波纹板式换热器传热与流动特性分析

通过数值模拟的方式,对一种用于低粘度流体的波纹板式换热器的传热特性和阻力性能进行分析,以控制变量法分析了流体速度对传热特性及阻力性能的影响;搭建了板式换热器测试平台,验证了数值模拟结果的正确性和可行性;用等速法拟合了Nu-Re与Eu-Re的相关准则式,并采用JF因子评价换热器综合性能。结果表明：模拟结果与实验结果相比误差在10%以内;当流体流速小于1.0 m/s时,换热器传热系数和压降随着流速的增大而增大,但综合换热性能逐渐变差,在此流速范围内,总传热系数随冷流体进口温度升高而增大。     

矩形通道内过冷流动沸腾传热特性试验研究

以高强化发动机缸盖材料蠕铁作为加热块材料,在矩形通道内开展了接近发动机冷却系统的不同流动工况下过冷沸腾传热特性的试验研究。流动工况取发动机常用范围:压力为0.10~0.25MPa,主流温度为60~95℃,流速为0.347~6m/s。研究结果表明:提高冷却液流速可以强化壁面对流换热强度,但是存在沸腾换热的低流速工况同样能够达到高流速工况下换热效果,系统压力和主流温度都会影响冷却液过冷度,进而影响沸腾传热效果。可视化结果表明:气泡直径增大、生长频率升高及气泡之间相互作用都会使气泡对边界层流体扰动增强,从而提高传热效率。在压力为0.2MPa、主流温度为95℃、流速为1m/s工况下,在壁温达到170℃以上时沸腾开始出现,在壁温达到210℃时,沸腾传热效率比单相对流换热提高了40%以上。     

竖直矩形细通道内水沸腾换热的数值模拟

对宽度为1和O.1 mm竖直矩形细通道内的沸腾换热展开研究,通过数值模拟的方法探索汽泡生成、长大和脱离的过程;用几何重构和界面追踪的方法获取相界面移动和变化对系统内压差以及平均表面换热系数的影响,计算中考虑了重力、表面张力和壁面黏性的作用.发现:通道宽度的不同对汽泡生长方式和汽泡形态产生很大影响,并由此导致临界热流密度...     

偏心方形管内超临界水传热特性数值研究

采用SSG湍流模型,对偏心方管内超临界水传热特性进行三维数值研究。研究了不同偏心距对通道内传热特性的影响及通道内的周向传热不均匀特性。研究结果表明:偏心方形管内存在着强烈的传热不均匀现象,在远离大比热区的低焓值区和高焓值区,偏心距离的增加对内壁温度和换热系数的影响不大。随着偏心距的增加,方形管壁面最高温度逐渐升高,边通道传热系数要强于角通道;超临界水的物性变化也会影响偏心方形管内周向不均匀性,从而影响传热特性。     

3×3棒束通道内低速流动沸腾传热特性的实验研究

以去离子水为实验介质,在入口温度为80~100℃、质量流速为0~100kg/(m2·s)、入口压力为0.1 MPa的参数范围内,对棒束通道内流动沸腾传热特性进行了实验研究,分析了质量流速和热流密度对流动沸腾传热系数的影响,考察不同子通道内的传热特性.根据实验工况选取了Liu-Winterton关系式、Kandlikar关系式、Gungor-Winterton关系式和Chen关系式对棒束通道内的流动沸腾传热系数进行预测,将实验结果与预测结果进行比较,并对4个关系式的预测结果进行统计评估.结果表明:Liu-Winterton关系式、Kandlikar关系式和Chen关系式的预测结果偏小,其中Chen关系式的预测结果误差最大,Gungor-Winterton关系式的预测结果更加准确.     

竖直矩形窄通道内流动沸腾局部换热特性实验研究

为了明确竖直矩形窄通道内各阶段流动沸腾的换热特性,优化换热器性能,以去离子水为工质,对尺寸为720 mm×250 mm×3.5 mm的单面电加热竖直矩形窄通道内的流动沸腾换热进行实验研究,分析了质流密度、进口温度、热流密度对流动沸腾局部换热特性的影响。并在已有流动沸腾传热关联式的基础上,对实验数据进行非线性回归分析,得到适用于实验工况下的新流动沸腾传热关联式。结果表明：质流密度增大对流动沸腾段换热特性有强化作用,对核态沸腾段换热特性有削弱作用;热流密度对核态沸腾影响剧烈,但对流动沸腾的影响不明显;入口温度越高,流体会越早进入过冷沸腾阶段,但对局部传热系数的影响不明显;新流动沸腾传热关联式与实验值的平均相对误差为23.87%,其中74.19%的预测值在±25%内,83.87%的预测值在±50%以内,能很好地预测本实验工况下矩形窄通道内流动沸腾的局部传热系数。     

窄通道内流动沸腾强化换热研究进展

窄通道具有结构紧凑、传热效率高等优点。随着科技发展,窄通道已经成为强化换热的常用结构形式之一,被广泛应用于各种换热设备。由于窄通道内间隙内气泡的尺寸受限,气泡在发展过程中会受到挤压而发生变形,带走大量的潜热,引起汽液界面的扰动,换热性能较常规通道有很大区别。本文综述了窄通道内的主要流型及转变准则;介绍了几何与工况参数变化对窄通道内换热效果影响的传热实验研究;分析了窄通道中传热机理以及两相摩擦压降机理,并对关联式进行了总结与评述;对窄通道内强化换热的机理与进一步强化换热的方法进行归纳总结;结合目前实验与理论研究总结了现存问题,为窄通道内流动沸腾强化换热的进一步研究提供了参考。     

发动机缸盖模拟冷却通道内过冷沸腾传热特性

搭建一套试验台架用来模拟发动机鼻梁区冷却通道内的传热状况,采用与发动机缸盖相同的铸铁材料作为加热块,并且安装3块石英玻璃用来观察沸腾发生时气泡运动状态.对不同流速、不同入口温度和不同系统压力状况下的沸腾传热特性做了相应的研究,为真实发动机冷却水腔内沸腾传热预测提供较为全面的试验数据.结果表明:沸腾起始点位置与通道流动参数有着直接关系,表现为速度越高、入口温度越低,沸腾起始壁面过热度越高;提高通道速度和降低冷却入口温度可以强化壁面对流换热程度,但对充分发展沸腾下的传热特性影响很小;增加系统压力,沸腾起始壁面温度越高,其增加幅度与饱和温度增幅大致相同.此外,控制系统压力是抑制沸腾过度发展的重要手段.     

一次表面回热器全程通道流动和传热分析

由于一次表面回热器全程通道流动与换热的复杂性,很难利用数值分析的方法确定传热系数,也给回热器的设计计算带来很大的困难。本文通过分析对比0°、60°和120°一次表面回热器换热单元的传热流动特性,提出了针对单元体与全程通道回热器之间换热流动关系的修正系数。并基于0°全程通道数值模拟结果,采用该修正系数完成了60°、120°全程通道回热器流动换热特性。     

导热油在单叶型空心桨叶换热器中的层流换热特性

采用数值模拟方法计算了导热油在单叶型空心桨叶换热器中的层流换热特性,并与实心桨叶换热器进行了比较。计算结果表明,空心桨叶换热器的导热油压降、传热率、总传热系数和总熵产率均显著高于实心桨叶换热器;对于空心桨叶换热器,流动孔直径和数量对导热油压降影响较大,但对传热率影响有限;在总熵产率组成,传热熵产率占主要部分。物料换热系数和总传热系数组合结果可用于桨叶换热器设计。     

超临界碳氢燃料流动换热的仿真研究

建立了适用于温度、压力大范围变化的超临界碳氢燃料换热特性研究的一维模型,试验验证了模型的可靠性,基于该模型分析了质量流率、换热状态转换和压力等因素对超临界碳氢燃料的换热特性的影响。结果表明:4.0 MPa的低压力工况下,在超临界碳氢燃料的拟临界温度附近存在传热强化现象,而7.0 MPa的高压力工况下,该传热强化现象消失;不论质量流率的大小,4.0 MPa工况下的传热系数始终高于7.0 MPa工况下的传热系数,压力的高低在超临界碳氢燃料的汽相换热区对传热性能的影响更为显著;40kg/(m2.s)的低质量流率条件下,由液相换热区向汽相换热区的换热状态转换将导致转换区附近的传热恶化,而增大质量流率则可避免该问题的发生。     

狭缝通道内核态沸腾中的气泡动力学研究

为加深对狭缝通道内核态沸腾气泡动力学机理的探索,对宽度为2 mm的I-形和Z-形两种不同截面形状的狭缝通道内核态沸腾展开研究,通过数值模拟的方法探究不同壁面接触角对气泡生成及长大过程的影响,不同狭缝形状与流动压降的关系,计算中考虑了重力、表面张力和壁面黏附的作用.发现:壁面接触角的不同对气泡的形态有很大影响,壁面接触角...     

Simulation of heat transfer with the growth and collapse of a cavitation bubble near the heated wall

The growth and collapse behaviors of a single cavitation bubble near a heated wall and its effect on the heat transfer are numerically investigated. The present study is designed to reveal the mechanism of cavitation enhanced heat transfer from a microscopic perspective. In the simulation, the time-dependent Navier-Stokes equations are solved in an axisymmetric two-dimensional domain. The volume of fluid (VOF) method is employed to track the liquid-gas interface. It is assumed that the gas inside the bubble is compressible vapor, and the surrounding liquid is incompressible water. Mass transfer between two phases is ignored. The calculated bubble profiles were compared to the available experimental data, and a good agreement was obtained. Then, the relationship among bubble motion, flow field and surface heat transfer coefficient was analyzed. On this basis, the effects of such factors as the initial distance between the bubble and the wall, the initial vapor pressure and the initial bubble nucleus size on the heat transfer enhancement are discussed. The present study is helpful to understand the heat transfer phenomenon in presence of cavitation bubble in liquid.     

Nucleate boiling in two types of vertical narrow channels

To explore the mechanism of boiling bubble dynamics in narrow channels, we investigate 2-mm wide I- and Z-shaped channels. The influence of wall contact angle on bubble generation and growth is studied using numerical simulation. The relationships between different channel shapes and the pressure drop are also examined, taking into account the effects of gravity, surface tension, and wall adhesion. The wall contact angle imposes considerable influence over the morphology of bubbles. The smaller the wall contact angle, the rounder the bubbles, and the less time the bubbles take to depart from the wall. Otherwise, the bubbles experience more difficulty in departure. Variations in the contact angle also affect the heat transfer coefficient. The greater the wall contact angle, the larger the bubble-covered area. Therefore, wall thermal resistance increases, bubble nucleation is suppressed, and the heat transfer coefficient is lowered. The role of surface tension in boiling heat transfer is considerably more important than that of gravity in narrow channels. The generation of bubbles dramatically disturbs the boundary layer, and the bubble bottom micro-layer can enhance heat transfer. The heat transfer coefficient of Z-shaped channels is larger than that of the I-shaped type, and the pressure drop of the former is clearly higher.     

Transition and flow boiling heat transfer inside a horizontal tube

Experiments on transition and flow boiling heat transfer with refrigerant R114 inside a horizontal tube were performed at bubble flow, critical heat flux and in the transition region between bubble flow and film boiling at mass fluxes between 1200 and 4000 kg/m² s and in the pressure range between 5 and 15 bar. In comparison with pool boiling bubble flow heat transfer depends essentially on the mass flow rates and on the vapor quality. The critical heat flux depends less on the temperature difference than in pool boiling heat transfer and exhibits a maximal and a minimal value as a function of the pressure. The critical heat flux increases with mass flow rate as already shown by Collier. In the region of transition boiling the heat flux over the difference between wall and saturation temperature approaches a horizontal curve. Therefore in this region an evaporator may always be operated under stable conditions and burn out does not occur.     

两相流振荡换热过程数值模拟分析

为深入了解振荡条件下的两相流运动过程与流动换热特性,采用数值仿真研究方法,摒弃活塞复杂的结构因素,以简化模型为研究对象,从基础问题出发探索各因素对两相流振荡换热过程的影响。通过一系列的数值模拟,直观地展现了油腔内两相流运动过程是高度紊乱的不对称流动。研究结果表明:除活塞二阶运动产生偏离外,油腔上下运动引起的压力变化也将引起油柱倾斜,造成喷油与油腔入口偏离;活塞运动产生的强制振荡是影响腔内流动换热波动的决定性因素,油腔表面传热系数、填充率等的波动频率都与活塞运动频率完全吻合。此外,研究还发现喷油速率、活塞转速等参数对填充率和传热系数都会产生显著的影响,但这些影响因素可能并不是单一作用而是相互耦合的。喷油油柱在进入油腔时的速度与活塞的最大运动速度之间的匹配度可能是影响油腔填充率及传热系数的一个关键因素;当喷油的绝对速度与活塞最大速度接近或大于活塞最大速度时,将达到较为理想的填充率和传热系数。     

矩形冷却通道内超临界RP-3航空煤油对流传热数值研究

对超临界压力下RP-3航空煤油在内截面宽为4mm、高为4mm、固体壁面厚为1mm、加热段长度为500mm的水平矩形冷却通道内的对流传热特性进行了数值模拟研究。分析了通道内速度场的分布规律,讨论了热流密度、压力、进口温度对传热的影响。计算结果表明:当主流温度处于拟临界温度附近时,流体物性参数变化剧烈,导致传热系数降低,传热出现恶化。在超临界压力下,较低的热流密度、增大压力、降低进口流体温度或提高质量流速均有利于改善冷却通道内的传热性能。     

Experimental investigation of heat transfer and pressure drop in serrated-fin tube bundles with staggered tube layouts

An experimental investigation of the heat transfer and pressure drop performance of ten finned tube bundles using serrated fins is presented. All tube bundles had staggered layouts, and the influence on varying tube bundle layout, tube and fin parameters are presented. The heat transfer coefficient experienced a maximum when the flow areas in the transversal and diagonal planes were equal. An increase in the fin pitch increased the heat transfer coefficient; the same was observed with an increase in fin height. The pressure drop coefficient showed no influence of the tube bundle layout for small pitch ratios, but dropped significantly for higher ratios. Increasing fin pitch reduced the pressure drop, whereas varying fin height had insignificant effect. None of the literature correlations were able to reproduce the experiments for the entire range of tested conditions. A set of correlations were developed, reproducing the experimental data to within ±5% at a confidence interval of 95%.     

百叶窗翅片传热与流动的三维数值模拟

本文对空气在百叶窗翅片内部流动和传热建立了三维数值计算模型.计算结果与文献所提供的实验数据进行了对比,在整个计算范围内,Re=0～1500,j和f的平均偏差分别为1.96%和10.5%.在深入揭示百叶窗翅片流动机理的基础上,进一步比较了百叶窗翅片开窗角度La和换向区长度S对其传热和流动阻力的影响,分析结果为百叶窗翅片的模具制作及其优化设计提供了依据.