【作者】 李娟；

【导师】 汪荣顺； 石玉美；

【作者基本信息】 上海交通大学 ， 制冷与低温工程， 2013， 博士

【摘要】 液氦由于其极低的温度、超流动性（HeII）和热机械效应，在航天、军工、机械、医疗等领域都有重要用途。在液氦甚至超流氦温度下，如果其中存在任何杂质例如氧、氮、水分、碳氢化合物等等，这些杂质在低温下将固化，而任何细小的固体颗粒一方面将在系统内部造成污染，另一方面可能导致低温阀门不能正常关闭，从而使得液氦和超流氦大量流失。因此，必须研制出可行的液氦净化方法，尽可能最大量地去除其中的固体颗粒杂质，达到指定的纯度，从而保证所有设备的正常运转并满足寿命要求。过滤是从固液混合物中分离固体颗粒最有效的方法之一。成饼过滤过程中，利用表面型过滤介质进行两相分离，其特点是固体颗粒被截留在过滤介质表面形成滤饼。烧结不锈钢丝网过滤器是利用多层丝网叠层的合理搭配和真空烧结等复合工艺制备而成的一种多孔功能材料，具有机械强度高、能承受热应力及冲击、可在较高压差下稳定工作、易于反吹清洗、寿命长等特点。本文从理论和试验两个方面对于烧结不锈钢丝网过滤器这种多孔材料应用于液氦净化的机理和特性进行研究，具体内容及结论包括：一、过滤器结构型式及布置设计根据设计参数确定烧结不锈钢丝网过滤器的结构尺寸与性能指标，针对过滤丝网褶皱结构与圆筒结构、过滤器水平布置与垂直布置，结合计算流体动力学理论和多孔介质过滤理论，分别建立过滤器以及过滤外壳的整体三维模型。其中，使用多孔介质压降方程作为Navier-Stokes方程的动量源项。利用CFD软件StarCCM+求解模型。通过模拟结果与试验结果的对比分析发现，在相同外径、相同液氦流量以及相同的出口压力下，圆筒结构的压降为褶皱结构压降的2.67倍。重力作用对于整个过滤过程压降的影响较小，但是过滤器垂直布置不利于研究滤饼特性。因此，本文选用褶皱结构并将过滤器以水平形式布置。以上结论可为过滤器结构型式以及布置设计提供理论依据。二、液氦过滤与再生试验研究根据试验目的及原理，搭建液氦过滤与再生试验装置台，分为九个独立的系统：液氦输送系统、杂质气体充注系统、气液混合系统、过滤单元、滤液成分分析系统、反吹再生系统、真空绝热系统、安全保护系统、压力和温度测量系统。对于液氦/固氮颗粒、液氦/固氧颗粒两种滤浆，各自分别实施恒速过滤和恒压过滤试验。试验结果验证了烧结不锈钢丝网过滤器应用于液氦净化的可行性。恒速过滤试验结果表明：滤饼形成过程对于过滤特性具有关键影响。而且，在相同的固体颗粒体积浓度下，滤浆流量越高时压降升高趋势更快，到达最大允许压降的时间更短。在相同的滤浆流量下，固体颗粒体积浓度较高时，压降和过滤效率增长速度也较快。恒压过滤试验结果表明：固体颗粒体积浓度越高，滤浆流速下降越快，达到最低流速值的时间越短，过滤效率达到最高值的时间也越短。在恒速工况和恒压工况下，固氮滤饼和固氧滤饼的平均比阻都随着滤浆中固体杂质颗粒体积浓度的增加而降低。两种滤浆具有相同的结论。针对固氮滤饼、固氧滤饼，各自分别实施内部反吹、外部反吹、内外部反吹三种方式下的过滤器再生试验。定义反吹速度，比较三种反吹方式的差异程度。结果表明：三种反吹方式均能吹除过滤器表面以及过滤外壳内部沉积的固氮和固氧颗粒；内外部同时反吹方式的反吹速度最高，内部反吹方式次之，外部反吹方式最低。实际应用中，内外部同时反吹是最佳的再生方式。三、过滤器渗透率测试试验及结果分析基于多孔介质Darcy压降定律、Bernoulli方程、沿程损失与局部损失方程，计算过滤介质渗透率。利用纯净的低温液氦、液氮、常温氦气、氮气和氧气五种流体作为试验工质，最终获得烧结不锈钢丝网过滤器在三种温度下的渗透率数值。结果表明，低温下的渗透率比常温下低得多。这是因为低温下烧结不锈钢丝网发生一定程度的冷缩，流道结构发生改变。以体积应变为基本变量，使用Matlab软件对三种温度下的五组试验结果进行拟合，得出渗透率与温度的经验关系式，为烧结不锈钢丝网过滤器的实际应用提供参考。四、过滤过程数值模拟及试验结果对比分析基于滤饼压缩及分层特性，将滤饼生成过程离散成等时间间隔以内的逐层压缩过程，分析每一层滤饼的压降和厚度，结合颗粒沉积因子，建立滤饼生成模型。模型求解时，建立滤饼比阻连续性方程，以便利用Darcy方程计算每一层滤饼的压降；建立滤饼孔隙率连续性方程，以便利用Carman-Kozeny方程、Happel Cell方程和Ergun方程计算每一层滤饼的压降。提出目标函数，结合液氦+固氮颗粒、液氦+固氧颗粒这两种滤浆分别在三种工况下的恒速试验数据，利用反抛物线插值法求解目标函数最小值从而求取滤饼比阻连续性方程和滤饼孔隙率方程中的经验参数、最佳沉积因子、最适合试验工况的压降方程，为液氦过滤特性以及滤饼性质提供一种预测方法。更多还原

【Abstract】 Liquid helium is widely applied in astronavigation, military project, mechanicalfield and medical aspect, etc. as an important medium due to its very low temperature,super-fluidity (HeII) and thermal-mechanical effect. At the temperature of liquidhelium and even super-fluid helium, if there is any impurity such as oxygen, nitrogen,water, hydrocarbon, etc. in it, such substances shall solidify while any small solidparticle shall, in one hand, lead to pollution inside the system, and in the other hand,may make cryogenic valve to function abnormally. On such condition, liquid heliumand super-fluid helium will loss in a large amount. Therefore, feasible method must bestudied to purify liquid helium, so as to remove solid particle in it as a maximum,achieve the specified purity, guarantee normal running of all equipments and satisfyrequirement for lifetime.Filtration is one of the most efficient methods to separate solid particle out ofsolid-liquid mixture. During cake filtration, two-phase separation is achieved bysurface-type filtration medium, in which solid particles are entrapped on the surfaceof filtration medium to form filter cake. Sintered stainless steel screen filter is a kindof porous functional material which is made by composite technology, includingrational combination of multi-layer screen and vacuum sintering, and it ischaracterized by high mechanical strength, capacity to bear thermal stress and impact,work stably under high pressure difference, tend to regeneration, long lifetime, etc.In this paper, theoretical and experimental researches are conducted to study themechanism and characteristics when sintered stainless steel screen filter is used inliquid helium purification. Following contents and conclusions are involved in details:1. Filter Structural Pattern Design and Layout DesignStructural dimensions and performance indexes of sintered stainless steel screenfilter are determined based on design parameters. For pleated structure and cylindricalstructure of filter screen, as well as horizontal layout and vertical layout of filter unit,combine computational fluid dynamics theory with porous medium filtration theory toestablish overall3D model for filter and the shell. Wherein, pressure drop equationfor porous medium is adopted as the momentum term in Navier-Stokes equation. CFDsoftware, Star CCM+is used to solve the model. It is found by comparison andanalysis of simulation results and experimental results that, in the case of the sameexternal diameter, the same flow of liquid helium and the same outlet pressure,pressure drop in cylindrical structure is2.6times of that in pleated structure. Effect of gravity on the pressure drop in total filtration process is relatively small, but verticallayout of filter is unbeneficial for research on filter cake characteristics. Therefore,pleated structure and horizontal layout of filter unit are applied. Such conclusions canprovide theoretical basis for design of filter structure and layout.2. Liquid Helium Filtration and Regeneration TestIn accordance with test purpose and principles, liquid helium filtration andregeneration test rig is established, which is divided into nine independent systems, i.e,liquid helium transportation system, impurity gas filling system, gas/liquid mixingsystem, filtration unit, filtrate composite analysis system, regeneration system,vacuum and thermal-insulating system, security preservation system, pressuregauging system and temperature gauging system. For two kinds of flow slurry, i.e.,liquid helium and solid nitrogen, liquid helium and solid oxygen, constant velocityand constant pressure drop filtration tests are conducted. Experimental results verifythe feasibility to apply sintered stainless steel screen filter in liquid heliumpurification.It is shown by constant velocity filtration results that, generation of filter cakehas key influence on filtration characteristics. Furthermore, in the case of the samevolumetric concentration of solid particle, pressure drop shall increase more quicklywhen flow rate of slurry is higher, and it shall achieve maximum allowable pressuredrop in a shorter duration. In the case of the same flow rate of slurry, when volumetricconcentration of solid particle is higher, pressure drop and filtration efficiency shallincrease more quickly. It is shown by constant pressure drop filtration results that,when volumetric concentration of solid particle is higher, flow rate of slurry shalldecrease and achieve the minimum value more quickly, and filtration efficiency shallachieve the maximum value in a shorter duration. For constant velocity filtration andconstant pressure drop filtration, average specific resistance of solid nitrogen cake andsolid oxygen cake both reduce with the increase of volumetric concentration of solidparticle in flow slurry.For solid nitrogen cake and solid oxygen cake, filter regeneration tests in themanner of internal regeneration, external regeneration, simultaneous internal andexternal regeneration are conducted respectively. Regeneration speed is defined todescribe the difference for such three kinds of regeneration manners. It is shown byexperimental results that, three kinds of regeneration manners can all removecryogenic solid nitrogen and oxygen particles on filter surface and inside filter shell;simultaneous internal and external regeneration manner has the largest regenerationspeed, the next one is internal regeneration, and the last one is external regeneration.In practical application, simultaneous internal and external regeneration is the bestmanner.3. Filter Permeability Test and Result AnalysisOn the basis of Darcy pressure drop law for porous medium, Bernoulli equation, on-way resistance and local resistance equation, permeability of filter medium iscalculated. Five kinds of pure fluid, i.e., cryogenic liquid helium and liquid nitrogen,helium gas, nitrogen gas and oxygen gas at normal temperature, flow through thesintered staninless steel screen filter respectively, permeability values of the filter atthree kinds of temperature are acquired. It is shown by the results that, permeability atcryogenic temeprature is much lower than that at normal temperature, due toshrinkage of sintered stainless steel screen at cryogenic temperature to some degree,which changes flow passage structure.Volumetric strain is adopted as the basic variable, and Matlab software is appliedto fit experimental results and obtain empirical relation expression betweenpermeability and temperature, which provide reference for practical application ofsintered staninless steel screen filter.4. Numerical Simulation of Filtration Process and Comparison withExperimental ResultsBased on filter cake compression and lamination characteristics, filter cakegeneration procedure is divided into compression procedures layer by layer in equaltime internal. Pressure drop and thickness in each layer of filter cake is analyzed, andparticle deposition factor is combined with it to establish cake generation model. Inorder to solve the model, establish continuity equation for specific filter cakeresistance, so as to apply Darcy equation to calculate pressure drop in each layer offilter cake; establish porosity continuity equation, so as to apply Carman-Kozenyequation, Happel Cell equation and Ergun equation respectively to calculate pressuredrop in each layer of filter cake. Put forward objective function, combine withexperimental filtration data of two kinds of feed slurry (liquid helium and solidnitrogen particle, liquid helium and solid oxygen particle) on three kinds of constantvelocity conditions, apply inverse parabola interpolation method to obtain minimumvalue of such objective function, so as to acquire empirical parameters in continuityequation for specific filter cake resistance and porosity, the most suitable values ofparticle deposition factor and the most applicable pressure drop equation finally. Thesimulation shall provide a prediction technique for liquid helium purificationperformance and filter cake characteristics.更多还原