Effect of CNT structures on thermal conductivity and stability of nanofluid

Thermal conductivity and stability of carbon nanotube (CNT) structures in water-based nanofluid, as well as their dependence to temperature and time variation are of a great concern. In order to investigate such dependence, five different structures, namely SWNT (single wall CNT), DWNT (double wall CNT), FWNT (few wall CNT) and two different multiwalls were applied in this study. The experiments reveal that the maximum UV–VIS absorbance of the solution corresponds to the dispersion of SWNT in the base fluid. The results from zeta size distribution and thermal conductivity demonstrate that as the number of nanotube wall increase, both stability and thermal conductivity decrease.     

The particle thermal conductivity influence of nanofluids on thermal performance of the microtubes

The paper presents the numerical analysis on microchannel laminar heat transfer and fluid flow of nanofluids in order to evaluate the suitable thermal conductivity of the nanoparticles that results in superior thermal performances compared to the base fluid. The diameter ratio of the micro-tube was D_i/D_o = 0.3/0.5 mm with a tube length L = 100 mm in order to avoid the heat dissipation effect. The heat transfer rate was fixed to Q = 2 W. The water based Al₂O₃, TiO₂ and Cu nanofluids were considered with various volume concentrations ϕ = 1,3 and 5% and two diameters of the particles d_p = 13 nm and 36 nm. The analysis is based on a fixed Re and pumping power Π, in terms of average heat transfer coefficient and maximum temperature of the substrate. The results reveal that only the nanofluids with particles having very high thermal conductivity (λ_Cu = 401 W/m K) are justified for using in microcooling systems. Moreover, the analysis is sensitive to both the comparison criteria (Re or Π) and heat transfer parameters (h_ave or t_max).     

Effective thermal conductivity of wet particle assemblies

The effective thermal conductivity of mono- and poly-dispersed random assemblies of spherical particles and irregular crystals, both dry and partially or fully saturated by wetting and non-wetting liquids, has been determined computationally by numerical solution of the Fourier’s law on 3-D reconstructed media and experimentally by the transient hot wire method. The effect of spatial distribution and volume fractions of the vapour, liquid, and solid phases on effective thermal conductivity was systematically investigated. A power-law correlation for estimating the effective conductivity, valid over a wide range of phase volume fractions and relative conductivities of components, has been proposed.     

基于纳米流体的光伏热联用装置及其理论分析

利用纳米流体对太阳能辐射分频吸收与高效传热的性能,结合太阳能直接吸收(DAC)技术、菲涅尔聚光技术,构建了基于纳米流体的新型聚光太阳能光伏热联用(CPV/T)装置,并建立了装置的物理模型和能量平衡模型。采用计算流体力学方法进行模拟仿真和对比实验,分析了不同流速及流体工质下光伏组件及纳米流体的温度场。研究结果表明:基于纳米流体的CPV/T装置能够充分利用纳米流体辐射分频吸收与高效传热特性,取得对太阳能光伏组件冷却的更好效果,也获得了更高的光热部分流体出口温度;与仅背板冷却的普通光伏热装置相比,流体出口温度有较大幅度的提升,从而提高了光热部分的利用能级。     

新的水导热系数方程

全面搜集整理了国内外已公开发表的水导热系数的实验数据,并比较分析筛选出了其中可靠的实验数据点作为基准数据.在此基础上,通过多元回归分析和加权最小二乘拟合方法,建立了水的导热系数方程.对4 539个基准数据点的计算结果表明,方程的精度优于IAPWS推荐的现行水的导热系数方程,特别是在高温高压区和临界区,方程的描述精度比原...     

Effect of particle size on the convective heat transfer in nanofluid in the developing region

An experimental investigation on the convective heat transfer characteristics in the developing region of tube flow with constant heat flux is carried out with alumina–water nanofluids. The primary objective is to evaluate the effect of particle size on convective heat transfer in laminar developing region. Two particle sizes were used, one with average particle size off 45 nm and the other with 150 nm. It was observed that both nanofluids showed higher heat transfer characteristics than the base fluid and the nanofluid with 45 nm particles showed higher heat transfer coefficient than that with 150 nm particles. It was also observed that in the developing region, the heat transfer coefficients show higher enhancement than in the developed region. Based on the experimental results a correlation for heat transfer in the developing region has been proposed for the present range of nanofluids.     

The effect of particle size on hydrolysis properties of Mg3La hydrides

The effect of particle size on hydrolysis properties of hydrogenated Mg₃La was investigated through fixing the grain size of in-situ formed LaH₃ and MgH₂ from hydrogenated Mg₃La. The hydrolysis rate and hydrogen yield were affected by its particle size. The hydrogenated Mg₃La with smaller particle size of [<12] μm had a higher hydrolysis yield of 863 ml g⁻¹ (7.70 wt.%) hydrogen. The surface area and defect of samples were increased through reducing the particle size and thus promoted the complete hydrolysis. Reducing the particle size is an effective and simple method to improve hydrolysis properties of magnesium hydrides-based materials.     

The role of different parameters on the stability and thermal conductivity of carbon nanotube/water nanofluids

It is obvious that the applicability and efficiency of nanofluids (suspensions contained nanoparticles) are related to their high heat transfer coefficients, especially thermal conductivity. Many parameters affect this property including size, shape and source of nanoparticles, surfactants, power of ultrasonic, time of ultrasonication, elapsed time after ultrasonication, pH, temperature, particle concentration and surfactant concentration. Some of these parameters may have interaction effects. An accepted way for obtaining the optimized condition is based on the design of experiments and statistical analysis. In this paper we investigate the stability and thermal conductivity of carbon nanotube (CNT)/water nanofluids and propose the optimum condition for the production and application of nanofluids. It has been shown that the significant factors on the thermal conductivity and stability are not precisely similar to one another.     

An experimental study on thermal performance of Al2O3/water nanofluid in a minichannel heat sink

In the present work, experimental efforts have been undertaken to explore the forced convective heat transfer performance of using Al₂O₃/water nanofluid to replace the pure water as the coolant in a copper minichannel heat sink. The minichannel heat sink fabricated consists of 10 parallel rectangular minichannels of length 50 mm with a cross-sectional area of 1 mm in width by 1.5 mm in height for each minichannel. Hydraulic and thermal performances of the nanofluid cooled minichannel heat sink have been assessed from the results obtained for the pumping power, the averaged heat transfer coefficients based on the inlet and bulk temperature difference, respectively, with the Reynolds number ranging from 133 to 1515. Compared with the results for the pure water, it was found that the nanofluid cooled heat sink has significantly higher average heat transfer coefficients and hence outperforms the water cooled heat sink. Meanwhile, the heat transfer efficacy of using the nanofluid in the heat sink was further evaluated against the accompanied pumping power penalty.     

Stabilizer effect on CHF and boiling heat transfer coefficient of alumina/water nanofluids

We measured the critical heat flux (CHF) and boiling heat transfer coefficient (BHTC) of water-based Al₂O₃ (alumina) nanofluids. To elucidate the stabilizer effect on CHF and BHTC of alumina/water nanofluids, a polyvinyl alcohol (PVA) was used as a stabilizer. The plate copper heater (10 × 10 mm²) is used as the boiling surface and the concentration of alumina nanoparticle varies 0–0.1 vol.%. The results show that the BHTC of the nanofluids becomes lower than that of the base fluid as the concentration of nanoparticles increases while CHF of it becomes higher. It is found that the increase of CHF is directly proportional to the effective boiling surface area and the reduction of BHTC is mainly attributed to the blocking of the active nucleation cavity and the increase of the conduction resistance by the nanoparticle deposition on the boiling surface.     

Investigation of thermal conductivity and viscosity of Fe3O4 nanofluid for heat transfer applications

Experimental investigations and theoretical determination of effective thermal conductivity and viscosity of magnetic Fe₃O₄/water nanofluid are reported in this paper. The nanofluid was prepared by synthesizing Fe₃O₄ nanoparticles using the chemical precipitation method, and then dispersed in distilled water using a sonicator. Both experiments were conducted in the volume concentration range 0.0% to 2.0% and the temperature range 20 °C to 60 °C. The thermal conductivity and viscosity of the nanofluid were increased with an increase in the particle volume concentration. Viscosity enhancement was greater compared to thermal conductivity enhancement under at same volume concentration and temperature. Theoretical equations were developed to predict thermal conductivity and viscosity of nanofluids without resorting to the well established Maxwell and Einstein models, respectively. The proposed equations show reasonably good agreement with the experimental results.     

强化导热相变材料对PV/PCM热控特性影响研究

文章建立了光伏/相变材料(PV/PCM)太阳能热控系统二维模型,并根据模拟结果研究了相变材料热导率对太阳电池热控特性的影响。模拟结果表明,当PCM热导率由0.3 W/(m·K)逐渐增加至1.1 W/(m·K)时,相变材料对太阳电池的热控效果越来越好。此外,文章设计了PCM热导率分别为0.8,1.1 W/(m·K)的PV/PCM太阳能热控系统实验装置,在模拟光源和自然光条件下,对太阳能热控系统实验装置的输出功率以及太阳电池的温度进行测试。实验结果表明:在模拟光源下,与无PCM太阳电池相比,PCM热导率分别为0.8,1.1 W/(m·K)的太阳电池的最高温度分别降低了4.6,10.8℃,平均输出功率分别提高了2.2%,4.1%;在自然光条件下,与无PCM太阳电池相比,PCM热导率分别为0.8,1.1 W/(m·K)的太阳电池的最高温度分别降低了9.7,12℃,平均输出功率分别提高了3.1%,5.98%。     

Heat transfer to a silicon carbide/water nanofluid

Heat transfer experiments were performed with a water-based nanofluid containing 170-nm silicon carbide particles at a 3.7% volume concentration and having potential commercial viability. Heat transfer coefficients for the nanofluid are presented for Reynolds numbers ranging from 3300 to 13,000 and are compared to the base fluid water on the bases of constant Reynolds number, constant velocity, and constant pumping power. Results were also compared to predictions from standard liquid correlations and a recently altered nanofluid correlation. The slip mechanisms of Brownian diffusion and thermophoresis postulated in the altered correlation were investigated in a series of heating and cooling experiments.     

Transient characteristics of initial droplet size distribution and effect of pressure on evolution of transient condensation on low thermal conductivity surface

The droplet size distribution evolution in the initial dropwise condensation process from formation of primary droplets to fully developed stage has been investigated by utilizing high speed camera and microscope. Focus was put on the transient characteristics of droplet size distribution in this duration, it has been revealed that the primary droplets just formed on the condensing surface satisfied Lognormal distribution, with coalescing among them (without departing from condensing surface), bimodal size distribution formed, finally the classical exponential size distribution showed on the condensing surface when the condensation became steady state. At the same time, the effect of steam pressure on the evolution of transient dropwise condensation on low thermal conductivity surface has been analyzed. All the investigations included in the present paper are for the first droplets cycle of dropwise condensation.     

Orthotropic thermal conductivity effect on cylindrical pin fin heat transfer

Analytical equations for temperature distribution and heat transfer rate from a cylindrical pin fin with orthotropic thermal conductivity, encountered in the use of thermally enhanced polymer composites, are derived and validated using detailed finite-element results. The thermal performance of such fins was found to depart from the classical fin solution with increasing radial conductivity-based Biot number. The in depth analysis of developed orthotropic axi-symmetric pin fin temperature and heat transfer rate equation is carried out to better understand the heat flow rate in such fins.     

一种高导热材料导热系数测试仪的研制

对经典周期热流法进行了研究和改进,采用实验测试和数值仿真相结合的研究方法,设计开发了一套简单实用,数据可靠的高导热材料导热系数测试装置。通过编程技术,实现了数据的自动采集及处理。应用有限容积法建立二维模型,借助Visual Basic语言,开发了该测试系统在线仿真的软件系统。最后根据实验结果和文献值对该测试系统进行评价。