Coding,evaluation, comparison,ranking and optimal selection of nanoparticles with heat transfer fluids for thermal systems

Performance of nanofluids is affected by various parameters and it is important to identify these parameters and their inheritance that determine the overall performance of nanofluids. The 125 attributes are identified and a coding scheme is developed for an in-depth understanding and visual comparison of nanofluids more precisely. A three-stage methodology named multi-attribute decision making (MADM) is used for the evaluation, comparison and ranking of nanofluids and it selects the optimum nanofluid for a given application in less time and effort. In the first stage of MADM, known as elimination search, a long list of alternatives is converged to a manageable list. Later, the procedure is followed for the ranking of nanofluids by employing the Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) approach. This ensures that the selected nanofluid is the closest to the hypothetical best nanofluid and the farthest from the hypothetical worst nanofluids. Finally, the optimum nanofluid is selected by considering the cost factor as well as the government rules for a particular application. This methodology has been explained by an example in which nanofluids are developed in the laboratory to evaluate the heat transfer characteristics for cooling devices such as microchannels. This scheme is user friendly and a simple mathematical procedure is required to use it.     

Stability,viscosity, thermal conductivity,and electrical conductivity enhancement of multi-walled carbon nanotube nanofluid using gum arabic

This experimental investigation discussed on the stability and rheological behavior of multi-wall carbon nanotubes (MWCNTs) nanofluids with and without gum arabic (GA). The stability of MWCNT in the presence of GA dispersant in solar glycol is systematically investigated by taking into account the combined effect of different parameters, such as sonication time, temperature, dispersant and particle concentration. The concentrations of MWCNT and GA have been varied from 0.2 to 0.6% volume concentration and from 0.25 to 1.25 wt%, respectively, and the sonication time has been varied in between 30 and 120 min. The effect of sonication time on viscosity was discussed. It was perceived that the shear thinning behavior is exhibited by all the nanofluid samples. The stability of nanofluid is measured in terms of MWCNT concentration as a function of sediment time using UV-Vis spectrophotometer. Rheological behavior of MWCNT nanofluids is measured using Bohlin CVO Rheometer in the temperature range of 30–50°C, with step sizes of 5°C. Optimal GA concentration is obtained for the entire range of MWCNT concentration and 0.25–1.25 wt% of GA is found to be sufficient to steady all MWCNT range in solar glycol. Rapid sedimentation of MWCNTs is observed at higher GA concentration and sonication time. The presence of MWCNT and GA enhanced the thermal conductivity of the nanofluids by 30.59% at 0.6 vol.% particle concentration and 1.25 GA wt% at 50°C. The electrical conductivity is enhanced in a linear manner with respect to the loading of MWCNT and GA. Nevertheless, the electrical conductivity is increased linearly with increasing the temperature of the nanofluid. At particle concentration of 0.6 vol.% of MWCNT and 1.25 wt% of GA, the electrical conductivity of the nanofluid is improved by 190.57% at a temperature of 50°C.     

Influence of ultrasonication energy on the dispersion consistency of Al2O3–glycerol nanofluid based on viscosity data,and model development for the required ultrasonication energy density

Achieving homogenised and stable suspensions has been one of the important research topics in nanofluid investigations. Preparing nanofluids, especially from the two-step method, is often accompanied with varying degrees of agglomerations depending on some parameters. These parameters include the physical structure of the nanoparticle, the prevalent particle charge, the strength of van der Waals forces of attraction and repulsiveness strength. Amongst the methods of deagglomeration, the use of ultrasonic vibration is most popular for achieving uniform dispersion. However, there are very few works related to its effect on the thermo-physical properties of nanofluids, and above all, standardising the minimum required ultrasonication time/energy for nanofluids synthesis. In this work, the optimum energy required for uniform and initially stable nanofluid has been investigated through experimental study on the combined influence of ultrasonication time/energy, nanoparticle size, volume fraction and temperature on the viscosity of alumina–glycerol nanofluids. Three different sizes of alumina nanoparticles were synthesised with glycerol using ultrasonication-assisted two-step approach. The viscosities of the nanofluid samples were measured between temperatures of 20–70?°C for volume fractions up to 5%. Based on the present experimental results, the viscosity characteristics of the nanofluid samples were dependent on particle size, volume fraction and working temperature. Using viscometry, the optimum energy density required for preparing homogenous nanofluid was obtained for all particle sizes and volume fractions. Finally, an energy density model was derived using dimensionless analysis based on the consideration of nanoparticle binding/interaction energy in base fluid, particle size, volume fraction, temperature and other base fluid properties. The model's empirical constants were obtained using nonlinear regression based on the present experimental data.     

Stability and thermal conductivity enhancement of carbon nanotube nanofluid using gum arabic

This experimental study reports on the stability and thermal conductivity enhancement of carbon nanotubes (CNTs) nanofluids with and without gum arabic (GA). The stability of CNT in the presence of GA dispersant in water is systematically investigated by taking into account the combined effect of various parameters, such as sonication time, temperature, dispersant and particle concentration. The concentrations of CNT and GA have been varied from 0.01 to 0.1?wt% and from 0.25 to 5?wt%, respectively, and the sonication time has been varied in between 1 and 24?h. The stability of nanofluid is measured in terms of CNT concentration as a function of sediment time using UV-Vis spectrophotometer. Thermal conductivity of CNT nanofluids is measured using KD-2 prothermal conductivity meter from 25 to 60°C. Optimum GA concentration is obtained for the entire range of CNT concentration and 1–2.5?wt% of GA is found to be sufficient to stabilise all CNT range in water. Rapid sedimentation of CNTs is observed at higher GA concentration and sonication time. CNT in aqueous suspensions show strong tendency to aggregation and networking into clusters. Stability and thermal conductivity enhancement of CNT nanofluids have been presented to provide a heat transport medium capable of achieving high heat conductivity. Increase in CNT concentrations resulted in the non-linear thermal conductivity enhancement. More than 100–250% enhancement in thermal conductivity is observed for the range of CNT concentration and temperature.     

Effect of rotation on a layer of micropolar ferromagnetic fluid heated from below saturating a porous medium

This paper deals with the theoretical investigation of the effect of rotation on a layer of micropolar ferromagnetic fluid heated from below saturating a porous medium subjected to a transverse uniform magnetic field. For a flat fluid layer contained between two free boundaries, an exact solution is obtained using a linear stability analysis theory and normal mode analysis method. For the case of stationary convection, the effect of various parameters like medium permeability, rotation, non-buoyancy magnetization, coupling parameter, spin diffusion parameter and micropolar heat conduction has been analyzed. The critical magnetic thermal Rayleigh number for the onset of instability are also determined numerically for sufficiently large values of magnetic parameter M₁ and results are depicted graphically. The principle of exchange of stabilities is found to hold true for the micropolar ferromagnetic fluid saturating a porous medium heated from below in the absence of micropolar viscous effect, microinertia and rotation. The oscillatory modes are introduced due to the presence of the micropolar viscous effect, microinertia and rotation, which were non-existent in their absence. The sufficient conditions for the non-existence of overstability are also obtained.     

Investigation on Stability and Optical Properties of Titanium Dioxide and Aluminum Oxide Water-Based Nanofluids

Water is regarded as a poor absorber of solar energy. This affects the efficiency of solar thermal systems. The addition of nanoparticles to heat transfer fluids used in solar thermal systems can enhance their optical properties. These new-generation heat transfer fluids are known as nanofluids. The present study investigates the stability and optical properties of three nanofluids, including aluminum oxide (13 nm and <50 nm) and titanium dioxide (21 nm) nanofluids. The stability of the nanofluids was observed through a photo-capturing method and zeta potential measurements. Ultraviolet–visible (UV–Vis) spectrophotometer was used to measure the absorbance and transmittance of the prepared nanofluids. The effect of factors such as type of particle, type of surfactant, and pH of the solution on the optical properties of the nanofluids was also investigated. We found that the titanium dioxide nanofluid had better optical properties but lower stability compared to aluminum oxide nanofluids.     

An experimental and theoretical study of the influence of surfactant on the preparation and stability of ammonia-water nanofluids

Two types of nanofluids are obtained by adding the mixture of carbon black nanoparticles with emulsifier OP-10, and Al₂O₃ nanoparticles with sodium dodecyl benzene sulfonate (SDBS) in the ammonia-water solution, respectively. The dispersion stability of the prepared nanofluids in different mass fractions of surfactants is investigated by the light absorbency ratio index methods. The results show that with the increase of mass fraction of surfactant, the stability of carbon black nanofluid is improved firstly and then is exacerbated, while the stability of Al₂O₃ nanofluid is exacerbated firstly, then is improved, and then is exacerbated again. The influences of surfactant on the stability of ammonia-water nanofluids abide by the monolayer adsorption theory or electric double layer adsorption theory. Finally, the theoretical surfactant mass fractions required in the preparation of ammonia-water nanofluids are calculated by simplifying the dispersion models and the results are in accordance with experimental results.     

Stability analysis of Al2O3/water nanofluids

ABSTRACT

This paper presents the stability analysis of Al₂O₃/water nanofluid. The stability is investigated with the help of zeta potential and visual inspection methods. The effects of pH and sonication time for the stability of nanofluids are studied in detail. The visual inspection method is used to calculate the stability period of nanofluids. The zeta potential is directly related to stability period of nanofluids; higher the absolute value of zeta potential, higher the stability period. The stability is also analysed by using sodium dodecyl sulphate, a surfactant, with respect to the time elapsed after the preparation of nanofluids.     

Experimental study on enhancement of ammonia-water falling film absorption by adding nano-particles

Based on the preparation of Al₂O₃, Fe₂O₃ and ZnFe₂O₄ nanofluid, the comparative experiments on the falling film absorption between ammonia-water and ammonia-water with various kinds of nano-particles are carried out. Experimental results show that the sorts and mass fraction of nano-particles, the viscosity and stability of nanofluid, as well as the mass fraction of ammonia in the basefluid are considered as the key parameters. The absorption of ammonia is weakened by only adding surfactants or adding poorly dispersed nano-particles. The increase of mass fraction of nano-particles with matched surfactants can improve the absorption rate of ammonia under the condition that the viscosity of nanofluid does not increase remarkably, and there is an optimal mass fraction for each kind of nano-particles and surfactant. With the increase in ammonia mass fraction of initial nanofluid, the absorption potential capacity decline, but the enhancing effect induced by the nanofluid is more obvious compared to that without nano-particles. The effective absorption ratio can be increased by 70% and 50% with Fe₂O₃ and ZnFe₂O₄ nanofluid respectively when the initial ammonia mass fraction is 15%. The absorption enhancement by the nanofluid is attributable to the heat transfer enhancement and the decrease in viscosity of nanofluid, which are strongly proved by the temperature differences in cooling water and nanofluids as well as the falling film flowing time.     

A simple single-step approach towards synthesis of nanofluids containing cuboctahedral cuprous oxide particles using glucose reduction

Enhancement of thermal properties of conventional heat transfer fluids has become one of the important technical challenges. Since nanofluids offer a promising help in this regard, development of simpler and hassle free routes for their synthesis is of utmost importance. Synthesis of nanofluids using a hassle free route with greener chemicals has been reported. The single-step chemical approach reported here overcomes the drawbacks of the two-step procedures in the synthesis of nanofluids. The resulting Newtonian nanofluids prepared contained cuboctahedral particles of cuprous oxide and exhibited a thermal conductivity of 2.852 W·m^-1·K^-1. Polyvinylpyrrolidone (PVP) used during the synthesis acted as a stabilizing agent rendering the nanofluid a stability of 9 weeks.     

A study of ZnO nanoparticles and ZnO-EG nanofluid

This study deals with the synthesis and characterisation of ZnO nanoparticles and suspension of ZnO-ethylene glycol (EG) nanofluid. Crystalline ZnO nanoparticles are synthesised using the chemical method. The nanofluids were synthesised by the dispersion of ZnO nanoparticles in EG solution using an ultrasonicator. The prepared ZnO nanoparticles are characterised by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and UV-Vis absorption spectrum. The particle size distribution and ultrasonic parameters of the synthesised nanofluid are measured with the help of acoustic particle sizer (APS-100) and ultrasonic interferometer. The observed features of ZnO nanoparticles and ZnO-EG nanofluids are discussed in correlation with known properties of other nanofluids.     

Cu-水纳米流体的分散行为及导热性能研究

通过测定Cu-水纳米悬浮液的Zeta电位和吸光度,采用Hotdisk热物性分析仪测量了其导热系数,探讨了不同pH值和分散剂浓度对Cu-水纳米悬浮液分散稳定性和导热性能的影响.结果表明,pH值和分散剂加入量是影响Cu-水纳米悬浮液分散稳定和导热系数的重要因素.最优化的pH值和分散剂加入量能显著提高水溶液中Cu表面Zeta电位绝对值,增大了颗粒间静电排斥力,悬浮液分散稳定性较好,导热系数较高.从分散稳定和导热系数提高两个方面来考虑,pH=9.5左右被选为最优化值,在0.1%Cu-H2O纳米流体中,0.07%SDBS被选为最优化浓度.另外,Cu-水纳米流体的导热系数随纳米粒子质量分数的增大而增大,呈非线性关系,且比现有理论(Hamilton-Crosser模型)预测值大.     

Thermal relaxation effect on magnetohydrodynamic instability in a rotating micropolar fluid layer heated from below

Summary. The effect of a vertical magnetic field on the onset of convective instability in a conducting micropolar fluid (Oldroyd fluid) layer heated from below confined between two horizontal planes under the simultaneous action of the rotation of the system and a vertical temperature gradient is considered. Linear stability theory and normal mode analysis are used to derive an eigenvalue system of order twelve, and an exact eigenvalue equation for natural instability is obtained. Under somewhat artificial boundary conditions, this equation can be solved exactly to yield the required eigenvalue relationship from which various critical values are determined in detail. The effects of magnetic field, the relaxation time and micropolar parameters on the critical Rayleigh number and critical wave number are discussed and presented graphically. The analysis presented in this paper is more general than any previous investigation.     

BN/EG纳米流体的制备及稳定性研究

通过两步法制备了氮化硼/乙二醇(BN/EG)纳米流体,研究了超声振荡时间、pH值、分散剂种类及添加量3种因素对其稳定性的影响。结果表明超声分散时间太长或太短都不利于流体的稳定性,实验中取30min最好;酸或碱的加入都会使BN/EG纳米流体稳定性急剧恶化;适量分散剂PVP的加入能够明显改善BN/EG纳米流体的稳定性。     

Free convection of hybrid Al2O3-Cu water nanofluid in a differentially heated porous cavity

Hybrid nanofluids are a new type of enhanced working fluids, engineered with enhanced thermo-physical properties. The hybrid nanofluids profit from the thermo-physical properties of more than one type of nanoparticles. The present study aims to address the free convective heat transfer of the Al₂O₃-Cu water hybrid nanofluid in a cavity filled with a porous medium. Two types of important porous media, glass ball and aluminum metal foam, are considered for the porous matrix. The experimental data show dramatic enhancement in the thermal conductivity and dynamic viscosity of the synthesized hybrid nanofluids, and hence, these thermophysical properties could not be modeled using available models of nanofluids. Thus, the actual available experimental data for the thermal conductivity and the dynamic viscosity of hybrid nanofluids are directly utilized in the present theoretical study. Various comparison with results published previously in the literature are performed and the results are found to be in excellent agreement. In most cases, the average Nusselt number Nu_l is decreasing function of the volume fraction of nanoparticles. The results show the reduction of heat transfer using nanoparticles in porous media. The observed reduction of the heat transfer rate is much higher for hybrid nanofluid compared to the single nanofluid.     

Progress of Nanofluid Application in Machining: A Review

A colloidal mixture of nanometer-sized (<100 nm) metallic and non-metallic particles in conventional cutting fluid is called nanofluid. Nanofluids are considered to be potential heat transfer fluids because of their superior thermal and tribological properties. Therefore, nano-enhanced cutting fluids have recently attracted the attention of researchers. This paper presents a summary of some important published research works on the application of nanofluid in different machining processes: milling, drilling, grinding, and turning. Further, this review article not only discusses the influence of different types of nanofluids on machining performance in various machining processes but also unfolds other factors affecting machining performance. These other factors include nanoparticle size, its concentration in base fluid, lubrication mode (minimum quantity lubrication and flood), fluid spraying nozzle orientation, spray distance, and air pressure. From literature review, it has been found that in nanofluid machining, higher nanoparticle concentration yields better surface finish and more lubrication due to direct effect (rolling/sliding/filming) and surface enhancement effect (mending and polishing) of nanoparticles compared to dry machining and conventional cutting fluid machining. Furthermore, nanofluid also reduces the cutting force, power consumption, tool wear, nodal temperature, and friction coefficient. Authors have also identified the research gaps for further research.     

Grinding of Ti-6Al-4V Under Small Quantity Cooling Lubrication Environment Using Alumina and MWCNT Nanofluids

Ti-6Al-4V is a difficult-to-grind material as chips tend to adhere to the grit materials of an abrasive wheel due to its chemical affinity. In the present work, it has been attempted to improve the grindability by application of small quantity cooling lubrication (SQCL) technology using nanofluids, namely, multiwalled carbon nanotube (MWCNT) and alumina nanofluid. The suitability of nanofluids was experimentally evaluated in reciprocating surface grinding using a vitrified SiC wheel. Substantial improvement in grindability under the influence of MWCNT nanofluid (SQCL) could be achieved compared to soluble oil (flood). Reduction of specific grinding forces and specific energy was observed due to the combined effect of superior heat dissipation and lubrication abilities; when the latter one was realized through on-site rolling of MWCNT strands, inter-tubular slip and solid lubrication of the film adhered onto the wheel surface. These outperforming characteristics of MWCNT nanofluid helped in retaining grit sharpness superiorly, thus resulting in better surface finish and less re-deposition of metal on the ground workpiece. On the contrary, Al2O3 nanofluid (SQCL) underperformed even soluble oil (flood). For an ageing effect, Al2O3 nanoparticles resulted in abrasive agglomerates, which led to its failure, despite its good heat dissipation capability.     

An effective thermal conductivity model of nanofluids with a cubical arrangement of spherical particles

The theoretical investigation of the effective thermal conductivities of nanofluids, a new class of solid-liquid suspensions, is important in both predicting and designing nanofluids with effective thermal conductivities. We have developed a new thermal conductivity model for nanofluids that is based on the assumption that monosized spherical particles are uniformly dispersed in the liquid and are located at the vertexes of a simple cubic lattice, with each particle surrounded by a liquid layer having a thermal conductivity that differs from that of the bulk liquid. This model nanofluid with a cubical arrangement of nanoparticles gives a more practical upper limit of thermal conduction than a model nanofluid with a parallel arrangement of nanoparticles. The new model unexpectedly shows a nonlinear relationship of thermal conductivity with particle concentration, whereas the conductivity-concentration curve changes from convex upward to concave upward with increasing volume concentration. The effects of particle and layer parameters on the effective thermal conductivities are also analyzed. A comparison of predicted thermal conductivity values and experimental data shows that the predicted values are much higher than the experimental data, a finding that indicates that there is a potential to further improve the effective thermal conductivities of nanofluids with more uniformly dispersed particles.     

Determination of effective specific heat of nanofluids

The effective specific heat of several types of nanofluids are measured by transient double hot-wire technique. Sample nanofluids are prepared by suspending 1–5 volume percentages of titanium dioxide (TiO₂), aluminium oxide (A1₂O₃) and aluminium (Al) nanoparticles in various base fluids, such as deionised water, ethylene glycol and engine oil. The effective specific heats of these nanofluids were found to decrease substantially with increased volume fraction of nanoparticles. Besides particle volume fraction, particle materials and base fluids also have influence on the effective specific heat of nanofluids. Except Al/engine oil-based nanofluid, predictions of the effective specific heat of nanofluids by the volume fraction mixture rule-based model showed reasonably good agreement with the experimental results. Based on the calibration results obtained for the base fluids, the measurement error is estimated to be within 2.77%.     

Experimental evaluation of the effect in the stability and thermophysical properties of water-Al2O3 based nanofluids using SDBS as dispersant agent

Nanofluids are thermo-fluids engineered by dispersing nanosized particles in conventional base fluids used in heat transfer applications. In this experimental study, the temporal stability and thermophysical properties of water-based alumina nanofluids, such as thermal conductivity, surface tension and viscosity, were evaluated at three different concentrations (0.1 wt%, 0.5 wt%, and 1.0 wt%) using sodium dodecyl benzene sulfonate (SDBS) as dispersant agent. The results showed that the nanofluid prepared with 1.0 wt% exhibits thermal conductivity enhancements between 11% and 15% compared to deionized water. Additionally, a pseudoplastic behaviour was identified by means of a rheological study, which increased as the nanoparticle concentration increased. Nevertheless, the nanofluids with SDBS showed a Newtonian behaviour and viscosity values close to those of water, which is suitable to avoid frictional losses in pump processes. In turn, surface tension increases with the amount of DI-water in Al₂O₃ concentrations, but it decreases with the addition of SDBS, which has an important effect on the boiling applications of nanofluids. This work was carried out to highlight the importance of nanofluids stability in function of the surfactant added and the nanoparticle concentration, in the measurement of significant thermophysical properties such as surface tension, viscosity and thermal conductivity, which could be used to explain the nanofluids behaviour in different thermal devices.