Effects of filling ratio and condenser temperature on the thermal performance of a neon cryogenic oscillating heat pipe

A cryogenic oscillating heat pipe (OHP) made of a bended copper capillary tube is manufactured. The lengths of the condenser section, adiabatic section and evaporator section are 100, 280 and 100 mm, respectively. Neon is used as the working fluid. Effects of liquid filling ratio and condenser temperature on the thermal performance of the OHP are studied. A correlation based on the available experimental data sets is proposed to predict the thermal performance of the neon cryogenic OHP with different filling ratios and condenser temperature. Compared with the experimental data, the average standard deviation of the correlation is about 15.0%, and approximately 92.4% of deviations are within ±30%.     

Theoretical analysis of the thermal performance of a plate heat exchanger at various chevron angles using lithium bromide solution with nanofluid

Nanofluids technology has been rapidly developing over the last two decades. In this paper, the performance of a lithium bromide (LiBr) solution with and without nanoparticles in plate heat exchanger (PHE) for various chevron angles and mass flow rates was investigated. As a result, the heat transfer rate and the overall heat transfer coefficient in 60°/60° PHE is over 100% higher than that of 30°/30° PHE, and the effectiveness of the PHE in 60°/60° PHE is about 70% higher than that of 30°/30° PHE. By using nanoparticle in the working fluid, the heat transfer performance can increase significantly. The heat transfer rate of 3 vol.% nanofluids increased about 3–8% compare to that of LiBr solution for all chevron PHEs. Besides, the 60°/60° PHE using 3 vol.% nanofluids produced the largest heat transfer rate and heat exchange effectiveness under given operating conditions.     

机柜服务器热管背板换热性能的试验研究

通过测试热管背板系统在不同进风温度、制冷剂冷凝单元（RCU）冷冻水进水温度及水流量工况下的换热能力,分析环境温度升高、系统供水不足等对高密度机柜服务器安全运行的影响,采用模拟负载机柜测试机房机柜安装热管背板后不同位置的换热能力。结果表明,热管背板系统对进风温度、冷冻水温度及流量具备良好的换热适应性,发热量大的服务器优先安装在机柜中下部,发热量小的服务器安装在机柜上部,可以最大限度发挥热管背板的换热能力。     

Effects of intermediate pressure on the heating performance of a heat pump system using R410A vapor-injection technique

Recent heat pumps are actively equipped with a refrigerant vapor-injection technique to acquire better performance in severe operating conditions. In this study, the target system which has an additional expansion valve at the outlet of condenser was presented. The concept of intermediate pressure was generated in the target system. Effects of the intermediate pressure on the heating performance with various injection ratios were measured and analyzed according to the compressor frequencies ranging from 60 to 100 Hz. Unlike conventional vapor-injection cycle, the maximum injection ratio was highly affected by the intermediate pressure. The high intermediate pressure provided high initial heating capacity and COP; but it restricted the available range of vapor-injection within narrow limits. The result indicates that a proper operating strategy is needed for the vapor-injection cycle.     

Effect of number of turns and configurations on the heat transfer performance of helium cryogenic pulsating heat pipe

The pulsating heat pipe (PHP) is a potential alternative to highly conductive metals such as copper for long distance heat transfer. Effective thermal conductivity and heat transfer capacity of a PHP are two of the most critical factors for practical applications. In this paper, a helium based PHP, which consists of 48 parallel tubing sections, was developed. The lengths of the evaporator, adiabatic and condenser sections are 50 mm, 100 mm and 50 mm respectively. The condenser section was thermally anchored to a Gifford-McMahon cryocooler (GM cryocooler) with a cooling capacity of 1.5 W at 4.2 K. A maximum effective thermal conductivity of 12330 W/m∙K was obtained when 1.1 W heat was applied to the evaporator section at a fill ratio of 70.5%. With the same geometric parameters and operational parameters, the effect of the number of turns on the heat transfer performance was figured out by comparing the 48-turn PHP with an 8-turn PHP. The results show that the temperature difference between the evaporator and condenser sections of the 48-turn PHP is much smaller than that of the 8-turn PHP. The dry-out temperature response, effective thermal conductivity and heat transfer capacity of them are obtained and analyzed. Furthermore, two configurations of the 48-turn PHP, a parallel configuration and a series configuration, are defined. An optimum configuration is proposed and makes a reference to the design of a cryogenic PHP for applications.     

Experimental investigation on the performance of a neon cryogenic oscillating heat pipe

An experimental investigation is conducted to study the performance of a cryogenic oscillating heat pipe (OHP) using neon as the working fluid. The stainless steel OHP with an inner diameter of 0.9 mm has 4 turns, and the lengths of the evaporator, condenser section and adiabatic section are 35 mm, 35 mm and 95 mm, respectively. The temperature of the evaporator and condenser and the pressure of the OHP are measured. The results show that the cooling down process of the OHP from room temperature to the working temperature can be significantly accelerated by charging with neon. During the pseudo steady-state operation process, the temperature of evaporator and the pressure of the OHP increase with increasing heat input. When the dry out appears, the temperature of evaporator rises quickly, and the pressure of the OHP drops sharply. In addition, the effective thermal conductivity of the OHP at the different heat inputs and the different filling ratios is calculated. It increases with increasing heat input, and there exists an optimum filling ratio which makes the maximum effective thermal conductivity. For this OHP, the optimum filling ratio is 24.5%, at which the effective thermal conductivity is 6100–22,180 W/m K.     

Enhancement of thermal performance in a sintered miniature heat pipe

Abstract

The performance of a sintered miniature heat pipe is enhanced. With the capillary limitation, porosity takes priority over the wick structure parameters that would affect the heat transfer capacity. Since sintered dendritic copper powder has higher porosity, it is used to mix with pore former (Na₂CO₃) in experiments for increasing porosity, and hence enhancing the thermal performance. The results show that, for a heat pipe with a 3mm outer diameter and 200 mm effective length, the heat transfer rate is up to 16.5W and the thermal resistance is 0.9°C/W. In comparison with the unmixed case, the performance increases about 40%.     

Operational performance of a cryogenic loop heat pipe with insufficient working fluid inventory

A cryogenic loop heat pipe (CLHP) has been developed for future aerospace applications at the Technical Institute of Physics and Chemistry (TIPC). It has been demonstrated that this CLHP, when placed horizontally, can operate in liquid-nitrogen temperature range and have a heat transfer capability of up to 12 W with proper working fluid inventory. This paper presents some particular characteristics of the CLHP when the compensation chamber is half-filled with liquid-phase working fluid before startup. The device has been tested at different orientations using nitrogen as the working fluid in order to compare its thermal behavior, specially related to the heat transfer capability, the operation temperature and the thermal resistance, as well as to investigate its operational characteristics under power level as low as 1 W. Tests were performed for the CLHP at horizontal position and with the liquid line 3.4 and 6.4 cm below the vapor line, respectively. The experimental results show the operationability of the CLHP tested at three orientations and tests with the liquid line 6.4 cm below the vapor line show lower operation temperatures and higher heat transfer capability.     

The heat transfer performance of horizontal tube bundles in large falling film evaporators

The performance of the horizontal heat transfer tube bundles in falling film evaporators in large compression refrigeration systems was investigated with numerical simulation in this paper. Four types of tubes, including plain tubes, and enhanced surface tubes of Turbo-B, Turbo-BII and Turbo-EHP, were employed in the simulation. Some factors, such as tube kind, tube pass arrangement, dry patch area on tube surface, liquid refrigerant flow rate, and number of flooded tubes, were analyzed based on simulated results. In the study, the maldistribution of liquid refrigerant flow caused by the distributor apparatus was discussed, which severely affects the performance of falling film evaporators according to the simulation. Some calculated results were verified by the experiment. These discussions and results can be used to guide the design of falling film evaporators under realistic flow conditions.     

槽道热管在加快低温回路热管主蒸发器降温过程中作用的实验研究

介绍液氮温区低温回路热管结构设计的1种方案.首次采用槽道热管作为低温回路热管的次蒸发器,并对其在加快主蒸发器降温过程中的作用进行了详尽的实验研究.这种次蒸发器的结构简单,并使低温回路热管的整体设计变得简易.实验表明,经过合理的阻力设计,槽道热管形式的次蒸发器能起到加快主蒸发器降温的作用,为研究低温回路热管的传热特性提供了前提条件.     

The development of a high-performance Ni-superalloy additively manufactured heat pipe

Additively manufacturing (AM) has been used to manufacture fine structures with structured/engineered porosity in heat management devices. In this study, laser powder bed fusion (LPBF) was used to manufacture a high-performance Ni-superalloy heat pipe, through tailoring LPBF process parameters to fabricate thin wall and micro-channel. By using novel laser scanning strategies, wick structure heat pipes with maximised surface-area-to-volume ratio, fine features size around 100 μm, and controlled porosity were successfully fabricated. Microscopy and X-ray microtomography (micro-CT) were used to investigate the 3D structure of the void space within the pipe. Wick test results showed that most of the heat pipes made by LPBF had better performance than the conventionally manufactured pipes. This study also investigated the influences of the process parameters on the porosity volume fraction and the feature size. The results showed that LPBF process could fabricate thin structure due to the change of melt pool contact angle. The relationship between process parameters and bead size reported in this study could help design and manufacture heat pipe with complex fine structure.The full text can be downloaded at https://link.springer.com/article/10.1007/s40436-022-00407-z     

Comparison of the heating performance of air-source heat pumps using various types of refrigerant injection

The performance degradation of air-source heat pumps cannot be avoided when they operate at both very low and high ambient temperatures. The refrigerant injection technique has rapidly developed in recent years due to its outstanding performance at low ambient temperatures. This study measured the heating performance of air-source heat pumps in which novel vapor injection techniques of a combined flash tank and sub-cooler (FTSC) cycle and a double expansion sub-cooler (DESC) cycle were applied. The performance of these cycles was compared with that of a flash tank (FT) and a sub-cooler (SC) cycle. The average heating capacities of the FT, FTSC, and DESC cycles were higher by 14.4%, 6.0%, and 3.8%, respectively, relative to that of the SC cycle, but the average COPs for the respective cycle options were very similar.     

表面活性剂对脉动热管传热特性的影响

本文试验研究了表面活性剂脉动热管的运行性能。采用浓度为0.125%的十六烷基三甲基溴化铵(CTAB)水溶液作为工质。试验结果表明,表面活性剂脉动热管的启动时间和启动温度随输入功率的增加而降低。脉动热管的传热特性与输入功率有关,且随着输入功率的增加而增强。与纯水脉动热管相比,充液率为50%的CTAB脉动热管在加热功率100 W时热阻降低52%。     

液氮温区重力辅助深冷回路热管的实验研究

给出了液氮温区重力辅助深冷回路热管结构设计方案,建立了实验系统,对其启动特性和工作性能进行了实验研究.深冷回路热管以高纯氮作为工作液体,工作温区为90 K～126 K.实验结果表明,深冷回路热管能在重力作用下快速启动,在气体管线高于液体管线20 mm的情况下,最大可传送的功率为11W.     

Entropy generation on MHD flow and convective heat transfer in a porous medium of exponentially stretching surface saturated by nanofluids

This paper examines the unsteady boundary layer magnetohydrodynamic flow and convective heat transfer of an exponentially stretching surface saturated by nanofluids in the presence of thermal radiation. The combined effect of stratifications (thermal and concentration) in the unsteady boundary layer flow past over a stretching surface embedded in a porous medium is analyzed. The system of coupled nonlinear differential equations are solved numerically by developing finite difference scheme together with the Newton’s linearization technique, which allows us to control nonlinear terms smoothly. The study shows that the thermal boundary layer thickness significantly increases with the increase of Brownian motion, thermophoresis number and magnetic field strength. The unsteadiness behavior of the flow of nanofluid has reducing effect on both momentum and thermal boundary layer thickness. The Brownian motion has controlling effect on nanoparticle migration. The entropy generation by means of Bejan number has strong impact on the applied magnetic field, dissipation of energy, thermal radiation and Biot number.     

微通道冷凝器低温回路热管的实验研究

设计了一套以乙烷作为工质,使用微通道冷凝器的低温回路热管原理样机,并对样机的降温过程、传热性能以及再启动特性进行了实验研究。结果表明:在5 W的驱动功率下该低温回路热管可实现快速降温,在降温过程中通过增大蒸发器的加热功率可以加速回路热管的降温;该样机可以在200 K时能够稳定传输50 W的热量,并且随着加热功率的增大,回路热管的热阻不断减小;在回路热管停止工作后,重新施加热量,该样机仍能够正常启动;该样机在本实验中的最大传热功率为54W,此时的热阻为0.46 K/W。     

Experimental thermal performance study of an inclined heat pipe heat exchanger operating in high humid tropical HVAC systems

In an earlier paper [Y.H. Yau, Application of a heat pipe heat exchanger to dehumidification enhancement in tropical HVAC systems – a baseline performance characteristics study, International Journal of Thermal Sciences 46 (2) (2007) 164–171], the author had established the baseline performance characteristics of the eight-row wickless heat pipe heat exchanger (HPHX) for a vertical configuration under a range of conditions appropriate for a tropical climate. Now, the same basic experimental set-up was to be used in the present research with the HPHX tilted 30°. In this configuration, the gravitational force would be expected to enhance drainage of any condensation forming on the extended fin surfaces of the HPHX evaporator section, and therefore, the effectiveness of the HPHX could be anticipated to be better than the vertical configuration, particularly when processing inlet air with high RH. The investigation has been carried out for 32 experiments with typically high RH and the results are presented in this paper. The results suggested that the possibly adverse influence of condensate forming on the fins of the HPHX was negligible, and therefore the HPHX in a typically-used vertical configuration could perform equally as well as it would if the HPHX was installed in an inclined position.     

The new thermo-ecological performance optimization of an irreversible three-heat-source absorption heat pump

The new thermo-ecological performance optimization of an absorption heat pump operating between three temperature levels with the losses of heat resistance, internal irreversibility and leakage is analyzed by taking the ecological coefficient of performance (ECOP) as an objective function. The new thermo-ecological criterion takes into account the first and second law of thermodynamics and is defined as the heating rate per unit loss rate of availability. The ecological coefficient of performance has been expressed and maximized in terms of the temperatures of the working fluid in the main components of the system. The corresponding optimal temperatures and other optimum performance parameters have been derived analytically, and the effects of the internal irreversibility, the heat leakage coefficient and the source temperature ratio on the global and optimal performances are discussed. The obtained results may provide a general theoretical tool for the ecological design of absorption heat pumps.     

U形热管换热器对空调箱机组除湿性能影响的试验研究

为分析U形热管换热器的工作性能,通过改变迎面风速和回风温度,测出U形热管换热器前后的参数,计算换热温差和换热效率,然后测试相同条件下无U形热管换热器的状况,分析得出增加热管换热器后空调箱机组除湿量可提高15%左右。     

高压绕管式换热器中换热管漏气后的返修

从某套空分设备上卸下的DR87XX型高压绕管式换热器的换热管胀接部分焊缝出现漏气现象,原因是胀接压力过大。制定了详尽的返修方案,具体实施后高压绕管式换热器恢复正常运行。文章简介了DR87XX型高压绕管式换热器中管板与换热管的"胀接+强度焊"模式,详细叙述了返修前的胀接试验、检验和返修工艺过程,最后总结了返修中的注意事项。