics of the flow under sinusoidal wall temperature conditions.Initially,a single phase flow and heat transfer study was performed in microchannels,and the results of the fully developed velocity profile and heat transfer rate were validated with benchmark analytical results.Then an optimal selection of the combination of sinusoidal thermal wall boundary conditions has been made for the two-phase slug flow study.Later,the effects of amplitude(0 bεb 0.03)and frequency(0 bωb 750πrad·s-1)of the sinusoidal wall temperature profile on the heat transfer have been studied using the optimal combination of the wall boundary conditions.The results of the numerical study using modulated temperature conditions on channel walls showed a significant improvement in the heat transfer over liquid-only flow by approximately 50%as well as over two-phase flow without wall temperature modulation.The non-dimensional temperature contours obtained for different cases of temperature modulation clearly explain the root cause of such improvement in the heat transfer.Besides,the results based on the hydrodynamics of the flow have also been reported in terms of variation of droplet shapes and film thickness.The influence of Capillary number on the film thickness as well as heat transfer rates has also been discussed.In addition,the measured film thickness has also been compared with that calculated using standard empirical and analytical models available in the literature.The heat transfer rate obtained from the numerical study for the case of unmodulated wall temperature was found to be in a close match with a phenomenological model to evaluate slug flow heat transfer having a mean absolute deviation of 7.56%.

Gravity-capillary waves modulated by linear shear flow in arbitrary water depth

Considering that the fluid is inviscid and incompressible and the flow is irrotational in a fixed frame of reference and using the multiple scale analysis method, we derive a nonlinear Schrodinger equation(NLSE) describing the evolution dynamics of gravity-capillary wavetrains in arbitrary constant depth. The gravity-capillary waves(GCWs) are influenced by a linear shear flow(LSF) which consists of a uniform flow and a shear flow with constant vorticity. The modulational instability(MI) of GCWs with the LSF is analyzed using the NLSE. The MI is effectively modified by the LSF. In infinite depth, there are four asymptotes which are the boundaries between MI and modulational stability(MS) in the instability diagram. In addition, the dimensionless free surface elevation as a function of time for different dimensionless water depth,surface tension, uniform flow and vorticity is exhibited. It is found that the decay of free surface elevation and the steepness of free surface amplitude change over time, which are greatly affected by the water depth, surface tension, uniform flow and vorticity.

Turbulence modulation model for gas–particle flow based on probability density function approach

he paper focuses on the turbulence modulation problem in gas–particle flow with the use of probability density function（PDF） approach. By means of the PDF method, a general statistical moment turbulence modulation model without considering the trajectory difference between two phases is derived from the Navier–Stokes equations. A new turbulence production term induced by the dispersed-phase is analyzed and considered. Furthermore, the trajectory difference between two media is taken into account. Subsequently, a new k–ε turbulence modulation model in dilute particle-laden flow is successfully set up. Then, the changes to several terms, including the turbulence production, dissipation, and diffusion terms, are well described consequently. The promoted model provides a more probable explanation for the modification of particles on the turbulence. Finally, we applied the model to simulate a gas–particle turbulence flow case in a wall jet, and found that the simulation results agree well with the experimental data.

Investigation of a Flow Modulation System for Siphonic Roof Drainage Systems

Siphonic roof drainage systems (SRDS’s) have been widespread used now for approximately 40 years and are an efficient method of removing rainwater rapidly from roofs. SRDS’s are designed to run full-bore, resulting in sub-atmospheric system pressures with high hydraulic driving heads and higher system flow velocities than conventionally guttered systems. Hence, SRDS’s normally require far fewer downpipes, and the depressurised conditions also mean that much of the collection pipework can be routed at a high level, thus reducing the extent of any underground pipework. But, they work properly at only one roof run-off rate and therefore suffer from sizing and operational problems including noise and vibration which limit their performance and adoption rate. Climate change is creating situations where normal ranges of rainfall intensity are being frequently exceeded, so the typical:storm ratios (rTS) are large increasing. Current SRDS’s typically operate within a small rTS range of 2. This may have an impact on the future uptake of SRDS’s. This paper describes the development of a novel SRDS which includes a small mobile cap at the roof of outlet appears to offer benefits and avoids sizing problems associated with current SRDS’s. The cap has the potential to avoid noise associated with making and breaking siphonic action through flow modulation. Laboratory scale tests demonstrate the basic feasibility of the cap system and indicate that the cap functions reliably. This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors. Basic on sizing and design optimiza-tion factors are suggested. The rTS range is increased from approximately 2 to approximately 6.

Transfer characteristics of dynamic biochemical signals in non-reversing pulsatile flows in a shallow Y-shaped microfluidic channel: signal filtering and nonlinear amplitude-frequency modulation

The transports of the dynamic biochemical signals in the non-reversing pulsatile flows in the mixing microchannel of a Y-shaped microfluidic device are ana- lyzed. The results show that the mixing micro-channel acts as a low-pass filter, and the biochemical signals are nonlinearly modulated by the pulsatile flows, which depend on the biochemical signal frequency, the flow signal frequency, and the biochemical signal transporting distance. It is concluded that, the transfer characteristics of the dynamic biochemical signals, which are transported in the time-varying flows, should be carefully considered for better loading biochemical signals on the cells cultured on the bottom of the microfluidic channel.

Turbulent Modulation in Particulate Flow: A Review of Critical Variables

A review of the main mechanisms influencing turbulent modulation in the presence of spherical and non-spherical particles is presented. The review demonstrates the need for more numerical and experimental work with higher accuracy than obtained so far and the need to resolve the flow near the surface of particles with the aim to re-evaluate the quantitative effect of different parameters on turbulent modulation. The review reveals that non-spherical particles have more adverse effect on turbulence as compared to spherical ones, for the same ambient conditions.

A nonlinear Schrodinger equation for gravity waves slowly modulated by linear shear flow

Assume that a fluid is inviscid, incompressible, and irrotational. A nonlinear Schr?dinger equation(NLSE) describing the evolution of gravity waves in finite water depth is derived using the multiple-scale analysis method. The gravity waves are influenced by a linear shear flow, which is composed of a uniform flow and a shear flow with constant vorticity. The modulational instability(MI) of the NLSE is analyzed, and the region of the MI for gravity waves(the necessary condition for existence of freak waves) is identified. In this work, the uniform background flows along or against wave propagation are referred to as down-flow and up-flow, respectively. Uniform up-flow enhances the MI, whereas uniform down-flow reduces it. Positive vorticity enhances the MI, while negative vorticity reduces it. Hence, the influence of positive(negative)vorticity on MI can be balanced out by that of uniform down(up) flow. Furthermore, the Peregrine breather solution of the NLSE is applied to freak waves. Uniform up-flow increases the steepness of the free surface elevation, while uniform down-flow decreases it. Positive vorticity increases the steepness of the free surface elevation, whereas negative vorticity decreases it.

AA3连续流动分析仪驱动马达带动转轴旋转模块设计及土壤中总氮测试

针对连续流动分析仪检测土壤中总氮时遇到的问题,研制了驱动马达带动转轴旋转模块.该模块可使混合的样品、试剂和空气保持在合适的温度区间进行中和反应,以利于后期利用连续流动分析仪在最优化条件下对样品进行检测分析,可将总氮的检出限从16.4µg/L降低至10.0µg/L,从而提高连续流动分析仪对样品的分析精度.

模块知识融合反应工程大作业设计的思考

化学反应工程是高校化学工程专业的重要核心课之一,致力于研究化学动力学并根据反应特点设计合适的反应器。课程教学团队根据多年教学经验,将课程内容分为理想反应器、非理想流动、非均相催化和工业反应器四个模块,并针对非理想流动与非均相催化模块的知识关联较弱而导致学生无法综合运用所学知识解决复杂反应工程问题的现象,设计了基于模块知识融合的课程大作业。本文阐述了课程大作业对加强模块间知识关联的重要作用,梳理了课程大作业的设计和实施要点,并以催化剂氧空位测量的课程大作业为例,介绍如何通过非理想流动和非均相催化模块知识的融合提升学生解决复杂工程问题的能力,最后总结了课程大作业实施过程中的共性问题并提出了改进建议。

叶片背面改型对轴流风机性能和压力脉动特性的影响

以小型轴流风机为研究对象,设计了光滑、凸槽、凹槽3种不同叶片背面形式的风机模型,并采用SST k-ω湍流模型研究其定常性能和非定常压力脉动特性。研究结果表明,3种轴流风机模型的全压-流量特性曲线基本一致,差别不明显,即叶片背面增加凸槽或者开槽对轴流风机的外特性影响较小。此外,叶轮和导叶各个监测点处的压力脉动规律一致,其脉动主频均位于叶频及其倍频处,脉动幅值随着倍频增加而减小。模型model 1由于凸槽叶型对流体的调制作用,叶轮流域内流体分布更加均匀,其压力脉动强度最低,而模型protype 0和model 2的叶片调制作用较弱,其内部流体脉动较为强烈,压力脉动的幅值也较高。综合轴流风机效率和噪声指标,模型model 1不仅能保证风机的全压性能,还能实现较小的脉动,其综合性能最佳。

一种利用ConvGRU解决交通流量预测问题的方法

准确的交通流量预测可协助交通管理部门工作,减少交通压力.但现有方法对交通流时间特征与空间特征学习不充分,将二者割裂讨论,忽视了时间与空间的动态相关性.针对以上问题,本文提出了基于ConvGRU的多时间尺度时空卷积交通流预测方法(MTSTC),设计浅层时空卷积模块对数据中的时空相关性进行初步提取;提出以ConvGRU为核心特征提取器的深层时空卷积模块,对数据的时空特征进行更深层次挖掘;并从3种时间尺度范围的数据中提炼交通流的周期性特征;结合注意力机制设计了时空注意力模块辅助模型训练,提升模型收敛速率.在公开数据集PEMS04和PEMS08上进行实验验证,结果表明采用MAE和RMSE评价指标时,本文方法的准确率相较基线方法在两个数据集上提升了3.23%~5.64%.

基于中空纤维的灌流培养细胞截留技术的研究

近年来由于生物制药行业对于蛋白产品需求的日益增加,连续型细胞培养工艺的灌流培养模式越来越受到重视.为了维持生物反应器中较高的细胞数量和生产力,收获纯度较高的蛋白产品,需要一种良好的分离装置来保留生物反应器中的细胞.本文在比较不同类型的细胞截留技术的基础上,总结了中空纤维过滤技术操作条件温和、通量大、过滤精度高且应用广泛的优势.为了缓解中空纤维过滤过程中存在的膜污染问题,延长膜组件寿命,本文还分析了灌流培养常用的中空纤维膜材质、孔径大小、组件内径、长度及填充密度等设计因素,讨论了不同的切向流过滤操作参数对膜组件的影响,以期为灌流培养工艺领域中细胞分离用中空纤维膜组件的设计与优化提供理论依据.

基于融合特征分布学习与图像重建的异常检测

无监督学习是当前工业产品缺陷检测领域的主流研究方向,目前主要分为基于重建和基于特征的两类方法。前者构建基于内容感知的映射以将异常区域映射为正常区域并通过残差图像来检测缺陷,注重于图像整体的表现。后者利用高层语义特征以实现定位异常,更加注重图像细节呈现。根据两种方法的优缺点,本文提出一种基于特征与重建融合的方法,有效结合两者优点互补其不足并实现统一的端到端的学习与推理。首先训练一个重建模型,然后采用归一化流模型以充分学习输入正常样本的高可能性数据概率分布,使其与重建模型相融合,有效地提高重建模型缺陷检测以及缺陷定位的准确率。在广泛使用的MVTec AD数据集上,提出的融合模型的平均图像级AUROC达到了98.7%,平均像素级AUROC达到了94.2%,特别地,相比单一重建模型提升了3.3%。提出的特征与重建网络融合模型显著提高了重建网络部分对于缺陷定位的不足,使结果更为精确。

混流式水电机组一次调频运行区分级划分方法——以拉西瓦水电站为例

混流式水电机组变水头宽负荷一次调频性能,是构建灵活电力平衡体系、保证频率稳定的重要内核。在水电站稳定性试验前提下,本文提出以一次调频电气特性为基准的运行区分级划分方法。该方法以调节时间、稳定时间和积分电量合格与否为分级划分基准,利用区域面积、区域中心和区域综合性能进行系统评价。以西北地区调频电厂拉西瓦水电站为例,通过与实验数据对比验证数学模型。在确认计算准确性基础上,以调频性能为优化目标,探究管径、转动惯量、积分增益等水机电参数对运行区分级划分的影响规律。最后,总结水机电参数对调频性能与运行区的贡献度占比,指出混流式水电机组变水头宽负荷一次调频运行优化的方向。

全向移动光伏清扫机器人清扫特性分析与优化

针对光伏组件灵活、安全清扫问题,设计一种全向移动式光伏清扫机器人构型,采用四轮差速建模法、轮组电磁制动分析,确定机器人构型的合理性。针对现有移动式清扫方式的局限性,研究新型滚刷除尘、管道吸尘同步清扫方法。通过CFD仿真方法,建立机器人吸尘管道、清扫机构流场仿真模型,完成结构模型的清扫特性分析与结构优化仿真,以及实际清扫效果对比评价试验。试验结果表明:设计的清扫机器人能实现86.5%清扫效率,同时改善了运动制动能力。

动力电池包局部复式模组热均匀性分析

乘用车动力电池包的模组布置方式通常具有单层布置、复式布置和局部复式布置3种不同的形式。局部复式布置结合了其他两种方式的特点,应用较为广泛。然而,通过对电动汽车火灾事故的研究发现,采用局部复式布置的电池包在自燃事故中所占比例较高,这表明该布置方式可能会对电池模组的热均匀性产生不利影响。鉴于此,本文选取了一款采用局部复式布置的电动汽车电池包作为研究对象,建立了电池包三维数值模型,并通过实验数据与仿真结果进行比较,验证了模型的准确性。利用经验证的模型,通过数值计算方法分析了电池包在快充及3种放电速率下的模组温度分布特征,揭示了局部复式布置模组在这些工况下的热均匀性,尤其是局部复式模组中双层模组的温度差异比单层模组更大。此外,探讨了冷却液入口温度和流速分别对模组热均匀性的影响。研究发现:试图降低冷却液入口温度来改善模组热均匀性的效果有限;而增大冷却液入口流速只能在高放电速率工况下减小模组温度差异,在低放电速率工况下的影响不明显。本研究为局部复式模组的电池热管理系统开发和设计提供了有意义的参考。

汽车在环境风洞内流场的数值模拟与实验研究

为研究汽车在环境风洞内的流场特性,建立了环境风洞和整车数值模型,考虑了风洞喷口和收缩段的阻塞效应、边界层抽吸以及实验设施等对汽车流场的干扰效应,对环境风洞内汽车前方、车身和车轮周围、冷却模块以及机舱内的流场进行了数值模拟。对车身表面的静压以及车身周围和车底部的风速等进行测试,通过数值模拟结果与实验结果的对比,表明数值风洞能准确预测汽车在环境风洞内的流场特性。研究结果显示:汽车前端气流的速度分布沿着气流方向发生显著变化,越接近前端冷却模块时风速的均匀性变得越差;汽车底部气流受地面、车底和轮胎旋转等的共同影响呈现规律性的变化,车底风速沿车身纵向先增大后减小。本方法对研究汽车在环境风洞内的热气动性能以及开发数值风洞提供了新的思路和参考。

融合知识图谱的时空多图卷积交通流量预测

现有的交通流量预测方法关注交通信息的时空相关性,未充分考虑外部因素对交通的影响,为此提出融合静态和动态知识图谱的时空多图卷积交通流量预测模型.基于道路交通信息和外部因素,构建城市交通知识图谱和4个不同语义的路网拓扑图,将城市交通知识图谱输入关系演化图卷积神经网络,实现知识嵌入;使用知识融合模块将车流量矩阵与知识嵌入融合;将4个路网拓扑图和融合知识的车流量矩阵输入时空多图卷积模块,提取时空特征,通过全连接层输出交通流量预测值.在杭州交通数据集上评估模型性能,与先进的基线模型对比,所提模型的性能提高了5.76%~10.71%.鲁棒性实验结果表明,所提模型具有较强的抗干扰能力.