玻璃钢及多层绝热材料真空放气速率试验研究

在低温储运时，由于真空夹层中材料的放气，夹层真空度会下降，热量从外界导入使得低温容器的蒸发率加大，低温液体损耗增加，真空寿命缩短。因此，对真空夹层中材料放气性能的研究非常重要。本文基于静态升压测量法，搭建了真空下材料放气率测试装置，进行了低温储罐用多层绝热材料和玻璃钢真空下放气速率测试研究，得到了多层绝热材料和D3848玻璃钢的单位面积放气速率分别为4.93×10^-8Pa·m³/(s·m²)和1.13×10^-7Pa·m³/(s·m²)，该结果可以为真空夹层吸附剂量的设计提供可靠依据。     

高真空多层绝热结构层间压力反演

高真空是液氢温区多层绝热的基础，通过不同抽气工艺获得的多层绝热材料层间压力分布，使绝热结构性能有显著差异。为了预测多层绝热材料层间压力分布，并作为抽气工艺的评价依据，基于逐层传热学模型，首先通过理论计算建立由不同层间压力分布对应的温度分布组成样本集，应用截断奇异值分解获得其正交基。再根据某绝热结构样品实测的多层材料层间温度分布，采用吉洪诺夫正则化方法对正交基系数进行回归预测，重构层间压力分布，即反演获得了多层绝热材料的层间压力。通过与给定算例的对比验证，结果表明，采用本方法可以从易于精确测量的层间温度分布反演获得层间压力分布，更精确地预测多层绝热材料的性能。为评价、比较和优化液氢容器高真空多层绝热结构的抽气工艺提供参考。     

非金属绝热材料低温热导率测试装置

建立了一套非金属绝热材料低温热导率测试装置,测试采用稳态轴向热流法,高真空绝热,通过可控气体热开关提高了样品的降温速度,运用该装置对聚氨酯隔热材料液氮到室温温区热导率进行了测量,并对装置的重复性以及漏热和误差进行了分析。     

航天器变密度多层绝热变工况漏热特性研究

航天器的热控技术在空间应用中至关重要。以探月需求为背景,基于逐层计算法建立了航天器多层绝热材料性能仿真模型,并通过实验进行了验证。对用内热外冷模型变密度填充方式改善多层绝热性能的机制和改善幅度进行了研究。结果表明变密度多层绝热相对于均匀密度多层绝热的漏热量可以减少37.3%。分析了冷边界温度、热边界温度和真空度等因素对绝热性能的影响。     

高真空多层绝热低温容器完全真空丧失后传热及绝热夹层内温度分布规律实验

在搭建了高真空多层绝热低温容器完全真空丧失传热研究实验台的基础上,分别利用干燥氮气、二氧化碳、氧气、氦气及空气为破空介质,进行了高真空多层绝热低温容器发生完全真空丧失事故后的传热实验研究.实验中通过流量计和温度采集系统测得了高真空多层绝热低温容器在发生完全真空丧失事故后的排放率和绝热夹层内的温度分布规律.实验结果表明,...     

低温下真空多层绝热材料导热系数的测试

介绍了低温下真空多层绝缘热材料导热系数的测试，在相同的条件下对各种材料作了对比试验，结果表明，由２３３材料组成的绝热全基热性能优于其它材料，因此如何改善和提高多层绝热材料的性能，是摆在我们面前的一个重要课题。     

聚合物多层微流控芯片超声波键合界面温度研究

针对聚合物多层微流控芯片键合,采用热辅助超声波键合方法实现了4层微流控芯片的键合,搭建了多界面温度测试装置,采用埋置热电偶的方法测试了三个被封接界面的温度场,研究了单独超声波作用和热辅助超声波键合法中各界面的温度并进行了比对.温度测试实验结果表明,在顶层热辅助温度70℃、6μm振幅、30kHz频率、100N超声波焊接压力和25s超声波作用时间下,基于热辅助的多层超声波键合方法可以使各键合界面的温度基本一致,从而实现多层微流控器件的多个界面键合质量一致.本文的研究为聚合物微流控器件的超声波多层键合机理研究提供了有益借鉴.     

HIAF薄壁二级铁真空室在线烘烤方案研究

针对强流重离子加速器装置小磁隙、周围磁元件温度有严格限制要求的薄壁二极铁真空室获得极高真空关键技术之一的在线烘烤方案进行研究。研究表明,烘烤系统采用加热带作为热源,纳米粉绝热材料作为主要的绝热层,镀铝聚酰亚胺薄膜作为保护层可以满足真空室在线烘烤的温度、时间和空间要求;在烘烤效果一定的情况下,烘烤的热力学温度与对数烘烤时间呈线性相关,并得到了达到预定烘烤效果选择不同的烘烤温度所需的烘烤时间;通过数值模拟得到了烘烤保温过程中的应力、变形和稳态热分布,并依此估算了烘烤功率和加热带的功率输入密度。其研究结果满足工程实际应用,证明了烘烤方案的可行性。     

用于低温存储系统的多层绝热性能分析

真空多层绝热的性能好坏直接影响到低温贮箱的安全性。根据修正的Lockheed模型,计算冷边界温度、热边界温度、层密度等对均匀层密度多层绝热性能影响,并对三区域变密度的多层绝热性能进行分析,最后针对在轨、地面状态时对低温贮箱漏热方面的要求提出采用复合多层绝热的概念,得出复合多层绝热具有优良的隔热性能。     

减少多层绝热中裂缝影响的系统研究第二部分 实验结果

在多层绝热（MLI）体上切割一系列不同宽度和各种形状的裂缝，测得的数据表明，每单位狭缝面积增加的热负荷有一个最大值≈135Wm^-1。如果狭缝宽度足够小，那么，热负荷的增加一般与裂缝下冷表面的处理情况，即发射率无关。裂缝附近的温度分布和当量热导率同没有裂缝的系统中的情况明显不同。实验也观察了热负荷和温度分布与真空压强的关系。为减小通过多层绝热体上的裂缝导入77K表面的热负荷，进行了覆盖裂缝“修补”法的系统研究。采用以下修补材料来确定在30层绝热体中补法的最佳分布：300 单面镀铝的皱纹迈拉（NRC—2）和1000 双面镀钼的平直迈拉。实验结果指出，几层中用一块补片同每层中用一块补片的效果几乎相同，把补片放在绝热体的外半部分比放在内半部分要好得多，并能把负荷减小到无裂缝时的值。把同样数量的补片放置在一个裂缝上面的效果要差得多。实验数据表明，用1000 的材料作补片较好。所有实验结果都同增强黑腔模型相符合。 作者问题     

Transient free convection about a vertical flat plate embedded in a porous medium

The problems of transient free convection in a porous medium adjacent to a vertical semi-infinite flat plate with a step increase in wall temperature and surface heat flux are considered in this paper. By assuming a temperature profile in each case, the governing equation for the boundary layer thickness is obtained by an integral method. These governing equations are first-order partial differential equations of the hyperbolic type that can be solved exactly by the method of characteristics and approximately by the method of integral relations. The results based on the method of characteristics clearly indicate that during the initial stage when the leading edge effect is not being felt, heat is transferred as if by transient 1-dimensional heat conduction. At a later time, depending on the vertical location, the heat transfer characteristics change from transient 1-dimensional heat conduction to steady 2-dimensional convection. The thickness of the boundary layer is shown to be increasing with time until it reaches steady state where its value remains constant thereafter. The growth rate of the boundary layer thickness exhibits a discontinuity at the end of the transient period and the beginning of the steady state period. On the other hand, the results based on the method of integral relations show that the boundary layer thickness grows continuously with time and approaches the steady state value asymptotically; the growth rate of the boundary layer thickness decreases from a finite value to zero continuously as the steady state is approached. Except between the end of the transient period and the beginning of the steady state period, the results based on the method of integral relations are in good agreement with those based on the method of characteristics.     

Performance of extended surface from a cryocooler for subcooling liquid nitrogen by natural convection

Natural convection of subcooled liquid nitrogen under a horizontal flat plate is measured by experiment. This study is motivated mainly by our recent development of cryocooling systems for HTS power devices without any forced circulation of liquid nitrogen. Since the cold surface of a GM cryocooler is very limited, the cooling plate immersed in subcooled liquid nitrogen is thermally anchored to the cryocooler located at the top in order to serve as an extended surface. A vertical plate generating uniform heat flux is placed at a given distance under the cooling plate so that subcooled liquid may generate cellular flow by natural convection. The temperature distributions on the plates and liquid are measured during the cool-down and in steady state, from which the heat transfer coefficients are calculated and compared with the existing correlations for a horizontal surface with uniform temperature. A fair agreement is observed between two data sets, when the heat flux is small or the plate temperatures are relatively uniform in horizontal direction. Some discrepancy at higher heat flux is explained by the cellular flow pattern and the fin efficiency of the extended surface, resulting in the non-uniformity of the horizontal plate.     

Interfacial Temperature Discontinuities in a Thin Liquid Layer during Evaporation

Recent measurements of the temperature profiles across the liquid-vapor interface of a steady evaporating liquid were performed in a thin planar liquid layer subjected to externally imposed horizontal temperature differences when the interface was open to air. Temperature discontinuities have been found to exist at the interface with an growing tendency as the imposed horizontal temperature difference increasing. Under the co-influence of thermocapillary convection and evaporation effect, a thin layer of 0.5 mm thick with approximate uniform temperature was found just below the liquid-vapor interface. Repeated experiments and further comparisons of the interfacial temperature profiles for different spatial positions along the streamwise center line and varying depths of the liquid layer were also carried out. And the temperature discontinuity was found related to the temperature in liquid phase, which was strongly influenced by the coupling of thermocapillary convection and evaporation effect.     

The temperature of liquid nitrogen in cryostat dewars

Measurements of the temperature distribution of liquid nitrogen contained in 15 and 7.5 cm diameter dewars show that the bulk of the liquid is at an approximately constant temperature .15 to .3 K above the surface temperature. About half the liquid in the dewar is in a superheated state and this state, allthough easily disturbed from equilibrium by thermal and mechanical perturbations, occurs in an unclean vessel and with liquid nitrogen likely to contain impurities. The usefulness of this phenomenon is demonstrated by showing that an immersed copper waveguide was within a volume (50 cm long by 15 cm diameter) of liquid nitrogen that was isothermal to ±10 mK.     

Analysis of a convective fluid flow with a concurrent gas flow with allowance for evaporation

The paper presents a theoretical analysis of a convective fluid flow with a concurrent gas flow accompanied by evaporation at the interface. The analysis of two-layer flows is based on a mathematical model taking into account evaporation at a thermocapillary boundary and effects of thermal diffusion and diffusion heat conduction in the gas–vapor layer. New exact solutions describing steady two-layer flows in a channel with the interface remaining undeformed and examples of velocity and temperature profiles for the HFE-7100 (liquid)–nitrogen (gas) system are presented. The influence of longitudinal temperature gradients along the channel boundaries, the gas flow rate, and the height of the fluid layer on the flow regime and evaporation rate is studied. A comparison of the calculated data with experimental results is performed.     

液氮浴中不锈钢板动态冷却过程试验及模拟分析

以液氮为介质，通过测量不锈钢平板试件在液氮浴中的动态温度分布，在实测降温曲线同根据经验公式模拟得到的降温曲线进行对比的基础上，对低温沸腾换热过程中临界热流密度和最小膜态沸腾热流密度所对应的温差进行了修正，为更准确地模拟深冷处理中的降温过程提供了指导。利用修正参数对不锈钢平板进行了模拟分析。     

Effects of Temperature and [S] on the Kinetics of Nitrogen Removal from Liquid Steel

The kinetics of denitrogenation from liquid steel was studied by using an oxygen-nitrogen analysis system(LECO TC-436) under 1600℃ similar to 2813℃ conditions. The results show that when [S]=0.005%, nitrogen removal was controlled by nitrogen transfer in liquid diffusion layer, when [S]=0.012% and 0.140%, it was controlled by both nitrogen transfer in liquid diffusion layer and the chemical reaction at the liquid-gas interface below 2250℃, and by nitrogen transfer in liquid diffusion layer under 2250℃ similar to 2813 degreesC conditions. The activation energy E-a was 57 kJ/mol for 0.0050%[S], 95 kJ/mol for 0.012%[S], 165 kJ/mol for 0.140%[S]. The resistance of sulphur on nitrogen removal decreased with the temperature rose, and disappeared at 2630℃. Based on the results obtained, it has been answered why the nitrogen in liquid steel can be decreased rapidly by carbon-oxygen reaction under very high oxygen and sulphur content conditions during the BOF, EAF, VOD and AOD steelmaking processes.     

Heat transfer in floating insulations for open cryogenic liquid containers

A transient heat transfer analysis of floating insulations in open cryogenic liquid containers is presented. The results are obtained in terms of three dimensionless parameters and are correlated to give the values of the cool down time, the total heat in-leak during cool down, the heat of sensible cooling of insulation and the steady state values of the temperature and heat flux at the exposed surface, as functions of the three parameters. The analysis facilitates the determination of the optimum insulation thickness for a given storage time. The utility of the analysis is demonstrated with a practical example of liquid nitrogen storage using expanded polystyrene beads as floating insulation.     

Study on effect of liquid level on the heat leak into vertical cryogenic vessels

The diminishing of heat leak into cryogenic vessels can prolong the storage time of cryogenic liquid. With the storage of cryogenic liquid reducing, the heat leak decreases, while the actual storage time increases. Compared with the theoretical analysis, the numerical simulation can more accurately calculate the heat transfer and temperature distribution in the vessel with complex structure. In this paper the steady state heat leak into cryogenic vessels with different liquid level height is analyzed using a finite element model. And liquid nitrogen boil-off method was adopted in experiments to validate the result of numerical simulation. Experimental results indicate favorable agreement with numerical simulation by ANSYS software. The effect of liquid level on heat leak into the cryogenic vessel can be considered in calculation of storage time and structure design.