板壳式换热器波纹倾角对换热和阻力性能影响

研究了波纹倾角（α=45°，60°，75°）对板壳式换热器单流道内单相流动与换热过程的影响，分析了波纹流道内的速度分布、湍动能分布、压力分布和温度分布；基于范宁摩擦阻力因子f和Nu与Re的关系，提出了板壳式换热器不同波纹倾角下换热特性和流动阻力特性的预测关联式；通过计算不同波纹倾角下的综合性能评价指标（performance evaluation criteria,PEC）和面积质量因子（j/f）综合评价了流动与换热性能。结果表明：波纹倾角是影响圆形板间流体流动形态的因素之一，随着波纹倾角增大，会出现“十字交叉流”向“曲折流”转变；所提出的关联式能够很好地预测板壳式换热器内阻力与换热性能，阻力特性偏差在±14%范围内，换热特性偏差在±7%范围内；α为45°时，j/f较大，流道内阻力相对小；α为60°时，PEC较大，流道内换热性能相对强。     

太阳能热电站容积换热器的数值研究

太阳能热电站容积换热器由以适当方式布置在封闭腔体内的圆柱阵列组成。其内部流动的复杂性增加了实验测量和数值场模拟的难度。本文采用连续模型对换热器内部的换热及压降进行了数值模拟。结果与实验数据相符良好     

小管径管翅式换热器气压胀接成形机理研究

管翅式换热器是制冷行业中最常用的换热器形式,其换热管的胀接性能决定了换热器的传热性能.本文提出了管翅式换热器的三维流-固耦合模型,采用单向流固耦合瞬态数值模拟方法,对小管径管翅式换热器的流体和固体域的流动和变形特征开展了数值研究.计算结果表明:根据换热管和翅片的胀接成形要求和胀后管径要求,气压胀接压力的合理范围为P=12.5 MPa,与理论公式推导值一致.根据管翅应力随时间变化的规律可知,换热管接头处应力远大于其屈服极限66 MPa,翅片接头处应力刚好略大于其屈服极限132 MPa,满足胀接成形要求.胀后的换热管直径随着压力的增加其管径增大,换热管的径向位移在水平方向较小,垂直方向较大,其最大和最小位移差约为0.03 mm.探究了管翅间残余接触压力随胀接压力的变化,残余接触压力随胀接压力的变化可分为三个阶段.结果表明当胀接压力使得翅片内孔发生屈服后,继续增大胀接压力会导致胀接不完全.最后研究了保压时间的影响,结果表明保压时间的增加对胀接效果并没有明显影响.相关结果可为工程实际中小管径管翅式换热器气压胀接工艺提供理论指导.     

波浪与新型双排开孔圆筒防波堤相互作用三维数值模拟

双排开孔圆筒防波堤是基于圆筒、板式结构的一种复合式新型结构型式;基于不可压缩两相流模型建立三维数值波浪水槽,通过RNG k-ε湍流模型进行湍流封闭,并采用TruVOF方法捕捉自由液面,开展波浪与双排开孔圆筒防波堤相互作用数值模拟,探究相对排间距、开孔率对新型双排开孔圆筒防波堤消浪性能的影响,分析了后排开孔圆筒防波堤附近的复杂水动力现象和流动特性.结果表明,在本文研究工况范围内,沿程平均波高随相对排间距的增大先增大后减小,随开孔率的增大而增大,周期对沿程平均波高的影响没有明显规律;当B/D=9, e=23.11%时,新型双排开孔圆筒防波堤消浪效果最优,反射系数在0.4～0.46之间,透射系数在0.3～0.35之间,耗散系数在0.8～0.85之间;自由液面破碎、水气掺混、环状涡运动演化是新型双排开孔圆筒防波堤紊动耗能消波的主要原因;相对排间距会引起后排防波堤附近涡量分布以及剪切层形态的变化,从而导致不同的紊动特性,影响双排开孔圆筒防波堤消浪特性.研究结果可以为新型双排开孔圆筒防波堤工程设计与消浪机理研究提供理论支撑.     

压缩载荷作用下复合材料开孔补强稳定性分析

运用有限元软件MSC.Nastran分别分析了压缩载荷作用下的复合材料板在无孔、开孔及开孔补强后的稳定性问题.在开孔复合材料板稳定性分析中,讨论了开孔孔径和开孔位置的影响;在补强中,分别选取不同开孔孔径和开孔位置研究补强参数对板稳定性的影响.研究表明:开孔降低了复合材料板的稳定性,通过补强可提高其稳定性;开孔孔径和开孔位置对复合材料板稳定性的影响较大,补强参数随开孔孔径和开孔位置改变而改变,且对复合材料板的稳定性有所影响;补强后板的稳定性要好于无孔复合材料板的稳定性.     

跨超声速风洞扩散段流动分离控制设计参数研究

为抑制跨超声速风洞扩散段的分离,提出了一种较为完备的设计方法。由于影响扩散段性能的参数较多,完全通过试验方法进行设计的成本过高,该方法通过数值模拟,结合适当的边界条件,详细描述了扩散段角度、分流锥角度与长度、孔板开孔率对扩散段性能的影响;从数值模拟的结果可以看出,孔板开孔率和扩开角对扩散段性能有显著影响,通过比较得出较为合理的参数匹配,提高了扩散段的防分离性能,并改善了出口气流质量。数值结果与试验结果结论一致,表明本文所用的方法用于扩散段气动设计是可行的,为数值模拟方法应用于风洞部段气动设计创造了一定的条件。     

加肋开孔柱壳混合有限条——有限元屈曲分析

本文提出了一种分析环加肋开孔柱壳屈曲问题的混合有限条──有限元法。环加肋柱壳作为一个构造上的正交各向异性壳处理，柱壳非开孔区采用有限条元离散，开孔区采用有限单元离散。在有限条元与有限单元交界面上，根据位移协调条件建立条元和单元的耦合关系，并据此构造一种特殊的过渡单元、联接条元和单元，进行整体分析。算例表明，这一方法对开孔柱壳屈曲问题的分折十分有效。     

考虑损伤效应的开孔浅球壳的非线性动力响应与动力屈曲

研究损伤对开孔浅球壳非线性动力响应与动力屈曲的影响.基于Talreja张量内变量损伤模型,建立了纤维增强复合材料板壳弯曲问题的损伤本构关系,导出了考虑损伤效应的具轴对称变形正交各向异性开孔浅球壳的非线性运动控制方程.对未知函数在空间域采用正交点配置法离散,时间域采用Newm ark-β方法离散.数值结果表明,由于损伤导致结构刚度不断削弱,结构振幅增大而频率减小,结构的动力临界屈曲载荷降低.     

球形爆炸容器开孔补强方法

参考压力容器设计标准,针对某实验用球形爆炸容器进行了开孔补强设计,对容器开孔处等效应变随接管壁厚、补强圈尺寸的变化规律进行了数值计算,确定了“5/3倍球壳厚度的接管”配合“与球壳等厚度、张角10°的补强圈”的开孔补强设计方案。并对数值计算结果和补强设计进行了实验验证。     

圆柱壳开孔的应力分析

本文研究圆柱壳开孔附近的应力分析问题。由于开孔问题的边界形状较为复杂,使用圆柱壳的 Morley 方程进行求解较为困难。本文在进行误差分析的基础上提出修正 Mo-rley 方程,并给出适合于开孔问题边界的一般解。以圆柱壳受轴向拉伸和内压为例分析了开孔附近的应力分布。给出了开孔率为 1/2以下的开孔解,并对 Donnell 方程解的适用范围给予了新的评价。     

Thermal-economic multi-objective optimization of shell and tube heat exchanger using particle swarm optimization (PSO)

Many studies are performed by researchers about shell and tube heat exchanger (STHE) but the multi-objective particle swarm optimization (PSO) technique has never been used in such studies. This paper presents application of thermal-economic multi-objective optimization of STHE using PSO. For optimal design of a STHE, it was first thermally modeled using e-number of transfer units method while Bell–Delaware procedure was applied to estimate its shell side heat transfer coefficient and pressure drop. Multi objective PSO (MOPSO) method was applied to obtain the maximum effectiveness (heat recovery) and the minimum total cost as two objective functions. The results of optimal designs were a set of multiple optimum solutions, called ‘Pareto optimal solutions’. In order to show the accuracy of the algorithm, a comparison is made with the non-dominated sorting genetic algorithm (NSGA-II) and MOPSO which are developed for the same problem.     

Sizing problem for a cross flow heat exchanger with wing-type vortex generator

In the present study, sizing of a single pass cross flow heat exchanger with unmixed fluid streams has been investigated. The heat exchanger is a cross flow heat exchanger. It has overall dimensions of 20 × 20 × 20 cm. Two the most common heat exchanger design problems are the rating and sizing problem. Sizing problems deal with designing an exchanger and determining its physical size to meet the specified heat duty, pressure drops and other considerations. It means the determination of the exchanger construction type, flow arrangement, heat transfer surface geometries and materials, and the physical sizes of an exchanger to meet specified heat transfer and pressure drop. In this study, the physical size (length, width, height, mass flow rates of both fluids and surface areas on each side of the exchanger) are determined. Inputs to the sizing problem are surface geometries, fluid mass flow rates, inlet and outlet fluid temperatures and pressure drop on each side. Dimensions of L _a , L _b , and L _c for the selected surfaces were investigated such that the design meets the heat duty and pressure drops on both sides exactly.     

An experimental and numerical investigation of air side heat transfer and flow characteristics on finned plate configuration

In this paper, a new type of finned plate heat exchanger (FPHE) is presented to recover the waste heat from exhaust flue gases. A finned plate configuration causes low pressure drop and it is especially appropriate for heat transfer at the flue gas side. Meanwhile, this paper presents a detailed experimental and numerical study of convection heat transfer and pressure drop of the new structure. Three-dimensional numerical simulation results using the CFD code FLUENT6.3 were compared with experimental data to select the best model. The heat transfer and pressure drop with different geometry pattern was then studied numerically using the selected model. And the velocity field and temperature distribution of air flow in the finned plate channel are presented with different geometry patterns. These results provide insight into improved designs of FPHEs.     

Numerical estimation of mixed convection heat transfer in vertical helically coiled tube heat exchangers

A numerical investigation of the mixed convection heat transfer from vertical helically coiled tubes in a cylindrical shell at various Reynolds and Rayleigh numbers, various coil‐to‐tube diameter ratios and non‐dimensional coil pitches was carried out. The particular difference in this study compared with other similar studies is the boundary conditions for the helical coil. Most studies focus on constant wall temperature or constant heat flux, whereas in this study it was a fluid‐to‐fluid heat exchanger. The purpose of this article is to assess the influence of the tube diameter, coil pitch and shell‐side mass flow rate on shell‐side heat transfer coefficient of the heat exchanger. Different characteristic lengths were used in the Nusselt number calculations to determine which length best fits the data and finally it has been shown that the normalized length of the shell‐side of the heat exchanger reasonably demonstrates the desired relation. Copyright © 2010 John Wiley & Sons, Ltd.     

Pressure drop and heat transfer comparison for both microfin tube and twisted-tape inserts in laminar flow

Experiments were carried out to compare pressure drop and heat transfer coefficients for a plain, microfin, and twisted-tape insert-tubes. The twisted-tape experiments include three different twist ratios each with two different widths. The data were taken at Reynolds numbers well in the laminar region. The heat transfer data were obtained in a single shell-and-tube heat exchanger where steam is used as a heat source to obtain a uniform wall temperature and the working fluid in the tube is oil. The twist ratio and the width of the tape seem to have a large effect on the performance of the twisted-tape insert. The results demonstrate that as the twist ratio decreases, the twisted-tape will give better heat transfer enhancement. The loose-fit (W=10.8 mm) is recommended to be used in the design of heat exchanger where low twist ratios (Y=5.4, and Y=3.6) and high pressure drop situations are expected since it is easier to install and remove for cleaning purposes. Other than these situations, the tight-fit tape gives a better performance over the loose-fit tape. For the microfin tube tested in this paper, the data shows a small increase in both heat transfer and pressure drop. This type of microfin tube is not recommended to be used in laminar flow conditions.     

The effect of flexible tube vibration on pressure drop and heat transfer in heat exchangers considering viscous dissipation effects

The pressure drop and heat transfer coefficient in tube bundle of shell and tube heat exchangers are investigated considering viscous dissipation effects. The governing equations are solved numerically. Because of temperature-dependent viscosity the equations should be solved simultaneously. The flexible tubes vibration is modeled in a quasi-static method by taking the first tube of the row to be in 20 asymmetric positions with respect to the rest of the tubes which are assumed to be fixed and time averaging the steady state solutions corresponding to each one of these positions .The results show that the eccentricity of the first tube increases pressure drop and heat transfer coefficients significantly comparing to the case of rigid tube bundles, symmetrically placed. In addition, these vibrations not only compensate the effect of viscous dissipations on heat transfer coefficient but also increase heat transfer coefficient. The constant viscosity results obtained from our numerical method have a good agreement with the available experimental data of constant viscosity for flexible tube heat exchangers.     

Further understanding of twisted tape effects as tube insert for heat transfer enhancement

Tube inserts are used as heat transfer enhancement tool for both retrofit and new design of shell and tube heat exchangers. This paper discusses and reviews the characteristics and performance of twisted tapes. The theory and application are also addressed. Industrial case study was selected to illustrate the behaviour effect that the twisted tapes impose at various laminar, transition and turbulent flow regions. This effect was demonstrated by changing the inside tube diameter and twist ratio through evaluating selected exchanger design parameters such as: local heat transfer coefficient, friction factor and pressure drop. Testing the exponent powers for Re and Pr at both laminar and turbulent regions were carried out. General design considerations are outlined for the use of twisted tapes in shell and tube heat exchangers.     

Numerical modeling of compact high temperature heat exchanger and chemical decomposer for hydrogen production

The present study addresses fluid flow and heat transfer in a high temperature compact heat exchanger which will be used as a chemical decomposer in a hydrogen production plant. The heat exchanger is manufactured using fused ceramic layers that allow creation of channels with dimensions below 1 mm. The main purpose of this study is to increase the thermal performance of the heat exchanger, which can help to increase the sulfuric acid decomposition rate. Effects of various channel geometries of the heat exchanger on the pressure drop are studied as well. A three-dimensional computational model is developed for the investigation of fluid flow and heat transfer in the heat exchanger. Several different geometries of the heat exchanger channels, such as straight channels, ribbed ground channels, hexagonal channels, and diamond-shaped channels are examined. Based on the results, methods on how to improve the design of the heat exchanger are recommended.     

Comparisons of the heat transfer and pressure drop of the microchannel and minichannel heat exchangers

The present study investigated the comparisons of the heat transfer and pressure drop of the microchannel and minichannel heat exchangers, both numerically and experimentally. The results obtained from this study indicated that the heat transfer rate obtained from microchannel heat exchanger was higher than those obtained from the minichannel heat exchangers; however, the pressure drops obtained from the microchannel heat exchanger were also higher than those obtained from the minichannel heat exchangers. As a result, the microchannel heat exchanger should be selected for the systems where high heat transfer rates are needed. In addition, at the same average velocity of water in the channels used in this study, the effectiveness obtained from the microchannel heat exchanger was 1.2–1.53 times of that obtained from the minichannel heat exchanger. Furthermore, the results obtained from the experiments were in good agreement with those obtained from the design theory and the numerical analyses.     

Experimental investigation of the flow pattern,pressure drop and void fraction of two-phase flow in the corrugated gap of a plate heat exchanger

The two-phase flow in the corrugated gap created by two adjacent plates of a plate heat exchanger was investigated experimentally. One setup consisting of a transparent corrugated gap was used to visualize the two-phase flow pattern and study the local phenomena of phase distribution, pressure drop and void fraction. Saturated two-phase R365mfc and an air-water mixture were used as working fluids.In a second experimental setup, the heat transfer coefficients and the pressure drop inside an industrial plate heat exchanger during the condensation process of R134a are determined. Both experimental setups use the same type of plates, so the experimental results can be connected and a flow pattern model for the condensation in plate heat exchangers can be derived. In this work the results of the flow pattern visualization, the two-phase pressure drop in the corrugated gap and the void fraction analysis by measurement of the electrical capacity are presented. A new pressure drop correlation is derived, which takes into account different flow patterns, that appear during condensation. The mean deviation of the presented pressure drop model compared to the experimental data and data from other experimental works is 18.9%. 81.7% of the calculated pressure drop lies within ±30% compared to the experimental data.