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.     

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。     

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.     

MHD convective heat transfer of nanofluids through a flexible tube with buoyancy: A study of nano-particle shape effects

This paper presents an analytical study of magnetohydrodynamics and convective heat transfer of nanofluids synthesized by three different shaped (brick, platelet and cylinder) silver (Ag) nanoparticles in water. A two-phase nanoscale formulation is adopted which is more appropriate for biophysical systems. The flow is induced by metachronal beating of cilia and the flow geometry is considered as a cylindrical tube. The analysis is carried out under the low Reynolds number and long wavelength approximations and the fluid and cilia dynamics is of the creeping type. A Lorentzian magnetic body force model is employed and magnetic induction effects are neglected. Solutions to the transformed boundary value problem are obtained via numerical integration. The influence of cilia length parameter, Hartmann (magnetic) number, heat absorption parameter, Grashof number (free convection), solid nanoparticle volume fraction, and cilia eccentricity parameter on the flow and heat transfer characteristics (including effective thermal conductivity of the nanofluid) are examined in detail. Furthermore a comparative study for different nanoparticle geometries (i.e. bricks, platelets and cylinders) is conducted. The computations show that pressure increases with enhancing the heat absorption, buoyancy force (i.e. Grashof number) and nanoparticle fraction however it reduces with increasing the magnetic field. The computations also reveal that pressure enhancement is a maximum for the platelet nano-particle case compared with the brick and cylinder nanoparticle cases. Furthermore the quantity of trapped streamlines for cylinder type nanoparticles exceeds substantially that computed for brick and platelet nanoparticles, whereas the bolus magnitude (trapped zone) for brick nanoparticles is demonstrably greater than that obtained for cylinder and platelet nanoparticles. The present model is applicable in biological and biomimetic transport phenomena exploiting magnetic nanofluids and ciliated inner tube surfaces.     

On the anharmonic contribution to the specific heat of monatomic face-centered cubic crystals

We have presented a method for an exact calculation of the two lowest-order, cubic (F ₃) and quartic (F ₄), perturbation terms in the Helmholtz free energy (F) of an anharmonic crystal in the high temperature limit. The method is applicable to a nearest-neighbor central force model of a monatomic fcc crystal for any phenomenological two-body potential (r). The calculation of F ₃ and F ₄ requires a knowledge of the six dimensionless Brillouin zone (BZ) sums, as a function of a parameter a ₁ depending on the first and second derivatives of (r). These sums are calculated to a high degree of accuracy for a mesh size of 308,000 points in the whole BZ in the range –0.1a ₁+0.1 in steps of 0.02. The linear temperature dependent anharmonic contribution to the specific heat at constant volume, calculated for the elements Pb, Ag, Ni, Cu, Al, Ca, and Sr, from the Morse and Rydberg potentials, is found to be positive in all cases, with the exception of Pb. In this case the Morse potential gives a negative sign. The predictions of theory are in agreement with experiments where the data is available (e.g., Cu, Al, and Pb).     

An analysis of the specific heat capacity of liquid Au-Sn alloy based on the ideal-associated solution model

The specific heat capacity of Au Sn liquid alloy was analyzed using the ideal associated solution model assuming associated compounds Au,Sn, AuSn, and AuSn₂. Based on this model, all of the specific heat capacity. heat of mixing and activity are described over a wide temperature range, Further, it became clear that (he summation of mole numbers of the species in the liquid alloy shows a minium at the stoichiometric composition of Au₃Sn, which implies that the liquid is the most ordered at this composition,Paper presented at the Twelfth Symposium on Thermophysical Properties. June 19–24, 1994, Boulder, Colorado, U.S.A.