真空闪蒸喷雾冷却中非等温液滴闪蒸特性的研究

真空闪蒸喷雾冷却是利用液体工质在真空环境下相变吸热来冷却加热表面的新型冷却手段。具有散热能力强、所需工质少,与加热表面没有接触热阻等优点,在航天器电子元器件冷却方面具有广阔的应用前景。液滴闪蒸是真空闪蒸喷雾冷却闪蒸过程的重要组成部分,要研究整个真空闪蒸喷雾冷却系统的闪蒸过程就必须对液滴的闪蒸特性进行研究。本文考虑液滴闪蒸过程中液滴内部存在的温度梯度和对流的影响,采用扩散控制蒸发模型并结合导热方程,对液滴的热导率进行修正,建立了热导率修正模型计算直径为微米级液滴在毫秒量级时间内的真空闪蒸特性,并通过实验验证。研究结果表明,导热模型较等温模型能更准确地预测液滴温度的变化;液滴闪蒸导致液滴到达被冷却表面时,其温度下降明显,但体积几乎没有变化;并且,环境压力越低,液滴的初始半径越小,液滴速度越大,液滴闪蒸对液滴温度的影响越明显。     

真空环境下乙酸液滴过冷凝固过程传热传质特性

利用真空闪蒸系统对乙酸液滴在真空环境下的过冷凝固过程进行了动态观测,建立了针对单个乙酸液滴在真空环境下过冷凝固过程的传热传质数学模型。模型考虑了液相和固相内的温度分布,引入过冷凝固的动力学条件以及表面升华的传质方程描述凝固界面与升华界面的运动。综合实验测量数据和模型计算结果,掌握了乙酸液滴在环境下过冷凝固过程中的温度变化,比较了乙酸液滴和纯水液滴过冷凝固过程中温度变化的特征,分析了环境压力对液滴温度变化的影响。结果显示:与纯水液滴相比,乙酸液滴具有更高的凝固平衡温度,在实验过程中更早开始凝固。由于乙酸凝固潜热较小,再辉阶段温度上升较为缓慢,凝固时间较短。环境压力越低,凝固过程中液滴过冷度越大,凝固速度越快,液滴最终温度越低。     

对流扩散-多相相变体系内柱状晶/等轴晶形成过程的数值模拟

采用扩散支配相变动力学方法对Fe-Bi-Mn系易切削合金侧向快速凝固过程进行数值研究。建立对流扩散-多相相变体系三维凝固模型,考虑固、液、气三相扩散流动相变对合金凝固的影响,模拟研究合金中MnS和Bi(易切削相)的柱状晶/等轴晶形成过程。结果表明:合金凝固过程中MnS和Bi的柱状晶/等轴晶形成模式强烈受对流扩散和多相相变影响;对流扩散为正值处,溶质的多相质量相变速率较大且富集程度较低,流动稳定易形成柱状晶;对流扩散为负值处,溶质的多相质量相变速率较小且富集程度较高,当晶尖处溶质富集到一定程度,对流扩散与多相相变产生的紊流使柱状晶尖端断裂,成为等轴晶形核中心,此处为等轴晶稳定形成区域。     

有无重力作用下Al-Bi偏晶合金凝固过程的模拟研究

基于二元Al-Bi偏晶合金难混溶区间凝固过程的液-液相分解及分离机制,采用欧拉法求解该合金显微组织演化的数学模型。该模型考虑了实际凝固过程中导致第二相宏观偏析的诸多复杂因素,包括形核、扩散长大、Stokes运动、Marangoni运动及重力沉降的影响。模拟结果表明,无重力条件下Marangoni力驱动L2相小液滴向中心高温区迁移,而重力条件下却呈现轴对称环流运动形式;随着冷却过程的进行,L2相在重力作用下发生沉降聚积,铸件底部L2相体积分数和液滴直径增加显著;与重力条件相比,无重力条件更有利于晶核的形成。     

基于液化天然气低温冷藏车的相变蓄冷实验

针对以水为相变工质通过铜质圆管壁与低温氮气换热发生固液相变问题,通过测温和可视化测量手段模拟研究固液相变贮存LNG冷量过程,获得管内低温气体、管外液相区温度分布及冰层图相,分析了管内换热和液相区自然对流综合影响下的冰层变化和分布特性,结果表明:该换热问题具有典型的变壁温变热流密度的热边界条件;冰层厚度在有限时间内近似线性增长,且沿管长锥状分布、冰层锥度随时问呈对数增长趋势;由于液相区水的密度反转效应使自然对流主流向发生改变,导致上下壁面冰层厚度发生反转.     

固液相变蓄能的数学模型和自然对流换热系数的实验研究

引入自然对流换热系数 ,将固液蓄能数学模型简化为仅用能量方程加以描述。并通过实验测得相变过程的实际温度场 ,证明了自然对流固液相变换热的影响不可忽略 ,验证了固液相变界面移动速率随自然对流换热系数的增大而增大的定性关系     

真空冷却过程中水分蒸发沸腾的数值模拟（Ⅰ）：模型的建立

真空冷却过程是复杂的相变传热传质过程。在真空冷却期间，当真空室内的真空压力低于或者等于熟肉温度对应的饱和压力时，熟肉内部的水分将会沸腾蒸发以产生冷却效应。在能量和质量守恒的基础上，经过适当的简化，建立了真空冷却过程中水分迁移的数学模型。此模型能够预测真空冷却过程中熟肉内部的温度、压力和水分含量分布。     

微观结构表面接触角模型及其润湿性

润湿性是固体表面的重要性质之一,固液界面润湿性与表面微观结构、界面能等有关.研究了微观结构固体表面固液界面润湿性以及界面能、液滴重力、微观结构参数对润湿性的影响.分析表明,Young、Wenzel、Cassie、Cassie-Baxter等4种接触角模型分别把固体表面看成光滑、粗糙(液滴完全填满)、粗糙(液滴不填充)以及粗糙(液滴部分填充),后3种模型可用于实际固体表面,并可在3种状态下实现转换.设计了方柱凹坑、圆柱凹坑等微观结构表面,调控横径比、纵径比、深径比等微观结构参数,可以改变固液界面润湿性.研究发现,液滴重力对表面接触角存在稍许影响,界面能的大小决定着材料表面的疏/亲水性.     

盐水液滴真空蒸发过程的热力学研究

基于质量与能量守恒原理、热力学理论和相变传热传质原理,建立了单个盐水液滴真空蒸发过程的数学模型,对比理论模拟结果与实验数据,验证了模型的有效性。通过模型计算探讨了液滴的无量纲面积,液滴内部浓度分布和温度分布随时间的变化,理论分析了盐水液滴真空蒸发过程中温度变化的影响因素。结果表明,抽真空过程中,盐水液滴内部存在显著温差,而浓度梯度很小。最终环境压力、液滴初始盐分浓度和初始直径对温度变化影响显著;而液滴初始温度对真空蒸发过程影响很小。     

制冷系统热回收装置相变材料的分析研究

分析了相变材料特性,并找出了适用于制冷系统热回收装置的相变材料,通过对热物性和工作性能的研究,选取Fecl3·6H2O为本装置的相变蓄能材料,同时建立蓄热体的物理模型及模拟简化模型,在自然对流的影响下,模拟相变材料的蓄、放热特性,找出固——液界面、温度场、速度场、液相比例、传热系数、热流密度、监测点的温度等随时间的变化规律,为制冷系统热回收装置的设计提供了理论依据。     

Theoretical Investigation on Rapid Evaporation of a Saline Droplet During Depressurization

This paper reports a theoretical investigation on rapid evaporation of a saline droplet during depressurization. A mathematical model was developed to simulate the droplet temperature variation by considering the ambient pressure change, the heat transfers due to evaporation and convection at the droplet surface, accompanying the heat and mass transfer inside the droplet. The component diffusion and the temperature gradient inside the droplet were mainly discussed by comparing the numerical droplet temperature with the experimental data. The result shows that, the variation of internal concentration is small, while the temperature gradient within the droplet is significant during the evaporation process. In addition, the influencing factors of the droplet temperature variation were analyzed, such as: the final ambient pressure, theinitial salt concentration and the initial droplet temperature. The present model calculations help to understand the thermodynamic process of rapid evaporation of a saline droplet during depressurization.     

真空法制取二元冰的理论分析

新的蓄冷介质越来越被人们重视,二元冰俗称冰浆是其中一个比较受到人们关注的一种蓄冷介质。制取冰浆的方法有很多种,其中真空法就是其中一种。因为它的制取步骤比较简单,不需要消耗太多的电力能源,同时制取二元冰的效率很高。通过对真空室内下落液滴的质量、速度、温度、下落距离建立微分方程,选定不同的初始条件和真空条件,模拟计算求解。通过对计算结果的比较、分析,明确指出在试验设计时应该选用喷射的液滴初始直径较小的喷头,同时保持真空室稳定在压力较小的环境下。     

熔模精铸型壳预热转移过程温度场及换热规律研究

目的 为了获得不同工艺条件对高温合金熔模铸造型壳在预热及转移过程中温度分布的影响规律。方法 根据实际工况设计了测温实验方案,采用热电偶测温的方法研究型壳在无保温措施、外加保温棉、填砂以及保温棉复合填砂4种工艺条件下对型壳在预热及转移过程中温度分布的影响。获得了型壳升温、保温和转移过程中的温度场变化曲线,并根据实际测温曲线采用ProCAST的反算模块对关键位置的界面换热系数进行了计算。结果 型壳外加保温材料后导致升温时间范围从原本的1.5 h升高到2.5~5.6 h,同时冷却时间也随之增加。结论 在转移过程中,外部材料的换热方式均从自然对流转变为强制对流,换热强度增大,静置于铸型室后,换热方式恢复至稳定的自然对流方式,界面换热系数趋于稳定,且冷却曲线呈线性降低。     

Coupled heat and mass transfer during nonisothermal absorption by falling droplet with internal circulation

We considered mass and heat transfer during nonisothermal absorption of a gas by a falling droplet with internal circulation. Gas phase is assumed to be free of inert admixtures and mass transfer is liquid phase controlled. Mass flux is directed from a gaseous phase to a droplet, and the interfacial shear stress causes a fluid flow inside the droplet. Droplet deformation under the influence of interface shear stress is neglected. Absorbate accumulation and temperature increase in the bulk of liquid phase are taken into account. The problem is solved in the approximations of a thin concentration and temperature boundary layers in the liquid phase. The thermodynamic parameters of the system are assumed constant. The system of transient partial parabolic differential equations of convective diffusion and energy balance with time-dependent boundary conditions is solved by combining the similarity transformation method with Duhamel's theorem, and the solution is obtained in a form of Volterra integral equation of the second kind which is solved numerically. Theoretical results are compared with available experimental data for water vapor absorption by falling droplets of aqueous solution of LiBr.     

往复振荡对活塞冷却油腔内纳米流体传热及流动特性的影响

内燃机工作过程中,燃烧产生的部分热能传给活塞。当活塞功率密度超过0.3 kW/cm~2时,必须采用冷却油腔进行冷却。为揭示纳米流体在冷却油腔内的传热及流动特性,对不同种类纳米流体在随活塞冷却油腔同步往复振荡状态下的传热和流动特性进行了对流换热和可视化实验。研究发现:最优传热充液率为53.4%;往复振荡频率、颗粒的水力半径与活塞冷却油腔内的对流换热系数成正比;转子转动角度在180°~270°范围时,工作流体混合效果最佳;纳米流体的流动紊乱度在整个往复振荡周期内均好于纯净水。     

A simple transient numerical model for heat transfer and shape evolution during the production of rings by centrifugal spray deposition

Centrifugal spray deposition, the atomisation of a liquid metal by centrifugal force and the subsequent collection of the atomised droplets on a reciprocating collector, is currently being developed for the production of high performance Fe, Ni and Ti based ring-shaped components for use in aerospace and gas turbine containment applications. The process combines the technical, economic and metallurgical benefits of more conventional gas-assisted spray forming techniques with the advantage that it can easily operate under vacuum, reducing potential problems from gas entrapment and thermally induced porosity. In order to aid process development, understanding and optimisation, a transient numerical heat and mass transfer model has been developed that is capable of predicting the evolution of the deposit temperature distribution during spraying. The model has been validated experimentally using thermocouple measurements obtained during the production of 35 kg (340 mm diameter) IN718 rings and qualitative correlations have been observed between the predicted data and the type/distribution of porosity and second phase precipitates in the deposit. The model is currently being further developed and integrated with droplet size distribution and cooling models to provide a better insight into the physics and operational parameters which control deposit shape and microstructure.     

Experimental study and modelling of the crystallization of a water droplet

Modelling the crystallisation of a water droplet into a cold humid airflow is the first step in modelling the behaviour of water droplets sprayed out of a snow gun. This modelling, which is based on an experimental study, deals with the behaviour of the droplet which is transformed successively from the supercooled liquid phase to the liquid–solid phase and then the solid phase. These three transformations are brought about by various exchanges: heat transfer with conduction and convection as well as mass transfer with evaporation and sublimation. The supercooling phenomenon is naturally observed during experiments and taken into account in the modelling.     

Nonsteady evaporation and growth of drops in gas(EOUS) medium

The process of heat and mass transfer during the evaporation or condensational growth of single drop in gas is investigated in quasistatic approach. The convection and diffusion in gas and heat conductivity in gas and drop are taken into account. For single drop in infinite gas medium the new analytical solution, which describes the asymptotic quasi-steady regime of phase transfer is obtained. There is the discussion of the numerically obtained distributions of the temperature and concentration and the laws of phase transfer and pressure change for particle in finite and infinite medium.     

Simulation Research of Vaporization and Pressure Variation in a Cryogenic Propellant Tank at the Launch Site

In order to improve depiction of pressure variation and investigate the interrelation among the physical processes in propellant tanks, a 2D axial symmetry Volume-of-Fluid (VOF) CFD model is established to simulate a large-sized liquid propellant tank when the rocket is preparing for launch with propellant loaded at the launch site. The numerical model is considered with propellant free convection, heat transfer between the tank and the external environment, thermal exchange between propellant and inner tank wall surfaces, gas compressibility, and phase change modeled under the assumption of thermodynamic equilibrium. Vaporization rate of the vented LH2 tank and prediction of pressure change in the tank pressurized with GHe are obtained through simulation. We analysis the distributions of phase, temperature, and velocity vectors to reveal interactions among the propellant’s own convection motion, heat transfer and phase change. The results show that the vaporization rate is mainly affected by heat leaks though the tank wall when the tank is vented, but it does not completely accord with the trend of the leakage because of convection motion and temperature nonuniformity of the liquid propellant in the tank. We also find that the main factors on pressure variation in the pressurized tank are the heat transfer on the tank wall surface bonding the ullage and propellant vaporization which has comparatively less influence.     

真空冷却过程的机理分析

真空冷却过程是复杂的相变传热传质过程,该过程涉及到扩散、传热、传质和相变等机理.在能量和质量守恒的基础上,经过适当的简化,建立了真空冷却过程中的数学模型.通过对结果分析发现,数值模拟的结果与实验数据吻合很好,误差在5%以内.数学模型可以准确预测真空冷却过程中的温度变化和质量损失,将有助于进一步理解和研究真空冷却机理.