入口流速对间断微通道内流型演变及流量分配特性影响

微通道内气液两相流动规律是影响微通道换热器换热系数和流场温度均匀性的主要因素。以N2和H2O为工质，对间断、并联矩形微通道换热器内气液两相流型的起始、发展、稳定过程的演化以及并联通道内流量分配不均匀特性进行了数值模拟研究。结果表明，不同的进口Re对微通道内流型的演变过程和流动周期有重要影响，当进口Re为450时，气相工质在均流腔内以离散的散团状形态脉动扩散至微通道中，并联通道内气相工质从弹状流型态逐渐转变为泡状流型态；当进口Re增至为1600时，气相工质在均流腔内以较连续的椭圆状型态扩散至微通道中，并联通道内气相工质从环状流型态逐渐转变为泡状流型态。通道结构还将影响并联通道间的流量分配的均匀性，间断微腔的存在使微通道内工质质量流量分布均匀性提升38.7%，通过研究通道内压力分布规律，发现通道内静压的分布不均匀是导致两相工质从均流腔进入微通道时发生不均匀分配的重要原因。     

CFD study on the flow distribution of an annular multi-hole nozzle

Annular multi-hole nozzles are commonly used to disperse one or more types of material in chemical processing, agricultural irrigation, firefighting, and so on. In this study, the LES (large eddy simulation) method combined with the VOF (volume of fluid) model was employed to investigate the impacts of inlet volume flow rate, hole diameter, and distance downstream of nozzle on the flow distribution of the nozzle. To quantify the uniformity of the flow distribution, the non-uniformity parameter, wetted area, and uniformity index were introduced. The sprays ejected from the nozzle are separated into two modes in accordance with their structure, namely partially impinging sprays (PIS) and fully impinging sprays (FIS). Compared with the three other types of nozzles investigated in this study, the nozzles with a 400 μm hole diameter can achieve the most uniform flow distribution of the nozzle outlet. In addition, the nozzle with larger holes and a higher inlet volume flow rate can easily achieve a large wetted area. However, the nozzle with smaller holes and a relatively low inlet volume flow rate is more inclined to achieve a uniform flow distribution. The larger the inlet volume flow rate, the larger the holes that should be adopted to achieve a relatively uniform flow distribution.     

气体入口角度和截面宽高比对微通道内泰勒气泡行为的影响

微通道的几何参数对于气泡或者液滴在其中的形成过程有着显著的影响。采用流体体积法(VOF法)研究了不同气体入口角度以及不同通道截面宽高比对微通道内气液两相流流动状况的影响。在所研究的操作范围内,各个微通道内的两相流流型均为泰勒流,并且在大多数情况下气泡长度分布均匀。在通道截面宽高比为0.5～2条件下,60°的气体入口角度有利于产生较短的气泡;如果通道截面宽高比达到4或8时,45°的气体入口角度更有利于形成较短的气泡。此外,随着通道截面宽高比的增大,通道内气泡的量纲1长度也随之增大,气泡长度分布的均匀性也逐渐变差。当通道截面宽高比增大到8时气泡长度分布变得很不均匀。     

Design of an Efficient Gas Distribution System for a Fluidized Bed Dryer

Fluid bed dryers are commonly used to process granular solids. The design of the gas distributor has a significant impact on the performance of heat and mass transfer with or without chemical reactions in fluidized beds because it affects the quality of the fluidization obtained. In this work, an extensive study was carried out to design an optimal gas distribution system for a fluidized bed dryer. The air distribution chamber, also called a plenum chamber or gas chamber, was modified to obtain uniform air distribution across the bed cross section. Percentage maldistribution of the flow is considered as the basic evaluation parameter to quantify uniformity of the fluidizing gas distribution. Effects of various relevant design parameters such as the ratio of the orifice diameter to plate thickness (d_o/t), percentage free area, superficial gas velocity, etc., were examined experimentally and via modeling. The gas chamber was redesigned by inserting different types of packings in the chamber. In addition, the effect of height-to-diameter ratio (H/D) of the chamber on flow distribution was studied. Chambers of different H/D ratios and different air inlet nozzle diameters at various positions were examined to evaluate their effect on flow uniformity across the distributor. Three-dimensional computational fluid dynamic studies were carried out to evaluate the effects of pertinent parameters on flow uniformity, which has a direct bearing on the quality of fluidization and hence heat and mass transfer rates obtained.     

高雷诺数下变截面分流歧管面积比对流体分配性能的影响

在高雷诺数(Re=5×105～30×105)下,对不同主管形状和面积比的分流歧管流体分配性能进行数值模拟.通过计算各支管出流流量比(Q/Qave)与流量分配均匀性指数(U),分析Re和主管形状以及面积比(AR)对分流歧管流量分配性能的影响.结果表明,与低Re下的分流歧管流量分配性能高度依赖于Re的规律不同,高Re(5×...     

流化床管式分布器内流场模拟和布气性能分析

以催化裂化装置（FCCU）再生器的管式气体分布器为研究对象,对流化床管式气体分布器的布气性能进行了分析。首先,对气体分布器分支管内的流场进行数值模拟,计算结果表明沿分支管内气体流动方向,压力逐渐增大,截面流量逐渐减少,沿程喷嘴流量逐渐增大;同时分支管上游入口还存在着明显的偏流现象,从而导致了上游喷嘴的出口流量小于设计流量,下游喷嘴的出口流量高于设计流量,造成流化床内非均匀布气。然后,依据分支管的变质量流动特点,将一般变质量流动的动量方程用于分析分支管内的流动过程,表明分支管的流动过程属于“动量交换控制模型”,具有始端静压低末端静压高的特点,固有压力分布不均匀的特征。这种不均匀的压力分布导致了喷嘴布气不均匀和磨损等系列问题。最后,结合流化床内的压力特点,综合分析气体分布器的分支管压降和喷嘴压降,明确了喷嘴出口流量与分支管压力分布的关系,喷嘴临界压降与设计压降的关系,结论表明分支管的结构改进可以优化和改善分布器的布气性能。     

Optimal design of heat exchanger header for coal gasification in supercritical water through CFD simulations

Heat exchangers play an important role in supercritical water coal gasification systems for heating feed and cooling products. However, serious deposition and plugging problems always exist in heat exchangers. CFD modeling was used to simulate the transport characteristics of solid particles in supercritical water through the shel and tube of heat exchangers to al eviate the problems. In this paper, we discuss seven types of exchangers (A, B, C, D, E, F and G), which vary in inlet nozzle configuration, header height, inlet pipe diameter and tube pass distribution. In the modeling, the possibility of deposition in the header was evaluated by accumulated mass of particles;we used the velocity contour of supercritical water (SCW) to evaluate the uniformity of the velocity dis-tribution among the tube passes. Simulation results indicated that the optimum heat exchanger had structure F, which had a rectangular configuration of tube pass distractions, a bottom inlet, a 200-mm header height and a 10-mm inlet pipe diameter.     

CFD Simulation of Flow Distribution in the Header of Plate‐Fin Heat Exchangers

The flow distribution through a plate‐fin heat exchanger is studied by using a computational fluid dynamics (CFD) code, FLUENT. The flow distribution through any heat exchanger affects its performance. In designing a heat exchanger, it is assumed that the fluid is uniformly distributed through the heat exchanger core. In practice, however, it is impossible to distribute fluid uniformly, because of an improper inlet configuration, imperfect design, and a complex heat transfer process. The CFD simulation of the flow distribution in the header of a conventional plate‐fin heat exchanger is presented. It is found that the flow maldistribution is very serious in the y‐direction of the header. A modified header is proposed and simulated using CFD. The modified header configuration has a more uniform flow distribution than the conventional header configuration. Hence, the efficiency of the modified heat exchanger is seen to be higher than that of the conventional heat exchanger.     

Refrigerant Droplet Size Measurements in Conjunction with a Novel Method for Improving Flow Distribution in Evaporators

Maldistribution of two-phase flow in evaporators is known to significantly affect their performance. To reduce refrigerant maldistribution, a pressure-swirl atomizer is used to distribute R-134a in the header of a typical evaporator. A phase-doppler particle analyzer is used to characterize the spray along the centerline of the header for 1, 2, and 3 g/s total mass flow rate. The thermodynamic quality is varied in the header from 0% to 15%. The centerline droplet size is found to decrease with increasing quality and to decrease with increasing distance downstream of the nozzle. Flow distribution is found to be more uniform when the atomizer is used as the expansion device when compared to a typical expansion valve and pipe inlet.     

封头结构对板翅式换热器物流分配不均匀性的影响

纵向传热、温度场与物流分配不均匀对换热器效能的影响具有耦合关系,其中物流分配的不均匀会加剧其内部温度场分布的不均匀,增大换热器的纵向传热,导致其效能的下降,所以物流分配的不均匀在三者中对换热器效能的影响最为重要.因此,对引起物流分配不均匀的因素进行研究是板翅式换热器优化设计的重要内容.针对板翅式换热器内部流动特性对封头结构的影响进行了深入研究.研究发现：封头结构的影响主要体现在垂直于总管流动方向上.发现了封头结构的改进方法,通过改变封头结构的设计可以有效地改善换热器内部的物流分配.     

陶瓷过滤器脉冲反吹用喷嘴优化设计

The experimental study was carried out to optimize the nozzle shape and dimension for the pulse cleaning of a ceramic filter candle. A bench scale unit of ceramic filter consisting of four commercial filter elements was used to measure the traces of the transient pressure around the nozzle and the overpressure in the filter cavity during the pulse-jet injection of pulse gas. Overpressure in the filter cavity is related to the pulse cleaning force. Nozzle design is concerned to increase the overpressure at the open end of filter element of pulse cleaning inlet, as well as to minimize the consumption of pulse gas. Convergent nozzle induces more secondary flow and generates higher pulse cleaning effect than straight nozzle. Nozzles of different convergent ratio （ratio of outlet to inlet diameter of nozzle） by changing the convergent angle and height were tested. The outlet diameter of convergent nozzle seriously influences the cleaning effect. The optimum convergent ratio increases with the increase of pulse gas pressure The nozzle position （distance of nozzle tip from the open end of filter inlet） is also important to decide the nozzle dimension. Nozzle of large outlet diameter accepts high pressure of pulse gas to provide large overpressure in the filter cavity of top position by applying long distance.     

Numerical Analysis of Header Structure Improvement in a Plate‐Fin Heat Exchanger

In order to improve the flow uniformity in a plate‐fin heat exchanger (PFHE), a modified header structure with a plain baffle inside is proposed. Flow distribution and pressure drop in the header were studied by numerical methods. The results show that the modified header is highly efficient in improving the flow uniformity with negligible pressure drop increase. A performance effectiveness factor was introduced to predict the effects of the modified structure on the performance of the PFHE. Influences of baffle structure parameters on modified structure performances were further discussed. The modified header structure can effectively improve the performance of a PFHE.     

板翅式换热器封头结构物流分配的PIV实验研究

在数值模拟的基础上,利用粒子图像测速仪(PIV)对板翅式换热器原始封头A和3种改进的顺排孔板型封头结构(B、C、D)的流场进行了可视化的实验,获得了封头内部不同截面处流场的速度流线分布图.比较了Re＝6.22×10⁴时原始封头A和一种改进型封头B的出口物流分配以及3种不同中孔面积比率的改进型封头对出口物流分配的影响.结果表明：改进型封头结构能有效地改善物流分配的均匀性;C型封头的不均匀参数S_U和最大流速与最小流速之比θ_U最低;流量分布和中孔面积比率之间存在着一个最佳值.     

平面防冲挡板式分离器冲击区非稳态湍流特性

为了获得较好的液液重力分离器沉降区流场,需要对初始进入的流体进行整流,使其进入分离区前流动平稳均匀。为此将平面防冲挡板改进为整高度矩形挡板,并采用VOF多相流模型对矩形截面分离器冲击区流动特性进行分析。考察了冲击区的喷嘴入口直径(d)、入口至防冲挡板间距(lb)、防冲挡板的宽度(w0)对挡板处和折流板处的速度相对不均匀度(Mf)及平均湍流强度(Iave)的影响。结果表明,综合考虑速度相对不均匀度Mf和平均湍流强度Iave,入口直径取d/w≥0.16较有利(w为沉降器宽度的二分之一);防冲挡板至入口间距的较佳取值范围为lb/w=0.33~0.53;防冲挡板宽度w0/w≥0.27时效果较好。对折流板处均匀度与湍流强度分别归一化后的相关数值进行统计分析,在分析加权前后的总区间变化率时应优先考虑d/w区间。在d/w区间挡板处与折流板处的湍流强度与均匀度都有较好的线性相关性,随着均匀度增加,湍流强度增加。     

CFD Simulation of Gas Distribution Performance of Gas Inlet Systems in Packed Columns

Packed columns are widely used in the chemical process industries. The optimum operation of these columns requires an even distribution of gas and liquid flows. This paper describes a method for modeling the flow pattern which develops above the gas inlet system using a computational fluid dynamics (CFD) approach. The uniformity of the gas flow through the packing is assessed by means of a maldistribution factor, M_F. Several factors which affect gas distribution, such as gas inlet type, gas inlet diameter and the distance between gas inlet and column bottom, were analyzed. It was found that gas distribution is more uniform as the inlet diameter and bottom distance are increased. Comparison of experimental data with a CFD simulation for several types of gas inlets, such as straight, slope and bend inlets, shows good agreement.     

进风角度对椭圆管翅式换热器传热性能影响

对两排椭圆管翅式换热器实验元件在不同进风角度下（30°、45°、60°和90°）的换热性能进行了实验研究,结果表明,在测试的迎风速度范围内随着进风角度的减小换热器换热性能减弱,并给出了测试工况范围内的换热性能经验关联式;对不同进风角度下空气侧换热性能进行了数值计算,与实验结果进行对比,符合良好;最后对不同进风角度下换热器内不同通道内空气平均速度的分布进行了研究,解释了换热性能差异的原因,为相应的工程应用提供理论参考。     

急冷锅炉入口联箱与扁圆管结构优化设计

采用ANSYS软件对急冷锅炉换热单元结构进行数值模拟,得到热负荷均匀情况下各炉管流量分配规律。根据辅助连接管的直径及位置提出六种结构方案,研究其流量分配规律,并获得了最优结构。采用数值模拟研究最优结构入口联箱和扁圆管的速度场、压力场分布规律,得出压力分布和速度分布均较原结构均匀,从而验证了最优结构结论的正确性。     

Research on rotary nozzle structure and flow field of the spinneret for centrifugal spinning

High-speed centrifugal spinning is a novel method of fabricating nanofibers by use of centrifugal force field. The neoteric designs and proper structural parameters of spinning nozzle will change the outlet velocity and velocity distribution uniformity of spinning solution, which could affect the quality of nanofibers. Four different spinning nozzles that can be used for high-speed centrifugal spinning are proposed. The flow of spinning solution in the different rotary nozzles is simulated based on the theoretical analysis. It can be found that the curved-tube nozzle is better for high-speed centrifugal spinning comparing with the velocity distribution of spinning solution in the four different spinning nozzles. The influence degree of the structure parameters of the curved-tube nozzle on the solution outlet-velocity is explored by orthogonal test method. The result shows that the curved-tube nozzle with an inlet diameter of 10 mm, conical transition pipe length of 3 mm, nozzle outlet diameter of 0.8 mm, curvature radius of 4 mm, and central angle of curved tube of 30°, can effectively enhance the outlet velocity of spinning solution. Finally, it is proved by centrifugal spinning experiment that the curved-tube nozzle after optimization can improve the morphology and quality of nanofibers.     

空心喷嘴喷淋特性实验研究

为使大型水平管降膜多效蒸发海水淡化装置的横管降膜蒸发器均匀布液,自主设计并搭建了一套离心喷嘴喷淋特性实验台及径向喷淋密度测量装置,通过高速摄影仪拍摄的喷淋照片对喷淋外缘进行标注测量,得到喷嘴各喷淋锥角,对旋流式空心喷嘴的流量、喷淋锥角和径向喷淋密度等喷淋参数随入口压力、喷淋高度的变化规律进行实验探究分析。结果表明,流量随入口压力增加而增加,但入口压力越大,流量增长速度越缓慢;正常工况下,喷淋锥角由喷嘴出口扩张段角度决定,与喷嘴扩张角保持一致,在重力作用下出现向内的收缩圆弧液膜边;喷淋形状为规则的环形喷淋,入口压力增加使有效喷淋区域整体向中心压缩,喷淋密度峰值变大,两波峰的对称性得到改善,压力为349 kPa时,喷淋密度基本完全对称,增加喷淋高度则情况相反。根据此喷淋密度分布特点,在设计横管蒸发布液器时合理控制工况压强,根据峰谷叠加原理可有效消除无效喷淋区域。     

封头结构对板翅式换热器温度分布特性的影响

在对板翅式换热器内部物流分配不均匀性研究的基础上,研究了各种封头结构对换热器换热性能的影响以及由于换热器内部物流分配不均匀引起换热器效能下降的问题.实验结果表明：不同封头结构对换热器流量分配和温度分布影响很大,改进型的封头结构不仅缩短了换热器从开启到稳定工况的时间,而且使得板束内部截面物流分配和温度分布更加均匀,并且通过实验研究得出了物流分配不均匀度与换热器整体效能变化的关系曲线和关系式.