基于RPI沸腾模型的液氮池内核态沸腾过程模拟与分析

为探究液氮的池内核态沸腾过程,使用RPI(Rensselaer Polytechnic Institute)沸腾模型对液氮池内核态沸腾阶段不同壁面过热度下的沸腾工况进行了数值模拟。计算得到的液氮核态沸腾区域的沸腾曲线与文献中的实验结果相吻合,验证了RPI沸腾模型和相关沸腾参数模型用于液氮核池沸腾模拟的可行性。依据气相分布云图讨论了不同过热度工况下沸腾发生过程中气液相分布的变化情况,并分析了气相的生成和上升过程。     

弦月形狭缝通道内液氮受迫流动沸腾传热强化的研究

研究了液氮在弦月形狭缝通道中受迫流动沸腾时的传热特性。发现液氮在弦月形狭缝通道中的受迫流动沸腾具有很高的换热系数,有显著的强化换热效果。详细分析了弦月形狭缝通道内液氮沸腾传热及流动的偏心特性。研究对于进一步理解狭缝通道沸腾传热强化的机理和狭缝强化传热技术在工程中的应用有着重要的意义。     

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。     

竖直多孔表面通道内液氮沸腾换热实验研究

在液氮自然循环流动时，对竖直多孔表面管管内沸腾换热及外管单面加热时竖直多孔表面套管内沸腾换热，进行了实验研究，分析并讨论了通道的当量半径、热流密度及含气率对沸腾换热的影响。     

微通道内流动沸腾的研究进展

微通道内的流动沸腾在能源、电子冷却、生物医疗等高新技术领域有着广泛的应用.对微通道内流动沸腾的研究进展进行了综述,研究工质涉及到水、制冷剂、液氮等,内容包括微通道与常规通道的划分,微通道的传热特性、临界热流密度、压降特性、主要流型以及流型转变、不稳定性的主要形式及产生机理等.同时指出了微通道内流动沸腾进一步的研究工作.     

沸腾表面对液氮临界热流密度的影响实验

采用液氮作为沸腾工质,通过可视化液氮沸腾实验台,对不同材料和直径的沸腾表面在液氮中进行相关稳态沸腾实验研究,总结分析沸腾表面材料、表面直径对临界热流密度及其对应过热度的影响规律.     

低温节流阀流动过程数值模拟和实验研究

对弯折型、旋转型两种节流阀进行数值模拟,研究不同工质下节流阀的温降以及流量特性,结果发现与液氮与液氧相比,甲烷在节流阀内的空化更为严重,汽相出现的较早,汽液分布较为混乱.搭建低温节流阀流动换热实验台,以液氮和甲烷为实验工质研究不同工况下的节流阀的流动特性,将进口过冷度作为影响因素提出新的质量流量关联式,可以准确预测节流...     

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.     

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.     

液氮核态池沸腾CFD模拟和可视化实验

为探究低温流体池内核态沸腾机理,对液氮池内核态沸腾进行了计算流体力学(CFD)建模及实验研究。除了探究过热度和热流关系,重点分析过热度对气泡脱离直径和频率影响。根据实验观测,将核态沸腾过程分为3个阶段:低热流阶段;过渡沸腾阶段;完全核态沸腾(FDNB)阶段。基于得到的沸腾过程气泡直径及频率,构建了核态沸腾CFD数值模型,得到的过热度及热流密度关系,与实验测量得到的数据吻合。     

Heat transfer and pressure drop for boiling nitrogen flowing in a horizontal tube: 1. Saturated flow boiling

Forced convection boiling of liquid nitrogen in a smooth horizontal copper tube with 14 mm id has been studied experimentally. The measured local heat transfer coefficients in nucleate boiling depend on the heat flux as well as on the mass flow rate. Furthermore, the influence of the vapour quality cannot be neglected.Our own experimental heat transfer data were correlated by an empirical equation. Mass flow rate, pressure, and diameter dependence of para-hydrogen data of other authors can also be correlated with this equation. A relationship for the critical heat flux is also given.     

Liquid nitrogen injection into water: Pressure build-up and heat transfer

This paper is concerned about the expansion of a small amount of liquid nitrogen injected into a relatively large pool of water and the heat transfer behaviour during the process. Both the transient pressure and temperature profiles are experimentally measured and analysed. The results show that the pressure and the rate of pressure rise increase approximately linearly with increasing injection pressure and reach, respectively, to 284 kPa and 500 kPa/s at a liquid nitrogen injection velocity of ∼0.85 m/s. The temperature varies little during the injection process due to relatively small amount of liquid nitrogen injected. A comparison of the experimental results with related work on surface boiling of cryogen suggests that the heat transfer of direct mixing be much stronger than boiling on smooth surfaces and flow boiling through smooth pipes, but comparable to the boiling on very rough surfaces and flow boiling in pipes with porous inserts. A comparison with the results generated by injecting a small amount of water into liquid cryogens shows that a higher pressure increase rate could be achieved if operating conditions are optimised to induce fragmentation. Implications of the results to cryogenic engine work output and ways to improve the performance of cryogenic engines are also discussed.     

Flow boiling heat transfer coefficients at cryogenic temperatures for multi-component refrigerant mixtures of nitrogen–hydrocarbons

The recuperative heat exchanger governs the overall performance of the mixed refrigerant Joule–Thomson cryocooler. In these heat exchangers, the non-azeotropic refrigerant mixture of nitrogen–hydrocarbons undergoes boiling and condensation simultaneously at cryogenic temperature. Hence, the design of such heat exchanger is crucial. However, due to lack of empirical correlations to predict two-phase heat transfer coefficients of multi-component mixtures at low temperature, the design of such heat exchanger is difficult.The present study aims to assess the existing methods for prediction of flow boiling heat transfer coefficients. Many correlations are evaluated against available experimental data of flow boiling of refrigerant mixtures. Silver-Bell-Ghaly correlation and Granryd correlation are found to be more suitable to estimate local heat transfer coefficients. A modified Granryd correlation is recommended for further use.     

Qualitative details of the complex flow in cryogenic vapour columns

A 2-dimensional flow visualization technique has revealed a double thermo-syphon type flow in low temperature vapour columns above boiling liquid nitrogen. The qualitative shape of the velocity profiles, the growth of the boundary layer at the wall of the containing vessel, and other flow parameters are presented. The thermo-syphon process appears to be responsible for a considerable source of heat flux into the cryogenic liquid.     

Design and development of a helium injection system to improve external leakage detection during liquid nitrogen immersion tests

The testing of assemblies for use in cryogenic systems commonly includes evaluation at or near operating (therefore cryogenic) temperature. Typical assemblies include valves and pumps for use in liquid oxygen-liquid hydrogen rocket engines. One frequently specified method of cryogenic external leakage testing requires the assembly, pressurized with gaseous helium (GHe), be immersed in a bath of liquid nitrogen (LN₂) and allowed to thermally stabilize. Component interfaces are then visually inspected for leakage (bubbles). Unfortunately the liquid nitrogen will be boiling under normal, bench-top, test conditions. This boiling tends to mask even significant leakage.One little known and perhaps under-utilized property of helium is the seemingly counter-intuitive thermodynamic property that when ambient temperature helium is bubbled through boiling LN₂ at a temperature of −195.8 °C, the temperature of the liquid nitrogen will reduce.This paper reports on the design and testing of a novel proof-of-concept helium injection control system confirming that it is possible to reduce the temperature of an LN₂ bath below boiling point through the controlled injection of ambient temperature gaseous helium and then to efficiently maintain a reduced helium flow rate to maintain a stabilized liquid temperature, enabling clear visual observation of components immersed within the LN₂. Helium saturation testing is performed and injection system sizing is discussed.     

Flow boiling of ammonia and hydrocarbons: A state-of-the-art review

A comprehensive review of flow boiling heat transfer, two-phase pressure drops and flow patterns of ammonia and hydrocarbons applied in air-conditioning, refrigeration and heat pump systems is presented in this paper. First, experimental studies of flow boiling of ammonia and hydrocarbons are addressed. Then, the prediction methods for flow boiling heat transfer, two-phase pressure drops and flow patterns are described. Next, comparisons of four flow boiling heat transfer and four two-phase pressure drop methods to the experimental data in smooth tubes derived from the available studies are presented. In addition, comparison of flow patterns to a flow map is presented. Based on the comparisons and analysis, recommendations on these methods are given. Furthermore, research needs on flow boiling and two-phase flow of ammonia and hydrocarbons have been identified. It is suggested that more experimental data be obtained through well conducted experiments and new prediction methods or modified ones based on the available methods be made for ammonia and hydrocarbons. In addition, the effect of oil on ammonia and hydrocarbon flow boiling and two-phase flow should be studied in order to have conclusive evidence of its effect.     

Heat transfer and pressure drop for boiling nitrogen flowing in a horizontal tube: 2. Pressure drop

The pressure drop of boiling two-phase nitrogen flow in a smooth horizontal tube has been studied experimentally. The results show in principle the well-known dependence of the local pressure drop from the vapour quality, the mass flow rate, and the heat flux.A comparison of the measured frictional pressure drop shows a sufficient agreement with predicted values using published correlations. The accelerational pressure gradient can also be predicted taking into account a suitable void fraction relationship.     

Visualization and void fraction measurement of decompressed boiling flow in a capillary tube

A capillary tube is often used as a throttle for a refrigerating cycle. Subcooled refrigerant usually flows from a condenser into the capillary tube. Then, the refrigerant is decompressed along the capillary tube. When the static pressure falls below the saturation pressure for the liquid temperature, spontaneous boiling occurs. A vapor-liquid two-phase mixture is discharged from the tube. In designing a capillary tube, it is necessary to calculate the flow rate for given boundary conditions on pressure and temperature at the inlet and exit. Since total pressure loss is dominated by frictional and acceleration losses during two-phase flow, it is first necessary to specify the boiling inception point. However, there will be a delay in boiling inception during decompressed flow. This study aimed to clarify the boiling inception point and two-phase flow characteristics of refrigerant in a capillary tube. Refrigerant flows in a coiled copper capillary tube were visualized by neutron radiography. The one-dimensional distribution of volumetric average void fraction was measured from radiographs through image processing. From the void fraction distribution, the boiling inception point was determined. Moreover, a simplified CT method was successfully applied to a radiograph for cross-sectional measurements. The experimental results show the flow pattern transition from intermittent flow to annular flow that occurred at a void fraction of about 0.45.