Subcooled Flow Boiling Heat Transfer to Water and Ethylene Glycol/Water Mixtures in a Bottom-heated Tube

Subcooled flow boiling heat transfer experiments were performed with water and ethylene glycol/water mixtures at five flow velocities and four fluid inlet temperatures. Both turbulent flow and laminar flow were tested in a special test facility with its experimental test section heated from the bottom surface only. Boiling curves and subcooled flow boiling heat transfer coefficients of the tested fluids were determined from the experimental measurements. Predictive correlations of the subcooled flow boiling heat transfer coefficients were developed based on the experimental data. Comparisons are presented of wall temperatures between the experimental measurements and the correlation predictions.     

Convective Heat Transfer of a Cu/Water Nanofluid Flowing Through a Circular Tube

Abstract

Fluids in which nanometer-sized solid particles are suspended are called nanofluids. These fluids can be employed to increase the heat transfer rate in various applications. In this study, the convective heat transfer for Cu/water nanofluid through a circular tube was experimentally investigated. The flow was laminar, and constant wall temperature was used as thermal boundary condition. The Nusselt number of nanofluids for different nanoparticle concentrations, as well as various Peclet numbers, was obtained. Also, the rheological properties of the nanofluid for different volume fractions of nanoparticles were measured and compared with theoretical models. The results show that the heat transfer coefficient is enhanced by increasing the nanoparticle concentrations as well as the Peclet number.     

Fully Developed Convective Heat Transfer of Power Law Fluids in a Circular Tube

We present a theoretical analysis for fully developed convective beat transfer in a circular tube for power law fluids by assuming that the thermal diffusivity is a function of temperature gradient. The analytical eolution is obtained and the heat transfer behaviour is investigated under a constant heat flux boundary condition. It is shown that the Nusselt number strongly depends on the value of power law index n. The Nusselt number sharply decreases in the range of 0 〈 n 〈 0.1. However, for n 〉 0.5, the Nusselt number decreases monotonically with the increasing n, and for n 〉 20, the values of Nusselt number approach a constant.     

管外高温熔盐的流动换热特性

本文实验研究熔盐蒸汽发生器管外高温熔盐的流动换热性能。测试不同熔盐流速下蒸汽发生器管外熔盐的传热系数,并拟合相应的实验传热关联式。考虑高温熔盐的变物性特征,利用黏度项对Wiegand关联式进行修正,得到用于管外熔盐传热的修正Wiegand关联式。研究发现,管外熔盐过渡流传热的修正Wiegand方程与实验测试结果平均偏差小于8%,而紊流传热修正Wiegand方程的平均偏差则小于9%。     

Experimental Study of Laminar Convective Heat Transfer and Pressure Drop of Cuprous Oxide/Water Nanofluid Inside a Circular Tube

Laminar convective heat transfer enhancement of cuprous oxide (Cu₂O)/water nanofluid flowing through a circular tube was investigated experimentally in the present work. A continuous closed loop was designed to measure heat transfer coefficients and pressure drop associated with the flow of Cu₂O/water nanofluid over a wide range of laminar flow conditions. Comparison of the nanofluid experimental results with those of pure water have shown significant enhancement for heat transfer coefficients. On average, a 10% increase in heat transfer coefficient was observed with 16% penalty in pressure drop.     

水平圆管内磁性潜热型功能流体对流换热特性的数值模拟

建立磁场作用下水平圆管内磁性潜热型功能流体对流换热的数学物理模型,分析磁场强度、磁性相变微胶囊体积分数、流体质量流量等因素对流体对流换热的影响.磁场对磁性潜热型功能流体的对流换热具有显著的强化作用,磁场强度愈大强化作用愈明显,强化原因是磁性相变微胶囊受到磁力作用产生扰动.     

最小煅耗优化在椭圆管换热中的应用

本文以(?)耗最小作为目标,以流动过程中的功率消耗一定作为约束条件,结合能量守恒方程和质量守恒方程得到了对流换热的优化场方程。通过数值计算,对优化场方程下的椭圆管对流换热问题进行了研究。结果表明,椭圆管优化速度场为多纵向涡结构,在这种优化场下换热能力增加的幅度远大于阻力增加的幅度,证明了以(?)耗最小作为目标的优化理论能够用于指导强化换热元件的设计。     

血液对流换热的数值计算

在血管壁施加第三类边界条件是计算血液与组织间对流换热的一种近似计算方法.为分析其可行性,用有限元数值模拟方法计算血管分支结构中血液与组织的对流换热,得到不同流速和半径下分支血管内血液的截面平均Nu数沿管长的变化曲线.结果表明,血管树中分支血管的Nu数变化幅度不大,且趋于稳定值的速度很快.以相同边界条件下包含简单血管系统的舌体为例,分别用近似方法和完全耦合计算方法,进行血液流场和舌体温度场模拟.通过比较计算结果,得出两种方法得到的温度场分布趋势基本相同;用完全耦合计算方法得到的舌体温度略高于用近似方法得到的舌体温度,两者差值小于0.2℃.     

含不同形状内热源的圆管内纳米流体自然对流及换热数值研究

采用格子玻尔兹曼方法对有三种恒温热源(圆形、三角形、方形)参与的圆管内纳米流体(铜-水)自然对流进行数值研究.主要研究瑞利(Ra)数,纳米颗粒体积分数以及热源几何形状等控制参数对纳米流体的流动与传热的影响.结果发现纳米颗粒体积分数的增加有利于强化传热,且在Ra数较小时,平均努塞尔(Nu)数增加的幅度要优于Ra数较大的情...     

Experimental and Numerical Investigation of Convective Heat Transfer in a Multiple-ventilated Enclosure with Discrete Heat Sources

The experimental and numerical investigation of convective heat transfer in a rectangular cavity of aspect ratio (L/H) = 5 has been presented in this study. The comparison of experimental results is done with a two-dimensional numerical model considered with steady and turbulent flow conditions. Six different arrangements (bottom/bottom/bottom, top/top/top, right/top/left, bottom/top/right, bottom/top/left, and bottom/right/left) were considered. The lowest and highest hotspot temperatures were observed in bottom/bottom/bottom and bottom/right/left arrangements, respectively. For similar configurations, the bottom/bottom/bottom arrangement displays better heat transfer performance, while heat transfer performance is inferior for the bottom/right/left arrangement than all other configurations.     

Experimental Study of CuO/Water Nanofluid Turbulent Convective Heat Transfer in Square Cross-section Duct

Ducts with a square cross-section are widely used in many industrial applications because of their high compactness, easy forming, and low pressure drop. But the thermal performance of a duct will be reduced when the circular cross-sectional shape is not used. In this study, the convective heat transfer for a CuO/water nanofluid through a square cross-section duct in the turbulent flow regime has been investigated. The Nusselt number of nanofluids for different nanoparticle concentrations, as well as various Peclet numbers, was obtained. The results show considerable enhancement in the heat transfer coefficient and Nusselt number by increasing the nanoparticle concentrations as well as the Peclet number.     

对流换热中的准(火积)耗散函数

通过分析对流换热过程中微元控制体内的(火积)流动平衡,得到类似于热传导过程中(火积)耗散函数表达形式的对流换热准(火积)耗散函数,表明对流换热过程中的(火积)耗散由热传导和对流形成的耗散共同构成。并通过分析对流换热过程的特点,对准(火积)耗散函数中对流项的影响进行了修正,使用圆管内热充分发展流动与无限宽平行通道内的对流换热过程检验,控制体内准(火积)耗散函数的计算结果与边界(火积)流的结果一致,表明文中提出的准(火积)耗散函数是可靠的。     

Heat Transfer and Pressure Drop Characteristics in Turbulent Flow Through a Tube

An experimental investigation has been carried out for turbulent flow through a tube with perforated strip inserts. Strips were of mild steels with circular holes of different diameters. Flow varies, with ranging Reynolds numbers from 15,000 to 47,000. Air velocity, tube wall temperatures, and pressure drops were measured for a plain and strip-inserted tube. The heat transfer coefficient and friction factor were found to be 2.80 times and 1.8 times, respectively, that of the plain tube. The heat transfer performance was evaluated and found to be 2.3 times that of the plain tube based on constant blower power.     

时域自适应精细算法求解对流热传导问题

应用时域自适应精细算法求解对流传热问题.通过展开技术,可更准确地描述变量随时间的变化,同时将时空耦合的初边值问题转化为一系列的空间边值问题,并采用有限元方法递推求解.自适应技术可弥补时间步长不同时可能造成的计算精度损失.并进行了数值检验.     

气相色谱法同时测定牙膏中二甘醇、乙二醇

建立了气相色谱同时检测牙膏中二甘醇、乙二醇含量的方法.牙膏样品用乙醇超声提取30min,静置、过滤,用外标法定量,结果表明各组分0.01-10mg/mL范围内线性关系良好,相关系数(r=0.99962-0.99998),加标回收率93.52％,相对标准偏差2.35％.该方法简便、准确,可作为牙膏中同时测定二甘醇、乙二醇含量的方法.     

Heat Transfer in a Tube Fitted with Vertical and Horizontal Wing-Cut Twisted Tapes

This article reports an experimental investigation on the heat transfer, friction factor, and thermal enhancement factor of a tube equipped with vertical wing-cut twisted tapes, horizontal wing-cut twisted tapes, and plain twisted tapes with twist ratios of y = 2.0, 4.4, and 6.0. The obtained results reveal that the heat transfer rate, friction factor, and thermal enhancement factor in the tube equipped with horizontal wing-cut twisted tapes are significantly higher than those in the tube fitted with vertical wing-cut twisted tapes, plain twisted tapes, and a plain tube. An empirical correlation is subsequently derived from the experimental results.     

全玻璃真空太阳能热水器强化换热机理分析

本文应用Ansys Fluent软件对加装导流板全玻璃真空太阳能热水器内自然对流流场进行三维数值模拟计算,并与实验进行对比分析,通过改变导流板长度和位置改善热水器内的流动和换热。结果表明,与无导流板结构相比较,加装导流板后管内涡流消失,流体流动稳定,有利于管内自然对流,增强了换热效果,水箱内温度分层效果明显,有助于提高换热效率;同时得出导流板长度最佳选择范围为1.4～1.6 m,导流板加装在玻璃管中间位置与距管中间位置偏下1/2之间范围内更有助于换热。     

水蒸气在竖直微细管外凝结传热特性研究

针对不同压力和不同流速下的饱和水蒸气在竖直微细圆管(内外径分别为0.571 mm和0.793 mm)外的凝结传热特性分别进行了实验研究,分析了蒸气压力和蒸气流速对凝结传热特性的影响。实验结果表明,凝结传热表面传热系数随着蒸气流速的增加而增加,在较高的蒸气压力下增加的更明显,且大于相同实验条件下的Nusselt理论分析解。在蒸气流速为2 m·s~(-1)时,凝结传热系数随压力的变化不大;在4 m·s~(-1)和6 m·s~(-1)时,随着蒸气压力的升高,凝结表面传热系数明显增大。     

Effects of Slip on Unsteady Mixed Convective Flow and Heat Transfer Past a Stretching Surface

Unsteady mixed convective boundary layer flow and heat transfer over a stretching vertical surface in the presence of slip axe investigated. It is noted that fluid velocity decreases due to the increasing velocity slip parameter resulting in an increase in the temperature field. The rate of heat transfer decreases with the velocity slip parameter while it increases with unsteadiness parameter. The same feature is also noticed for thermal slip parameter.     

Heat Transfer Characteristics for Condensation of R134a in a Vertical Smooth Tube

In this study, condensation of pure refrigerant R134a vapor inside a smooth vertical tube was experimentally investigated. The test section was made of a copper tube with inside diameter of 7.52 mm and length of 1 m. Experimental tests were conducted for mass fluxes in the range of 20–175 kg/m²s with saturation pressure ranging between 5.8 and 7 bar. The effects of mass flux, saturation pressure, and temperature difference between the refrigerant and tube inner wall (ΔT) on the heat transfer performance were analyzed through experimental data. Obtained results showed that average condensation heat transfer coefficient decreases with increasing saturation pressure or temperature difference (ΔT). In addition, for the same temperature difference (ΔT), heat can be removed from the refrigerant at a higher rate at relatively low pressure values. Under the same operating conditions, it was shown that average condensation heat transfer coefficient increases as mass flux increases. Finally, the most widely used heat transfer coefficient correlations for condensation inside smooth tubes were analyzed through the experimental data. The best fit was obtained with Akers et al.'s (1959) correlation with an absolute mean deviation of 22.6%.