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.     

Experimental Investigation on the Performance of Heat Pump Operating with Copper and Alumina Nanofluids

In the present study, an attempt is made to enhance the performance of heat pump by utilizing two types of nanofluids namely, copper and alumina nanofluids. These nanofluids were employed around the evaporator coil of the heat pump. The nanofluids were used to enhance the heat input to the system by means of providing an external jacket around the evaporator coil. Both the nanofluids were prepared in three volume fractions 1%, 2% and 5%. Water was chosen as the base fluid. The performance of the heat pump was assessed by calculating the coefficient of performance of the system when it was operated with and without nanofluid jacket. A significant enhancement in the coefficient of performance was noticed when copper and alumina nanofluids were employed in the system. Also, the coefficient of performance was found to have a direct relationship with the tested volume fractions. For the highest volume fraction of 5%, the performance of the heat pump was found to enhance by 23% with alumina nanofluid, while for copper nanofluid, a very significant enhancement in performance by 72% was observed. Thus, utilizing of nanofluids in heat pumps can be very beneficial towards performance enhancement and the idea can also be extended to other thermal systems such as steam power plant, automobile radiator, industrial heat exchangers and refrigeration systems.     

Improvement of thermosyphon performance by employing nanofluid

The presented study aims to make nanofluids applicable for thermosyphons. Experiments employing a vertical thermosyphon are carried out utilising deionised water, water based titanium dioxide and gold nanofluids with different concentrations as working fluids. A maximal reduction of the thermal resistance of about 24% can be achieved when nanofluids are employed. An optimum is reached at concentrations between 0.2 vol. % and 0.3 vol. %, whereas at higher concentrations the thermal resistance remains either unchanged or increases again. A nanoparticle layer on the evaporator surface seems to cause the found changes. Experiments with the gold nanofluid indicate that no nanoparticles are transported with the vapour phase and deposited on the condenser surface. Long term experiments carried out with 0.3 vol. % indicate a massive aging of the porous layer built of nanoparticles on the evaporator surface.     

平板型mLHP实验研究

研制了一套以铜为壳体、不锈钢丝网为毛细芯、丙酮为工质的小型平板型环路热管,实验研究了其系统倾角为10°、50°、90°,工质充灌量为50%、60%和70%条件下的运行特性.实验显示铜一不锈钢一丙酮平板型mLHP具有良好的启动和变工况特性.mLHP系统在热负荷低于18W和高于42W时均能达到稳定运行,而在热负荷介于18～42W的某些工况,系统出现温度波动现象,且其温度波动的波幅和周期与系统倾角、充灌量及热负荷有着密切的关系.系统在变工况运行时具有很快的响应速度,一般在2～3min内达到新平衡状态.此外,在不同工况下系统热阻介于0.45～2.8℃/W之间.     

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.     

γ-Fe_2O_3/EG纳米流体热输运性质的研究

选用乙二醇(EG)为基液,运用两步法制得稳定性良好的γ-Fe2O3纳米流体。测量并研究了γ-Fe2O3纳米流体的导热系数和粘度等热输运性质。结果表明,γ-Fe2O3纳米粒子的加入使得纳米流体的导热系数较基液提高了,纳米流体的粘度在低温下较大,并随着温度的升高而减小,纳米流体在强化传热领域有着潜在的应用前景。     

Theoretical investigations of hydrogen,nitrogen, and oxygen homogenous and annular wick heat pipes

In any heat pipe, the capillary pressure developed at the liquid-vapour interface balances the sum of the various pressure drops throughout the pipe. This study analyses the different contributions to pressure drop for both homogenous wick and annular wick heat pipes operating at low temperatures. The pressure drop in the wick structure is of primary importance for a homogenous wick heat pipe. The heat transfer capacity of an annular-wick heat pipe is strongly affected by the interphase pressure drop due to non-zero evaporation and condensation rates at the liquid—vapour interfaces. Theoretical heat transfer rates as functions of the vapour temperature have been computed for both homogenous and annular wick structures of heat pipes using hydrogen, nitrogen, and oxygen as working fluids. The heat transfer capacity of the annular wick design is more than an order of magnitude higher than that of the corresponding homogenous wick design.     

Study of viscosity and specific heat capacity characteristics of water-based Al2O3 nanofluids at low particle concentrations

In this paper, the specific heat capacity and viscosity properties of water-based nanofluids containing alumina nanoparticles of 47 nm average particle diameter at low concentrations are studied. Nanofluids were prepared with deionised water as base fluid at room temperature by adding nanoparticles at low volume concentration in the range of 0.01%–1% to measure viscosity. The effect of temperature on viscosity of the nanofluid was determined based on the experiments conducted in the temperature range of 25°C to 45°C. The results indicate a nonlinear increase of viscosity with particle concentration due to aggregation of particles. The estimated specific heat capacity of the nanofluid decreased with increase of particle concentration due to increase in thermal diffusivity. Generalised regression equations for estimating the viscosity and specific heat capacity of nanofluids for a particular range of particle concentration, particle diameter and temperature are established.     

动力型分离式热管设计与试验研究

为了研发数据机房用蒸气压缩/热管复合型环控系统,进行动力型分离式热管模块的设计和性能试验。热管系统采用平行流冷凝器和管片式蒸发器,由屏蔽泵提供循环动力,并由调节阀控制供液量。在标准焓差法试验室进行热管模块的制冷性能试验,结果表明：1热管系统在设定工况的制冷性能达到设计指标,验证热管系统参数匹配的合理性和实用性;2热管模块的制冷量、EER与室内外温差近似为线性关系;3液泵驱动的动力型分离式热管系统运行稳定、可靠,解决了重力型分离式热管的运行稳定性问题,简化了系统的安装,易于实施流量调节和能量控制。     

Can the Temperature Dependence of the Heat Transfer Coefficient of the Wire–Nanofluid Interface Explain the “Anomalous” Thermal Conductivity of Nanofluids Measured by the Hot-Wire Method?

It is suggested that a possible explanation for the “anomalous” thermal conductivity of nanofluids measured by the hot-wire method is the positive temperature dependence of the heat transfer coefficient of the hot-wire–nanofluid interface, which results from the positive temperature dependence of the energy exchanged during the particles’ collisions with the wire. It is shown qualitatively that this effect can result in an overestimate of the nanofluid’s thermal conductivity. It is concluded that the interpretation of the experimental data for the thermal conductivity of nanofluids, obtained by the hot-wire method, requires independently determined data for the heat transfer coefficients and their temperature dependence of the hot-wire nanofluid and the particle–fluid interfaces.     

Experimental study on the double-evaporator thermosiphon for cooling HTS (high temperature superconductor) system

A cryogenic thermosiphons is an efficient heat transfer device between a cryocooler and a thermal load that is to be cooled. This paper presents an idea of thermosiphon which contains two vertically-separated evaporators. This unique configuration of the thermosiphon is suitable for the purpose of cooling simultaneously two superconducting bearings of the HTS (high temperature superconducting) flywheel system at the same temperature. A so-called double-evaporator thermosiphon was designed, fabricated and tested using nitrogen as the working fluid under sub-atmospheric pressure condition. The interior thermal condition of the double-evaporator thermosiphon was examined in detail during its cool-down process according to the internal thermal states. The double-evaporator thermosiphon has operated successfully at steady-state operation under sub-atmospheric pressure. At the heat flow of 10.6 W, the total temperature difference of the thermosiphon was only 1.59 K and the temperature difference between the evaporators was 0.64 K. The temperature difference of two evaporators is attributed to the conductive thermal resistance of the adiabatic section between the evaporators. The method to reduce this temperature difference has been investigated and presented in this paper. The proper area selection of condenser, evaporator 1, and evaporator 2 was studied by using thermal resistance model to optimize the performance of a thermosiphon. The superior heat transfer characteristic of the double-evaporator thermosiphon without involving any cryogenic pump can be a great potential advantage for cooling HTS bulk modules that are separated vertically.     

一种提高低温环路热管主蒸发器降温速率的新方法及实验结果

总结了两类加快低温环路热管(CLHP)主蒸发器降温速率的措施,并且介绍了一种新颖的驱动措施:取消次蒸发器,修改冷凝器的设计,将螺纹槽道添加到冷凝器的设计中,使新的冷凝器兼具有类似于次蒸发器的功能.基于这种设计思路,设计了使用氮作为工质的低温环路热管,并且在实验中验证了这种设计具有加速主蒸发器降温的作用.     

Pressure-drop viscosity measurements for gamma-Al2O nanoparticles in water and PG-water mixtures (nanofluids)

Nanofluids have attracted wide attention because of their promising thermal applications. Compared with the base fluid, numerous experiments have generally indicated increases in effective thermal conductivity and convective heat transfer coefficient for suspensions having only a small amount of nanoparticles. It is also known that with the presence of nanoparticles, the viscosity of a nanofluid is greater than its base fluid and deviates from Einstein's classical prediction. However, only a few groups have reported nanofluid viscosity results to date. Therefore, relative viscosity data for gamma-Al2O3 nanoparticles in DI-water and propylene glycol/H2O mixtures are presented here based on pressure drop measurements of flowing nanofluids. Results indicate that with constant wall heat flux, the relative viscosities of nanofluid decrease with increasing volume flow rate. The results also show, based on Brenner's model, that the nanofluid viscosity can be explained in part by the aspect ratio of the aggregates.     

Thermal behavior of a cryogenic loop heat pipe for space application

This paper discusses a prototype of cryogenic loop heat pipe (CLHP) working around 80 K with nitrogen as the coolant, developed at CEA-SBT in collaboration with the CAS/TIPC and tested in laboratory conditions. In addition to the main loop it features a pressure reduction reservoir and a secondary circuit which allow cooling down the loop from the room temperature conditions to the nitrogen liquid temperature and transferring the evaporator heat leaks and radiation heat loads towards the condenser. The general design, the instrumentation and the experimental results of the thermal response of the CLHP are presented, analyzed and discussed both in the transient phase of cooling from room temperature (i) and in stationary conditions (ii). During phase (i), even in a severe radiation environment, the secondary circuit helped to condense the fluid and was very efficient to chill the primary evaporator. During phase (ii), we studied the effects of transferred power, filling pressure and radiation heat load for two basic configurations of cold reservoir of the secondary circuit. A maximum cold power of 19 W with a corresponding limited temperature difference of 5 K was achieved across a 0.5 m distance. We evidenced the importance of the filling pressure to optimize the thermal response. A small heating power (0.1 W) applied on the shunted cold reservoir allows to maintain a constant subcooling (1 K). The CLHP behaves as a capillary pumped loop (CPL) in such a configuration, with the cold reservoir being the compensation chamber of the thermal link. The radiation heat loads may affect significantly the thermal response of the system due to boiling process of liquid and large mass transfer towards the pressure reduction reservoir.     

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%.     

Heat transfer in the evaporator of an advanced two-phase thermosyphon loop

As heat generation from electronic components increase and the limit of air-cooling is reached, the interest for using liquid cooling for high heat flux applications has risen. Thermosyphon cooling is an alternative liquid cooling technique, in which heat is transferred as heat of vaporization from evaporator to condenser with a relatively small temperature difference.The effect of fluid properties, the structure of wall surfaces, and the effect of system pressure was investigated and reported previously by the author. In this paper, the influence of heat flux, system pressure, mass flow rate, vapor fraction, diameter of evaporator channel and tubing distance between evaporator and condenser on the heat transfer coefficient of an advanced two-phase thermosyphon loop is reported. The tested evaporators were made from small blocks of copper with 7, 5, 4, 3 and 2 vertical channels with the diameters of 1.1, 1.5, 1.9, 2.5, and 3.5 mm, respectively and the length of 14.6 mm. Tests were done with isobutane at heat fluxes ranging between 28.3 and 311.5 kW/m².     

过冷度对地热发电非共沸工质有机朗肯循环热力性能的影响

有机朗肯循环（organic Rankine cycle,ORC）是利用中低温地热能（< 150℃）发电的主要途径,在实际运行中,非共沸工质往往会冷凝至过冷状态。分析了冷凝过冷度对非共沸工质ORC热力性能的影响,建立了ORC、内回热（internal heat exchanger,IHE）ORC的热力学模型,以净输出功最大为目标函数优化了工质的蒸发压力,并开展了系统的㶲分析。结果表明：过冷度影响了工质与冷源换热流体间的温度匹配特性,受夹点温差的限制,随着过冷度的增加,工质的冷凝压力上升;过冷度亦改变了预热器和蒸发器的热量分摊,随着过冷度的增加,最佳蒸发压力亦上升。混合工质异丁烷/异戊烷的质量配比为0.4：0.6时,净输出功受过冷度的影响最大,当过冷度为2℃时,净输出功下降了4.36%。IHE回收膨胀机排汽的余热,提高了预热器入口温度,可提高过冷ORC系统净输出功0.55%。过冷度增大了冷凝器的㶲损失;采用内回热冷凝器的㶲损失降低了24.7%。     

Measurement of volumetric thermal expansion coefficient of various nanofluids

Experiments were carried out for studying volumetric thermal expansion behavior of various nanofluids in order to evaluate their potential application in heat removal systems employing natural convection as mode of heat removal. For this purpose, various nanoparticles such as Al₂O₃, CuO, SiO₂ and TiO₂ were used, which were suspended in the base fluid (water) by ultrasonication. All nanofluids had the same concentration of 1 wt %. Each nanofluid was heated from room temperature to a maximum of about 60°C and the increase in volume due to heat addition was recorded. The volumetric thermal expansion due to heating for each nanofluid was compared to that for the base fluid for same increase in the temperature. The volumetric thermal expansion coefficient was evaluated from the measured data. Surprisingly, it was found that the nanofluids have greater volumetric thermal expansion coefficients as compared to that of the base fluid. 1The text was submitted by the authors in English.     

Experimental investigation on R134a vapour ejector refrigeration system

The experimental investigation of the performance of a vapour ejector refrigeration system is described. The system uses R134a as working fluid and has a rated cooling capacity of 0.5 kW. The influence of generator, evaporator and condenser temperatures on the system performance is studied. This kind of system can be operated with low grade thermal energy such as solar energy, waste heat, etc. The operating conditions are chosen accordingly as, generator temperature between 338 K and 363 K, condenser temperature between 299 K and 310.5 K, and evaporator temperature between 275 K and 285.5 K. Six configurations of ejectors of different geometrical dimensions are selected for the parametric study. The performance of the refrigeration system at different operating temperatures is presented.     

Analysis of the heat transfer and friction factor correlations influence in the prediction of evaporating flows inside tubes

A methodology for analysing the influence of the heat transfer and friction factor correlations in the prediction of the two-phase flows inside horizontal ducts under evaporation phenomena is presented. An experimental unit based on single stage vapor compression refrigerating system with two parallel evaporation devices has been built to work under real refrigeration conditions. The first evaporation device consists of a double pipe evaporator which allows determining the heat flux through the pipe. The second device is an electrically heated pipe evaporator with uniformly distributed temperature and pressure sensors along the fluid path. The experimental data of temperature and pressure distribution along the smooth heated duct is compared with a selected set of heat transfer and friction factor correlations through a detailed numerical evaporation model. The aim of this paper is to determine possible criteria to select the most suitable heat transfer and friction factor correlations available.