并联管组内流量均匀分布的数学模型

为解决并联管组中流体流量分布不均的问题,提出改变集管内局部阻力配置的方法,建立了基于动量衡算法的流量均布离散化数学模型,利用解析法求解得到阻力配置的通用公式,并根据公式计算所得的加阻位置和阻力系数在太阳能集热器阵列上进行试验验证.结果表明:即使存在由于简化假设和参数近似拟合而导致的诸多误差,增加恰当的阻力元件仍可使流量分布不均匀度下降一个数量级,最大流量与最小流量之比从约等于7降至1.5以下,流量分布改善效果明显.     

集管系统压力与流量分布的研究--(Ⅰ)U型布置时的分析解

根据基本的质量和动量守恒定律建立Ｕ型布置时集管系统压力与流量分布的数学解析模型。将微分方程式无量纲化并从中导出一个综合反映集管系统流量分布状态的性能参数β。对于联箱阻力远远低于支管阻力的情况,得出相应于不同β的分析解。     

集管系统压力与流量分布的研究(II)——Z型布置时的分析解

建立了Z型布置时集管系统压力与流量分布的数学模型 ,在对微分方程进行无量纲化的过程中导出了一个重要的综合性无量纲性能参数。方程解的结果表明利用这个综合参数可以快速确定在联箱摩擦阻降相对很小条件下集管系统的流量分布状态和总压降 ;对集管系统采用Z型与U型布置导致的流量分布特性的差异作了分析 ;解的结果对压力恢复型集管系统的设计和性能预测具有指导意义     

太阳热水系统等温流动特性的研究

太阳热水器或热水系统是一个多支管并联管组。本文根据流体力学的基本原理,建立了描述热水系统等温流动的离散型数学模型,用计算机算得了集箱静压分布和支管流量分布的数值结果,并初步讨论了热水器结构尺寸对流动分布的影响,对四片集热器组成的热水系统进行了试验测定,发现流量分布是不均匀的:逆流布置时支管流量分布极不均匀,有短路现象;顺流布置时流量分布虽比逆流布置有所改善,但系统阻力增大。     

集管系统压力与流量分布的研究（II）—Z型布置时的分析解

建立了Z型布置时集管系统压力与流量分布的数学模型,在对微分方程进行了无量纲化的过程中导出了一个重要的综合性无量纲性能参数,方程解的结果表明利用这个综合参数可以快速确定在联箱摩擦阻降相对很小条件下集管系统的流量分布状态和总压降,对集管系统采用Z型与U型布置导致的流量分布特性的差异作了分析,解的结果对压力恢复型复集系统的设计和性能预测具有指导意义。     

柴油机壁流式过滤体内的流动分析及模拟计算

对柴油机壁流式过滤体内的气体流动进行了分析，建立了壁流式过滤体阻力特性的计算模型，对过滤体内的阻力特性和微粒捕集器内部的气流运动进行了模拟计算。试验和计算结果均表明，在较小流量时，壁流式过滤体的阻力与流量近似呈线性关系，在较大流量时，两者之间为二次曲线关系；微粒捕集器的扩张角越小，越有利于过滤体内流速和压力的均匀分布。     

并联管组流量分配的离散型算法及其应用

建立了锅炉过热器、再热器U、Z形布置联箱的流量分配离散化计算模型,确立了在并联管组中迭代计算各根管子流量的方法。将离散型算法应用于某厂包墙过热器爆管事故的分析和解决工程实践中,对锅炉包覆系统的蒸汽阻力、流量偏差、侧包墙出口联箱静压分布进行了校核计算,得出了比较精确的流量分布。根据理论计算和论证后确定的改造方案取得了良好效果。实践表明离散型算法与传统计算标准算法相比,更符合并联管组流动特性,具有更好的精确性,对解决相关的工程问题具有一定的指导意义。     

三通集箱过热器压力分布与流量分配的数值模拟

为了解三通结构对过热器流量分配的影响从而解决三通集箱过热器的爆管问题,利用Fluent软件对过热器模型进行了数值模拟,研究了三通结构集箱过热器内的静压分布、速度分布、各支管流量分配及各支管入口处流体的流速分布和静压分布.结果表明:三通结构附近流体的压力低且速度大,而远离该区域的流体则压力高且速度小;三通中涡流区域的支管入口有小涡流,而其他区域的支管入口没有出现小涡流;集箱内涡流下部的支管流量偏小,正对集箱入口处的支管流量最大;将第7管屏和第11管屏的入口形状改为圆形,改造后支管的流量明显增大.     

电站锅炉三通集箱流场的数值研究

根据电站锅炉三通集箱系统的结构特点,采用基于三维实体模型的建模方案,对径向引入引出集箱系统的流场分布进行了数值模拟。采用FLUENT软件分析了集箱流场的分布特点及受热面沿程阻力对集箱速度、静压分布的影响,详细讨论了三通结构、受热面沿程阻力对受热面流量分配的影响。研究结果表明,集箱流场呈三维分布,分为径向引入管下游的冲击射流区和周围的强迫对流区,三通结构直接影响集箱的速度分布;受热面的流量分配直接受三通结构的影响,沿程阻力影响受热面流量偏差的大小。     

并联管组流体特性分析及均流模型设计

对某采油厂太阳能集热系统并联管组进行实地调研,基于流体力学基本原理和FLUENT软件对该并联管组进行理论分析和仿真,提出通过增大集管直径可提高并联管组的流量分布均匀性。将集管直径改变前后并联管组内流体的压力与速度分布、支管流量进行对比,计算得出该并联管组的最大流量偏差系数Δη从33.57%减小到1.43%,为今后并联管组的设计提供一种方法。     

Flow distribution in an eight level channel system

In this paper, a computational model was developed to predict and analyze flow behavior in an eight level channel system in a parallel configuration between two headers. The combined effects of non-uniform heat flux distribution within the channel levels and in the channel system were accounted for. Simulations with a five channels system were also done to prove the effectiveness of the proposed approach for flow distribution prediction. The results achieved clearly demonstrate the effect of non-uniform heat load on the flow distribution in a multi-channel system. Also, results from uniform and non-uniform channel heat flux were compared.     

Modeling of flow field in polymer electrolyte membrane fuel cell

Isothermal two- and three-dimensional polymer electrolyte membrane (PEM) fuel cell cathode flow field models were implemented to study the behavior of reactant and reaction product gas flow in a parallel channel flow field. The focus was on the flow distribution across the channels and the total pressure drop across the flow field. The effect of the density and viscosity variation in the gas resulting from the composition change due to cell reactions was studied and the models were solved with governing equations based on three different approximations. The focus was on showing how a uniform flow profile can be achieved by improving an existing channel design. The modeling results were verified experimentally. A close to uniform flow distribution was achieved in a parallel channel system.     

Unsteady forced convection heat/mass transfer around two spheres in tandem at low Reynolds numbers

This paper presents a computational study of the forced convection heat/mass transfer from two spheres placed in a uniform viscous flow parallel to their line of centers. The temperature/concentration inside the spheres are assumed spatially uniform but not constant in time. Axisymmetric, slow, viscous flow (Stokes flow) around the spheres was considered. The appropriate energy/chemical species balance equations were solved numerically in bispherical coordinates. The finite difference method was used to discretize the mathematical model equations. Various spheres spacing, sizes and physical properties were considered at moderate Pe numbers.     

ISOTHERMAL FLOW DISTRIBUTION IN HEADER SYSTEMS

Flow distribution in header systems is studied both numerically and experimentally in the present work. A discrete numerical model describing the flow behavior is formulated in terms of mass and momentum balance equations at every segment of the header system. The numerical results show that the header systems can be categorized as pressure regain type and pressure decrease type according to the static pressure distribution along the dividing manifold. A header system may transfer from pressure regain type to pressure decrease type while the resistance of the dividing manifold increases. The flow distribution of pressure regain header system is much more uniform than that of pressure decrease header system. It is found that the numerical results obtained by the present model are in good agreement with the experimental results for both types of header systems.

A parametric study on the performances of header systems is also presented in this paper.     

MASS FLOW RATE DISTRIBUTION AND PHASE SEPARATION OF R-22 IN MULTI-MICROCHANNEL TUBES UNDER ADIABATIC CONDITION

The present study investigated mass flow rate distribution and phase separation of R-22 in multi-microchannel tubes under adiabatic condition. The test section consisted of inlet and outlet headers with the inner diameter of 19.4 mm and 15 parallel multi-microchannel tubes. Each microchannel tube had 8 rectangular ports with hydraulic diameter of 1.32 mm. The key experimental parameters were the orientation of the header (horizontal and vertical), flow direction of refrigerant into the inlet header (in-line, parallel and cross flow), and inlet quality (0.1, 0.2, and 0.3). The effect of inlet quality on the mass flow rate distribution and phase separation in the microchannel tubes was negligible. The effect of the orientation of the header on the mass flow rate distribution and phase separation was the largest among the test parameters. Horizontal header showed better mass flow rate distribution and phase separation characteristics than vertical header. Both parallel and cross-flow conditions showed better mass flow rate distribution and phase separation than in-line flow condition.     

Analysis and optimization of flow distribution in parallel-channel configurations for proton exchange membrane fuel cells

Parallel channels have many advantages, such as low pressure drop and easy fabrication, but they may cause flow maldistribution which would result in low reaction efficiency. This study presents an analytical model to calculate the flow distribution of the parallel channels based on the assumption of the analogy between fluid flow and electrical network. The model, which ultimately releases from the solution of a set of nonlinear equations, is validated by comparing with the results obtained from three-dimensional computational fluid dynamics (CFD) simulations. Consequently, the model is used to optimize the geometric dimension of a parallel plate to obtain a uniform flow field distribution.     

Modeling and analysis of flow distribution in an A-type microchannel reactor

Flow distribution among microchannels is a fatal factor affecting the performance of laminated microchannel reactors for hydrogen production. Homogeneous flow strongly depends on the structural design of the microchannel reactor. The present work concentrates on improving the flow distribution in microchannel reactors for hydrogen production by optimization of the structural design. An innovative A-type microchannel reactor for hydrogen production with one inlet/two outlets was developed and analyzed. The equivalent electrical resistance network model was used to calculate the flow distribution in the microchannel reactor which was validated by computational fluid dynamics (CFD). The influences of structural parameters on flow distribution in the A-type were investigated quantitatively. The calculated results showed that longer microchannels with a higher aspect ratio and a small side length in the manifolds were beneficial for attaining uniform flow distribution in the A-type microchannel reactor. Furthermore, it was found that flow distributions among the microchannels in the A-type were much more uniform than those in the conventional Z-type microchannel reactor with one inlet/one outlet. Finally, an optimization strategy was proposed to optimize the manifold geometries to obtain a comparatively even flow distribution among microchannels.     

凝汽器喉部蒸汽流动的三维数值模拟

应用具有超粒子模型的直接模拟蒙特卡罗方法,对汽轮机凝汽器喉部进行区域分解和数学建模,及对具有典型结构的凝汽器喉部蒸汽流动进行了三维数值模拟,重点分析了凝汽器喉部流场的流动分布情况,揭示了喉部流场不均匀性流动特点及其产生原因。     

Hydrodynamics in a circulating fluidized bed with annular furnace and six parallel cyclones

Systematic measurements were conducted on a cold CFB with annular furnace and six parallel cyclones to study gas-solids flow in the annular furnace and flow non-uniformity among six cyclones.The results show that axial solids holdup in the annular furnace decreases exponentially with height,similar to the conventional rectangular furnace.The uniform transverse distribution of solids holdup suggests a good gas-solids mixing in the annular furnace.The annular furnace presents the core/double-annulus flow structure,and it results in enhanced gas-solids back-mixing than the conventional core/annulus flow structure.The gas-solids flow of the inner wall-layer and the outer wall-layer is very close at most part of the furnace height,and the wall-layer thickness decreases with height.Flow non-uniformity exists among six parallel cyclones in the annular furnace CFB.But non-uniform distribution of solids circulating rates and cyclone pressure drops show no regularity,and the flow non-uniformity is no larger than the CFBs with conventional furnace.Under typical operating conditions,the relative deviation of six solids circulating rates is 8.0％.     

Network Analysis of Nonuniformly Heated Evaporative Microchannel Systems

A network analysis was established to model the array of evaporative microchannels with possible nonuniformity heating as well as branching of the channels. Iterative solution of the evaporative microchannel network can be obtained using the Hardy–Cross method together with accurate two-phase head-loss correlations. Based on the experimental evidence, cross-cutting microchannels reduce the uneven flow distribution in parallel microchannels at nonuniform heating. Through this network analysis, it is also evident that cross-cutting grooves on parallel microchannels are effective in reducing nonuniform heating effects and enhancing the uniform wall temperature distribution.