基于两流体模型的流动沸腾瞬态数值模拟程序

基于两流体模型与固壁非稳态导热模型,结合相关关联式组合,建立了流道内流动沸腾传热的瞬态数值模拟程序。通过不同入口瞬态下流道两相流动沸腾过程的算例计算分析,确认了程序进行流动沸腾瞬态模拟的能力。通过对不同固壁加热条件下流动沸腾行为的算例计算,检验了该程序进行流壁耦合行为模拟的功能。程序可进一步向系统分析程序和子通道程序发展。     

基于分子动力学理论界面传质的过冷沸腾汽泡生长特性研究

基于分子动力学理论的准平衡态界面处界面蒸发/冷凝因素,以及汽泡底部微液层传热因素建立综合传热传质相变模型,对窄通道内汽泡过冷流动沸腾条件下的生长情况进行模拟。相变模型体现了汽泡底部微液层蒸发、近壁过热液体传热、汽泡顶部主流冷凝等多方面机制对汽泡生长的影响。模拟结果体现了汽泡底部微液层厚度的变化情况,与实验结果相吻合;微液层蒸发机制在汽泡生长初期对汽泡生长有较大影响,流道壁面效应对汽泡生长有显著影响。     

竖直环隙流道内沸腾换热启动时的动态特性

对常压下竖直环隙流道内沸腾换热启动阶段壁温的瞬态变化特性和流动不稳定性进行了实验研究。结果表明环隙流道壁温变化经历跃升回落、各截面平均温度基本不变和快速和快速升三个阶段，存在强烈的流动不稳定性和壁温波动，并伴随着局部干湿交替现象。产生流动不稳定性的主要原因是流道中间歇生成长聚合汽泡，引起介质的突然加速、停滞或倒流。影响壁温波动的主要因素有环隙宽度、加热热流密度、初始水温、进出口附近大容积液体的过冷     

严重事故下反应堆压力容器下封头耦合烧蚀传热分析

核电站严重事故发生后，反应堆压力容器（RPV）固壁在熔池作用下会发生烧蚀、减薄。开展RPV下封头耦合烧蚀传热分析对堆坑注水有效性论证和RPV剩余壁厚确认有重要的理论指导意义。本文以CPR1000反应堆压力容器为研究对象，在FLUENT 17.2平台下，基于动态网格方法和UDF二次开发，构建了综合考虑RPV固壁瞬态烧蚀与导热、RPV内壁热流密度再分布及RPV外壁过冷沸腾的全耦合计算模型，获取了9 000 s内的堆坑两相流场分布和RPV固壁烧蚀温度场，分析确定了最小剩余壁厚和发生位置。结果表明：使用动态网格捕捉壁面烧蚀的方法可行，本文全耦合计算模型在分析RPV固壁瞬态烧蚀过程方面有一定优势。     

竖直环形流道内欠热沸腾时的气泡行为研究

一般在沸腾传热实验中壁面的加热方式有电加热和流体加热两种,流体加热方式下的沸腾传热研究进行得很少.在水加热条件下,对水在竖直环形流道内欠热流动沸腾时的气泡行为进行了可视化研究.环隙宽度为5mm和3mm两种,质量流速分别为16.8～55.3kg/(m2·s)和15.3～62.1kg/(m2·s).可视化实验结果表明:在贴近壁面的区域存在气泡运动层,大部分气泡在气泡运动层内运动.在宽度为3mm的流道中,气泡在脱离壁面前一般会滑动;滑动距离不超过2～3倍气泡直径,并且存在反复胀缩的现象.5mm流道内的气泡则较少发生滑动,往往在脱离壁面后会被弹回壁面.气泡的滑动和离开壁面后又返回壁面的运动方式是沸腾具有高强度传热能力的原因之一.     

矩形窄通道内汽泡滑移与冷凝前期生长模型研究

矩形窄流道内汽泡生长会直接改变相界面浓度,从而影响流道的传热传质性能。为获得适用于窄流道内不同类型的汽泡生长模型,基于通体可视的实验本体,开展壁面沸腾流动换热实验。基于传热能量方程,研究过冷沸腾中汽泡滑移与冷凝前期两种情况下汽泡生长模型。实验结果表明汽泡呈现两种形式的生长,即汽泡滑移生长以及冷凝前期生长。建立了两种情况下的汽泡生长模型,实验数据验证模型误差在20%以内。因此,本研究能为沸腾两相数值模拟提供更加精细化的汽泡生长模型,从而提高汽泡行为的预测精度。     

竖直窄矩形通道内过冷沸腾传热模型

通过引入池式沸腾-流动沸腾汽泡脱离直径比对沸腾抑制因子S进行了修正,并将修正后的S引入Lee-Mudawwar过冷沸腾CHF模型,通过结合竖直窄矩形通道内的汽泡行为进行分析,建立了适应于竖直窄矩形通道的过冷流动沸腾传热模型,探讨了影响过冷沸腾传热系数的主要因素,并通过将模型预测值与实验值进行对比,验证了模型的可靠,表明当前模型可用于计算竖直窄矩形通道内的过冷沸腾传热特性。     

在双面加热的窄缝环形流道中流动沸腾换热研究

在窄缝流道内发生沸腾换热现象时,由于沸腾产生的汽泡受窄缝流道的限制,受压变形而消除了汽泡表面张力对传热的影响。因此对此现象进行基础性理论研究具有很重要的意义。本文在常压下用蒸馏水对窄缝间隙为 0.75mm的垂直环形流道,进行了流动沸腾传热实验研究。实验段的有效加热长度为 900mm,其加热方式为内外侧双面加热,实验的流量变化范围在 1.67× 10－ 5～ 5.83× 10－ 5m3/s。通过实验得到了在不同质量流速和热流密度下双面加热的窄缝流道中内外侧沸腾换热系数随干度变化的分布和特点。研究结果表明,由于在窄缝流道中存在着大量的运动聚合受压变形汽泡,因此使内外侧沸腾换热系数都很高 (可达 105W· m－ 2· K－ 1以上 )。     

矩形通道干涸后过渡沸腾传热试验研究

在流动传热基础试验平台上进行了矩形通道干涸后过渡沸腾传热试验.对过渡沸腾的传热特性进行分析;对进口含汽率、质量流速、系统压力等各热工水力参数对过渡沸腾传热的影响进行试验研究.结果表明:干涸后过渡沸腾是一个不稳定的传热过程,壁面温度发生明显的脉动,过渡沸腾会引起质量流速和流道进出口压降等流动参数的脉动;进口含汽率的增加会...     

水冷包层模块第一壁流动传热特性初步分析

本文基于我国聚变工程实验堆水冷包层优化设计与安全分析的要求,针对水冷包层模块第一壁的流动传热特性进行三维数值模拟研究。采用计算流体力学方法,建立了水冷包层模块第一壁的三维数值模型,研究流量分配的特点以及温度分布情况,分析与评估在稳态工况、瞬态工况及失流事故下的水冷包层模块第一壁传热能力。研究结果表明,不同冷却管间存在流量分配不均匀的现象;在稳态工况下,水冷包层模块第一壁具有较好的传热能力,瞬态工况下水冷包层模块能够有效地导出反应堆热量;失流事故下冷却管内温度短时间上升至系统压力下的饱和温度,有待进一步研究。相关研究为优化包层第一壁传热设计提供参考,并为今后聚变堆的安全分析提供依据。     

自然循环系统摇摆条件下棒束通道内传热特性研究

棒束燃料元件子通道间流体存在搅混与横向二次流,流动及阻力特性相较矩形通道、圆管等简单通道更为复杂。核动力舰船、船舶、小型浮动核电站等会受到海浪影响,经常处于倾斜、摇摆、垂荡等瞬变运动下。目前的相关研究多集中在低压工况的研究领域,高温高压自然循环运动条件下的研究较少。本文采用实验研究方法,对自然循环系统摇摆条件下棒束通道内流动传热特性进行了研究,获得了过冷沸腾和饱和沸腾两种条件下摇摆角度和摇摆周期对棒束壁面温度变化和传热系数的影响,并获得了摇摆周期内棒束通道内的传热系数计算关系式。结果表明,饱和沸腾传热系数变化比过冷沸腾的剧烈;在本文实验工况范围内,棒表面传热系数波动幅值随着摇摆幅度的增大而增大;摇摆条件下棒束通道过冷沸腾和饱和沸腾工况时均传热系数基本不变。     

基于CFD方法的矩形窄缝通道内过冷流动沸腾模型研究

板状燃料元件中的矩形窄缝通道具有宽高比大的几何特征,高度方向速度梯度大、分布陡峭,发生过冷沸腾时,近壁面汽泡运动行为将受其影响而改变,其中汽泡滑移现象对沸腾换热影响较大。本文针对矩形窄缝通道中的汽泡滑移行为,构建了包含滑移热流的壁面热流分配模型,并建立机理性的汽泡受力模型和滑移模型计算汽泡脱离直径、浮升直径和滑移距离等辅助参数,开发了一套适用于矩形窄缝通道内向上流动沸腾的壁面沸腾模型。选用Nuthel窄缝通道沸腾实验进行数值模拟验证,结果表明：本文模型可以较好地预测1～4 MPa中低压工况窄缝通道向上流动沸腾的壁面过热度,最大误差相比RPI模型由80%降低至17%;蒸发热流份额和近壁面空泡份额相比RPI模型更低。     

Study on characteristics of vapor–liquid two-phase flow in mini-channels

To explore the mechanism of boiling bubble dynamics in narrow channels, two types of channels are investigated which have I- and Z-shaped with width of 2 mm. Using VOF model and self-programming, the whole flow field is simulated with two different kinds of media, namely, water and ethanol. The influence of wall contact angle on the process of bubble generating and growth is studied, and the relationship between different channel shapes and the pressure drop is also investigated taking into account the effects of gravity, viscosity, surface tension and wall adhesion. The bubble generation, growth and departure processes are analyzed through numerical simulation and self-programming, and the influence of interface movements and changes on internal pressure difference and average surface heat transfer coefficient is investigated by using geometry reconstruction and interface tracking. It is found that wall contact angle has a great influence on the morphology of bubble. The smaller the wall contact angles are, the more round the bubbles are, and the less time the bubbles take to depart from the wall. The variation of contact angle also has effect upon the heat transfer coefficient. The greater the wall contact angle is, the larger the bubble-covered area is, thus the wall thermal resistance gets higher, and bubble nucleation is suppressed, and the heat transfer coefficient becomes lower. The role of surface tension in the process of boiling heat transfer is much more important than the gravity in narrow channels. The generation of bubbles dramatically disturbs the boundary layer, and the bubble bottom micro-layer can enhance the heat transfer. The heat transfer coefficient of Z-shaped channels is larger than that of I-shaped ones, while the pressure drop of the former is obviously higher. In addition, surface tension and viscosity significantly impact the pressure drop of boiling system, and different specific heat and boiling point values result in different heat transfer coefficients. The simulation results in this paper match well with the experimental data revealed in other sources, both show that the heat transfer coefficient of water is higher than that of ethanol and Z-shaped channels have better heat transfer capability.     

人工神经网络在预测流动沸腾曲线中的应用

选用20世纪60年代以来的实验数据，应用人工神经网络分析入口欠热度、质量流速、压力等主要参数对沸腾曲线的影响。在整个传热区内，热流密度随入口欠热度的增加而增大；在过渡沸腾和膜态沸腾区，热流密度随质量流速的增加而增加；压力起重要的作用，除膜态沸腾区外，增加压力能强化传热。除泡核沸腾外，稳态和瞬态的流动沸腾曲线的差异很小。     

Transient film boiling under transient conditions related to vapor explosion (effects of transient flow and fragmentation under a shock pressure)

Two fundamental phenomena are significant when a shock pressure interacts with the large scale coarse mixing state. One is an intensive flow and the other is the surface area enhancement due to the disintegration of the hot drops. The effects of these phenomena on the transient heat transfer and behavior of vapor film under a shock pressure are investigated. Transient heat transfer of film boiling from an electrically heated platinum ribbon 2.5 mm wide and 0.15 mm thick was measured immediately after passage of a shock pressure from 0.1 to 0.7 MPa. The heater was set horizontally in a vertical shock tube which was filled with vapor liquid bubbly mixture and kept initially in the film boiling state. That is, the heater corresponds to a typical hot drop and the bubbles around it correspond to the coarse mixture around the drop. The liquid was Freon-113 with an initial void fraction in the range from 0 to 3%. When the shock wave arrives at the heater, intensive transient flow occurs due to collapse of bubbles around the heater. First, the effects of the initial void fraction, the intensity of the shock and the heated wall temperature on the transient heat fluxes and collapse of the vapor film were investigated experimentally and analytically under the shock pressure. Compared with a heated wall in the liquid alone, the transient heat flux at the heated wall increases and the collapse of the vapor film becomes easier in the bubbly mixture due to the transient flow. Effects of surface enhancement during the fragmentation process on the heat transfer rate and transient behavior of vapor film are investigated analytically by application of the newly proposed surface stretch model. It is made clear when the surface area is increasing, the vapor film is apt to collapse and the transient heat transfer is enhanced by the surface stretch.     

Experimental study on convective boiling heat transfer in narrow-gap annulus tubes

Since convective boiling or highly subcooled single-phase forced convection in micro-channels is an effective cooling mechanism with a wide range of applications, more experimental and theoretical studies are required to explain and verify the forced convection heat transfer phenomenon in narrow channels. In this experimental study, we model the convective boiling behavior of water with low latent heat substance Freon 113 (R-113), with the purpose of saving power consumption and visualizing experiments. Both heat transfer and pressure drop characteristics were measured in subcooled and saturated concentric narrow gap forced convection boiling. Data were obtained to qualitatively identify the effects of gap size, pressure, flow rate and wall superheat on boiling regimes and the transition between various regimes. Some significant differences from unconfined forced convection boiling were found, and also, the flow patterns in narrow vertical annulus tubes have been studied quantitatively.     

中低压条件下矩形窄缝通道两相流动传热试验研究

在中低压条件下,对矩形窄缝通道两相流动传热进行试验研究,分析两相流动传热的变化规律,拟合出饱和沸腾传热系数计算关系式,并采用简化的一维分析方法对两相压降进行分析计算。试验结果表明:在相同热平衡含汽率(x)情况下,两相流动压降随系统压力(p)的降低而增大,随系统流量的增大而增大的变化规律;p越低,两相流动压降随x的增加而增大越剧烈;流量越大,两相流动压降随x的增加而增大越剧烈。通过数据回归方法得到汽相湿周长比例因子F并拟合了计算关系式,其计算值与试验值符合得较好。矩形窄缝通道内饱和沸腾平均传热系数受p、质量流量及热流密度的影响较大。     

蒸汽发生器传热管一、二次侧耦合换热及管外过冷沸腾数值研究

采用两流体欧拉数学模型,结合气相和液相之间的界面传热、传质和动量交换封闭模型以及RPI壁面沸腾模型,利用ANSYS CFX 12.0对蒸汽发生器局部传热管束二次侧的过冷沸腾进行数值研究。数值研究结果与单管内过冷沸腾实验数据对比验证符合良好。结果表明,采用壁面沸腾模型能准确预测沸腾起始点的位置,同时梅花孔板的存在对二次侧流动换热特性影响显著。