自然循环蒸汽发生器倒U型管内倒流特性研究

以自然循环压水反应堆一回路系统的蒸汽发生器为对象,对蒸汽发生器并联倒U型管内单相水发生的倒流机理及其特性进行了分析,确定了倒流发生的条件和范围,以及发生倒流的倒U型管结构等.在此基础上,建立了分析自然循环系统蒸汽发生器倒U型管内单相水倒流的数学模型,并进行了计算,得出了蒸汽发生器倒U型管内正流和倒流流量、正流和倒流管分布等,并使用实验结果对计算结果进行了检验.研究结果表明,倒流对系统的自然循环能力有重要影响.     

自然循环蒸汽发生器倒U型管内单相水流动及传热特性分析

本文以模拟压水反应堆一回路系统在自然循环条件下运行时的蒸汽发生器为对象,提出了分析蒸汽发生器并联倒U型管内单相水的流动及传热流特性的数学模型及相应的数值计算方法,计算结果表明,在一定的回路系统自然循环流动条件下,并联倒U型管内会同时发生正流和倒流。并发现,蒸汽发生器倒U型管内的倒流对一回路系统的自然循环流量有重要影响。     

垂直U型沸腾管传热恶化特性的研究

本文阐述垂直U型管弯管段中,沸腾传热恶化特性的试验结果,试验在高压电加热水回路装置上进行.管子内径21mm,弯管半径700mm,压力P=45—144bar,质量流速G=800—2000 kg/m~2s,热负荷q=80—330 kw/m~2,试验得出了沿弯管段周界壁温分布特性。管子内侧与外侧之间的壁温差随质量流速、热负荷、含汽率和压力而变.△t的变化反映了两相流流型转变的影响,根据本文结果,为确保U型管内外侧壁温均匀,应避免在层状流与间歇状流动工况下工作。     

毛细芯热管蒸汽流动特性研究

热管内蒸发和凝结时工质会径向迁移,导致蒸汽流动特性与管流有很大不同。对热管内蒸汽流动的速度分布和压降特性进行了分析,比较了现有热管蒸汽压降的计算方法。同时考虑蒸汽流动时黏性力和惯性力的作用,提出了计算蒸汽流动速度和压降的改进方法,消除了前人方法应用范围的限制。利用此计算方法,蒸汽速度分布随径向雷诺数变化较大。与钠热管的数值模拟结果对比表明,此计算方法应用范围广,准确度较高。     

ZrO_2-水纳米流体管内层流流动特性的实验研究

实验研究了粒径为50 nm,质量分数为0.2%,0.4%,0.8%,1.2%的ZrO_2-水纳米流体分别在弯管和直管内的流动特性以及纳米流体的稳定性.实验结果表明:通过加入分散剂使纳米流体稳定性得到显著提高,在相同温度的层流状态下,弯管的流动阻力系数大于直管.纳米流体的黏度随着质量分数的增大而增大,在相同Re下,纳米流体在层流区内阻力系数随纳米流体质量分数的增加而增大.     

超临界CO_2在内螺纹管内换热特性研究

建立了外径为3 mm的内螺纹管三维实体模型,使用Fluent软件研究了在不同的进口雷诺数和操作压力下超临界二氧化碳在水平内螺纹管内的流动与传热特性。研究表明:不同截面局部换热系数和相同流体局部平均温度下的局部换热系数均随着冷却压力的增大而增大;相同流体局部平均温度下的局部换热系数随着进口雷诺数的增加而增大;不同冷却压力和进口雷诺数下,流体局部平均温度越接近超临界二氧化碳的临界点温度,局部换热系数也就越大。     

闪电消散过程等离子体温度衰减规律的理论研究

利用流体运动方程、连续性方程和能量守恒方程,对自然闪电回击后的消散过程建立数学模型,计算了等离子体温度随时间和空间的变化规律.结果分析表明：在相同的通道半径处,前期温度随时间衰减较快,后期较慢,并且,初始温度越高,衰减越快;随着半径的增大,温度的衰减幅度逐渐减小.同一时刻,半径小的位置温度梯度较小,半径大的位置温度梯度较大.由衰减到NO冻结温度的时间和位置,初步推断氮氧化物（NO_x）生成主要在闪电冲击波之后50 ms内、半径R=9 mm的等离子体通道内. 关键词： 闪电消散过程 等离子体 温度     

含内热源多孔介质中的混合对流

本文用数值及解析方法分析了含有内热源的竖直多孔通道中的混合对流,通道的两个壁面可维持不等的温度。计算了不同情况下通道中的流速、温度及热流量分布,结果表明,由:]:不同的壁温及内热源的自然对流效应,在两个壁面附近均可能出现流动倒流区,并向下游内部扩展直到“稳定”。数值计算可确定倒流初始出现的位置及发展规律。随着内热源强度的增加,倒流区将向上游移动。而FDF理论分析可得出一个壁面(冷壁)或两个壁面均出现倒流的判断准则。     

超临界压力航空燃料热声不稳定流动实验研究

对竖直圆管内超临界压力航空煤油的热声不稳定流动进行了实验研究。考察了不稳定流动工况壁温的振荡特征和发展过程,分析了运行参数对不稳定流动的影响机制,获得了不稳定流动的边界,实现了临界热流密度的预测,弥补了对其发展过程和边界预测研究的不足。结果表明:高热流密度下通道内煤油依次经历三种拟沸腾流动不稳定过程.振荡声响分为三类,基本处于低频范围。根据临界热流密度随质量流量、进口温度和进口压力的变化规律,通过无量纲化建立了不稳定流动的起始预测准则。     

微细管碳纳米管悬浮液强制对流换热实验研究

测量了水平微细圆管内蒸馏水和不同质量浓度的水基多壁碳纳米管纳米流体在低雷诺数下的强制对流换热特性。实验结果表明,与蒸馏水相比,纳米流体的对流换热系数显著提高,且随质量浓度和管内雷诺数的增大而增大;并且研究了流体管内流动阻力特性,得到的泊肃叶数f·Re值随着雷诺数的变化不明显,但纳米流体的f·Re值要明显小于纯水。     

托卡马克真空室内线圈水冷管道接头强化二次流的数值研究

基于计算流体力学(CFD)理论,研究了不同曲率半径的螺旋导流片的托卡马克真空室内线圈水冷管道接头。利用湍流数值模拟方法,分析了线圈管道接头导流片曲率半径比、冷却水入口流速对线圈管道内流体平均雷诺数分布的影响。结果表明,不同导流片曲率半径比的线圈管道内的流体雷诺数分布曲线相似,平均雷诺数随入口流速的增加而增大,管道接头出口雷诺数随导流片曲率半径比的增大而减小,导流片曲率半径比小的管接头更适用于线圈水冷曲线管的二次流强化。此外,还为导流片曲率半径比为0.2的管接头拟合了管接头出口雷诺数与入口流速的关系式,为进一步研究类似于托卡马克真空室内线圈管道的曲线管接头的二次流强化提供理论基础。     

综述180°回转型弯头对U型管内的气液两相流流型压降与传热的影响

从流型,压降和传热三个方面综述了以前研究者在U型管中所做的研究。指出了180°弯头对整个U型管的流型及其流型转变,压降损失及其预测方法和增强换热的影响。同时提出了今后关于U型管可能的研究方向:(1)细化曲率比等几何因素对流型压降传热的影响;(2)关注低温介质的U型管中的流动与传热;(3)以180°弯头加上距其一定距离的上下游为一个单元考虑其整体的压降损失;(4)量化气液两相流在U型管中的传热。     

Direct numerical simulation of transitional flow in a finite length curved pipe

Transition to turbulent flow in a curved pipe has been well studied through experiments and numerical simulations. Numerical simulations often use a helical pipe with an infinite length such that the inlet and outlet boundary conditions can be modelled as periodic which greatly reduces computational time. In this study, we examined a finite length curved pipe with Poiseuille flow imposed at the inlet and a stress-free boundary condition at the outlet. Direct numerical simulation of the Navier-Stokes equations for rigid walls and a Newtonian fluid was performed using nek5000. Straight extensions were added to the inlet and outlet such to diminish the impact of boundary conditions on the flow field in the region with curvature. The examined model has a pipe radius of curvature that is three times the pipe radius. The model has ～355 million nodes and required an order of magnitude greater computational time when compared with an infinite length curved pipe. Results show that the critical Reynolds number, the lowest value with instabilities present in the flow, is much greater than that of a straight pipe and occurs near Re=5000–5200. This is larger than the critical Reynolds number typically reported for an infinite length curved pipe (Re=4200–4300).     

翅片管束式管壳式换热器三维数值模拟研究

本文提出了运用多孔介质模型、分布阻力模型和k-ε湍流模型对壳侧为翅片管束的壳管式换热器壳侧速度场与温度场进行三维数值模拟的方法,并对一相应类型换热器壳侧的流动与换热进行了数值模拟,得出了壳侧流场参数的图示以及壳侧进出口压降,温差,换热量随壳侧Re变化的特性曲线。     

Thermo-Hydraulic Behavior of Microchannel Heat Exchanger System

This article presents an experimental study of thermo-hydrodynamic phenomena in a microchannel heat exchanger system. The aim of this investigation is to develop correlations between flow/thermal characteristics in the manifolds and the heat transfer performance of the microchannel. A rectangular microchannel fabricated by a laser-machining technique with channel width and hydraulic diameter of 87 μm and 0.17 mm, respectively, and a trapezoidal-shaped manifold are used in this study. The heat sink is subjected to iso-flux heating condition with liquid convective cooling through the channels. The temporal and spatial evolutions of temperature as well as total pressure drop across the system are monitored using appropriate sensors. Data obtained from this study were used to establish relationships between parameters such as longitudinal wall conduction factor, residence and switching time, and thermal spreading resistance with Reynolds number. Result shows that there exist an optimum Reynolds number and conditions for the microchannel heat exchanger system to result in maximum heat transfer performance. The condition in which the inlet manifold temperature surpasses the exit fluid temperature results in lower junction temperature. It further shows that for a high Reynolds number, the longitudinal wall conduction parameter is greater than unity and that the fluid has sufficient dwelling time to absorb heat from the wall of the manifold, leading to high thermal performance.     

Laminar-Turbulent Transition: The Change of the Flow State Temperature with the Reynolds Number

Representing the fluid flow as a collection of coherent structures of various size, the statistical temperature of the flow state is determined as a function of the Reynolds number. It is shown that at small Reynolds numbers, associated with laminar states, the temperature is positive, while at large Reynolds numbers, associated with turbulent states, it is negative. At intermediate Reynolds numbers, the temperature changes from positive to negative as the size of the coherent structures increases, similar to what was predicted by Onsager for a system of parallel point-vortices in an inviscid fluid. It is also shown that in the range of intermediate Reynolds numbers the temperature exhibits a critical divergence.     

Evaluation on the heat transfer performance of a vertical ground U-shaped tube heat exchanger buried in soil–polyacrylamide

Ground source heat pump (GSHP) systems have been applied widely because of their environmental-friendly, energy-saving, and sustainable nature. In this work, heat transfer performance of a single vertical small-scale U-shaped tube ground heat exchanger under hot climatic condition is addressed considering the influences of inlet water temperature, Reynolds number, and backfill materials (raw soil; soil–polyacrylamide (PAM) blend (0.27% blending ratio for PAM). The backfill materials had an important effect on the heat transfer of the ground heat exchanger. At an inlet water temperature of 45°C and Reynolds numbers of 3104 and 4656, the temperature drops of water in the tube in the soil–PAM blend increased by about 0.3 and 0.4°C compared to that in the raw soil. Within Reynolds number from 3104 to 6208, the average surface heat transfer coefficients of the water in the tube in the soil–PAM blend and raw soil at an inlet water temperature of 45°C were 411 and 231 W m^?2K^?1, respectively. The results suggest that adding the PAM into soil can be an effective manner for enhancing the heat transfer of the ground heat exchanger. The dimensionless surface heat transfer correlation of the water in the U-tube heat exchanger in the soil–PAM blend was obtained. The model could better fit the experimental data within ±10% deviation.     

微柱群阻力特性实验研究

以去离子水为工质,流经直径为0.5 aim,高度分别为1.0 mm、0.75 mm、0.5 mm和0.25 mm的圆柱组成的柱群板,其宽度与长宽分别为3.5 mm和40 mm,测量通道进出口压差及流量,研究微柱群内部分别在叉排和顺排时液体流动的阻力特性.研究表明,微柱群内流动阻力系数f,随Re数的增大而逐渐减小,当Re数大于500时,f基本不变;微柱高度和直径之间存在一个有利于流动的最佳比例,该值介于1到1.5之间;顺排微柱群的f明显小于叉排微柱群,其,值为叉排微柱群的0.5倍.     

具有隔板的平行通道内空气混合对流换热数值模拟

本文以锅炉干排渣装置为背景,对抽象的理论模型具有隔板的平行通道内空气混合对流换热进行了数值模拟.数值计算表明,在Re＞1000时应采用非稳态数学模型进行数值模拟;在Re＞500时,自然对流机制对流动和换热的影响基本可以忽略.数值计算给出了不同Re时的进出口无量纲压差、局部的Nux和平均Nu以及流线图.这些结果可为深入研究干排渣装置中流动和换热特性提供参考.     

Experimental Study on Flow and Heat Transfer in a 19.6-μm Microtube

The flow and the heat transfer characteristics in a quartz microtube with an inner diameter of 0.0196 mm are investigated experimentally. Measuring the pressure drop between the inlet and outlet of the microtube and the average temperature of the microtube wall heated by steam when the working fluid is distilled water, the corresponding friction factors and Nusselt number are obtained. The experimental results show the friction factors in the microtube exceed those of the Hagen–Poiseuille prediction due to the predominance of the effects of the electrical double layer and the entrance. Also, the experimental Nusselt number is less than the classical laminar at Reynolds number < 500 due to the effect of the variation of the thermophysical properties with the temperature.