低温冷凝可视化实验装置设计

低温流体与常规室温流体冷凝存在显著差异,简述了国内外报道的主要低温冷凝实验装置,指出要准确描述翅片通道内冷凝及两相流动过程,必须借助先进的可视化手段。为解决空分装置中主冷凝蒸发换热器翅片通道内低温流体冷凝热阻大、换热效率低的难题,设计搭建了一套低温冷凝可视化实验装置,实验段采用带光窗的可拆密封设计,便于研究平壁表面及各种型号翅片通道内冷凝换热过程。经初步测试,该实验装置运行稳定,借助高速摄像机可清楚地观察到氮冷凝两相流动的现象。     

低温流体的空间输送技术

介绍了美国宇航局刘易斯研究中心有关低温流体空间输送方面的最新研究情况，包括设备研制、实验结果、新方案和新技术等。描述了低温流体空间输送技术的效益；最后讨论了这些技术的改进与发展。     

低温流体光-摄同轴同侧可视化装置设计

为了获得清晰、稳定的低温冷凝图像以更深入地研究低温冷凝的传热与流动机理,提出了一种面向对象的光-摄同轴同侧可视化方法。该方法采用环形光源内置、高速摄像仪外置的同轴布置方式进行低温系统深井结构的可视化观察,借助双轴滑轨使高速摄像仪精确定位,以研究不同工况下的低温流体冷凝流型。     

脉动热管实验与理论研究进展

脉动热管(PHP/OHP)是一种新型的高效传热元件,在航天领域、电子器件冷却以及节能技术方面极具应用潜力。这里首先介绍了脉动热管的特点和工作原理,然后分别从实验研究、理论研究和实际应用等方面介绍了目前该领域的研究现状。实验研究方面着重介绍了流动可视化应用以及纳米流体和功能流体通过强化换热提高脉动热管性能等相关研究热点。同时指出,目前脉动热管的理论分析受限于两相流理论的发展,主要研究重点在于非线性分析;数值模拟方面,特别是同纳米流体以及功能流体应用相结合将会成为下一个研究热点。     

大型空间环境试验设备中的低温技术

对国内外大型空间环境模拟设备中的低温技术应用情况进行了介绍。重点介绍了我国一台大型空间环境试验设备中的低温技术应用情况介绍了低温技术对大型空间环境设备性能的影响。对大型空间环境模拟设备中的低温传热和流动现象进行了讨论。     

工质粘度对单相流体回路工作性能影响

当航天器所需热控单相流体回路排散的热负荷减小时,在太空对辐射器的作用下,流体回路内工质温度降低(处于低温工况),与设计工况相比,因温度降低而使得工质粘度增加,导致单相回路流动阻力损失增加.本文工作在对工质粘度对单相流体回路影响理论分析的基础上,开展了不同温度条件下的流体回路实验研究,对理论分析结果进行验证的同时,获得了大量的实验数据和有益结论,这些将对热控单相流体回路优化设计、工质选择等提供参考.     

新型CO2固气两相流循环制冷系统的可视化实验

本文研究了一种采用自然工质CO₂在固气两相流条件下,进行升华换热实现低温制冷的新型制冷系统。相应的过程在CO₂三相点-56.6℃以下通过相变的方式发生,从而能够在低温下达到良好的制冷效果。初步实验表明,实际可达到的稳定制冷温度可以在-62.0℃以下。研究从实验的角度探讨了相应的相变过程参数变化和固气两相升华流动的基本特性。实验设计了可视化方案,实现对于低温下复杂多变的两相流动性质和状态进行研究。特别是针对可视化实验中发现的管路控制和干冰颗粒沉积、堵塞的问题进行了多工况实验和讨论。研究期待对于相关低温制冷系统的进一步研发和实验优化提供有价值的参考。     

基于烟线技术的脉管制冷机流动可视化实验研究

脉管制冷机在低温部分没有运动部件,具有振动低、寿命长等突出优点,在空间技术、超导磁体冷却等方面应用广泛。但是脉管制冷机工作机理复杂,内部为交变流动,很多参数难以测量。常规方法无法显示实验过程中制冷机内部气体的流动情况,给学生理解脉管制冷机原理和工作特性带来了困难。基于烟线技术的可视化实验,可以直观地反映脉管制冷机内部气体的流场,能够直接观察制冷机运行时内部气体的流动情况。     

工质粘度对单相流体回路工作性能的影响

当航天器所需热控单相流体回路排散的热负荷减小时，在太空对辐射器的作用下，流体回路内工质温度降低（处于低温工况），与设计工况相比，因温度降低而使得工质粘度增加，导致单相回路流动阻力损失增加。在对工质粘度对单相流体回路影响理论分析的基础上，开展了不同温度条件下的流体回路实验研究，对理论分析结果进行验证的同时，获得了大量的实验数据和有益结论，这些将对热控单相流体回路优化设计、工质选择等提供参考。     

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。     

Visualization techniques applied to thermo-fluid phenomena in cryogenic fluids

A variation of visualization techniques such as Shadowgraph, Schlieren and holographic interferometry, has been so far applied to visualize thermo-fluid phenomena in cryogenic fluids, superfluid helium (He II) and supercritical nitrogen, by some researchers. This paper is a review of these visualization techniques used in cryogenic fluids as well as an introduction of visualization techniques.     

Visualization in cryogenic environment: Application to two-phase studies

This paper reviews recent technical developments devoted to the study of cryogenic two-phase fluids. These techniques span from simple flow visualization to quantitative measurements of light scattering. It is shown that simple flow pattern configurations are obtained using classical optical tools (CCD cameras, endoscopes), even in most severe environments (high vacuum, high magnetic field). Quantitative measurements include laser velocimetry, particle sizing, and light scattering analysis. In the case of magnetically compensated gravity boiling oxygen, optical access is used to control the poistioning of a bubble subject to buoyancy forces in an experimental cell. Flow visualization on a two-phase superfluid helium pipe-flow, performed as a support of LHC cooldown studies, leads to flow pattern characterization. Visualization includes stratified and atomized flows. Thanks to the low refractive index contrast between the liquid and its vapor, quantitative results on droplet densities can be obtained even in a multiple scattering regime.     

Capacitance-based liquid holdup measurement of cryogenic two-phase flow in a nearly-horizontal tube

Liquid holdup measurement of cryogenic fluids is an area of considerable significance because of its inevitable occurrence in LNG transportation, rocket propellant delivery and superconducting equipment cooling, etc. To measure the liquid holdup of cryogenic two-phase flow, a capacitance sensor was carefully designed, which consists of a pair of optimized concave-electrode form with the electric circuit for the small capacitance detection. Four flow patterns were realized to evaluate the performance of the sensor in visualization experiments with liquid nitrogen and vaporous nitrogen. An image method was employed to calibrate the capacitance sensor, which led to a mathematical relationship between the capacitance and the liquid holdup. The results indicated that the obtained correlation between liquid holdup and capacitance satisfactorily coincided with the measured data.     

Two-Phase Flow and Heat Transfer During Chilldown of a Simulated Flexible Metal Hose Using Liquid Nitrogen

For many industrial, medical and space technologies, cryogenic fluids play irreplaceable roles. When any cryogenic system is initially started, it must go through a transient chill down period prior to normal operation. Chilldown is the process of introducing the cryogenic liquid into the system, and allowing the system components to cool down to several hundred degrees below the ambient temperature. The chilldown process is an important initial stage before a system begins functioning. The objective of this paper is to investigate the chilldown process associated with a flexible hose that was simulated by a channel with saw-teeth inner wall surface structure in the current study. We have investigated the fundamental physics of the two-phase flow and quenching heat transfer during cryogenic chilldown inside the simulated flexible hose through flow visualization, data measurement and analysis. The flow pattern developed inside the channel was recorded by a high speed camera for flow pattern investigation. The experimental results indicate that the chilldown process that is composed of unsteady vapor-liquid two-phase flow and phase-change heat transfer is modified by the inner wall surface wavy structure. Based on the measurement of the channel wall temperature, the teeth structure and the associated cavities generally reduce the heat transfer efficiency compared to the straight hose. Furthermore, based on the measured data, a complete series of correlations on the heat transfer coefficient for each heat transfer regime was developed and reported.     

Numerical study of flow and heat-transfer characteristics of cryogenic slush fluid in a horizontal circular pipe (SLUSH-3D)

Cryogenic slush fluids, such as slush hydrogen and slush nitrogen, are two-phase, single-component fluids containing solid particles in a liquid. Since their density and refrigerant capacity are greater than those of liquid-state fluids alone, there are high expectations for use of slush fluids as functionally thermal fluids in various applications, such as fuels for spacecraft engines, clean energy fuels to improve the efficiency of transportation and storage, and as refrigerants for high-temperature superconducting equipment. In this research, a three-dimensional numerical simulation code (SLUSH-3D), including the gravity effect based on the thermal non-equilibrium, two-fluid model, was constructed to clarify the flow and heat-transfer characteristics of cryogenic slush fluids in a horizontal circular pipe. The calculated results of slush nitrogen flow performed using the numerical code were compared with the authors’ experimental results obtained using the PIV method. As a result of these comparisons, the numerical code was verified, making it possible to analyze the flow and heat-transfer characteristics of slush nitrogen with sufficient accuracy. The numerical results obtained for the flow and heat-transfer characteristics of slush nitrogen and slush hydrogen clarified the effects of the pipe inlet velocity, solid fraction, solid particle size, and heat flux on the flow pattern, solid-fraction distribution, turbulence energy, pressure drop, and heat-transfer coefficient. Furthermore, it became clear that the difference of the flow and heat-transfer characteristics between slush nitrogen and slush hydrogen were caused to a large extent by their thermo-physical properties, such as the solid–liquid density ratio, liquid viscosity, and latent heat of fusion.     

Influence of cryogenic coolant on turning performance characteristics: A comparison with wet machining

Machining of 17-4 Precipitation Hardenable Stainless Steel (PH SS) is one of the difficult tasks because of its high cutting temperatures. Conventional cutting fluids are used to overcome the high cutting temperatures, but these are not acceptable from the health and environmental sustainable points of view. Cryogenic cooling is one of the potential techniques to overcome such problems. In the current work, comparison is made of cryogenic turning results, such as tool flank wear, cutting forces (feed force, main cutting force), cutting temperature, chip morphology and surface integrity characteristics with wet machining during machining of heat-treated 17-4 PH SS. The result showed that in cryogenic machining, a maximum of 53%, 78%, 35% and 16% reductions was observed in tool flank wear, cutting temperature, surface roughness and cutting force, respectively, when compared with wet machining. It was also evident from the experimental results that cryogenic machining significantly improved the machining performance and product quality even at high feed rates.     

Heat transfer performance of cryogenic oscillating heat pipes for effective cooling of superconducting magnets

The cryogenic oscillating heat pipe (OHP) for conduction cooling of superconducting magnets was developed and the function was demonstrated successfully. OHP is a highly-efficient heat transfer device using oscillating flow of two-phase mixture. The working fluids that are employed in the present research are Nitrogen, Neon and Hydrogen, and the operating temperatures are 67–91 K, 26–34 K and 17–27 K, respectively. The estimated effective thermal conductivities from the measurement data of the OHP were higher than one of the solids such as copper at low temperature. These results revealed that the cryogenic OHP can enhance the performance of cooling system for magnets.     

Structured packing liquid holdup in cryogenic systems

Experimental results and a theoretical analysis are presented on structured packing liquid holdup under cryogenic conditions. The experimental tests were performed on a novel laboratory unit incorporating a gas lift pump to recirculate the cryogenic liquid in a flow visualization dewar. Holdup is measured by sensing the difference in level in the dewar when the liquid flow is shut off after a constant operation at a given liquid loading. A dimensional analysis is performed to identify the important system properties that affect the liquid holdup. The preliminary experimental data are compared against a correlation developed from the dimensional analysis.     

Cryogenic Boiling and Two-Phase Flow during Pipe Chilldown in Earth and Reduced Gravity

For many industrial, medical and space technologies, cryogenic fluids play indispensable roles. An integral part of the cryogenic transport processes is the chilldown of the system components during initial applications. In this paper, we report experimental results for a chilldown process that is involved with the unsteady two-phase vapor-liquid flow and boiling heat transfer of the cryogen coupled with the transient heat conduction inside pipe walls. We have provided fundamental understanding on the physics of the two-phase flow and boiling heat transfer during cryogenic quenching through experimental observation, measurement and analysis. Based on the temperature measurement of the tube wall, the terrestrial cryogenic chilldown process is divided into three stages of film boiling, nucleate boiling and single-phase convection that bears a close similarity to the conventional pool boiling process. In earth gravity, cooling rate is non-uniform circumferentially due to a stratified flow pattern that gives rise to more cooling on the bottom wall by liquid filaments. In microgravity, there is no stratified flow and the absence of the gravitational force sends liquid filaments to the central core and replaces them by low thermal conductivity vapor that significantly reduces the heat transfer from the wall. Thus, the chilldown process is axisymmetric, but longer in microgravity.      

Development of density and mass flow rate measurement technologies for slush hydrogen

Slush hydrogen is a two-phase solid-liquid cryogenic fluid consisting of solid hydrogen particles in liquid hydrogen. Compared to liquid hydrogen, the density is about 16% greater at a solid mass ratio (solid fraction) of 50%, and the cryogenic heat capacity (enthalpy) is about 18% higher. Various applications are anticipated, including fuel for reusable space shuttles, coolant for cold neutron generation, as well as the transport and storage of hydrogen as a clean energy source. At a solid fraction of within 50%, piped transport can be conducted in the same way as for normal fluids. This paper reports on the slush hydrogen technology in terms of the measurement of the density and the mass flow rate.