气—液—固三相流载气蒸发传热研究

本文在一由垂直设置的紫铜电加热棒与一玻璃套管构成的同心环隙内,对气—液—固三相流载气蒸发传热进行了实验研究.考察了载气进口速度,固体颗粒性质,热负荷等因素对传热的影响.结果表明:引入载气与固体颗粒,对环隙内对流沸腾传热均有显著强化作用,并使物料在加热壁面上的过热度明显下降.本文还分析了其传热强化的机理,并获得了计算传热膜系数的关联式.     

强化管管外升膜蒸发换热特性实验

升膜蒸发是利用微细槽道对液体的毛细抽吸作用,在强化管外表面覆盖一层薄液膜,进而以薄膜蒸发的形式实现强化换热.本文针对强化管在水中的浸入深度,蒸发压力,加热壁面过热度等因素对升膜蒸发换热性能的影响展开实验研究.实验结果表明随着管外液位的降低升膜蒸发换热系数明显提高,此外,蒸发压力和加热壁面过热度因素对升膜蒸发换热性能也有着显著的影响.     

自然循环型气液固三相流载气蒸发传热的实验研究

符号表Cs固体颗粒在液体中的含量vol．％dp固体颗粒直径mmde实验段当量直径mmh表面传热系数kw／m2Kk液体导热率W／mKq热通量kw／m2r液体汽化潜热kJ／kgTw加热壁面温度℃ug载气表现速度mm／sul循环液速m／sρg载气的密度kg／m3ρl液体的密度kg／m3ρs固体颗粒的密度kg／m3μg气相粘度mPa·sμl液相粘度mPa·sBo沸腾准数Nu努塞尔准数Reg载气雷诺数Rel液体雷诺数沸腾与蒸发装置内换热壁面上的结垢与结疤是降低换热效率的重要因素之一。换热装置的防垢抗垢一直受到重视。近年来，一些研究者将固体颗粒引入换热器的加热管内，形成流化床换热…     

加热表面粗糙度对高热流密度沸腾传热影响的多尺度数值模拟

本文以沸腾固气液界面为研究对象,建立了包括孔穴活化、液膜蒸发、气泡生长与脱离、壁面热传导等子过程的耦合模型,以探究固气液界面传热对高热流密度沸腾过程的影响。为了能够分辨微米量级的孔穴,模型中10 mm×10 mm的沸腾表面被划分为诸多子区域,每一个子区域中孔穴大小和数量随机分布,当子区域的过热度大于孔穴活化的临界过热度时,一部分孔穴活化生成气泡。进一步结合大液膜蒸发模型获得沸腾传热热流密度,并将其作为边界条件分析加热器热传导特性,从而通过对不同过程的多尺度耦合模拟不同表面粗糙度条件下高热流密度区的核态沸腾曲线,并进一步分析了孔穴数量及分布对加热壁面温度的影响。结果表明:预测所得沸腾曲线与实验结果基本相符,加热表面孔穴数量的增加使沸腾曲线左移,同时,孔穴数目的增多还会使活化点密度对壁面温度波动更为敏感,从而产生交替出现的长短周期。     

微细通道内蒸发薄液膜区壁面滑移及微流动特性

基于扩展Young-Laplace方程和动力学理论研究微通道中蒸发薄液膜区固液界面附近流动和传热特性,考虑压力特征、壁面滑移和温度跳跃建立物理模型.利用边界层近似,提出一种计算固液界面吸附微液层热阻的方法,导得固液界面的热阻和温度.数值模拟结果表明,壁面微流动会降低毛细压力梯度,增加壁面热阻,降低液相过热度,恶化液膜铺展和传热性能,在薄液膜区不可忽略.阐明壁面微流动含义,指出滑移系数与吸附流动微液层厚度的关系.     

液滴在不同润湿性表面上蒸发时的动力学特性

基于润滑理论,采用滑移边界条件建立了二维液滴厚度的演化模型和移动接触线动力学模型,利用数值计算方法模拟了均匀加热基底上固着液滴蒸发时的动力学特性,分析了液-气、固-气和液-固界面张力温度敏感性对壁面润湿性和液滴动态特性的影响.结果表明,液滴的运动过程受毛细力、重力、热毛细力和蒸发的影响,重力对液滴铺展起促进作用,而毛细力、热毛细力则起抑制作用;通过改变界面张力温度敏感性系数,可使液滴蒸发过程中的接触线呈现处于钉扎或部分钉扎模式,且接触线钉扎模式下的液滴存续时间低于部分钉扎模式;提高液-气与液-固界面张力温度敏感系数均可改善壁面润湿性能,加快液滴铺展速率;而增大固-气界面张力温度敏感系数则导致壁面润湿性能恶化、延缓液滴铺展过程;通过改变固-气界面张力温度敏感系数更有利于调控处于蒸发状态下的液滴运动.     

微重力核态沸腾近壁面汽泡生长模型

建立二维近壁面汽泡生长模型,研究过冷度,壁面过热度,壁面物性,接触角,核化点尺寸等因素对汽泡生长的影响.本文建立的汽泡生长模型在低壁面过热度条件下趋近于微液层蒸发控制的汽泡生长模型,在高壁面过热度条件下趋近于无限过热液体中传热控制球形汽泡生长模型.     

VOSET方法计算膜态沸腾的不同流态

本文在VOSET界面捕捉方法的基础上对控制方程进行修正,使其能够计算带相变的两相流问题,然后用这种方法计算水平壁面上的膜态沸腾。模拟得到的平均Nu数与Klimenko的关联式的计算结果基本一致。计算结果表明,在较低的壁面过热度下,膜态沸腾呈现气泡状流动;在较高的壁面过热度下,膜态沸腾呈现气带状流动。     

凝汽器管内插转子的热力与抑垢性能研究

针对凝汽器背压高和管内结垢问题,研发了具有自清洁能力的管内扰流物。为研究凝汽器管内置扰流物(内插转子)的综合性能,搭建了流动及抑垢实验台,建立了MRF数值模型。通过数值和实验方法,分析了内插转子的管内流动、换热及抑垢特性。结果表明,与传统转子相比,新型转子在阻力降低1/2的同时,增强换热10%～15%。其中空的结构形式对中心区域扰动较弱,能防止阻力增加过大。而对壁面附面层的层流底层的扰动,可增加壁面处的温度梯度有利于强化换热。内插转子可使污垢长期处于诱导期,避免管内污垢过快附着和增长。应用于某300 MW机组凝汽器,测算数据表明具有较高的运行稳定性和可靠性,在不同的负荷下均能提高真空度,起到节能降耗的效果。     

液滴在气固交界面变形移动问题的光滑粒子流体动力学模拟

为准确模拟液滴在气固交界面变形移动问题,对基于连续表面张力模型的表面张力光滑粒子流体动力学方法进行了改进.改进方法采用新的边界处理方式和界面法向修正方法,即将固体边界虚粒子色函数值根据液面的位置进行相应设定以保证气-液-固三相交界处流体粒子的界面法向沿接触线法线方向,引入Brackbill提出的壁面附着力边界条件处理方法,对在气-液-固三相交界处的流体粒子及部分固体边界虚粒子的界面法向进行修正,修正前后保持法向模值不变,得到了含壁面附着力边界条件的表面张力算法.模拟了受壁面附着力影响的水槽中液面的变化过程、液滴润湿壁面过程和剪切气流驱动液滴在固体表面变形脱落过程,并与流体体积函数方法进行了对比.结果表明,该方法在处理壁面附着力问题时精度较高,稳定性较好,适合处理工程中液滴在气固交界面变形移动问题.     

垂直套管环隙内汽液固三相流动沸腾传热的实验研究

符号表C。固体颗粒在液q热通量kw／mZu。固体颗粒与液体中的含量voL％，液体汽化游热kJ／kg体的滑移速度m／s小固体颗粒直径mm或mTi流体温度”CP；液体的密度kg／m’D实验段当量直径mm或mTw加热壁面温度“CP。固体颗粒的密度kg／m’h传热膜系数kw／m’K。1循环液速m／sH液相粘度mPa。k液体导热系数W／inK1前言自MatchL．P．等人首次将固体颗粒引入换热器应用于地热利用山后，对这种流化床换热器的研究开始增多【‘－‘］。研究结果均表明，固体颗粒的引入，可显著强化传热，并有较好的防垢抗垢性能。本文的研究是在液体在垂直套管环…     

毛细微槽内的相变传热的实验研究

本文对矩形毛细微槽竖直板的相变传热特性进行了实验研究。结果表明毛细微槽对相变换热具有很大的促进作用。当壁面过热度较小时,相变换热形式主要是三相接触线附近的蒸发换热机制。而当过热度较大时,微槽内发生剧烈的沸腾。微槽内相变换热的临界热负荷有两种产生机理:其一是当微槽长度较大时微槽内由于流动阻力而产生的液体输运临界;另一机理是当微槽长度较小时的池内沸腾临界现象,亦即由动态微液层模型决定的临界机理。实验还得到了微槽强化传热的最佳优化尺寸。     

烧结/堆积床多孔介质流动沸腾换热实验研究

本文实验对比研究了0.3 mm、0.5mm、0.7 mm三种粒径的铜颗粒烧结与堆积床多孔介质中的流动沸腾换热,主要研究了入口流速、热流密度、加热方位及粒径对流动沸腾换热的影响,以及多孔介质中的沸腾滞后。实验结果表明:大入口流速、低热流密度、下方加热以及小粒径时加热壁面的过热度较低,即有利于沸腾换热;本实验所用烧结多孔介质壁面过热度高于堆积床多孔介质,其原因是内部含有闭孔。     

尺寸效应对微通道内固液界面温度边界的影响

采用非平衡分子动力学方法模拟不同浸润性微通道内液体的传热过程,分析了尺寸效应对固液界面热阻及温度阶跃的影响.研究结果表明,界面热阻随微通道尺寸的变化可分为两个阶段,即小尺寸微通道的单调递增阶段和大尺寸微通道的恒定值阶段.随着微通道尺寸的增加,近壁区液体原子受对侧固体原子的约束程度降低,微通道中央的液体原子自由移动,固液原子振动态密度近似不变,使得尺寸效应的影响忽略不计.上述两种阶段的微通道尺寸过渡阈值受固液作用强度与壁面温度的共同作用:减弱壁面浸润性,过渡阈值向大尺寸区域迁移;相较于低温壁面,高温壁面处的过渡阈值更大.增加微通道尺寸,固液界面温度阶跃呈单调递减趋势,致使壁面温度边界和宏观尺度下逐渐符合.探讨尺寸效应有助于深刻理解固液界面能量输运及传递机制.     

Ignition of single nickel-coated aluminum particles

A thin coating of nickel on the surface of aluminum particles can prevent their agglomeration and at the same time facilitate their ignition, thus increasing the efficiency of aluminized propellants. In this work, ignition of single nickel-coated aluminum particles is investigated using an electrodynamic levitation setup (heating by laser) and a tube reactor (heating by high-temperature gas). The levitation experiments are used for measurements of the ignition delay time at different Ni contents in the particles. The high-temperature gas experiments are used to measure the critical ignition temperature. It is reported that the Ni coating dramatically decreases both the ignition delay time of laser-heated Al particles and the critical ignition temperature of gas-heated Al particles. A heat balance analysis of the levitated particles shows that the lower ignition temperature of Ni-coated Al particles is the most probable reason for the observed reduction in the ignition delay time. Exothermic intermetallic reactions between liquid Al and solid Ni are considered as the main reason for the lowered ignition temperature of Ni-coated Al particles.     

Numerical Simulation Study on Flow Laws and Heat Transfer of Gas Hydrate in the Spiral Flow Pipeline with Long Twisted Band

The natural gas hydrate plugging problems in the mixed pipeline are becoming more and more serious. The hydrate plugging has gradually become an important problem to ensure the safety of pipeline operation. The deposition and heat transfer characteristics of natural gas hydrate particles in the spiral flow pipeline have been studied. The DPM model (discrete phase model) was used to simulate the motion of solid particles, which was used to simulate the complex spiral flow characteristics of hydrate in the pipeline with a long twisted band. The deposition and heat transfer characteristics of gas hydrate particles in the spiral flow pipeline were studied. The velocity distribution, pressure drop distribution, heat transfer characteristics, and particle settling characteristics in the pipeline were investigated. The numerical results showed that compared with the straight flow without a long twisted band, two obvious eddies are formed in the flow field with a long twisted band, and the velocities are maximum at the center of the vortices. Along the direction of the pipeline, the two vortices move toward the pipe wall from near the twisted band, which can effectively carry the hydrate particles deposited on the wall. With the same Reynolds number, the twisted rate was greater, the spiral strength was weaker, the tangential velocity was smaller, and the pressure drop was smaller. Therefore, the pressure loss can be reduced as much as possible with effect of the spiral flow. In a straight light flow, the Nusselt number is in a parabolic shape with the opening downwards. At the center of the pipe, the Nusselt number gradually decreased toward the pipe wall at the maximum, and at the near wall, the attenuation gradient of the Nu number was large. For spiral flow, the curve presented by the Nusselt number was a trough at the center of the pipe and a peak at 1/2 of the pipe diameter. With the reduction of twist rate, the Nusselt number becomes larger. Therefore, the spiral flow can make the temperature distribution more even and prevent the large temperature difference, resulting in the mass formation of hydrate particles in the pipeline wall. Spiral flow has a good carrying effect. Under the same condition, the spiral flow carried hydrate particles at a distance about 3–4 times farther than that of the straight flow.     

等离子体反应器中传热与流动的三维数值模拟

本文采用我们新发展的三维计算机程序,对有载气和颗粒从单个喷孔侧向喷射的等离子体反应器中的传热与流动以及颗粒的运动轨迹与加热历程进行了三维数值模拟,并与相应的二维数值模拟结果进行了比较。模拟结果表明侧向喷入的载气所引起的三维流动效应相当明显,不同的颗粒在反应器内的运动轨迹与加热历程也有明显差别。     

Mechanism of heat transfer in heterogeneous droplets of water under intense radiant heating

Thermocouple measurements of temperature have been performed at three main points of heterogeneous water droplet–high-temperature gases system: on the surface and in the depth of a solid inclusion, as well as on the free surface of the water droplet. Investigations have been carried out for water droplets of an initial volume of 5–15 μl with single inclusions of cubic graphite particles of a typical size of 1 mm. The gas temperature varied from 700 K to 1200 K, which corresponds to the main practical applications: thermal purification of water from solid and liquid impurities, fire extinguishing, treatment of heat-loaded surfaces of power equipment, etc. A hypothesis about the dominant role of radiant heat transfer in vaporization within heterogeneous water droplets has been grounded. It has been shown that in a short period (a few seconds), the surface temperature of an opaque solid inclusion within a droplet can reach the boiling point of water. A significant change in the optical properties of water with increasing temperature has been revealed, i.e., water became partially transparent to the infrared radiation. Presence of an opaque heterogeneous inclusion enhances this effect due to intensification of the heating of the water film. The heat and mass transfer characteristics obtained in the experiments were used for designing a model that takes into account the radiative properties of water film and adequately reproduces the results of thermocouplemeasurements. Based on the findings of the investigations, a conclusion has been formulated that models of high-temperature evaporation of water droplets should be developed with due account of changes in the optical properties of water and formation of a vapor buffer layer around inclusions.     

Heat and mass transfer at gas-phase ignition of grinded coal layer by several metal particles heated to a high temperature

Characteristics of gas-phase ignition of grinded brown coal (brand 2B, Shive-Ovoos deposit in Mongolia) layer by single and several metal particles heated to a high temperature (above 1000 K) have been investigated numerically. The developed mathematical model of the process takes into account the heating and thermal decomposition of coal at the expense of the heat supplied from local heat sources, release of volatiles, formation and heating of gas mixture and its ignition. The conditions of the joint effect of several hot particles on the main characteristic of the process–ignition delay time are determined. The relation of the ignition zone position in the vicinity of local heat sources and the intensity of combustible gas mixture warming has been elucidated. It has been found that when the distance between neighboring particles exceeds 1.5 hot particle size, an analysis of characteristics and regularities of coal ignition by several local heat sources can be carried out within the framework of the model of “single metal particle / grinded coal / air”. Besides, it has been shown with the use of this model that the increase in the hot particle height leads, along with the ignition delay time reduction, to a reduction of the source initial temperatures required for solid fuel ignition. At an imperfect thermal contact at the interface hot particle / grinded coal due to the natural porosity of the solid fuel structure, the intensity of ignition reduces due to a less significant effect of radiation in the area of pores on the heat transfer conditions compared to heat transfer by conduction in the near-surface coal layer without regard to its heterogeneous structure.