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.      

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.     

An experimental and numerical study of He II two-phase flow in the TESLA test facility

We report on measurements of the liquid level and temperature corresponding to different local heat loads at several sections of the He II two-phase flow channel in the TESLA (Tera-eV Energy Superconducting Linear Accelerator) Test Facility phase I (TTF1) during its operation. The measurements show that under normal operating conditions saturation between He II and its vapor can be maintained even in the transient process of heat transfer. A computer code for He II stratified two-phase flow analysis has been developed for the numerical simulation of the He II and vapor flow in the configuration of the cryogenic cooling channel in TTF1. Comparison with the measurement shows the prediction by the code agrees well with the experimental results. The code also predicts the maximum heat load under which the two-phase tube in TTF1 would locally dry out. In its application, the code is helpful to evaluate the impact on the flow behaviour resulting from changes to the TTF1 configuration.     

Transient cryogenic chill down process in horizontal and inclined pipes

Experimental results are presented that describe the parametric effects of inclination of transfer line and mass flux on cryogenic chill down process. Experiments were performed in a pressurized liquid nitrogen transfer line made of stainless steel. Fluid and wall temperatures were measured at various axial locations of the test section to monitor the chill down process. The local heat transfer coefficient and heat flux were predicted for the transient chill down period using an inverse heat transfer technique. The results show that the chill down period is characterized by three distinct flow regimes at all mass flux rates. However the variation in chill down time is more predominant at low mass fluxes. Heat transfer coefficient and heat flux calculated using the inverse heat transfer technique further confirmed this and showed that peak heat flux increases with increase of mass flux. It is found that the inclination of the chilling line displayed similar temperature profile but accompanied with variation in chill down time. Results suggest the existence of an optimum upward line inclination minimizing the chill down time.     

Investigation of two-phase heat transfer coefficients of argon–freon cryogenic mixed refrigerants

Mixed refrigerant Joule Thomson refrigerators are widely used in various kinds of cryogenic systems these days. Although heat transfer coefficient estimation for a multi-phase and multi-component fluid in the cryogenic temperature range is necessarily required in the heat exchanger design of mixed refrigerant Joule Thomson refrigerators, it has been rarely discussed so far. In this paper, condensation and evaporation heat transfer coefficients of argon–freon mixed refrigerant are measured in a microchannel heat exchanger. A Printed Circuit Heat Exchanger (PCHE) with 340 μm hydraulic diameter has been developed as a compact microchannel heat exchanger and utilized in the experiment. Several two-phase heat transfer coefficient correlations are examined to discuss the experimental measurement results. The result of this paper shows that cryogenic two-phase mixed refrigerant heat transfer coefficients can be estimated by conventional two-phase heat transfer coefficient correlations.     

低温储罐预冷过程预测

为了能输送和存储单相低温液体,保证输送管道和低温储罐的安全,预冷过程不可或缺。对预冷时间、预冷介质消耗量进行预测有利于指导实际操作。从热力学基本理论出发,采用稳态与变热导率的热力学分析方法,建立储罐预冷时间与预冷介质消耗量数学模型,推导了储罐温度随时间的变化关系,探讨进口流量与预冷时间、预冷介质消耗量之间的相互影响。将不同的计算方法相结合,指导选取最佳预冷流量。     

Temperature distribution and heat transfer in a laminar incom-pressible annular-channel flow with energy dissipation

Expressions are obtained for the temperature distribution over the section, the heat flow through the channel wall, and the coefficient of heat transfer from the fluid to the wall for the case of a laminar flow in an infinite annular channel with constant specific heat flux at the outer wall of the channel and a thermally insulated inner wall taking energy dissipation into account.     

The measurement of thermodynamic performance in cryogenic two-phase turbo-expander

Liquid fraction measurement in cryogenic two-phase flow is a complex issue, especially for an industrial cryogenic system. In this paper, a simple thermal method is proposed for measuring the liquid fraction in cryogenic two-phase turbo-expander by an electric heating unit in experimental study. The liquid fraction of the cryogenic two-phase flow is determined through the heat balance built at the outlet of the turbo-expander (inlet of heating unit) and the outlet of the heating unit. Liquid fractions from 1.16% to 5.02% are obtained from five two-phase expansion cases. Under the same turbo-expander inlet pressure and rotating speed, five superheated expansion cases are tested to evaluate the wetness loss in two-phase expansion. The results show that the proposed method is successful in measuring the liquid fraction of cryogenic two-phase expansion for turbo-expander in an industrial air separation plant. The experimental isentropic efficiency ratio and the tested Baumann factor decrease with the increasing mean wetness. Based on prediction of Baumann rule, the cryogenic turbo-expander with low liquid fraction in two-phase expansion cases suffers from more severe wetness loss than that with the higher liquid fraction.     

用于环形正负电子对撞机探测器超导磁体的小型氦虹吸回路实验与数值模拟研究

在探测器超导磁体的低温系统预研中,搭建了小型的氦虹吸冷却回路实验系统,进行了气-液两相沸腾和传热特性实验研究,并利用VOF多相流模型模拟计算了氦的热虹吸自然冷却循环过程和过热烧干过程。在相同条件下,模拟计算结果与实验数据进行了比较,模拟计算得到的的温度分布、璧面过热度与实验测量数据符合度较好。     

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.     

Theoretical Study on the Interaction Between Constant-Pressure Specific Heat and Nonequilibrium Phase Change Process in Two-Phase Flow

In two-phase flow, the constant-pressure specific heat of a mixture correlates with the flow and the heat transfer processes. In this paper, the air-water-vapor system is taken as an example, and the behavior of the constant-pressure specific heat during a nonequilibrium phase change process in a two-phase flow system is deduced using the theory of two-phase flow and thermophysics; corresponding calculations are employed to the actual two-phase flow process. The results show that the flow and the phase change heat transfer processes determine the variation and magnitude of the specific heat. Vice versa, the specific heat affects the flow and the phase change heat transfer processes.     

B型LNG模拟舱内低温流体两相流动与相变传热数值研究

本文以内容积为40 m³,绝热层(聚氨酯)厚度为400 mm的新型独立B型液化天然气船模拟舱为研究对象,对模拟舱内低温流体的两相流动及相变传热问题进行了非稳态三维CFD模拟。采用流体体积函数(VOF)模型追踪气液相界面,利用Lee模型作为相变模型,在相变模型中考虑了静压的影响,对模拟舱的温度分布及静态BOG生成速率进行了计算。研究了在不同液位以及绝热层存在破损的情况下模拟舱的温度分布及静态BOG生成速率的变化,同时研究了当模拟舱密闭时的增压特性。对比模拟结果与实验结果,偏差在10%以内,模型对模拟舱内的低温液体的蒸发过程模拟较好,可为新型独立B型液化天然气实船的设计和改进提供参考。     

基于AMESim的液氮可控传输性能分析

针对超低温冷却加工液氮可控传输难题,分析了热流量、管路压降等复杂因素对液氮可控传输的影响机制,提出了基于AMESim的液氮可控传输性能分析方法,建立了受热管道液氮两相流动传热数值模型,并在此基础上,研制出一套液氮可控传输原理性系统。通过对比实验表明,提高系统的输入压力能够增大低温流体的流量,缩短系统进入热平衡状态的时间,提高输出流体的干度和流型的稳定性;研制出的液氮可控传输原理性系统在输入压力为1.3 MPa时,在一定的开口范围内,能够稳定输出流量可控的低干度流体,且符合超低温冷却加工的要求。     

低温真空腔体结构设计及传热分析

为适应未来航天低温光学系统的需要,研制了1套测量光学元件低温场和低温变形的系统。该系统由真空机组、低温真空腔体、防振系统、测量装置等主要部分组成。其中低温真空腔体是该系统中关键的装置,为光学元件的低温真空实验提供了必要的条件。对低温真空腔体的结构设计、真空度和传热进行了分析探讨,并且对梯形薄壁绝热支撑进行了详细结构热分析和COSMOSWORK软件的传热分析,结果表明可以满足低温真空的实验要求。     

Operational Characteristics of the 200-m Flexible Cryogenic Transfer System

A proper cryogenic environment is essential for the operation of superconducting devices. A test area for the superconducting radio-frequency modules (SRF) has been established in the RF laboratory at National Synchrotron Radiation Research Center in Taiwan; these modules require much liquid helium during conditioning and performance tests; a cooling capacity of 120 W is expected for the acceptance test of the SRF module. The cryogenic environment of the test area is completed on transferring the liquid helium over a remarkable length of 205 m from the two cryogenic plants at Taiwan Light Source, with a valve box located at each end to control and to measure the cryogenic flow. Flexible cryogenic transfer lines of concentric four-tube type are chosen for both the supply of liquid helium and the return of cold helium gas. Functional examination of this long transfer system was first achieved with a 500-L Dewar in the radio-frequency laboratory; an SRF module was then installed in the test area for practical operation. The primary concern about the cryogenic transfer system is the heat loss; a measurement technique based on the principle of thermodynamics is developed and proposed herein. With the available sensors inside the valve boxes and the heaters inside the 500-L Dewar and the test SRF module, this technique has proved promissing from the measured results.     

HTR-10螺旋管式直流蒸汽发生器的动态数学模型

为满足 10MW高温气冷堆 (HTR 10 )控制系统分析设计的需要 ,分析了HTR 10螺旋管式直流蒸汽发生器内两相流动和传热的实际过程 ,根据建模的要求和可能获得的结构数据 ,选择四方程漂移模型来描述螺旋管内两相流现象 ;经过仿真试算与实验总结 ,确定了模型中关键的结构关系式和传热工况的判断逻辑 ,以此为基础 ,建立了可反映直流蒸汽发生器在正常功率运行范围内动态特性的动态数学模型。通过对典型工况的动静态仿真计算和研究分析 ,表明所得的结果与理论定性分析和实验吻合 ,从而证明了该模型的正确性。所建立的动态数学模型还可以用于其他螺旋管式直流蒸汽发生器的研究设计     

A survey of correlations for heat transfer and pressure drop for evaporation and condensation in plate heat exchangers

Plate Heat Exchangers (PHEs) are used in a wide variety of applications including heating, ventilation, air-conditioning, and refrigeration. PHEs are characterized by compactness, flexible thermal sizing, close approach temperature, and enhanced heat transfer performance. Due to their desirable characteristics, they are increasingly utilized in two-phase flow applications. Detailed research on heat transfer and fluid flow characteristics in these types of exchangers is required to design and use plate heat exchangers in an optimal manner. This paper reviews the available literature on the correlations for heat transfer and pressure drop calculations for two-phase flow in PHEs as an initial process step in order to understand the current research status. Comparative evaluations for some of the existing correlations are presented in the light of their applicability to different refrigerants. Overall, there is a significant gap in the literature regarding two-phase heat transfer and fluid flow characteristics of these types of exchangers.     

Pressure drop characteristics of cryogenic mixed refrigerant at macro and micro channel heat exchangers

Mixed Refrigerant-Joule Thomson (MR-JT) refrigerators are widely used in various kinds of cryogenic systems these days. The temperature glide effect is one of the major features of using mixed refrigerants since a recuperative heat exchanger in a MR-JT refrigerator is utilized for mostly two-phase flow. Although a pressure drop estimation for a multi-phase and multi-component fluid in the cryogenic temperature range is necessarily required in MR-JT refrigerator heat exchanger designs, it has been rarely discussed so far. In this paper, macro heat exchangers and micro heat exchangers are compared in order to investigate the pressure drop characteristics in the experimental MR-JT refrigerator operation. The tube in tube heat exchanger (TTHE) is a well-known macro-channel heat exchanger in MR-JT refrigeration. Printed Circuit Heat Exchangers (PCHEs) have been developed as a compact heat exchanger with micro size channels. Several two-phase pressure drop correlations are examined to discuss the experimental pressure measurement results. The result of this paper shows that cryogenic mixed refrigerant pressure drop can be estimated with conventional two-phase pressure drop correlations if an appropriate flow pattern is identified.     

Certain peculiarities of heat transfer in nucleate boiling in capillary tubes

Boiling cryogenic fluids in capillary tubes of varying geometry are used to study heat transfer. It was found that nucleate boiling in a capillary channel, when the heat flux density on the wall is increased, is followed by slug boiling. The optimal cross section for the system is also calculated.     

An experimental study of heat transfer characteristics of a two-phase nitrogen thermosyphon over a large dynamic range operation

Heat transport characteristics of a cryogenic two-phase nitrogen thermosyphon have been experimentally investigated in this study. The thermal resistance and the maximum heat transfer rate were mainly investigated over a wide dynamic range from near the triple point to the critical point. The experimental data suggests that the nominal thermal resistance does not have pressure dependence in the high pressure and high temperature region. The present experimental result is well explained by the theoretical prediction. From the experimental result of the operating limit of the thermosyphon, it is found that the maximum heat transfer rate is governed by the interaction between the vapor flow and the returning liquid film flow along the wall in the evaporator section, even near the critical point.