Effect of thermophysical properties models on the predicting of the convective heat transfer coefficient for low concentration nanofluid

The term of nanofluid refers to a solid–liquid mixture with a continuous phase which is a nanometer sized nanoparticle dispersed in conventional base fluids. In order to study the heat transfer behavior of the nanofluids, precise values of thermal and physical properties such as specific heat, viscosity and thermal conductivity of the nanofluids are required. There are a few well-known correlations for predicting the thermal and physical properties of nanofluids which are often cited by researchers to calculate the convective heat transfer behaviors of the nanofluids. Each researcher has used different models of the thermophysical properties in their works. This article aims to summarize the various models for predicting the thermophysical properties of nanofluids which have been commonly cited by a number of researchers and use them to calculate the experimental convective heat transfer coefficient of the nanofluid flowing in a double-tube counter flow heat exchanger. The effects of these models on the predicted value of the convective heat transfer of nanofluid with low nanoparticle concentration are discussed in detail.     

Preparation and thermophysical properties of nanoparticle-in-paraffin emulsion as phase change material

In this study, phase change material (PCM) embedded by nanoparticles was prepared by emulsifying alumina (Al₂O₃) nanoparticles in paraffin (n-octadecane) by means of a non-ionic surfactant. The formulated nanoparticle-in-paraffin emulsions contain the nanoparticles of 5 wt.% and 10 wt.%, respectively; their effective thermophysical properties, such as latent heat of fusion, density, dynamic viscosity, and thermal conductivity, were investigated experimentally. The experimentally measured density of the emulsions agrees excellently with that predicted based on the mixture theory. The measured thermal conductivity and dynamic viscosity for the nanoparticle-in-paraffin emulsions formulated show a nonlinear increase with the mass fraction of the nanoparticles compared with that for the pure paraffin, depending on the temperature.     

高水分烟气的热物理性质

在燃用废弃物的特种锅炉中,烟气的水分容积含量ｒＨ２Ｏ达到或超过了常用锅炉热力计算标准２５％ 的上限,需要对原有的计算方法及公式进行检验和修正。本文使用线性规则和混合物规则分别计算了不同水分含量的气体混合物的热物理性质以及相应的对流换热特性,得到了物性随温度及水蒸汽容积含量变化的曲线。分析表明,在锅炉尾部低温区的对流换热受热物性影响较大,对高水分的烟气需进行修正。     

Evaluation of srk equation of state in calculating the thermophysical properties of gas turbine combustion gases

The purpose of this work is to evaluate the thermophysical properties of combustion gases of the gas turbine engine using the Soave-Redlich-Kwong equation of state and to compare the results with those obtained from the virial equation of state and the experimental values obtained from experiment and the generalized charts. The properties which have been considered in this work were, density, specific heat at constant pressure, enthalpy, entropy, viscosity and thermal conductivity. The temperature range was (200–2600 K) theoretically, while the pressure range was (3–12 atmospheres). The Soave-Redlich-Kwong (SRK) equation of state generally predicted better values for thermophysical properties than those predicted by the virial equation of state. A computer program, to evaluate the departure of thermophysical properties using virial and SRK equations of state, was used.     

Estimation of thermophysical properties of fouling using inverse problem and its impact on heat transfer efficiency

At high temperature, the circulation of fluid in heat exchangers provides a tendency for fouling accumulation to take place on the internal surface of tubes. In brief, the deposits on heat exchanger tubes are caused by the presence of inorganic salts, of small quantities of organic materials and products of corrosion in the water. From thermophysical point of view, the deposited fouling has harmful effects on the heat exchanger efficiency. Indeed, it increases the thermal resistance which can raise the energy consumption. This study shows an experimental and a theoretical process of estimation of thermophysical properties of the fouling deposited on a section of a heat exchanger and its effects on the heat transfer efficiency. The estimation method is based on the Gauss-Newton algorithm that minimizes the ordinary least squares function comparing a measured temperature and a theoretical one. The temperature response is measured on the rear face of a bi-layer system composed of a section of a heat exchanger and the fouling deposited on during and after a finite width pulse heat flux on its front face. The theoretical temperature, that is a function of the unknown thermophysical properties of the bi-layer system, is calculated by the resolution of the one-dimensional linear inverse conduction problem, and by the use of the quadrupole formalism.The results of the estimation procedure show, on the one hand the efficiency and the stability of the optimization algorithm to estimate the thermophysical properties of the fouling. On the other hand they underline the necessity of the maintenance of fluid circulating tubes at high temperature.     

Numerical simulation of supercritical carbon dioxide flows across critical point

Numerical study of supercritical carbon-dioxide flows across the critical point is presented. The present numerical method is based on the preconditioning method developed by Yamamoto and mathematical models of thermophysical properties for carbon dioxide programmed in the program package for thermophysical properties of fluids, developed by Kyushu University. First, the two-dimensional natural convection of carbon dioxide between two parallel plates is calculated while changing the bulk pressure. The calculated thermophysical properties of the carbon-dioxide flow under supercritical pressure are compared with those in a gas condition. Next, the natural convection of carbon dioxide in an O-shaped cyclic channel is calculated, and the effect of the density difference induced by the phase change to the flow is investigated. For application to high-speed flows, supercritical carbon dioxide flows through a nozzle with free-jet expansion (known as the process of rapid expansion of supercritical solutions) process are calculated. The calculated shock distance to the Mach disk generated in the free jet is compared with experiments and the density variations in the nozzle while changing the inlet temperature are numerically predicted.     

Energy software programs for educational use

A package of six fully interactive energy software programs was developed for educational use. The first three programs can be used to simulate energy systems, while the rest three can be used to calculate some characteristic values, which can be used separately or in conjunction with the first three programs. The first one is a multimedia program for an active solar hot water system. It accounts for a large number of parameters and can be used to investigate their effect on the thermal efficiency of the system. The second program is a transient simulation for thermal behavior of a building. It can be used to predict the temperature and relative humidity inside a building along with the heating/cooling load required to keep the temperature at a preset point. The third program has been developed for sizing solar thermal systems. The fourth program is a graphical representation of the apparent motion of the sun on the celestial vault. The fifth program calculates the thermophysical properties of a number of gases and liquids for a desired temperature. The sixth program represents graphically the psychrometric chart. The above-mentioned programs, are useful tools for education in energy, suitable for high school students and Universities. It can also be used for an extensive set of exercises in the tertiary education sector.     

Synthesis and characterization of Bi31Cr5O61.5, a new bismuth chromium oxide, potential mixed-ionic-electronic conductor for solid oxide fuel cells

Single crystals, as well as pure powder of the new bismuth chromate, Bi₃₁Cr₅O_61.5 were synthesized by slow cooling of a mixture of Bi₂O₃-Cr₂O₃ and then fully structurally characterized using X-ray diffraction. It crystallizes in the monoclinic space group P2₁/n with unit cell parameters a = 23.5794(8), b = 11.6189(4), c = 24.3629(8) Å and β = 108.02(1)°, Z = 4. The final conventional agreement factors converged to R = 0.0540 and wR = 0.0561 for 27520 independent reflections. The +VI chromium oxidation state was proved by thermogravimetric and magnetic measurements. The structure can be described by the association of isolated CrO₄ tetrahedra surrounded by 11 or 12 Bi atoms forming truncated or not cuboctahedra. The stability of both pure powder sample and pressed sintered pellets was studied at elevated temperature by X-ray diffraction and by dilatometry previously to ionic conductivity measurements done by impedance spectroscopy.     

The influence of binary mixture thermophysical properties in the analysis of heat and mass transfer processes in solar distillation systems

Although a substantial amount of research work has already been devoted to various aspects of modeling the convective and mass transport processes in solar distillation systems, it appears that the role of thermophysical and transport properties of the working medium and their effect on the thermal behavior and performance analysis of such systems has been left almost completely unnoticed. The working medium in these systems, which is a binary mixture of water vapor and dry air in equilibrium, appears to exhibit a completely different set of properties than dry air, especially at saturation conditions and at the higher region of the solar still operational temperature range. An analysis is presented aiming to signify the effect of binary mixture thermophysical properties on the transport processes and the associated quantities and evaluate the thermophysical properties of the working medium in these systems, based on contemporary data for dry air and water vapor. The derived results, in the form of convenient algebraic correlations, are employed to investigate the effect of using the appropriate thermophysical properties on the calculation of the convective heat and mass transfer, as well as the distillate mass flow rates. According to the results from the present investigation, although the use of improper dry air data leads to a significant overestimation of the convective heat transfer coefficient, the errors associated with the use of improper dry air properties is a moderate overestimation of distillate output which is estimated to be up to 10% for maximum average still temperatures of 100 °C.     

Investigation of texturization for monocrystalline silicon solar cells with different kinds of alkaline

In this paper, monocrystalline silicon was textured with different kind of etchants for solar cells, respectively. It was found that, only with sodium hydroxide (NaOH) or sodium acetate anhydrous (CH₃COONa) solution, the textural results were very weak, resulting in high reflectance of silicon surface. However, if using the mixture solution of NaOH and CH₃COONa, the reflectance was noticeably decreased. Moreover, the dependence of reflectance on the etching time showed that longer etching time was necessary for texturization in the NaOH+CH₃COONa+H₂O system. And it was also found that the addition of isopropyl alcohol (IPA) to this mixture solution had a detrimental effect on the texturization. All these results suggested that acetate (CH₃COO⁻) plays a similar role as IPA for alkaline texturization, but they cannot coexist. Finally, the mechanisms of texturization with different kinds of etchant were discussed in detail.     

Optical properties of tungsten and titanium oxide thin films prepared by plasma sputter deposition

Tungsten oxide and titanium oxide thin films were prepared by RF reactive magnetron sputter deposition. The stationary and rotating substrate holders were applied to analyze the rotating effect. The optical properties and thicknesses of oxide films were determined by a proposed optical model and the measured transmittance spectra. The dispersed refractive indices of thin films have a wide range distribution in different sputtering conditions. In the situation of rotating substrate holder, the refractive index was lower than that of the stationary substrate holder. Also, amorphous TiO₂ structure can be prepared by using rotating substrate holder. The transmittance spectrum of crystalline TiO₂ reveals that the textured structure on the film surface affects the transmittance characteristic.     

Calculation of temperature field in a thin moving sheet heated with laser beam

A temperature field in a thin moving sheet heated with a laser beam is calculated. The power density in the beam is distributed according the Gauss function. Cooling effects caused by the free or forced convection in ambient gas is taken into account. The problem becomes two dimensional by averaging the temperature field over the sheet thickness. Two-dimensional integral Fourier transformation on space coordinates is applied to solve the problem. It gives an analytical solution of the problem in the Fourier space. The inverse Fourier transformation is fulfilled and the solution is represented via an integral having exponential asymptotes at infinity.     

High performance light trapping textures for monocrystalline silicon solar cells

Two novel texture schemes for the front of a c-Si silicon wafer solar cell are presented. The “bipyramid” texture is of two inverted pyramids of similar sizes laid out in alternating order. The “patch” texture uses a checkerboard layout of blocks of parallel grooves, with the grooves of alternating blocks perpendicularly oriented to each other. We estimate that these textures, which almost fully trap light for the first six passes through the substrate, can deliver better optical performance than the standard inverted pyramid texture, especially in narrow-band applications.     

Development of a pyrolysis model for corrugated cardboard

The current study outlines a general approach to construct a one-dimensional pyrolysis model based on milligram-scale and bench-scale test data collected systematically to isolate a specific process in each test. This approach is demonstrated by developing a model of burning for corrugated cardboard. Thermogravimetric analysis and differential scanning calorimetry were conducted on pulverized cardboard to determine the thermal degradation mechanism, the enthalpy of decomposition reactions, and the heat capacities of apparent species. Data collected in pyrolysis-combustion flow calorimetry tests were analyzed to assign a heat of combustion to the volatiles evolved from each reaction. Bench-scale tests were conducted with a cone calorimeter on samples in a horizontal orientation to observe the flaming combustion of this material at external heat fluxes ranging from 20 to 80 kW m⁻². Condensed phase temperature data was collected in these tests to measure and infer thermal transport properties and to characterize property changes associated with thermal degradation through an iterative inverse analysis. All the parameters determined through analysis of the milligram-scale and bench-scale tests were used to construct a one-dimensional pyrolysis model that predicted the average mass loss and heat release rates from cone calorimeter tests to within, on average, 17% and the times to ignition to within 2 s.