水平窄空间沸腾换热的数理模型研究

窄空间只有在间距小于汽泡脱离直径时，对沸腾传热强化才有比较显的效果。窄空间沸腾强化传热的机理在于较大的泡底微层加速了蒸发传热和窄空间中被加热的液体周期性地与池液进行容积交换。水平圆盘窄空间中的汽泡生长分为性质完全不同的自由生长期和抑制长大期；在一个周期内，加热面的总传热量等于壁面传导给窄空间液体的热量与通过合体泡底微层蒸发潜热之和。在对圆形水平窄空间的沸腾传热的现象和机理进行分析的基础上，提出了窄空间的沸腾换热过程的数理模型；进而对窄空间沸腾的本质规律在理论上进行了初步探索，并得到分析解。理论计算结果与实验数据比较表明，该分析解适合于中低壁面过热度的情形。由于问题的复杂性，该模型仍需不断完善。     

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.     

高饱和蒸发压力氨喷雾冷却沸腾传热特性

利用冷却工质的相变蒸发带走大量热量的喷雾相变冷却技术成为大功率电子元件散热需求的最佳途径.建立了双喷嘴阵列氨喷雾相变冷却实验系统,研究了饱和蒸发压力以及进口流量对氨喷雾相变冷却传热特性的影响规律.实验结果表明:在氨喷雾相变冷却过程中,维持较高的饱和蒸发压力有利于传热系数提高,过热度降低;流量对传热特性影响较大,低流量时...     

Optimal analysis of the exothermic process of a Mg/MgH2 thermochemical heat storage system

基于镁/氢化镁热化学储热系统,建立了二维非稳态数学模型.对吸氢放热过程中的传热传质现象进行了数值模拟,主要研究了壁面温度和反应床当量导热系数对系统反应速率的影响.结果表明,放热过程中存在最佳的壁面温度使反应速率达到最快,过高或者过低的壁面温度都将使反应床的温度偏离理论上的最佳值,从而降低反应速率.针对不同当量导热系数的反应床,最佳壁面温度也不相同;反应床的当量导热系数并非越大越好,应该根据具体的边界温度以及氢气压力情况进行合理的选择以获得最佳的反应速率.     

一种熔融铝液辐射余热回收用集热器的性能仿真研究

以回收熔融铝液辐射热量的集热器为研究对象,建立了集热器热力学过程理论模型,并采用Workbench和Fluent软件对集热器辐射传热过程和对流传热过程进行仿真分析。结果表明：当辐射距离为400~900 mm时,壁面平均温度在190~350 ℃之间;出口压力和壁面温度增加导致对流传热系数减小,而流量增加则使对流传热系数增加;流量和出口压力增加导致出口温度降低;系统流量增加造成压缩空气压降增大,出口压力增大导致压降减小,而壁面温度对压降没有影响。     

下降管反应器内颗粒流动升温过程数值研究

采用数值模拟的手段研究了下降管反应器内包含不同尺寸及密度冷热颗粒混合物的流动传热特性。双流体模型及离散单元法分别被用于描述颗粒混合物的流动过程并与实验结果进行了对比。反应器内气固相间传热,颗粒混合物间碰撞传热,以及壁面与气/固两相间的热量传递采用计算流体力学和离散单元法相耦合的方式进行了模拟,对颗粒到达反应器出口前影响温度变化趋势的因素展开了分析研究。模拟结果表明:在V型下降管反应器内,粒度较小的颗粒以沿壁面向下滑动为主;较大尺寸颗粒向下流动过程中在反应器截面上分布区域较广;当反应器壁面热边界条件发生变化时,颗粒升温过程变化明显,采用恒温壁面冷颗粒升温速率明显提高;同时热载体颗粒数目越多,冷颗粒在下降管反应器内升温越快。     

闪急沸腾条件下的乙醇气泡生长数值模拟

采用能量方程和动量方程耦合求解的方法,开展了闪急沸腾条件下的乙醇气泡生长数值模拟研究.通过与过热水和三氟三氯乙烷气泡生长的实验数据对比,验证了数值模拟方法的准确性.在此基础上,通过改变环境压力和过热度,研究了不同初始状态下乙醇气泡半径、生长速度、生长加速度、不同的力、热边界层温差和厚度等参数随时间的变化规律.结果表明,乙醇气泡生长过程中表现出的生长特性是抑制生长的表面张力、黏性力、流动阻力和促进生长的气泡内外压差及热反馈效应相互竞争的结果.雅各比数Ja对不同阶段的气泡生长特性有较大影响,随雅各比数增大,乙醇气泡生长过渡阶段由热传递控制逐渐转变为惯性力控制;环境压力一定时,雅各比数越大,乙醇气泡在表面张力控制阶段的生长延迟时间越短,最大加速度越大,在热传递控制阶段的生长速度也越快.     

热回收式CO2制冷系统性能研究

对热回收式CO2制冷系统性能COP进行了计算和分析,结果表明:蒸发温度、气体冷却器出口CO2温度、热水加热器入口水温是影响其COP的主要因素;回热器出口过热度对COP的影响较小,对压缩机的排气温度影响较大;随着排气压力的升高,COP是否出现峰值,取决于气体冷却器入口制冷剂的特征温度;在相同工况下,蒸发温度、气体冷却器出口CO2温度、回热器出口过热度对最佳排气压力的影响较小,热水加热器入口水温是影响最佳排气压力的主要因素。     

锅炉停炉后的持续供汽能力计算

建立了锅炉汽包的物理与数学模型,采用Visual Basic对研究对象进行编程计算。探讨了锅炉停炉后的闪蒸供汽特性随初始水位和初始压力的变化规律以及压降速率对供汽特性的影响。研究结果表明:汽包内初始压力越高,水位下降越快。在闪蒸过程中存在临界水位并对汽包内的最终压力和最终水温影响较大。当初始压力为5.72MPa、4.97MPa和4.12MPa时,对应临界水位为160mm、140mm和120mm。初始压力变化时,闪蒸效率随初始水位的变化规律不同。闪蒸后供汽时间与初始压力和初始水位密切相关。压降速率不会对最终水位、压力、水温和供汽时间等参数产生较大影响。     

多孔表面新型复杂结构优化沸腾传热的实验研究

报道了R11在烧结多孔表面开槽时沸腾传热的实验研究，实验发现，与普通槽道和双空隙层多孔表面相比，沸腾换热增强，沸腾表现为液体灌注、槽道起泡、底部蒸干三个区，对特定的多孔层，开槽可获得更好的换热效果。带槽道的多孔表面实验件与均匀多孔表面相比，在相同壁面过热度（θ）条件下，热流密度（q）提高2－10倍，临界热流密度提高2－4倍。     

The dynamics of spherical bubble growth

Spherically symmetric bubble expansion in uniformly superheated infinite pools of liquid have been simulated numerically. Bubble growth curves have been generated for a range of Jakob numbers, 3?Ja?3167, by altering the initial metastable state of the system facilitated by changes in the initial superheat and system pressure. The detailed physics of vapour bubble growth is studied through delineation of the parameters governing the changes in the growth dynamics from surface tension, to inertia dominated, to diffusion controlled, and the domains between them.     

Some experimental observations on the single and multi-phase flow patterns in a model flash evaporation chamber

Flow characteristics inside a model flash evaporation chamber for Multi-stage Flash (MSF) desalination have been investigated using Particle Image Velocimetry (PIV). The back flow region near the inlet gate, the free water surface shape, and the overall velocity distribution were measured so as to identify the characteristics of the flow pattern at different water levels in the chamber. The results show that a large recirculation region with several vortices embedded would be generated at a higher water level (approximately twice the inlet gate height). The water level remained uniform in spite of the presence of the vortices. Three flash evaporation modes at different superheats of the water in the flash chamber were also studied using a high speed CCD camera. The bubble behaviors such as the size, velocity, and growth rate were estimated and analyzed. It was found that the multi-phase flow pattern during the flash evaporation was mainly determined by the liquid superheat and the nuclei distribution.     

Effect of an electric field on bubble growth rate

A uniform electric field elongates spherical bubbles to prolate spheroids. This results in a larger surface area to volume ratio for a given bubble size, and more efficient heat transfer from a superheated liquid to the bubbles during nucleate boiling. Hence bubbles should grow faster in an electric field. This paper studies the bubble growth rate in an electric field using the heat diffusion controlled growth model.     

Simulation on nucleate boiling in micro-channel

Using the VOF multiphase flow model, numerical simulations are conducted to investigate the nucleate boiling of water in micro-channels. The Marangoni heat transfer through the bubble surface is analyzed, and is compared with the incipient heat flux at the onset of nucleate boiling in micro-channels. The bubble growth in the channel is divided into two stages. At the initial stage, bubble growth is controlled by surface tension, while at the second stage the incipient heat transfer dominated the boiling process. In the results, the full process of bubble generating, growing, departing, combining, and shrinking in the channel is displayed. The simulated results with similar condition are agreed well with some experimental results in references. The method and discussion in the paper are helpful to the investigation of the mechanism of micro-scale two-phase flow and heat transfer.     

黏性液体中锐孔处气泡的形成

考察一定流量气体,通过锐孔在静止黏性液体中连续溢出气泡的过程。应用动力学平衡半经验关系式,综合考虑气泡受力,分析气泡形成过程,给出合理假设,预测气泡直径。分析表面张力、气体流速、锐孔直径及液相物性对气泡脱离尺寸的影响,找到影响气泡脱离尺寸的主要因素。计算预报值与实验结果符合良好。     

Explosive vapor bubble growth in uniformly superheated liquids: R-113 and mercury

Based on experimental and analytical work conducted previously with R-113, the results of an analytical study of the vapor bubble dynamics for mercury associated with nucleation and growth are presented here. The simulations show that a growing mercury vapor bubble can be substantially unstable under sufficiently high superheat, as with a superheat level of 100 °C, and that surface tension tends to stabilize the process. At a superheat level of 30 °C, the growth is marginally stable due to the high surface tension, and quite stable in the very early stages of the growth. For most cases, the wavelength of growing perturbations appear to be relatively large compared to that for water, again due to the large surface tension of mercury. As a result of the high liquid-to-vapor density ratio, the growth rates of mercury vapor bubbles can be quite high, reaching a diameter of 1 m in 1 s, with an initial superheat of 100 °C and pressure of 0.1 atm. The effects of system pressure are also considered here.     

Mechanical nonequilibrium considerations in homogeneous bubble nucleation for unsteady-state boiling

With thermal and mechanical nonequilbrium taken into consideration, the classical kinetic theory of boiling is modified to study unsteady-state homogeneous nucleation processes. Based on this newly developed model, the degree of superheat and the maximum nucleation rate corresponding to different rates of temperature rise in water are calculated and presented. For the first time, the initial nonequilibrium vapor pressure and the initial growth rate of bubble nuclei with different initial embryo sizes and different rates of temperature rise are accurately modeled. The resulting algorithm provides a method by which the details of bubble nucleation in a superheated liquid can be predicted, leading to a better understanding of the kinetics of boiling. Model validation, accuracy and application are also presented and discussed.     

Lattice Boltzmann Simulation of Growth and Deformation for a Rising Vapor Bubble Through Superheated Liquid

Combining with a lattice Boltzmann thermal model, a lattice Boltzmann multiphase model with a large density ratio can be extended to describe phase change with mass and heat transferring through the interface. Based on the Stefan boundary condition, the phase change is considered as a change of phase order parameter and is disposed as a source term of the Cahn-Hilliard equation. This hybrid model is applied to simulate the motion and growth of a rising vapor bubble through a uniformly superheated liquid. Meanwhile, the parametric effect on the bubble growth, deformation and rising in the different surface tension forces and kinetic viscosities are also presented.     

湿蒸汽非均质高速凝结流动的数值研究

基于冠状成核机理,建立了湿蒸汽两相非均质凝结流动数值模型,对缩放喷管、汽轮机叶栅和汽轮机级内湿蒸汽两相非均质凝结流动进行了数值模拟．结果表明：与自发凝结相比,非均质凝结流动中杂质颗粒改变了凝结过程;杂质颗粒减小了喷管中凝结激波强度,改变了汽轮机叶栅中的压力分布,降低了蒸汽过冷度,减少了不平衡热力学损失;在汽轮机级内,非均质凝结流动的动、静叶进、出口汽流角接近过热蒸汽流动的动、静叶进、出口汽流角,其动叶前压力高于过热蒸汽的动叶前压力,但级反动度偏离过热蒸汽流动数值．     

Confined growth of a vapour bubble in a capillary tube at initially uniform superheat: Experiments and modelling

Bubble growth was triggered in a capillary tube closed at one end and vented to the atmosphere at the other and initially filled with uniformly superheated water. Measurements of the rate of axial growth and the varying pressure at the closed end were used to test under these simplified conditions assumptions employed in one-dimensional models for bubble growth applicable to the more complex conditions of confined-bubble flow boiling in micro-channels. Issues included the thickness of the liquid films round confined bubbles and changes in saturation temperature due to the changes in pressure generated by bubble motion. Modelling features requiring further attention were identified, such as the possibility of “roll-up” of the liquid film due to a large dynamic contact angle.