Effect of particle concentration,temperature and surfactant on surface tension of nanofluids

Heat transfer performance with nanofluids depends on the thermo physical properties of the suspension. Surface tension is an important property for heat transfer calculation. In this paper, various parameters that effect on the surface tension of nanofluids such as nanofluid preparation method, effect of volume fraction, temperature, and surfactants on nanofluids have been studied. Additionally, precise assessments on the theoretical correlations related to the surface tension of nanofluids have also been included. Based on the existing experimental results, surface tension augments respectively with volume fraction intensification. Surface tension of nanofluids decreases accordingly with the increase of temperature and surfactant concentration. Nevertheless, there have been some contradictory results on the effect of volume fraction and surfactant on surface tension of nanofluids.     

Molecular dynamic simulation: Studying the effects of Brownian motion and induced micro-convection in nanofluids

ABSTRACT

Nanofluids are suspensions of nanoparticles into convectional heat transfer fluid to enhance the thermal conductivity of its base fluid. The roles of Brownian motion of nanoparticles and induced micro-convection in base fluid in enhancing the thermal conductivity of nanofluids were investigated using molecular dynamic (MD) simulation. The roles were determined by studying the effect of particle size on thermal conductivity and diffusion coefficient. Results show that the Brownian motion and induced micro-convection have insignificant effects on enhancing the thermal conductivity. The hydrodynamic effect is restricted by an amorphous-like interfacial fluid structure in the vicinity of the nanoparticle due to its higher specific area.     

Simulation of heat transfer enhancement in nanofluids using dissipative particle dynamics

The current paper applied dissipative particle dynamics (DPD) approach to investigate heat transfer within nanofluids. The DPD approach was applied to study natural convection in a differential heated enclosure by considering the viscosity and the thermal conductivity of the nanofluid to be dual function of temperature and volume fraction of nanoparticles. Experimental data for viscosity and thermal conductivity are incorporated in the current DPD model to mimic energy transport within nanofluids. This incorporation is done through the modification of the dissipative weighting function that appears in the dissipative force vector and the dissipative heat flux. For the entire range of Rayleigh number considered in this study, it was found that the DPD results show a deterioration in heat transfer in the enclosure due to the presence of nanoparticles for φ > 4%. However, some slight enhancement is shown to take place for small volume fraction of nanoparticles, φ ≤ 4%. The DPD results experienced some degree of compressibility at high values of Rayleigh number Ra ≥ 10⁵.     

Modeling the effect of particle size on the activation energy and ignition temperature of metallic nanoparticles

The present work reports a simple theoretical model to calculate the effect of the particle size on the activation energy and the ignition temperature of metallic nanoparticles. The activation energy was deduced from the particle cohesive energy and the ignition temperature was calculated using the condition that the heat generated by the combustion reactions is sufficient to counterbalance the particle heat loss to the surrounding. Heat loss was assumed to be in the transient regime and the combustion heat generation was calculated using the simplest Arrhenius-type model. Using aluminum as an example, the results showed that for particles of sizes larger than 50 nm, increasing the particle size had a little effect on the number of the surface atoms, the activation energy and the ignition temperature. As the particle size decreases the number of the surface atoms increases and the corresponding activation energy, E_d/E_∞ and the ignition temperature decrease. As the particle size decreased to about 5 nm and smaller, the activation energy could reduce to 20% or 50% of the bulk value and an ignition temperature as low as 800 K was obtained from the calculation depending on the ratio of the coordination number.     

The effect of particle size and base liquid on thermo-physical properties of ethylene and diethylene glycol based copper micro- and nanofluids

Nanofluid (NF) is a fluid containing nanometer-sized particles. The present work investigates, experimentally and theoretically, on fabrication and thermo-physical properties evaluation of ethylene glycol and diethylene glycol (EG/DEG) based nanofluids/microfluids (NFs/MFs) containing copper nanoparticles/microparticles (NPs/MPs) with focus on the effect of the particle size and the base liquid. A series of stable Cu NFs and MFs with various NP/MP concentration (1, 2 and 3 wt%) were fabricated by dispersing Cu NPs and Cu MPs in EG and DEG as the base liquids. The physicochemical properties of Cu NFs and MFs were analyzed by various techniques including X-Ray diffraction (XRD), Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM) and Dynamic Light Scattering (DLS). The thermo-physical properties including thermal conductivity (TC) and viscosity of EG/DEG based Cu NFs/MFs were measured at 20 to 40 °C. The results for TC and viscosity of EG based Cu NF/MFs were compared to the same NFs/MFs with DEG base liquid with focus on the impact of the particle size as well as the base liquid. The experiments showed that EG based NFs/MFs exhibit more favorable characteristics than that of DEG based ones. Moreover, NFs with Cu NPs revealed higher TC than those MFs containing Cu MPs at the same particle concentration and temperature (effect of NP size). As the best result, a TC enhancement of ~ 4.7% was achieved for EG based NF with 3 wt% Cu NP while maximum increase in viscosity of ~ 1.8% was observed for the same NF at 20 °C. To compare the experimental results with the estimated values, Maxwell predictive correlation and Corcione model were employed while Einstein equations as well as Kriger-Dougherty correlation were applied for TC and viscosity of NFs/MFs, respectively.     

Experimental investigation of titanium nanofluids on the heat pipe thermal efficiency

The enhancement heat transfer of the heat transfer devices can be done by changing the fluid transport properties and flow features of working fluids. In the present study, therefore, the enhancement of heat pipe thermal efficiency with nanofluids is presented. The heat pipe is fabricated from the straight copper tube with the outer diameter and length of 15, 600 mm, respectively. The heat pipe with the de-ionic water, alcohol, and nanofluids (alcohol and nanoparticles) are tested. The titanium nanoparticles with diameter of 21 nm are used in the present study which the mixtures of alcohol and nanoparticles are prepared using an ultrasonic homogenizer. Effects of %charge amount of working fluid, heat pipe tilt angle and %nanoparticles volume concentrations on the thermal efficiency of heat pipe are considered. The nanoparticles have a significant effect on the enhancement of thermal efficiency of heat pipe. The thermal efficiency of heat pipe with the nanofluids is compared with that the based fluid.     

不同Lennard-Jones模型参数对分子动力学模拟的影响

针对分子动力学模拟中应用广泛的Lennard-Jones模型中的参数对压力、比热容等宏观参数和表面张力的影响进行了比较和分析,发现采用不同原始数据拟合得到的参数在模拟远离临界态的压力和比热容时,各系统差别不大。与实验值的相对误差在5％;但在临界态附近,只有黏度系数和维里系数一起拟合的参数精度较高,对系统的适应性也较好并且模拟的表面张力值精度比其他参数高很多,相对误差在3．5％。最后根据参数对径向分布函数的影响,给出反应机理的解释。     

Effect of using nanofluids on efficiency of parabolic trough collectors in solar thermal electric power plants

In this paper, forced convection heat transfer nanofluid flow inside the receiver tube of solar parabolic trough collector is numerically simulated. Computational Fluid Dynamics (CFD) simulations are carried out to study the influence of using nanofluid as heat transfer fluid on thermal efficiency of the solar system. The three-dimensional steady, turbulent flow and heat transfer governing equations are solved using Finite Volume Method (FVM) with the SIMPLEC algorithm. The results show that the numerical simulation are in good agreement with the experimental data. Also, the effect of various nanoparticle volume fraction on thermal and hydrodynamic characteristics of the solar parabolic collector is discussed in details. The results indicate that, using of nanofluid instead of base fluid as a working fluid leads to enhanced heat transfer performance. Furthermore, the results reveal that by increasing of the nanoparticle volume fraction, the average Nusselt number increases.     

Comparative study on heat transfer enhancement of nanofluids flow in ribs tube using CFD simulation

This study presents, a numerical investigation of two‐dimensional turbulent nanofluids flow in different ribs tube configurations on heat transfer, friction, and thermal performance coefficients using ANSYS‐FLUENT software version‐16. Governing equations of mass, momentum, and energy have been solved by means of a finite volume method (FVM). Four types of nanoparticles namely; Al₂O₃, CuO, SiO₂, and ZnO with volume fraction range (1%‐4%) and different size of nanoparticles (dp = 30 nm, 40 nm, 50 nm, and 60 nm) with various Reynolds number (10 000‐30 000) in a constant heat flux tube with rectangular, triangular, and trapezoidal ribs were conducted for simulation. The results exhibit that Nusselt number for all cases enhanced with Reynolds number and nanofluid volume fraction increases. Likewise, the results also reveal that SiO₂ with volume fractions of 4% and diameters of nanoparticles of 30 nm in triangular ribs offered the highest Nusselt number at Reynolds number of Re = 30 000. In addition, the higher value of thermal performance factor was obtained at Reynolds number of Re = 10 000.     

Effects of raw material particle size on the briquetting process of rice straw

Biomass feedstocks need to be milled or chopped into particles before briquetting, and the particle size has great effects on the energy consumption and product quality. In this study, the effects of the particle size on the rice straw briquetting process were investigated. The raw materials were milled or chopped into four different sized test materials. Experiments were carried out with an electronic universal testing machine and a self-designed single pellet unit on the basis of a simplex-centroid design. Several parameters, including briquetting time, energy consumption, maximum extrusion force, product compressive strength, and product density, were tested and recorded. The experimental data were processed by the methods of regression analysis and variance analysis. Finally, effects of raw material particle size on the briquetting process, energy consumption, maximum extrusion force, product compressive strength, and product density were obtained. Results showed that, compared with simple sized materials, mixed materials achieved lower energy consumption, higher product compressive strength, and higher product density.     

Numerical simulation of particle size distribution evolution during pulverized coal combustion

Two numerical simulations of particle size distribution (PSD) evolution during ash-free char combustion are presented to help determine the sensitivity of measured coal PSD evolution to fragmentation. The first simulation is based on percolation theory, and it builds the PSD evolution from an ensemble of individual particle size time histories. The second simulation is population balance that operates on the entire distribution as a unit. Inputs to the simulations come from experimental data available in the literature, and results of the simulations are discussed in conjunction with these data.     

Experimental investigation of the effect of particle deposition on pool boiling of nanofluids

Research on pool boiling of nanofluids has shown contradicting trends in the heat transfer coefficient (HTC). Such trends have been attributed, in part, to nanoparticle deposition on the heater surface. An experimental investigation of the transient nature of nanoparticle deposition and its effect on the HTC of pool boiling of nanofluids at various concentrations has been carried out. Pool boiling experiments have been conducted on a horizontal flat copper surface for alumina (40–50 nm) water based nanofluids at concentrations of 0.01, 0.1 and 0.5 vol.%. Nanofluids boiling experiments have been followed by pure water boiling experiments on the same nanoparticle-deposited (NPD) surfaces. This technique has been employed in order to separate the effect of nanoparticle deposition from the effect of nanofluids properties on the HTC. Contrary to what was expected, boiling of pure water on the NPD surface produced using the highest concentration nanofluid resulted in the highest HTC. A closer look at the nature of the NPD surfaces explained such trend. A new approach using a transient surface factor in Rohsenow correlation has been proposed to account for the transient nature of nanoparticle deposition. The applicability of such approach at different concentrations has been investigated.     

Effect of grain size on the thermoelectric conversion efficiency of semiconductor alloys at high temperature

The theoretical increase in the thermoelectric figure of merit of silicon-germanium alloy at 1000 K which would accompany a reduction in material grain size is calculated. Assuming that acoustic phonon scattering is the dominant scattering mechanism, the improvement in the conversion efficiency compared with single crystal values is estimated to be 14 per cent and 30 per cent for n-type Si₈₀Ge₂₀ alloy with grain sizes of 10 μm and 0·1 μm, respectively.     

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).     

纳米通道内纳米流体流动特性的分子动力学模拟

利用分子动力学方法对铜-氩纳米流体和基础流体在不同剪切速度下的纳米尺度的Couette流进行模拟计算。结果表明:在纳米尺度通道内,纳米流体流动过程中颗粒存在旋转运动和平移运动,从而加强湍流效果,强化传热并影响整个流动区域内的流动速度分布,造成纳米流体速度呈非线性分布。壁面和纳米颗粒表面都会形成一层排布更为规则的液体原子吸附层,吸附层内液体分子在流体流动过程中一直伴随着壁面和纳米颗粒进行运动,且吸附层具有"类固"特性,可以增强纳米流体的传热能力。     

Direct numerical simulation of thermal conductivity of nanofluids: The effect of temperature two-way coupling and coagulation of particles

An Eulerian–Lagrangian based direct numerical simulations (DNS) model was developed to investigate the effective thermal conductivity of nanofluids. A two-way coupling term to resolve the temperature interactions between the solid particles and fluid field was considered. The model also considered various forces acting on the nanoparticles. Cu/water nanofluids with 100 nm particles and Al₂O₃/water nanofluids with 80 nm particles were simulated at different volume fractions and the effective thermal conductivity of nanofluids was calculated. The present results suggest that the particle conductivity and forces acting on nanoparticle are necessary while predicting the effective thermal conductivity of nanofluids.     

Measurement of high gas-stream temperature using dynamic thermocouples

The dynamic probe technique using the transient response of a thermocouple is one of the methods of measuring high temperature flowing gases. In this paper, the complete dynamic response of a thermocouple has been solved consisting of convective, conductive, and radiative terms. The solution has been used to arrive at correction factors for actual experimental data. The use of dynamic thermocouples in the measurement of temperature profiles has also been illustrated by experiment. The model is verified at lower temperatures using a bunsen flame.     

Effects of rice husk particle size on biohydrogen production under solid state fermentation

As a renewable energy source bio-hydrogen production from lignocellulosic wastes is a promising approach which can produce clean fuel with no CO₂ emissions. Utilization of agro-industrial residues in solid state fermentation (SSF) is offering a solution to solid wastes disposal and providing an economical process of value-added products such as hydrogen.In this study three different particle size of rice husk (<2000 μm, <300 μm, <74 μm) was subjected to batch SSF with a Clostridium termitidis: Clostridium intestinale ratio of 5:1. C. termitidis is a cellulolytic microorganism that has the ability to hydrolyze cellulosic substances and C. intestinale is able to grow on glucose having a potential of enhancing hydrogen production when used in the co-culture. 5 g dw rice husk with 75% humidity was used as substrate in SSF under mesophilic conditions. The highest HF Volume (29.26 mL) and the highest yield (5.9 mL H₂ g⁻¹ substrate) were obtained with the smallest particle size (<74 μm). The main metabolites obtained from the fermentation media were acetic, butyric, propionic and lactic acids. The second best production yield (3.99 mL H₂ g⁻¹ substrate) was obtained with the middle particle size (<300 μm) rice husk with a HF of 19.71 mL.