Application of CNT nanofluids in a turbulent flow heat exchanger

Nanofluids have received much attention since its discovery owing to its enhanced thermal conductivity and heat transfer characteristics which makes them a promising coolant in heat transfer application. In this study, the enhancement in heat transfer of carbon nanotube (CNT) nanofluids under turbulent flow conditions is investigated experimentally. The CNT concentration was varied from 0.051 to 0.085 wt%, respectively. The nanofluid suspension was stabilised by gum arabic through a process of homogenisation and water bath sonication at 25 °C. The flow rate of cold fluid (water) is varied from 1.7 to 3 L/min, while flow rate of the hot fluid is varied between 2 and 3.5 L/min. Thermal conductivity, density, and viscosity of the nanofluids are also measured as a function of temperature and CNT concentration. The experimental results were validated with theoretical correlations for turbulent flow available in the literature. Results showed an enhancement in heat transfer between 9% and 67% as a function of temperature and CNT concentration.     

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.     

Determination of the thermal diffusivity and specific heat using an exponential heat pulse,including heat-loss effects

The one-dimensional heat diffusion equation has been solved analytically for the case of a heat pulse of the form F(t) = exp(–t/)/ applied to the front face of a homogeneous body including the effects of heat loss from the front and back faces. Approximate expressions are presented which yield a simple, accurate technique for the determination of the thermal diffusivity and specific heat, suitable to a wide range of heat-pulse time constant and heat-loss parameters, without recourse to graphical techniques or requiring further computer analysis. A procedure is described for the determination of an effective time constant to allow application of the present results to the case of a nonexponential heat pulse. Experimental results supporting the theoretical analysis are presented for five samples of silicon germanium alloys of various thicknesses, determined using a xenon flash tube heat-pulse exhibiting an exponential dependence. Proper consideration of the experimental heat pulse shape is shown to lead to reliable corrections to the apparent thermal diffusivity, even for relatively long heat-pulse times.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

Experimental study on heat transfer and rheological characteristics of hybrid nanofluids for cooling applications

The enhancement of heat transfer and rheological behaviour of hybrid nanofluids (HyNF) flowing through the tubular heat exchanger system were experimentally analysed. In this study, the effects of Nusselt number, Peclet number, Reynolds number, heat transfer coefficient and pressure drop were investigated for various volume concentrations of copper-titania hybrid nanocomposite (HyNC). The experiments were performed for various HyNC volume concentrations in the base fluid (cold water) ranging from 0.1% to 1.0%. The experimental results showed that the convective heat transfer coefficient of the HyNF increased by 59.3% for the particular volume concentration of 0.7% of HyNC. The friction factor and pressure drop of HyNF for 1.0% volume concentration were expected to be 0.8% and 5.4%, respectively. This implies for experiencing penalty in the pumping capacity. The experimental measurements, on the other hand, were validated using a newly developed correlation. For all the volume concentrations of HyNF, the deviation obtained for the experimental data and the prediction was observed to be +8% and ?8%, respectively. The present correlation has been found to be in good agreement with the experimental data, which can be helpful in predicting the heat transfer characteristics of the HyNF.     

圆环腔内纳米流体自然对流的数值研究

采用Fluent软件对圆环封闭腔内的Ag-水纳米流体自然对流传热进行数值模拟,着重分析在不同瑞利数下Ag纳米颗粒的添加量和圆环内外壁半径比对圆环传热性能的影响.研究结果表明,随着瑞利数的增加,圆环间的换热强度不断加剧,换热由热传导逐渐向对流转变.添加纳米颗粒降低了换热性能,且随着颗粒浓度的增加换热效果不断恶化;同时,圆环半径比对换热有很大的影响,对一定的瑞利数而言随着半径比的减小,换热性能逐渐增强,且增大的趋势越来越显著.     

封闭腔内MWCNT纳米流体自然对流传热的数值模拟

采用Fluent软件对封闭腔内纳米流体层流自然对流换热进行了数值模拟研究.重点分析了Ra数和纳米颗粒的体积分数对自然对流换热特性的影响.数值模拟结果表明:在机油中添加多壁碳纳米管(MWCNT)粒子并没有提高基液的自然对流传热特性;对于给定的Ra数下,随着纳米颗粒体积分数的增大,纳米流体的传热特性也随之减弱;对于给定的体积分数,随着Ra数的增大,纳米流体的传热特性显著增强,但纳米流体的传热性能比机油的要弱,且在同一体积分数下随着Ra数的增大,传热性能减弱的程度要减小.     

Fluctuations contribution to the specific heat of a van hove superconductor

The fluctuation contribution to the specific heat of a two-dimensionai superconductor has been calculated, taking into consideration a logarithmic density of states. The specific heat correction contains a term proportional to (T-T _c)^-1 and a logarithmic contribution. It has been shown that this contribution represents a very small correction. The specific heat correction is analyzed in the critical region where the fluctuations interact strongly.     

NiTi合金热容的测定及传热的研究

使用调制式差示扫描量热器（MDSC）精确测定了Ni-Ti合金的比热容。研究了以不同的速度加热和冷却合金时相变点的变化,并用试验测定的热容值,计算了电流加热的NiTi合金丝的温度变化历程,这将为科学合理地计算加热时合金的温度场提供参考信息。     

Estimation and experimental study of the density and specific heat for alumina nanofluid

This study analyses the density and specific heat of alumina (Al₂O₃)/water nanofluid to determine the feasibility of relative calculations. The Al₂O₃/water nanofluid was produced by the direct-synthesis method with cationic chitosan dispersant served as the experimental sample, and was dispersed into three concentrations of 0.5, 1.0 and 1.5?wt.%. This experiment measures the density and specific heat of nanofluid with weight fractions and sample temperatures with a liquid density meter and a differential scanning calorimeter (DSC). To assess the availability of these equations, it then compares the experimental data with the calculated results according to the concepts of mixing theory and statistical mechanism. Comparing the calculated results of density and specific heat with the experimental data, the deviation of density fell within the range of ?1.50% to 0.06% and 0.25% to 2.53%, whereas the deviation of specific heat fell within the range of ?0.07% to 5.88% and ?0.35% to 4.94%, respectively. Calculated results of density and specific heat show a trend of greater deviation with an increased concentration of nanofluid. However, two kinds of density and specific heat of the calculated results fall within an acceptable deviation range in this study.     

An evaluation of specific heat measurement methods using the laser flash technique

Different methods for adapting the laser flash technique to measure simultaneously specific heat have been proposed in the literature. Among them are the coating method, the absorbing disk method, the double-specimen method, the pulse heating-cooling method, and the cavity method. These methods are briefly reviewed, and their merits and demerits are evaluated.     

Determination of thermal conductivity from specific heat and thermal diffusivity measurements of plasma-sprayed cermets

The thermal conductivities of three plasma-sprayed cermets have been determined over the temperature range 23–630°C from the measurement of the specific heat, thermal diffusivity, and density. These cermets are mixtures of Al and SiC prepared by plasma spray deposition and are being considered for various applications in magnetic confinement fusion devices. The samples consisted of three compositions: 61 vol% Al/39 vol% SiC, 74vol% Al/26vol% SiC, and 83 vol% Al/17 vol% SiC. The specific heat was determined by differential scanning calorimetry through the Al melt transition up to 720°C, while the thermal diffusivity was determined using the laser flash technique up to 630°C. The linear thermal expansion was measured and used to correct the diffusivity and density values. The thermal diffusivity showed a significant increase after thermal cycling due to a reduction in the intergrain contact resistance, increasing from 0.4 to 0.6 cm²·^–1 at 160°C. However, effective medium theory calculations indicated that the thermal conductivities of both the Al and the SiC were below the ideal defect-free limit even after high-temperature cycling. The specific heat measurements showed suppressed melting points in the plasmasprayed cermets. The 39 vol% SiC began a melt endotherm at 577°C, which peaked in the 640–650°C range depending on the sample thermal history. Chemical and X-ray diffraction analysis indicated the presence of free silicon in the cermet and in the SiC powder, which resulted in a eutectic Al/Si alloy.Paper presented at the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

金属氧化物纳米流体的导热性能研究

采用瞬态热线法测量了4种不同种类、不同体积份额配比的纳米流体的导热系数,分析了纳米颗粒属性、体积分数、悬浮稳定性及温度等因素对纳米流体导热系数的影响.实验结果表明,在流体中加入纳米颗粒将显著提高流体的导热系数.     

Low temperature specific heat of Dow Corning Silicon Oil No 704 between 2 and 18 K

The specific heat of Dow Corning Silicon Oil No 704 was measured between 2 and 18 K. Below 5 K it was found to follow a T³ law. Throughout the range it could be fitted with a 5th order polynomial in T.     

Stress dependence of specific heat: observations on the TERSA technique

Recently a new method of residual stress measurement, TERSA, was described₁and it was suggested that there was a contribution to the effects reported from the stress dependence of specific heat. In this paper, experimental and theoretical evidence is presented demonstrating that the magnitude of the change of specific heat with stress is too small to make a significant contribution to the change in temperature resulting from a stress increment applied to a laser irradiated object.     

Simultaneous measurements of specific heat and total hemispherical emissivity of chromel and alumel by a transient calorimetric technique

Using a transient calorimetric technique, the specific heat and total hemispherical emissivity of chromel and alumel were measured simultaneously in the temperature range 360–760 K. Two types of specimens for each material were prepared. To obtain reliable experimental values of specific heat and total hemispherical emissivity, an expression for the time history of the temperature of the specimens was developed; this expression is accurate over the whole temperature range. An error analysis is made and the uncertainty (the total error) in the values of specific heat and total hemispherical emissivity is estimated to be 3.1% for the well-designed specimens.

     

Spin wave contribution to the specific heat of ferromagnetic terbium at low temperatures

The magnon contribution to the specific heat of ferromagnetic terbium is evaluated and the results are compared with the measurements from 90 K down to absolute zero.     

Accurate determination of specific heat at high temperatures using the flash diffusivity method

The flash diffusivity method can be extended, very simply, to measuring simultaneously thermal diffusivity and specific heat and thus obtaining the thermal conductivity directly. This was accomplished by determining the amount of heat absorbed by a sample with a well-known specific heat and then using this to determine the specific heat of any other sample. The key to using this technique was to have identically reproducible surfaces on the standard and the unknowns. This was achieved earlier by sputtering the surfaces of the samples with a thin layer of graphite. However, the accuracy in determining the specific heat was within ±10% and there was considerable scatter in the data. Several improvements in the technique have been made which have improved the accuracy to ±3% and increased the precision. The most important of these changes has been the introduction of a method enabling the samples to be placed in exactly the same position in front of the light source. Also, the control of the thickness and the application of the graphite coating have turned out to be very important. A comparison of specific heats obtained with this improved technique and with results obtained using other techniques has been made for two materials.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

Magnetic specific heat and susceptibility of cupric oxide (CuO) single crystals

We investigated the magnetic specific heat and entropy of a CuO single crystal in the wide temperature range from 1·5 K to 1000 K and determined the anisotropy of its magnetic susceptibility below 300 K. CuO behaves like an anisotropic low-dimensional antiferromagnet. The magnetization discontinuity is most clearly visible atT _N2=214 K for fields in theb-c plane. The measured magnetic entropy at 1000 K,S _m(1000)=5·67 J/mol K, approaches the expected value ofS _m=Rln2=5·76 J/mol K.     

Radiative heat transfer in transient hot-wire measurements of thermal conductivity

New measurements of the thermal conductivity of liquid toluene between 300 and 550 K have been used to study the importance of radiative heat transfer when using the transient hot-wire technique. The experimental data were used to obtain the radiation correction to the hot-wire temperature rises. Radiationcorrected values of thermal conductivity are reported. This study shows that the transient hot-wire method is much less affected by radiation than steady-state techniques.     

Energy efficient and cost effective method for generation of in-situ silver nanofluids: Formation,morphology and thermal properties

In the present work, silver nanoparticles (AgNPs) are generated using the Micro-Electro Discharge Machining process (micro-EDM). Nanofluids are synthesised in dielectric fluids such as polar fluid like Deionized water (DI), Deionized water with 4 wt% of Poly-Vinyl Alcohol (DI + PVA), and non-polar fluids like Ethylene Glycol (EG) and Kerosene (KR). Low energy consumption, in-situ nanofluid synthesis, cleaner work environment, non-essential chemical post-processing during the synthesis using micro-EDM are the significant reasons creating broad scope for this process exploration. To understand the process, theoretical approach is explored to study the effect of dielectric fluids on particle formation mechanism, critical radius and nucleation rate of nanoparticles. In the experimental approach, silver nanoparticles are generated and characterized for the particle concentration, morphology and size distributions in all four dielectric fluids. High Resolution Scanning Electron Microscopy (HRSEM), Dynamic Light Scattering (DLS), and UV–Visible spectroscopy (UV–Vis) are used for the study. Nanofluid's decomposition temperatures and latent heat of vaporization are investigated using ThermoGravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC), respectively. Particles generated in DI + PVA fluid found to be smaller mean size of (30.06 ± 1.12) nm followed KR fluid of (40.32 ± 1.29) nm, EG fluid of (47.85 ± 1.24) nm, and in DI fluid of (149.04 ± 1.93) nm. Also polar liquids yielded wider and non-polar liquids yielded narrower particle distribution. KR nanofluid is thermally stable followed by DI + PVA, DI, and EG nanofluid. With the spark energy of 1.15 mJ, the in-house developed micro-EDM process yielded highest nanoparticles concentration of 2.68 g/L in KR fluid followed by DI + PVA fluid of 2.13 g/L, DI fluid of 2.09 g/L and least by EG fluid of 1.04 g/L.