液氮速冻对火龙果块微生物和品质的影响研究

本文以火龙果块为原料,研究了-40℃、-60℃、-80℃和-100℃喷淋式液氮速冻及-18℃冰箱缓冻处理对火龙果微生物和品质的影响,并筛选了最佳液氮速冻温度。结果表明:样品冻结速率随液氮速冻温度的降低而加快,冻结时间由1. 25 min降至0. 367 min,但-100℃出现低温断裂现象;液氮速冻处理对样品的多酚氧化酶有抑制作用,其中,-80℃液氮速冻处理后样品的多酚氧化酶活性降至47. 27%,而缓冻处理后的酶活升高至301. 09%;相比于缓冻处理,液氮速冻处理后的样品的质构保持更好,其汁液流失率在8. 88%～10. 50%之间,而缓冻处理则高达38. 29%;随着液氮速冻温度的降低,样品的可滴定酸、可溶性固形物、总酚、花色苷、抗坏血酸等品质保持更好。所以在不同温度液氮速冻处理中,-80℃对样品的品质保持最佳。     

柱面压缩液氮低温靶的设计与实验研究

研制了柱面加载压缩的液氮温区低温靶,该低温靶以液氮为冷源将样品室冷却到所需的温度,并保持该温度,然后对注入的样品气体液化,为柱面等熵压缩提供条件。根据加载方式和保温时间要求确定了低温靶的初步结构和尺寸,在此基础上分析计算了靶体的漏热、样品室的温度分布和保温时间,对结构尺寸进行优化。在自行搭建的实验台上对柱面压缩液氮低温靶的漏热、降温、温度调控及不同环境条件下的保温特性、样品气体（氩气）液化过程的温度变化进行了试验。试验结果表明：柱面压缩液氮低温靶的温度可在一定范围进行调节控制;借助样品室顶部针阀内部的温度计,结合分批充注样品气体的方法可以很好地观测样品室液体充满状况;成功地将氩气液化,充满样品室;断开冷源和真空泵后,样品室温度可以维持93 min。     

液氮冷藏箱性能实验研究

基于液氮制冷技术,设计了一种用于冷链物流末端的食品冷藏箱。对冷藏箱的降温性能进行实验研究,分析了自然对流和风机强制对流两种方式下,冷藏箱的降温特性及液氮消耗量。实验结果表明,冷藏箱无负荷时箱内各层温度分布较为均匀,有负荷时样品中心温度仅在水平方向上有所差别。冷藏箱采用风机强制对流的方式可使箱内降温速度更快,但同时也会消耗更多的液氮。     

日本生产充液氮的水果饮料罐头

据日本消息报道,日本麒麟啤酒公司生产一种充液氮的水果饮料罐头. 充液氮的原因是:原先的水果饮料罐头,在封罐时要加热,因此当罐冷却时,就会因罐内物质体积收缩而造成负压,从而出现罐体凹瘪,为此罐体壁要做得较厚.而充液氮可以防止这一现象发生,并可减少罐壁厚度,节约用料.此外,在水果饮料罐头中充人液氮,可排除部分空气,有利于饮料的贮存.据试验,每罐饮料大约只需0.7克液氮.其中,实际封人罐内的液氮只有0.1克,其余的均在充填过程中汽化了.     

油罐呼吸损耗量计量新模型的构建与研究

通过研究油罐在静置存储及装卸作业过程中罐内气体空间和油品内部温度的变化规律,以及温度对油品蒸汽压的影响,建立了新的呼出罐外空气量计算方法。结合Fick扩散定律、连续性方程和罐内油蒸汽实际分布特点,构建求解呼气浓度的数值模型。实际呼吸损耗量可用呼出空气量与呼气浓度的乘积得到。与实测数据进行对比,证实该浓度模型、空气量计量办法可靠,新构建模型的平均误差约为其它模型的38%。     

冰球式蓄冰罐蓄冷过程的数值研究

建立了冰球式蓄冰罐蓄冷过程的数值仿真模型,冰球侧采用焓方程模型并考虑了冰球内水的过冷现象。求解此模型可以得出蓄冷过程中任意时刻冰球内部温度分布、冰层厚度以及蓄冰罐内载冷剂温度分布。进行了三组工况的计算,求解结果与实验结果吻合较好,模型可用于模拟仿真以及冰球式蓄冰罐的性能研究。     

直接接触式蓄冷传热特性的试验研究

设计了直接接触式蓄冷试验台,观察了蓄冷罐内的蓄冷现象,对直接接触式蓄冷系统蓄冷罐内的传热特性进行了研究。通过试验对蓄冷罐水位、制冷节流阀的开度、罐内水的初温及系统充灌量对蓄冷罐内容积换热系数及温度分布迸行了研究,研究表明,蓄冷罐内的容积换热系数随罐内水位升高而降低,随初始水温的升高而增加,制冷节流阀的开度的增大将会增大蓄冷罐内的容积换热系数,而制冷剂的充罐量对容积换热系数影响不大。     

真空多层绝热性能测试装置及初步实验验证

建立了一套低温用真空多层绝热材料性能测试装置,用于研究变密度多层绝热材料的特性规律。以内密外疏的层密度分布方式为例,在测试装置充注液氮之后,通过稳态量热法实验研究了真空条件下该布置方式绝热层内不同层位置的温度瞬态和稳态分布规律、单点的降温规律、以及单位面积漏热率情况。考察了稳态温度分布以及单位面积热流随热边界温度的变化特性,并将理论温度分布曲线与实验曲线进行了对比,从而验证层与层模型的准确性。     

垂直上升管内气液两相流动特性的数值模拟

某空间模拟器的液氮制冷系统采用了结构简单、可靠性高、最节能的重力自循环系统.基于气液两相流理论,建立了垂直上升管内液氮气液两相流动的一维均相流模型;数值模拟不同入口压力下的管内压力、质量流量、管壁温度和质量含气率等参量,研究其变化规律.计算结果表明,建立的数学模型能够详细描述重力自循环系统垂直上升管内液氮流动的流体动力特性,为空间模拟器液氮制冷系统的研制提供了重要的基础数据和理论参考依据.     

液氮直接蒸发制冷系统模糊PID控制方法的数值模拟与实验研究

为了满足液氮直接蒸发制冷系统大温区范围内的高精度温度控制需求,本文提出一种应用于该类型系统控制需求的模糊PID控制策略,并建立了液氮直接蒸发制冷系统的一维动态分布参数模型,分别进行了-50、-80、-140℃等不同目标温度工况下的数值模拟与实验,结果表明:模拟与实验结果最大相对误差为7. 8%,提出的模糊PID控制策略取得了较好的控制效果,在-50、-80、-140℃目标温度下,其温度波动范围分别为±0. 5、±1. 0、±2. 0℃,实现了较宽低温区的高精度控温。     

PVT gauging with liquid nitrogen

Experimental results are presented for pressure–volume–temperature (PVT) liquid quantity gauging of a 0.17 m³ liquid nitrogen tank pressured with ambient temperature helium in the normal gravity environment. A previously reported PVT measurement procedure has been improved to include helium solubility in liquid nitrogen. Gauging data was collected at nominal tank fill levels of 80%, 50% and 20% and at nominal tank pressures of 0.3, 1.0, and 1.7 MPa. The test tank was equipped with a liquid pump and spray manifold to circulate and mix the fluid contents and therefore create near-isothermal conditions throughout the tank. Silicon diode sensors were distributed throughout the tank to monitor temperatures. Close-spaced arrays of silicon diode point sensors were utilized to precisely detect the liquid level at the nominal 80%, 50%, and 20% fill levels. The tests simulated the cryogenic tank-side conditions only; helium mass added to the tank was measured by gas flowmeters rather than using pressure and temperature measurements from a dedicated helium supply bottle. Equilibrium data for cryogenic nitrogen and helium mixtures from numerous sources was correlated to predict soluble helium mole fractions. Results show that solubility should be accounted for in the PVT gauging calculations. Mole fractions predicted by Dalton’s Law were found to be in good agreement with the compiled equilibrium data within the temperature–pressure range of interest. Therefore, Dalton’s Law was deemed suitable for calculating ullage composition. Gauging results from the PVT method agreed with the reference liquid level measurements to within 3%.     

液氮辅助冻干机冷阱室内温度场模拟与分析

对液氮辅助制冷冻干机的冷阱进行数值模拟,建立冻干机冷阱的三维几何模型和冷阱内流体流动控制方程及边界条件.采用Fluent模拟软件对冷阱室内温度场模拟分析,得到了冷阱室内温度场分布图,通过对冷阱内部温度场分析,为指导冷阱室内结构设计,提高冷阱捕水能力提供参考.     

超导电缆循环过冷液氮温度场模拟

建立了循环过冷液氮在恒温器内迫流冷却超导电缆的模型,利用Fluent软件模拟了30 m电缆系统内液氮温度场在不同流量、终端漏热、交流损耗等工况条件下的变化情况。仿真结果表明,在终端漏热和交流损耗一定时,液氮出口断面的最高温度随流量的增大而降低;在流量和交流损耗一定时,液氮出口断面的最高温度随终端漏热的增大而升高;在流量和终端漏热一定时,液氮出口断面的最高温度随交流损耗的变化不明显。     

An automatic liquid nitrogen filling system

Spectroscopic measurements at low temperature require liquid nitrogen in some type of research dewar. Often the research dewar must be refilled from a reservoir dewar several times during the course of an experiment. A system is described which automatically refills a research dewar from a reservoir dewar. Two sensors are used for detecting the liquid nitrogen level in the research dewar, and an external nitrogen gas supply is used to provide the pressure to effect the transfer of liquid nitrogen.     

Subcooled liquid nitrogen refrigerator for HTS power systems

Subcooled liquid nitrogen is a good cooling medium of high temperature superconducting (HTS) electric power systems such as an electric power line and a power transformer. To produce subcooled liquid nitrogen, a cryocooler is used and a circulation pump is installed in the system. Several subcooled liquid nitrogen circulation systems were constructed and tested. Those are used as a refrigerator for HTS power systems. The pressure of subcooled liquid nitrogen is maintained at atmospheric pressure (0.108 MPa) and the working temperature is 68 K. One system of HTS power transformer was tested in distribution power line. In each case, the temperature of the cold head of the cryocooler is kept at 64 K little above nitrogen freezing temperature. For the stable operation, the system must work even in the case of shaking condition by earthquake, the pressure must be stable and be kept at atmospheric pressure.     

液氮预冷管道表面温度的数值模拟研究

基于FLUENT建立传感器温度测量以及管道内液氮流动的数值计算模型,计算并分析对于与管壁不同接触面积、管道上不同安装位置、液氮不同流速以及不同管长条件下温度传感器所测温度变化规律以及影响温度传感器测量出的温度值与管内流体温度之间差异的因素。     

液氮发动机循环分析

新型零排放液氮发动机存在朗肯循环和布雷顿循环两种做功循环方式,为了了解两种循环方式对液氮发动机匹配设计的要求,基于有限时间热力学理论,对比了两种循环对液氮发动机输出功率的影响,研究了液氮发动机在变温热源和恒温冷源之间朗肯循环和布雷顿循环的性能,着重分析了导热率对循环性能及输出功率的影响,为液氮发动机换热器的匹配提供依据.得出换热器的热导率和热容率的匹配决定朗肯循环的最佳循环功率,而与朗肯循环相比布雷顿循环能更好的实现发动机的功率特性的结论.     

Wicking of liquid nitrogen into superheated porous structures

Evaporation in porous elements of liquid–vapor separation devices can affect the vapor-free cryogenic propellant delivery to spacecraft engines. On that account, the capillary transport of a cryogenic liquid subjected to evaporation needs to be understood and assessed. We investigate wicking of liquid nitrogen at saturation temperature into superheated porous media. A novel test facility was built to perform wicking experiments in a one-species system under non-isothermal conditions. A setup configuration enabled to define the sample superheat by its initial position in a stratified nitrogen vapor environment inside the cryostat. Simultaneous sample weight and temperature measurements indicated the wicking front velocity. The mass of the imbibed liquid nitrogen was determined varying the sample superheat, geometry and porous structure. To the author’s extent of knowledge, these are the first wicking experiments performed with cryogenic fluids subjected to evaporation using the weight–time measurement technique. A one-dimensional macroscopic model describes the process theoretically. Results revealed that the liquid loss due to evaporation at high sample superheats leads to only a slight imbibition rate decrease. However, the imbibition rate can be greatly affected by the vapor flow created due to evaporation that counteracts the wicking front propagation.     

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.     

大型贮槽内液氮纯度下降的原因分析

分析了引起大型贮槽内液氮纯度下降的几个主要原因 ,并提出了预防措施。同时 ,还讨论了贮槽内的液氮受氧污染后的置换问题