Unconditional nonlinear stability for double-diffusive convection with temperature- and pressure-dependent viscosity

A linear and nonlinear stability analyses are carried out for a double-diffusive chemically reactive fluid layer with viscosity being a function of temperature and pressure. The linear stability analysis is studied when the stabilizing salt gradient acts against the destabilizing thermal gradient. The effect of reaction parameters and variable viscosity on the stability of the system is studied for heated below, salted above, and the heated and salted below models with Rigid–Rigid boundary conditions. Chebyshev pseudospectral method is applied to determine the numerical solutions.     

地下环境CO2盐水溶液密度的变化及状态方程

利用可以在高温高压条件下，对密闭容器内的流体密度进行直接测量的电磁悬浮式天平对地下盐水的密度进行了测量。实验结果表明，在CO2地下储存30～50℃、10～20MPa时，CO2盐水溶液的密度随着温度的升高而减小，随着CO2的质量分数的增加而增大，随着压力的增加呈线性增加。CO2盐水溶液密度与盐水密度之比与压力无关，但随着CO2的质量分数的增加而增大，随着温度的升高而减少。     

Marangoni Convection Heat and Mass Transport of Power-Law Fluid in Porous Medium with Heat Generation and Chemical Reaction

This article presents a study for Marangoni convection of power-law fluid in a porous medium driven by a power-law temperature and a power-law concentration with heat generation and first-order chemical reaction. It is assumed that the surface tension varies linearly with both the temperature and the concentration. The effects of power-law viscosity on temperature and concentration fields are taken into account bya modified Fourier law and Darcy's Law for power-law fluid. An approximate analytical solution is obtained using a homotopy analytical method, which is verified by numerical ones with good agreement. The transport characteristics of velocity, temperature, and concentration fields are analyzed in detail.     

The effect of computational processing of temperature- and concentration-dependent parameters on non-Newtonian fluid MHD: Applications of numerical methods

This investigation aims to introduce a new solution with aid of numerical methods to the blood flow of Carreau–Yasuda fluid through a microvessel. Swimming of gyrotactic microorganisms with nanoparticles is considered. A resulting system of partial differential equations is simplified by the meaning of low Reynolds number and long wavelength. This system of partial differential equations was formulated and transformed mathematically using new theories of differential transform method. Variable nonndimensional physical parameters effects, such as numbers of bioconvection Peclet and bioconvection Rayleigh, and so forth on velocity, temperature, and concentration distribution as well as oxytactic microorganism and oxygen concentration profiles are studied. All results are constructed in two cases of viscosity on the same figure, one of them in the case of variable parameters and the other in constant parameters. The existing study assured that the microorganism density in the direction near to the hypoxic tumor tissues regions grows with a rise in oxygen concentrations and the blood viscosity diminutions.     

Thermophysical properties of fly ash–Cu hybrid nanofluid for heat transfer applications

The effect of temperature and concentration on the thermophysical properties of fly ash–copper (80% fly ash and 20% Cu by volume) water-based stable hybrid nanofluid is studied. The experiments are conducted for the volume concentration range of 0 to 0.5% in the temperature range of 30 to 60°C. The nanoparticles have been characterized by transmission electron microscopy and dynamic light scattering to determine an average nanoparticle diameter of 15 nm. The stability of nanofluid in the presence of surfactant Triton X-100 is examined with the help of zeta potential. The maximum enhancement in thermal conductivity and viscosity is 19% and 22%, respectively. The outcome of the present study showed that density, thermal conductivity, and viscosity of the hybrid nanofluid increased, whereas specific heat decreased with an increase in the nanofluid concentration. In addition, the specific heat and thermal conductivity increase, there is a decrease in density and viscosity of the hybrid nanofluid with an increase in temperature.     

Research and application of molten salts for sensible heat storage

熔融盐具有液体温度范围宽,黏度低,流动性能好,蒸汽压小,对管路承压能力要求低,相对密度大,比热容高,蓄热能力强,成本较低等诸多优点,已成为一种公认的良好的中高温传热蓄热介质.本文对熔融盐显热蓄热技术原理和发展现状进行了简要概述,包括熔融盐的种类,熔融盐显热蓄热技术的原理,关键技术,研发现状及其在太阳能热发电和间歇性余热利用中的应用.认为开展高温熔融盐传热蓄热介质制备,热性能表征和熔融盐流动与传热性能研究,进而完善整个熔融盐蓄热系统,提高蓄热效率,降低管路腐蚀性,提高系统可靠性仍将是未来熔融盐蓄热技术的研究重点.     

Experimental–numerical study of heat flow in deep low-enthalpy geothermal conditions

This paper presents an intensive experimental–numerical study of heat flow in a saturated porous domain. A temperature and a flow rate range compared to that existing in a typical deep low-enthalpy hydrothermal system is studied. Two main issues are examined: the effect of fluid density and viscosity on heat flow, and the significance and effect of thermal dispersion. Laboratory experiments on a saturated sand layer surrounded by two impermeable clay layers, subjected to different flow rates under cold and hot injection scenarios, and for both vertical and horizontal flow directions, are conducted. A temperature range between 20 °C and 60 °C is studied. The finite element method is utilized to analyze the experimental results. Backcalculations, comparing the numerical results to the experimental results, are conducted to quantify the magnitude of thermal dispersion. A constitutive model describing thermal dispersion in terms of fluid density, viscosity and pore geometry, taking into consideration different injection scenarios, is developed. This study demonstrates the importance of taking the variation of formation water density and viscosity with temperature into consideration in predicting the lifetime of deep low-enthalpy geothermal systems. It shows that if ignored, the lifetime of a system with hot injection will be overestimated, and that with cold injection, will be underestimated.     

Conductivity of libob-based electrolyte for lithium-ion batteries

This work reports the use of mixtures of γ-butyrolactone (GBL) and ethyl acetate (EA), with and without ethylene carbonate (EC), as solvents for lithium bis(oxalato)borate (LiBOB) salt as potential electrolytes for Li-ion cells. The effects of salt concentration, ethylene carbonate (EC) content, and temperature on the conductivity and viscosity of the mixture are reported. Results indicate that the best electrolyte for high-temperature application is that which contains 1 kmol m⁻³ LiBOB in GBL + EA + EC of composition 1:1:0.1 (wt). For low-temperature applications, the best electrolyte is that which contains 0.7 kmol m⁻³ LiBOB in GBL + EA + EC of composition 1:1:0 (wt). The product of conductivity with viscosity was essentially independent of temperature but was dependent on solvent composition showing that at fixed salt concentration, the viscosity is the major criteria affecting electrolyte conductivity rather than dielectric constant.     

The testing of the effects of cooking conditions on the quality of biodiesel produced from waste cooking oils

In this study, the effects of cooking conditions on the cold flow properties and kinematic viscosity of biodiesel produced from cooking oils were investigated. Sunflower, corn and canola oils were used as vegetable oils. Salt content, water content, cooking time and cooking temperature were selected as the experimental parameters. Some of the physical properties such as kinematic viscosity, density, cloud point and pour point were examined. In addition, total polar material contents, heating values and acid values of biodiesel produced from waste cooking oils were analysed. The results of the study revealed that increase in salt and water content, cooking time and temperature led to deterioration in the physical properties and cold flow properties of B100 biodiesel samples from waste cooking oils of sunflower, corn and canola oils. On the other hand, the heating values of all biodiesels were found to improve with the increasing salt content.     

Study on design of molten salt solar receivers for beam-down solar concentrator

Systems using molten salt as thermal media have been proposed for solar thermal power generation and for synthetic fuel production. We have been developing molten salt solar receivers, in which molten salt is heated by concentrated solar radiation, in the Solar Hybrid Fuel Project of Japan. A cavity shaped receiver, which is suitable for a beam-down type solar concentration system, was considered. In order to design molten salt solar receivers, a numerical simulation program for the prediction of characteristics of receivers was developed. The simulation program presents temperature distributions of a receiver and molten salt with the use of heat flux distribution of solar radiation and properties of composing materials as input data. Radiation to heat conversion efficiency is calculated from input solar power and heat transferred to molten salt. The thermal resistance of molten salt and the maximum discharge pressure of molten salt pumps were taken into account as restrictions for the design of receivers. These restrictions require control of maximum receiver temperature and pressure drop in the molten salt channel. Based on the incident heat flux distribution formed with a 100 MWth class beam-down type solar concentration system, we proposed a shape of solar receiver that satisfies the requirements. The radiation to heat conversion efficiency of the designed receiver was calculated to be about 90%.     

Experimental investigation of the thermophysical properties of AL2O3-nanofluid and its effect on a flat plate solar collector

Experimental investigations have been carried out for obtaining the thermophysical properties of 60:40 (by mass) ethylene glycol/water mixture and water based alumina nanofluids. The effect of density and viscosity on the pumping power for flat plate solar collector has been investigated as well. Nanofluids of 0.05–0.1%v/v concentrations were prepared and characterized. Water based alumina nanofluids were found more preferable against sedimentation and aggregation than ethylene glycol/water mixture based nanofluids. The measured thermal conductivities of both types of the nanofluids increased almost linearly with concentration and are consistent in their overall trend with previous works done at lower concentrations by different researchers. In contrast to thermal conductivity, viscosity measurements showed that the viscosity of the Al₂O₃–water nanofluids exponentially decreases with increasing temperature. Furthermore, the measured viscosities of the Al₂O₃–water nanofluids showed a non-linear relation with concentration even in the low volume concentration except 0.05%v/v at below 40 °C. On the other hand, Al₂O₃–EG/water mixture exhibited Newtonian behavior. Existence of a critical temperature was observed beyond which the particle suspension properties altered drastically, which in turn triggered a hysteresis phenomenon. The hysteresis phenomenon on viscosity measurement, which is believed to be the first observed for EG/water-based nanofluids, has raised serious concerns regarding the use of nanofluids for heat transfer enhancement purposes. Results suggest that nanofluids can be used as a working medium with a negligible effect of enhanced viscosity and/or density. Results also show that the pressure drop and pumping power of the nanofluid flows are very close to that of the base liquid for low volume concentration.     

Effect of sloping walls on salt concentration profile in a solar pond

The effect of sloping walls on the salt concentration profile in solar ponds is studied. The variation of the area of the pond at different depths is expressed in terms of the top surface area and a single non-dimensional parameter defined in terms of the geometrical characteristics of the pond. This variation is then introduced into the differential equation governing the upward salt diffusion. The dependence of the molecular diffusivity of salt on temperature and the resulting vertical variation of the molecular diffusivity in solar ponds with sloping walls is also considered. The differential equation is then solved and the general solution for the salt concentration as a function of depth is obtained. Results for different pond configurations and also different top and bottom salt concentrations are presented and discussed. It is shown that as a result of sloping walls the density gradient in the top region assumes a smaller value than at the bottom of the solar ponds. If this effect is not considered in the design of solar ponds the density gradient in the top region may decrease well below the stability limit which can then result in an undesired growth of the top mixed layer.     

物性参数对硅单晶Czochralski生长过程的影响

为了了解硅单晶C zochra lsk i(C z)法生长时物性参数对熔体流动和氧传输过程的影响,利用有限元法对炉内的传递过程进行了全局数值模拟,假定熔体和气相中的流动都为准稳态轴对称层流,熔体为不可压缩流体,C z炉外壁温度维持恒定。结果表明:熔体的导热系数及发射率对熔体流动、加热器功率、结晶界面形状、晶体内轴向温度梯度和氧浓度有重要影响,而熔体的密度、黏度系数及熔解热对硅单晶C z法生长过程影响较小。     

含磷共聚物钻井液降黏剂合成及性能评价

以丙烯酸(AA)、2-丙烯酰胺基-2-甲基丙磺酸(AMPS)和含磷化合物共聚合成了抗220℃高温的钻井液降黏剂,对含磷化合物用量、引发剂(过硫酸铵)用量和滴加温度等对钻井液降黏剂特性黏度和降黏率的影响进行了考察。结果显示,钻井液降黏剂的最佳合成条件为:含磷单体用量为反应单体总质量的6%,过硫酸铵用量为反应单体总质量的2%,滴加溶液温度为55℃。利用红外光谱(IR)对钻井液降黏剂进行了表征,并借助热分析考察了钻井液降黏剂的热稳定性,同时对其在泥浆中的降黏性能进行了室内评价。研究结果表明:研制的钻井液降黏剂可以显著降低泥浆的黏度和切力,加入0.5%降黏剂的淡水基浆,高温降黏率可以达到91.4%;具有较好的抗盐、抗钙性能,在饱和盐水泥浆和含钙泥浆中也具有很好的降黏作用和分散作用;具有较好的热稳定性。     

Numerical study of the natural convection of nanofluids based on mineral oil with properties evaluated experimentally

This paper deals with the numerical simulation of natural convection inside a closed cavity of multi-walled carbon nanotubes and diamond nanoparticles dispersed in pure mineral oil. The data of the thermal conductivity and viscosity of the nanofluids were obtained experimentally, while specific heat and density were evaluated by the first law of thermodynamic and mass conservation principle, respectively. The physical model considered is a 2D cavity with adiabatic horizontal walls, where the left vertical wall with high temperature and the right vertical wall with low temperature and all walls with no-slip boundary conditions. The simulations were performed in a numerical code based on the Finite Volume Method-FVM, where second order temporal and spatial schemes were used. The results showed that only the Nusselt number for MWCNT based nanofluids increased in comparison to the mineral oil for the same Grashof number. However, the comparison between the convective coefficient for nanofluids and mineral oil showed that nanofluids presented better convection characteristics, for the same Grashof number. Furthermore, the diamond nanofluid with higher volumetric concentration presented the maximum increase of the convection coefficient, about 23%, though this nanofluid did not presentthe higher Nusselt number in comparison to the other nanofluids analyzed.     

Experimental exploration and theoretical certainty of thermal conductivity and viscosity of MgO-therminol 55 nanofluid

In this study, a combination of thermal conductivity, viscosity, and density characteristics are experimentally probed for attaining maximum heat transfer using MgO-Therminol 55 as nanofluid is reported. Recent studies proved that nanofluids have miserable properties that make them feasibly useful in many applications in heat transfer compared to base fluid.MgO-Therminol 55 nanofluid is synthesized by diffusion of MgO nanoparticles of size 160–190 nm in Therminol 55 at different concentrations (0.05%–0.3%). Thermal conductivity and viscosity are calculated at a temperature range of 30–60°C using kd₂ analyzer and Fenske viscometer. Data obtained from the experimental results reveals that when volume concentration is increased with respect to that thermal conductivity increases, viscosity decreases and density decreases at different temperatures. The proposed models were supportive to the experimental data.     

Effects of temperature and salt concentration on distilled water production

The paper studies the influence of salt concentration and temperature on the distillation flow rate in a single effect multi-stage system for small water distillation units for low purchasing communities of 50 to 100 people. The distillation process has been evaporation–condensation under variable conditions: temperature of the evaporation and salt concentration. The results have shown there is a dependence of the amount of the distilled water on temperature and salt concentration whose effects are more significant as the temperature rises and salt concentration lowers. The amount of the distilled water has been increased with the use of the multistage system, 103% for sea water, and 56% for brackish water. The ratio of the quantities of distilled water for both cases, sea water and brackish water, has increased from 15.8% for one single stage to 55.7% for the four stages.     

Laminar free-convection over a vertical isothermal plate with uniform blowing or suction in water with variable physical properties

The majority of studies for laminar free-convection over a vertical isothermal plate with uniform blowing or suction concern gases and air. The existing results for water have been produced assuming a linear relationship between fluid density and temperature and constant viscosity and thermal conductivity. However, it is known that the density-temperature relationship for water is non-linear at low temperatures and viscosity and thermal conductivity are functions of temperature. In this study the problem of laminar free-convection over a vertical isothermal plate with uniform blowing or suction in water has been investigated in the temperature range between 20 and 0 °C taking into account the temperature-dependence of μ, k and ρ. The results are obtained with the numerical solution of the boundary layer equations. The variation of μ, k and ρ with temperature has a strong influence on the results.     

Effect of salt additives on film boiling heat transfer and mechanism of quenching temperature rise

A high‐temperature stainless‐steel sphere was immersed into various salt solutions to investigate the film boiling behavior at vapor film collapse. The film boiling behavior around the sphere was observed with a digital video camera. Both surface temperature of the sphere and solid–liquid contact behavior were measured. Results of the experiment showed that salt additives enhanced condensation heat transfer, and the observed vapor film was thinner. Furthermore, the frequency of direct contact between the sphere surface and coolant increased. The quenching temperature increased with increased salt concentration, and was highly correlated with ion molar concentration, which represents the density of ions regardless of the type of salt. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/htj.20327     

Heat Transfer and Pressure Drop Characteristics of Dilute Alumina–Water Nanofluids in a Pipe at Different Power Inputs

This work addresses the effect of temperature on the thermophysical properties (i.e., density, viscosity, thermal conductivity, and specific heat capacity) of alumina–water nanofluid over a wide temperature range (25°C–75°C). Low concentrations (0–0.5% v/v) of alumina nanoparticles (40 nm size) in distilled water were used in this study. The pressure drop and the effective heat transfer coefficient of nanofluids were also estimated for different power inputs and at different flow rates corresponding to Reynolds numbers in the range of 1500–6000. The trends in variation of thermophysical properties of nanofluids with temperature were similar to that of water, owing to their low concentrations. However, the density, viscosity, and thermal conductivity of nanofluids increased, while the specific heat capacity decreased with increasing the nanoparticle concentration. The convective heat transfer coefficient of the nanofluid and the pressure drop along the test section increased with increasing the particle concentration and flow rate of nanofluid. Results showed that the heat transfer coefficient increases, while the pressure drop decreases slightly with increasing the power input. This is because of the fact that increasing power input to heater increases the bulk mean temperature of nanofluids, resulting in a decreased viscosity. The prepared nanofluids were found to be more effective under turbulent flow than in transition flow.