低温液氧储罐预冷过程数值模拟

为了避免低温储罐受到强烈热冲击,在投入运行之前,必须对其进行预冷。采用计算流体力学(CFD)的方法,对低温液氧储罐的预冷过程进行了数值模拟分析。建立了带保温层厚度与不锈钢罐壁厚度的低温液氧储罐计算模型,通过相变模型修正及时间步长验证,考虑固液耦合传热,获得预冷过程中储罐内气体温度、压力变化及气液分布情况,获得储罐降温规律,并探究不同进口流量对预冷过程的影响,以便对预冷过程进行参数优化。     

液氧煤油火箭发动机排放预冷数值仿真

针对预冷过程中输送系统和发动机流道内沸腾传热和两相流动过程,建立一维二流体模型。以某液氧煤油发动机为原型,对不同气枕压力、排放管径和空中排放时间条件下排放预冷过程进行仿真,分析进出口温度以及流量特性的变化情况,结果表明增大排放管径和排放压力均可以增强预冷效果,而空中排放预冷时达到同样预冷效果所需的推进剂量则与管径大小无关,并且预冷效果受到地面状态的影响。     

低温容器的研制

着重介绍了低温高压液氢容器和全包式预冷夹套低温容器的结构，容器分为三导：外容器是真空容器，夹套容器是预冷介质容器，内容器是低温介质容器。容器可用液氮预冷，大大节省预冷液氢，全包式的预冷夹套液氢容器，使预冷夹套发挥最大作用，其关键是使用了锥筒式支承，锥筒小端支承在内容器颈管上，大端支承在外容器上，在内容器上没有任何支耳和凸台，透用于带预冷夹套低温容器和多层绝热低温容器。     

预冷冷库内冰浆式湿冷蒸发器的性能实验研究

对果蔬预冷应用场合,本文提出一种冰浆式湿冷蒸发器,并将其应用于小型模拟冷库。采用娃娃菜为被冷却对象,以库内温度和相对湿度为指标,改变填料类型（金属、塑料、纸质填料）和载冷介质（冰浆、冷水）,在冷库空载条件下进行负载预冷实验。结果表明：金属、纸质填料的换热能力较塑料填料好;与冷水相比,以冰浆为载冷介质,预冷时间缩短了1/6并保持更低的库内温度,库内相对湿度稳定后均能保持在90%以上;预冷后样品失重率较冷水的小0.19%;冰浆式湿冷蒸发器可在冷库中稳定运行,且较冷水为载冷介质的湿冷蒸发器,可更高效地实现预冷过程的低温（3~6 ℃）高湿（>90%RH）环境,适用于果蔬预冷和保鲜。     

基于DCS的EAST低温系统预冷过程自动控制

为避免热应力对EAST托卡马克装置的损坏,超导磁体与冷屏的300-80 K预冷过程需要严格控制降温温差;同时低温系统还需提供合适的流量与压降,以保证各冷质部件同步快速降温.通过低温控制流程分析,明确了预冷控制回路与控制需求,设计了预冷操作模式及其顺序控制流程.针对预冷过程中温差-压降耦合问题,设计了分阶段的解耦控制方案...     

黄瓜压差预冷过程中的阻力特性实验研究

建立差压预冷箱阻力特性测试装置,并对压差预冷过程中黄瓜的阻力特性进行实验研究,通过对不同流量的空气流过黄瓜与预冷箱所产生的压降进行测试分析,获得了空气流量与压降的函数关系,并进一步对影响差压预冷箱及其内部货物(黄瓜)流动阻力系数S的因素进行分析,为货物的差压预冷箱以及差压预冷工艺设计提供指导.     

BOG气体对LNG输送管道预冷的数值模拟

针对DN1 000 mm的长距离LNG输送管道BOG(闪蒸气)预冷过程,建立一维流动传热模型,对预冷过程进行了模拟分析.整体壁面温度下降速率最大不超过10 K/h,计算时间步长取10s,计算得出761.2 m的LNG输运管道冷却到154.5 K(-118.65℃)左右所需时间为30.25 h.分析了整个预冷过程中,管道壁面温度,管内BOG气体温度和BOG气体进口质量流量的变化趋势,还分析了输运管道中管道变径处温度阶跃突变的现象和原因,同时分析不同因素对管道预冷过程的影响.从以上影响预冷过程的主要因素的分析得出整个预冷过程的规律.     

基于液氮冷源超导磁体氦气循环预冷系统设计

为了提高超导磁体300-80 K预冷过程中的降温效率和安全性,开发了一种新的预冷方法.设计了一台以液氮为冷源、氦气为循环介质的可控温预冷装置,对其内部结构进行了优化设计,包括低温风机、板式换热器、气动调节阀、翅片换热器等主要组成部分,整个装置与磁体构成一个闭合循环系统.在预冷装置的作用下,该超导磁体从300 K到80 ...     

低温管路系统预冷过程仿真研究

低温液体在通过管路系统传输前,需对管路系统进行充分预冷,以避免低温液体在输运过程中出现气液两相,造成潜在的危害。基于多学科仿真平台AMESim中的Steiner流动沸腾换热模型,采用液氮作为预冷工质分析管路系统的预冷过程,重点研究进口压力、脉冲流频率及脉冲比率对管路系统预冷效果的影响。分析表明,在计算范围内,随着进口压力的增大,管路的预冷效率逐渐降低。因此,实际预冷过程中在满足流体输送要求时可以适当降低进口压力来提高预冷效率;脉冲流的预冷效率随脉冲频率的增大,先增大后趋于稳定,随脉冲比率的增大而减小。研究结果对于提高预冷效率、降低资源消耗、确保预冷过程安全进行,具有一定的指导意义。     

开架式气化器换热管预冷过程传热数值模拟

换热管作为开架式气化器的基本换热单元,工作时存在从环境温度降至低温的预冷过程,采用数值模拟的方法,对开架式气化器换热管预冷过程进行数值模拟,得到管内流体温度场和速度场变化情况。分析发现:预冷过程中,海水膜厚度越厚,换热管内流体平均温度和平均速度越高;进入稳定换热后,海水膜厚度不再影响换热管内流体温度场和速度场的分布。结果有助于指导实际的预冷操作,对缩短预冷时间,提高预冷效率,减小海水用量,降低运行成本有意义。     

An experimental study on flow patterns and heat transfer characteristics during cryogenic chilldown in a vertical pipe

In the present paper, the experimental results of a cryogenic chilldown process are reported. The physical phenomena involve unsteady two-phase vapor–liquid flow and intense boiling heat transfer of the cryogenic fluid that is coupled with the transient heat conduction inside pipe walls. The objective for the present study is to compare the chilldown rates and flow patterns between the upward flow and downward flow in a vertical pipe. Liquid nitrogen is employed as the working fluid and the test section is a vertical straight segment of a Pyrex glass pipe with an inner diameter of 8 mm. The effects of mass flow rate on the flow patterns, heat transfer characteristics and interface movement were determined through experiments performed under several different mass flow rates. Through flow visualization, measurement and analysis on the flow patterns and temperature variations, a physical explanation of the vertical chilldown is given. By observing the process and analyzing the results, it is concluded that pipe chilldown in a vertical flow is similar to that in microgravity to some extent.     

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.      

低温推进剂输送系统循环预冷非稳态数值模拟研究

针对某带引射装置的循环预冷系统,基于一维非平衡两流体模型,结合引射器模型建立采用交错网格与两步求解法,数值模拟并分析了整个自然循环(含引射增强)预冷非稳态进程,定量评价了循环流动特性、引射增强及预冷效果.数值模拟并探讨了循环预冷系统参数对预冷状态与过程的影响.     

基于差压法的飞机液冷系统冷却液流量测量

飞机液冷系统冷却液流量是评定系统性能的一项重要指标。利用液冷系统中的变径导管作为节流件,可以采用差压法对冷却液流量进行测量。利用地面校准系统获得的试验数据,拟合得到了不同温度、不同压力下的冷却液流量-差压特性曲线,通过插值得到了可用于流量计算的修正公式。试飞结果表明,采用这种方法测量飞机液冷系统冷却液流量是可行的,解决了飞机液冷系统不便加装流量计的问题。     

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.     

蓄冷平板堆积床动态蓄冷性能研究

本文建立了描述蓄冷平板堆积床动态蓄冷性能的数理模型,并与实验结果进行了比较,两者吻合得较好。分析了载冷剂流量对充冷时间的影响,得到了载冷剂出口温度和蓄冷剂温度随时间的变化规律。该模型可为蓄冷平板堆积床的设计和优化提供依据     

An experimental study on ice formation around horizontal long tubes

The results of an experimental study are presented where the growth rate of ice on the outside of cooled copper tubes was studied. The tubes, which were immersed in water in an insulated vessel, were internally cooled by circulating glycol through them.It was found that axial growth rate of ice is distinct at low values of the coolant Reynolds number and short freezing times. The slope of the ice thickness with axial distance showed moderate dependency on time but varied with coolant flow rate, and with Stanton and Biot numbers.A key result from the experiments is the abrupt ice thickness enlargements on the surface of tube bends. This anomaly may be attributed to internal flow disturbances of the coolant, and creation of local eddies inside the bends that enhance growth of ice. The effect was evident for a low Reynolds number (Re = 251.9 and Bi < 1), and fades out for large Reynolds number flows.     

Heat and mass transfer characteristics of a constrained thin-film ammonia-water bubble absorber

A study of absorption of ammonia vapour bubbles into a constrained thin-film of ammonia-water solution is presented. A large-aspect-ratio microchannel constrains the thickness of the weak solution film and ammonia vapour bubbles are injected from a porous wall. A counter flowing coolant in a minichannel removes the generated heat of absorption. Experiments and a simple one-dimensional numerical model are used to characterize the absorber performance at a nominal system pressure of 6.2 bar absolute. Effect of varying the mass flow rate of the weak solution, vapour flow rate, solution inlet temperature, and coolant inlet temperature on absorption heat and mass transfer rates and exit subcooling are discussed. Two absorber channel geometries, each of 600 μm nominal depth, are considered: 1) a smooth-wall channel, and 2) a stepped-wall channel that has 2-mm deep trenches across the width of a channel wall. Results indicate that the reduction in coolant inlet temperature significantly enhances the mass transfer rates in both absorber geometries. While the stepped-wall geometry exhibits higher mass transfer rates at lower coolant inlet temperatures of 30 °C and 40 °C, the smooth-wall channel shows higher mass transfer rates at the highest coolant inlet temperature of 58 °C. Both absorption limited and residence time limited conditions are observed with variation of weak solution flow rate at fixed vapour flow rates.     

Characterization of a dry ice heat exchanger

Here, a novel multipurpose heat exchanger was designed and fabricated based on dry ice-ethylene glycol bath as the coolant mixture in order to characterize the operating parameters for fast chilling of the fluids with low electrical consumption. Moreover, it was shown that the cooling rate and chilling temperature strongly depend on the initial fluid temperature, loading rate, heat rate and the volume ratio of the glycol coolant to the fluid accordingly.     

Effects of Cooling and Dressing Methods on Grinding of Glass

Experiments were designed and conducted to evaluate the effects of cooling and dressing methods and five other factors on the grinding of glass. Surface roughness of the ground samples was measured and microscopic studies of the ground glass surfaces were performed. The column effect method and plotting method were applied to analyze and interpret the experimental results. The results showed that the average surface roughness heights of ground glass were improved by 5-36% and 42-52%, respectively, when an improved coolant system and dressing device were employed. The average surface roughness heights decreased by 52-58% and 30-37%, when smaller grit sizes and a resin-bond grinding wheel rather than a metal-bond grinding wheel were used, respectively. In addition, the surface roughness averages decreased by 18-23% and 13-18% when a smaller table-feed rate was used and truing of grinding wheels was performed, respectively. Moreover, compared with the surface roughness heights of the sample that was ground using the existing coolant system and manual dressing method, the surface roughness heights of the sample ground using both the improved coolant system and device-dressing method were improved by 46-70%, because of the positive interaction of these two factors.