Stability and thermal conductivity enhancement of carbon nanotube nanofluid using gum arabic

This experimental study reports on the stability and thermal conductivity enhancement of carbon nanotubes (CNTs) nanofluids with and without gum arabic (GA). The stability of CNT in the presence of GA dispersant in water is systematically investigated by taking into account the combined effect of various parameters, such as sonication time, temperature, dispersant and particle concentration. The concentrations of CNT and GA have been varied from 0.01 to 0.1?wt% and from 0.25 to 5?wt%, respectively, and the sonication time has been varied in between 1 and 24?h. The stability of nanofluid is measured in terms of CNT concentration as a function of sediment time using UV-Vis spectrophotometer. Thermal conductivity of CNT nanofluids is measured using KD-2 prothermal conductivity meter from 25 to 60°C. Optimum GA concentration is obtained for the entire range of CNT concentration and 1–2.5?wt% of GA is found to be sufficient to stabilise all CNT range in water. Rapid sedimentation of CNTs is observed at higher GA concentration and sonication time. CNT in aqueous suspensions show strong tendency to aggregation and networking into clusters. Stability and thermal conductivity enhancement of CNT nanofluids have been presented to provide a heat transport medium capable of achieving high heat conductivity. Increase in CNT concentrations resulted in the non-linear thermal conductivity enhancement. More than 100–250% enhancement in thermal conductivity is observed for the range of CNT concentration and temperature.     

Thermal Conductivity of Nanofluids – Experimental and Theoretical

The thermal conductivity of nanofluids has been studied experimentally using the transient hot-wire method, and it is shown that a significant increase can be obtained. Existing methods for the prediction and correlation of the thermal conductivity are discussed. It is shown that a lot of work still needs to be done in this area.Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.     

多壁碳纳米管水基纳米流体的对流换热特性

实验研究了纳米粉体浓度、雷诺数Re和热流密度对多壁碳纳米管水基纳米流体(MWNTs/H2O)对流换热性能的影响。纳米粉体浓度分别为0.05 g/L、0.1 g/L、0.2 g/L和0.4 g/L,雷诺数Re为500~900,热流密度为10~20 k W/m2。结果表明:1)纳米流体对流换热系数随着纳米粉体浓度、Re、热流密度的增加而增加。如在Re为631且纳米粉体浓度为0.4 g/L时,纳米流体对流换热系数比基液增大了17.6%;2)纳米流体对流换热系数的提高率明显大于对应的导热系数提高率,当纳米粉体浓度为0.05g/L时,其对流换热系数和导热系数的提高率分别为7.4%和0.15%;3)在Eubank-Proctor方程的基础上,建立了适合于低Re条件下的混和对流换热的实验关联式。     

载银二氧化钛纳米流体强化气液传热传质实验研究

在碳中和背景下,减少能源消耗、提高暖通运行能效和净化效率是在新时代亟需解决的问题。本文研究了载银二氧化钛(TiO₂-Ag)纳米流体的气液传热传质性能,探索了纳米流体在鼓泡、喷雾、填料等不同流动状态下的传热传质特性,分析了系统参数对传热传质效果的影响。结果表明：鼓泡工况相对于喷雾与填料工况的气液传热效果更好,其中气体流量的设定应同时考虑纳米流体稳定性和气液接触时间等因素来获得最佳效果,因此在本实验的设定中,选定最佳进口气体流量为4 L/min。     

基于纳米粒子/相变石蜡乳状液的研究

介绍了纳米流体的制备方法,探讨了纳米流体强化传热的机理.结合本实验室的研究方向,首次提出了在相变储热石蜡乳状液中添加纳米粒子强化石蜡乳状液传热性能的方法,并制备了纳米铝/石蜡乳状液.分析了该悬浮液的性能,实验结果表明,将0.1%(质量分数)的纳米铝粉分散于石蜡乳状液中,悬浮液的导热系数提高了29.4%,大大提高了石蜡乳状液的传热速率.与水作为储热、传热介质相比,该新型相变纳米流体具有储热密度大、换热能力强的优点.最后,指出了该新型相变纳米流体研究存在的问题并展望了其应用前景.     

Synthesis of individual ultra-long carbon nanotubes and transfer to other substrates

In this article, we report the synthesis of ultra-long carbon nanotubes (CNTs) by thermal chemical vapour deposition method. Ultra-long, individual and aligned CNTs were directly grown on a flat silicon substrate. The orientation of the nanotubes was found parallel to the gas flow direction. The ultra-long CNTs were grown with different transition metallic salts, such as nickel chloride, iron (III) chloride, cobalt acetate and ruthenium acetate, as the catalysts. The influence of the growth conditions, such as growth temperature, reactive gas flow on the length and alignment of the CNTs was studied in detail. By using different catalysts, ultra-long single-walled carbon nanotubes (SWCNTs) or multi-walled carbon nanotubes (MWCNTs) were successfully grown. These ultra-long CNTs were transferred to other substrates by two methods. (1) The first method is to use polydimethylsiloxane as a stamp. (2) The second method is to use KOH as an etching agent. The diameter and length of the CNTs were characterised by transmission electron microscope, scanning electron microscope, atomic force microscope and Raman spectroscopy. The results indicate that the length of the CNTs can reach up to 4?mm. The diameter of the SWCNTs is in the range of 0.7–2.1?nm and the diameter of the MWCNTs is approximately 150?nm.     

二氧化碳翅片管式蒸发器模拟与分析

运用分布参数法建立采用CO2的翅片管式蒸发器的数学模型，分析制冷剂侧和空气侧温度、压力和换热的变化情况。同时讨论迎面风速和制冷剂质量流量对蒸发器换热和流动性能的影响，结果表明提高迎面风速可以增加换热效果，但增加的趋势趋于平缓。制冷剂侧压降则成近似线性增大；随着管内工质流量的增大，蒸发器总换热量和制冷荆侧压降都成近似线性增大。这些工作有助于进一步了解CO2在翅片管式蒸发器中的换热和流动特性，并为换热器的优化设计和系统的匹配提供理论依据。     

水基碳管纳米流体制备及其热物性实验研究

采用添加表面活性剂阿拉伯胶(GA)的方法制备碳管纳米流体,并对不同长径比以及经球磨、酸化处理的碳管纳米流体热物性进行了研究。通过透射电子显微镜和扫描电子显微镜观察表明,所制备的碳管纳米流体具有很好的分散性和稳定性。碳管纳米流体热物性实验结果表明,碳管的比表面积和直线度是碳管长径比影响纳米流体热导率的主要因素。碳管经球磨处理时,随球磨时间延长,碳管长径比和直线度先后对纳米流体热导率提升起主导作用,碳管酸化处理后,改善了其分散性并降低了接触热阻,这是纳米流体热导率提高的主要因素。但随着碳管酸化时间的延长,碳管长径比起主导作用。碳管纳米流体的粘度主要受碳管分散性和直线度的影响。     

智能发电路面技术现状及发展

为给发电路面的研究及推广应用提供理论指导与技术支持,全面调查国内外发电路面研究成果及应用现状并进行系统分析,确定了不同类型发电路面的适用性,提出了国内外现有发电路面技术存在的主要问题,从基于压电沥青混凝土发电路面、基于压电元件发电路面电能测试两个方面探索了智能压电发电路面新技术,并展望了智能压电发电路面未来发展趋势及研究方向.     

A numerical and experimental study of turbulent flow through the evaporator coil in an air-conditioning unit

Computational fluid dynamics (CFD) and heat transfer has matured to the stage where state-of-the-art commercial codes can address real engineering problems. However, the advances in finite volume numerical capability which permit this, such as full three-dimensionality, generalised coordinate systems and multiblocking, require convincing validation procedures to demonstrate quantitative engineering potential. In this paper a two-dimensional incompressible turbulent flow in an industrial packaged air-conditioning system is investigated numerically, with specific reference to the air flow through the tube banks of the evaporator coil. The system modelled comprises an outer casing containing the coil which has three rows of 20-mm diameter tubes in staggered formation, each row having tubes, and a condensate tray with complex geometry. The analysis is based on the Reynolds-averaged Navier-Stokes equations in conjunction with the standard k-ε turbulence model. The finite-volume method (FVM) is used to solve the governing partial differential equations on a non-staggered grid. In order to treat the detailed fin tube geometry, a novel method called ‘masking’ has been developed to construct body-fitted curvilinear grids quickly and efficiently. In the computation, several differencing schemes of various orders are outlined and their accuracy are examined.In a parallel experimental investigation conducted on the actual air-conditioning unit, both the mean velocity profiles and turbulence properties of the flow were obtained from triple hot-wire anemometry measurements. These were then used to compare with the computational results and validate the mathematical models developed. It is found that the predictions were generally in good agreement with the measurements, especially for the higher-order differencing scheme. An immediate practical effect of the study was that it revealed the flow disturbance resulting from the condensate tray. This has been suspected but its magnitude not anticipated. The consequent amendment of the design of this component in the commercial unit led to an improved performance of the evaporator.     

弦月形狭缝通道内液氮受迫流动沸腾传热强化的研究

研究了液氮在弦月形狭缝通道中受迫流动沸腾时的传热特性。发现液氮在弦月形狭缝通道中的受迫流动沸腾具有很高的换热系数,有显著的强化换热效果。详细分析了弦月形狭缝通道内液氮沸腾传热及流动的偏心特性。研究对于进一步理解狭缝通道沸腾传热强化的机理和狭缝强化传热技术在工程中的应用有着重要的意义。     

添加剂在电冰箱制冷系统上应用试验研究

在电冰箱制冷系统氟利昂中加入极少量的一种添加剂 ,增强了蒸发器和冷凝器氟利昂侧传热。试验表明 ,这种添加剂在电冰箱上应用 ,制冷效果明显好转 ,其工作时间系数和功耗下降 ,值得推广应用     

Symmetry-Preserving Discretization of Heat Transfer in a Complex Turbulent Flow

Convective and diffusive operators are discretized such that their symmetries are preserved. The resulting discretization inherits all symmetry-related properties of the continuous formulation. It is shown that a symmetry-preserving discretization is unconditionally stable and conservative. A fourth-order, symmetry-preserving discretization method is developed and tested for the numerical simulation of turbulent (flow and) heat transfer in a channel with surface-mounted cubes, where the temperature is treated as a passive scalar. The Reynolds number (based on the channel width and the mean bulk velocity) is Re=13,000. The results of the numerical simulation agree well with available experimental data.     

Enhancement of heat transfer by water–Al2O3 and water–TiO2 nanofluids jet impingement in cooling hot steel surface

Two different nanofluids, namely water–Al₂O₃ and water–TiO₂, were impinged in the form of jet on hot steel surface to remove high heat flux, and their performance was compared. The dimension of the test steel sample was 120 mm × 120 mm and 4 mm thickness. Four K-type thermocouples were embedded on the bottom surface of the plate to measure the transient temperature distribution. The time-temperature data were recorded by the help of a data acquisition system (make: CHINO, model: KR2000), and the results were analysed by ZAILA application software. Effect of impinging nanofluids with weight concentrations of 0.01%, 0.03%, 0.05% and 0.07% Al₂O₃ and TiO₂ nanoparticles on heat transfer from the hot surface was tested. The surface heat transfer coefficient (HTC) was computed from the time-temperature history recorded during experimentation. Experimental results revealed that addition of nanoparticles to the base fluid (water) surprisingly enhanced the heat transfer rate and HTC as expected. The heat transfer rate increased up to certain limit of nanoparticle concentrations, and then declined. Application of nanofluids for the steel strip cooling was found very effective in terms of heat transfer phenomena as compared to the conventional fluid cooling methods.     

平板太阳能集热器强化传热应用研究进展

集热器作为平板太阳能热水器的关键部分,其传热性能决定着热水器集热效率的高低。介绍了平板太阳能集热器的典型结构以及传热方式,综述了平板太阳能集热器目前所采用的强化传热方式及今后强化传热技术的发展趋势。     

绿色阻垢剂在开放式换热器中的抗垢强化传热性能研究

针对一种蒸发式冷凝器系统换热器盘管表面的易结垢现象，采用高精度摄像仪观察法、红外温度摄像仪及热电偶温度测量手段，对绿色阻垢剂（聚天东冬氨酸PASP）抗垢强化传热性能进行了实验研究。结果表明，在管内放热实验中，采用绿色阻垢剂后的管表面温度平均比污垢管表面温度高4～8℃，前者喷淋循环冷却水温度升高基本维持在0．5～0．6℃／min的水平，而污垢管实验中喷淋循环水温升维持在0．3～0．4℃／min的水平。说明在循环水中加入绿色阻垢剂后，污垢热阻大大减少。     

应用二氧化钛纳米流体提高冷却壁换热能力的实验研究

着重研究了纳米流体在的小型冷却壁内流动时强化换热能力,分别测试了不同浓度的纳米流体在热交换装置内Nu准数,以及纳米流体随着Re数的变化情况,结果显示加入纳米颗粒后,流体的换热系数得到了明显的提高,且纳米流体的强化对流换热系数随着颗粒浓度的增加以及在管内流动的Re数增加而增加。