微槽平板热管的流动分析

通过建立微槽平板热管的数学模型,得到了热管内部毛细流动和传热的数据,分析了热管在多种工况下工作特性的变化规律,考察了汽液界面上的剪切作用对热管传热量的影响,并且进一步在理论上预测了该型热管的毛细和沸腾极限。     

平板热管的研究现状与最新进展

平板热管由于质量轻、良好的启动性和均温性的优势,而成为目前电子元件散热方面的热点研究,在国外已经得到应用。目前国内的研究成果很少。综述了平板热管目前国外的主要研究趋势、结构和加工方法的改进、参数优化、内部流动与传热的研究、毛细限和沸腾限及对多个热源冷却的研究等,并对这些前沿的研究结论进行概述,指出了下一步的研究趋势和面临的主要挑战。     

平板热管的黏性极限的理论分析与研究

通过建立合理的理论研究模型,在传统热管的基础上,对平板热管的黏性极限作了较为深入的理论研究,最终得到平板热管的粘性传热极限的解析表达式。     

分离式CO_2热管传热性能分析

用CO_2替代R22应用于分离式热管系统对于环境保护很有意义。从目前已有的实验结果来看,CO_2的流动沸腾和凝结传热系数要明显高于常规制冷剂,说明其在提高热管系统传热性能方面具有潜力。但考虑到热管内工质的沸腾和凝结换热系数较高,相对来说,热管系统的主要热阻集中在管外空气侧或水侧的对流换热热阻。因此,尽管CO_2替代常规制冷剂时管内沸腾凝结换热系数可以成倍提高,但热管系统整体传热性能的提高可能较为有限。本文通过实验对比了CO_2热管和R22热管的传热性能,并结合相关的传热模型,分析了分离式热管中的各部分热阻,结果显示,由于CO_2的管内沸腾凝结换热热阻小于R22,使得CO_2热管的整体传热性能优于R22热管,其总热阻比R22热管降低22%~25%。     

单面波浪平板脉动热管的传热性能

对一种单面波浪平板脉动热管的传热性能进行了实验研究,分析在空气强制对流冷却条件下充液率、加热功率、倾角等因素对其传热性能的影响。研究结果表明,除0°倾角外,脉动热管的最佳充液率为20%～30%,倾斜角度对脉动热管传热性能的影响很小,但90°时相对最好。脉动热管在0°放置时其传热性能较差,在低充液率的情况下甚至丧失脉动效果,主要是工质回流不畅的原因,与平板脉动热管的槽道设计有很大关系。此外低加热功率时热管传热性能存在波动,有时甚至不能启动。     

重力铯热管等温性能研究

为了验证铯热管研制的关键技术,通过测量热管外壁温度,在定温条件下研究了重力铯热管的等温特性和启动性能;同时,分析了冷凝段长度对铯热管等温性能的影响。实验结果表明:当加热炉温度在330~630℃,铯热管均能正常启动;加热炉温度越高,启动越快;在该温区,铯热管具有优良的传热性能;然而,当冷凝段长度为300mm时,铯热管壁面温度出现锯齿状周期波动。从过热度的角度,分析了铯热管内部的沸腾相变传热机理;选择合适长度的冷凝段可避免周期性间歇沸腾的产生。实验结果同时也了证明铯热管在330~630℃温区可作为高效的传热元件,非常适合复现ITS-90国际温标锌凝固点。     

小型平板式环路热管的反重力运行特性

为满足电子设备高效冷却的需要,本文从提高毛细压力、降低流动阻力、强化蒸发器壁面传热能力以及降低热泄漏方面入手,设计了一种具有新型蒸发器结构的平板环路热管。利用集成蒸气通道的一体式双孔毛细芯替代传统的机械加工微槽道,并在反重力条件下对其传热性能进行实验研究。结果表明:该环路热管具有良好的反重力运行性能,能够在20 W热负荷下实现快速、低温启动,且无温度波动现象;在200 W热负荷下,能够维持蒸发器最高温度在80℃以下,环路热管的热阻仅为0.15 K/W。由于新型蒸发器结构缩短了热传递路径,降低了蒸发/沸腾的强化以及热泄漏,与传统结构的环路热管对比,能够有效提升环路热管的反重力运行能力。     

微槽平板热管传热性能的实验研究

系统地研宄小充液率条件下重力对微槽平板热管传热性能的影响，分析了工作温度、冷却方式等影响因素．发现重力对热管径向液膜的分布影响比较小，而对轴向的影响比较明显，从而使得倾角较大地影响了热管的传热能力．进一步证明了深槽平板热管具有良好的传热性能．     

新型平板热管流动与传热特性的理论分析

提出了一种新型平板热管，这种热管整体成矩形平板状，其内部结构由许多并列的小矩形长条单元组成。对这种热管的流动特性和传热特性进行了理论建模分析，得到了热管蒸气腔内的速度分布，压力分布及温度分布的解析表达式。     

平板微热管阵列的研究现状与进展

平板微热管阵列作为一种新型高效传热元件,解决了单一微热管热通量和热输运能力有限的问题,提升了热管整体的可靠性,克服了传统圆热管在应用中出现的接触面积小、形状限制、接触热阻等局限。本文总结了平板微热管阵列的内部结构、吸液芯种类、制造和封装工艺,介绍了针对平板微热管阵列性能进行的实验、理论研究及建立的数学模型,阐述了平板微热管阵列在多个领域的应用现状,并提出了进一步的研究重点,为平板微热管阵列的研究提供参考。     

Thermal performance of cold storage in thermal battery for air conditioning

This article studies, experimentally and theoretically, the thermal performance of cold storage in thermal battery for air conditioning. Thermal battery utilizes the superior heat transfer characteristics of heat pipe and eliminates drawbacks found in the conventional thermal storage tank. Experimental investigations are first conducted to study the cold storage thermal performance in two experimental systems: the ratio of distance between heat pipes to outer diameter of heat pipe W/D=6 and 2. Different heat transfer mechanisms including nucleate boiling, geyser boiling and natural convection are identified in different experimental systems with various liquid fills. A theoretical model to determine the thermal characteristics of the thermal battery has also been developed. Comparisons of this theory with experimental data show good agreements in the nucleate boiling stage of cold storage process.     

Partial Nucleate Pool Boiling at Low Heat Flux: Preliminary Ground Test for SOBER-SJ10

Focusing on partial nucleate pool boiling at low heat flux, SOBER-SJ10, one of 27 experiments of the program SJ-10, has been proposed to study local convection and heat transfer around an isolated growing vapor bubble during nucleate pool boiling on a well characterized flat surface in microgravity. An integrated micro heater has been developed. By using a local pulse overheating method in the experimental mode of single bubble boiling, a bubble nucleus can be excited with accurate spatial and temporal positioning on the top-side of a quartz glass substrate with a thickness of 2 mm and an effective heating area of 4.5 mm in diameter, and then grows under an approximate constant heat input provided by the main heater on the back-side of the substrate. Ten thin film micro-RTDs are used for local temperature measurements on the heating surface underneath the growing bubble. Normal pool boiling experiments can also be carried out with step-by-step increase of heating voltage. A series of ground test of the flight module of SOBER-SJ10 have been conducted. Good agreement of the measured data of single phase natural convection with the common-used empirical correlation warrants reasonable confidence in the data. It is found that the values of the incipience superheat of pool boiling at different subcooling are consistent with each others, verifying that the influence of subcooling on boiling incipience can be neglected. Pool boiling curves are also obtained, which shows great influence of subcooling on heat transfer of partial nucleate pool boiling, particularly in lower heat flux.     

Heat transfer to subcritical and supercritical helium in centrifugal acceleration fields 1. Free convection regime and boiling regime

Effect of centrifugal acceleration on heat transfer from the circular flat surface (6.1 mm diameter) of copper to helium was investigated by placing the test piece in a small cell which is revolving about a displaced axis. Free convection data were obtained in the temperature range 4.2–4.5 K, the pressure range 0.1–0.7 MPa (1–7 atm) by imposing centrifugal accelerations of 1-300 ‘g’ towards the surface. Data in the nucleate and film boiling regimes were also obtained at near atmospheric pressures.The new correlation is suggested for free convection heat transfer. However, the existing correlations for conventional fluids in the gravity field seem to be also useful for the design work of superconducting generators, once the gravity term in the Rayleigh number is replaced by a centrifugal acceleration. Nucleate boiling heat transfer is found to be little affected by rotation, although increase in the maximum and minimum heat flux were observed. Also, heat transfer in the film boiling regime was improved.     

Influence of oil on nucleate pool boiling heat transfer of refrigerant on metal foam covers

Nucleate pool boiling heat transfer characteristics of refrigerant/oil mixture on metal foam covers were experimentally investigated. The refrigerant is R113, and the oil is VG68. The copper foams, having ppi (pores per inch) of 10 and 20, porosity from 90% to 98%, and thickness of 5 mm, are selected in this study. Experimental conditions include a saturation pressure of 101 kPa, oil concentrations from 0 to 5%, and heat fluxes from 0 to 80 kW m⁻². The experimental results indicate that the nucleate pool boiling heat transfer coefficient on copper foam covers is larger than that on flat heated surface by a maximum of 160% under the present experimental conditions; the presence of oil deteriorates the nucleate pool boiling heat transfer on copper foam covers by a maximum of 15% under the present experimental conditions, and the deterioration of oil on nucleate pool boiling heat transfer on copper foam covers is lower than that on a flat heated surface. A correlation for predicting the nucleate pool boiling heat transfer coefficient of refrigerant/oil mixture on copper foam cover is developed, and it agrees with 95% of the experimental data within a deviation of ±20%.     

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.      

泡沫金属中池沸腾传热的研究进展

介绍了泡沫金属的结构特性,总结了泡沫金属中池沸腾的气泡生长速度、气泡直径和气泡生长现象等传热特点,以及泡沫金属的孔隙率、孔密度等参数对池沸腾传热的影响,并指出了泡沫金属中沸腾传热的研究方向。     

Heat transfer in subcooled liquid cryogens

Experimental study of heat transfer in helium, hydrogen and nitrogen has been carried out over a wide range of pressures and subcooling. Empirical correlations are obtained to calculate heat transfer coefficients at nucleate boiling, film boiling and single-phase convection. Conditions are determined for transitions from one heat transfer mode to another.