Particle agglomeration and properties of nanofluids

The present study demonstrates the importance of actual agglomerated particle size in the nanofluid and its effect on the fluid properties. The current work deals with 5 to 100 nm nanoparticles dispersed in fluids that resulted in 200 to 800 nm agglomerates. Particle size distributions for a range of nanofluids are measured by dynamic light scattering (DLS). Wet scanning electron microscopy method is used to visualize agglomerated particles in the dispersed state and to confirm particle size measurements by DLS. Our results show that a combination of base fluid chemistry and nanoparticle type is very important to create stable nanofluids. Several nanofluids resulted in stable state without any stabilizers, but in the long term had agglomerations of 250 % over a 2 month period. The effects of agglomeration on the thermal and rheological properties are presented for several types of nanoparticle and base fluid chemistries. Despite using nanodiamond particles with high thermal conductivity and a very sensitive laser flash thermal conductivity measurement technique, no anomalous increases of thermal conductivity was measured. The thermal conductivity increases of nanofluid with the particle concentration are as those predicted by Maxwell and Bruggeman models. The level of agglomeration of nanoparticles hardly influenced the thermal conductivity of the nanofluid. The viscosity of nanofluids increased strongly as the concentration of particle is increased; it displays shear thinning and is a strong function of the level of agglomeration. The viscosity increase is significantly above of that predicted by the Einstein model even for very small concentration of nanoparticles.     

Survey on nucleate pool boiling of nanofluids: the effect of particle size relative to roughness

Pool boiling heat transfer using nanofluids (which are suspensions of nano-sized particles in a base fluid) has been a subject of many investigations and incoherent results have been reported in literature regarding the same. In the past, experiments were conducted in nucleate pool boiling with varying parameters such as particle size, concentration, surface roughness etc. and all sort of results ranging from heat transfer enhancement, deterioration and no effect were reported. This work tries to segregate a survey on pool boiling of nanofluids with respect to particle concentration. This is due to the fact that a major drift in heat transfer behavior is observed at higher and lower particle concentration. But upon deep perusal it has been found that deterioration in heat transfer coefficient are mainly observed at higher particle concentrations (4–16% by weight) and enhancements mainly at lower particle concentrations (0.32–1.25% by weight). Moreover, the relative size of the particle with respect to the surface roughness of the heating surface seems to play an important role in understanding the boiling behaviour. Also, recent works have reported that change in ‘surface wetting’ of the heating surface due to nanofluids and the formation of a porous layer modifiying nucleation site density can be of importance in predicting nucleate pool boiling characteristics of nanofluids. In the present paper, attempts are made to make systematic analysis of results in literature and try to bring out a common understanding of the results in literature.     

Phase change heat transfer of liquid nitrogen upon injection into aqueous based TiO<Subscript>2</Subscript> nanofluids

This paper reports an experimental study on the cryogenic phase change behaviour of liquid nitrogen upon injection into a relatively large pool of aqueous based titanium dioxide nanofluids under ambient temperatures. Stable TiO₂ are formulated at concentrations between 1% and 4% by particle weight, and characterized in the experiments. Both transient pressure and temperature profiles are measured and analysed in the range of liquid nitrogen Reynolds number of 2,200–11,000 and Webber number of 9–220. The results, however, do not show obvious influence of nanoparticles’ concentration on the pressure build up and heat transfer under the condition of this work. Very similar results are observed for liquid nitrogen injection into pure distilled water and into different concentrations of TiO₂ nanofluids; both the pressure and the rate of pressure rise during the injection process increase linearly with injection velocity irrespective of nanoparticle concentrations. Further discussion shows that a higher rate of pressure rise could be achieved if operating conditions were optimised to induce the fragmentation and subsequent vapour explosion.     

NMR velocimetry studies of the steady-shear rheology of a concentrated hard-sphere colloidal system

NMR velocimetry has been used to observe the steady-shear rheological behaviour of a concentrated suspension of hard-sphere like 370 nm diameter PMMA core-shell latex particles at the volume fraction Φ = 0.46, the liquid core of the spheres rendering possible NMR observation of the particles themselves. Rheological measurements in a cone-and-plate geometry indicate that when aged (i.e. left at rest for two weeks), the material exhibits yield stress behaviour at very low shear rates. For shear rates greater than 1 s ^{- 1} a transition to liquid-like behaviour was observed, leading to a rejuvenated fluid state which exhibits shear-thinning behaviour over a wide range of shear rates. A similar yield stress behaviour was reflected in NMR velocimetry measurements in a Couette geometry, where the solid-to liquid transition could be clearly observed. Under steady-state flow, the fluid state inside the radius at which yield stress was observed, exhibited shear-thinning behaviour with a power law exponent n slowly approaching unity with increasing shear rate. This behaviour has some similarities with a model of Derec et al. in which aging and rejuvenation effects compete. Substantial wall slip was observed both at the inner and at the outer wall, an effect which disappeared as the shear rate was increased. No radial particle migration from the high-shear region at the inner wall was observed.     

Dispersion stability and thermal conductivity of propylene glycol-based nanofluids

The dispersion stability and thermal conductivity of propylene glycol-based nanofluids containing Al₂O₃ and TiO₂ nanoparticles were studied in the temperature range of 20–80 °C. Nanofluids with different concentrations of nanoparticles were formulated by the two-step method and no dispersant was used. In contrast to the common belief, the average particle size of nanofluids was observed to decrease with increasing temperature, and nanofluids showed an excellent stability over the temperature range of interest. Thermal conductivity enhancement for both studied nanofluids was a nonlinear function of concentration and was temperature independent. Theoretical analyses were also performed using existing models compared with experimental results. The model based on the aggregation theory appears to be the best.     

Investigation into the antibacterial behaviour of suspensions of ZnO nanoparticles (ZnO nanofluids)

The antibacterial behaviour of suspensions of zinc oxide nanoparticles (ZnO nanofluids) against E. Coli has been investigated. ZnO nanoparticles from two sources are used to formulate nanofluids. The effects of particle size, concentration and the use of dispersants on the antibacterial behaviour are examined. The results show that the ZnO nanofluids have bacteriostatic activity against E. coli. The antibacterial activity increases with increasing nanoparticle concentration and increases with decreasing particle size. Particle concentration is observed to be more important than particle size under the conditions of this work. The results also show that the use of two types of dispersants (Polyethylene Glycol (PEG) and Polyvinylpyrolidone (PVP)) does not affect much the antibacterial activity of ZnO nanofluids but enhances the stability of the suspensions. SEM analyses of the bacteria before and after treatment with ZnO nanofluids show that the presence of ZnO nanoparticles damages the membrane wall of the bacteria. Electrochemical measurements using a model DOPC monolayer suggest some direct interaction between ZnO nanoparticles and the bacteria membrane at high ZnO concentrations. On visiting from the Tianjin University of Science & Technology, Tianjin, P.R. China.     

Characterization of TiO<Subscript>2</Subscript> nanoparticle suspensions for electrophoretic deposition

The rheology of suspensions is critically important for the successful achievement of defect-free TiO₂ deposits by electrophoretic deposition (EPD). The rheological behaviour of TiO₂ nanoparticle suspensions in acetylacetone with and without iodine was investigated over a broad solid-concentration range (0.3–2.5 wt.%) and at different shear rates ( = 10–250 s⁻¹). The influence of these parameters on the quality of TiO₂ films obtained by EPD on stainless steel substrates was assessed. The pure solvent and the 1 wt.% TiO₂ nanoparticles suspension without iodine exhibited shear-thickening flow behaviour. For other concentrations, the suspensions showed shear-thinning behaviour followed by an apparent shear-thickening effect at a critical shear rate (100 s⁻¹). For the suspension with 1 wt.% TiO₂ containing iodine, a shear-thickening flow behaviour was observed over the whole shear rate range investigated. The maximum solids fraction (ϕ_m) was experimentally determined from a linear relationship between solid concentration and viscosity. The estimated value was ϕ_m = 7.94 wt.% for this system. Using a suspension with 1 wt.% concentration, good-quality TiO₂ deposits on stainless steel planar substrates were obtained by EPD at constant voltage condition. The influence of pH on suspension stability was determined in the range pH = 1–9, being pH ≈ 5 the optimal value for this system in terms of EPD results.     

Fabrication, characterization, and measurement of viscosity of α-Fe2O3-glycerol nanofluids

Magnetic nanofluids, ferrofluids, are a special category of smart nanomaterials, consisting of stable dispersion of magnetic nanoparticles in different fluids. In this study, magnetic nanoparticles of hematite, α-Fe₂O₃, were prepared by solvothermal method using Fe(NO₃)₃ as a starting material. The nanoparticles were characterized by X-ray diffraction (XRD) and transmission electronic microscope (TEM).To the best of our knowledge, this is the first research on the rheological properties of nanofluids of α-Fe₂O₃ nanoparticles and glycerol. The experimental results showed that the viscosity of α-Fe₂O₃-glycerol nanofluids increases with increasing the particle volume fraction and decreases with increasing temperature. Our results clearly showed that the α-Fe₂O₃-glycerol nanofluids are non-Newtonian shear-thinning and their shear viscosity depends strongly on temperature. The experimental data were compared with some theoretical models. The measured values of the effective viscosity of nanofluids are underestimated by the theoretical models.     

Study on Steady Shear Rheological Behavior of Concentrated Suspensions of Sulfonated Polyacrylamide/Na-Montmorillonite Nanoparticles

Concentrated suspensions of sulfonated polyacrylamide (SPA)/Na⁺-montmorillonite (Na-MMT) were prepared and their stability and steady shear rheological properties were described as a function of nanoparticle and polymer concentration and temperature. The results showed that the Na-MMT nanoparticles suspensions were stable in the absence and presence of SPA and no sedimentation was seen. The Z-average particle sizes for the SPA/Na-MMT suspensions increased in the presence of SPA. Rheological investigations showed that the SPA solutions and SPA/Na-MMT suspensions displayed non-Newtonian behavior in almost the whole range of shear rate. All the suspensions exhibited a shear-thinning flow character as shear rate increased. The flow curves indicated the shear viscosity and stress of the samples were decreased with increasing nanoparticles concentration up to 1.5 wt%, but for Na-MMT loading greater than 1.5 wt% there was an increase in shear viscosity and stress of the suspensions. Increasing of SPA concentration had more effect on increasing the rheological properties of SPA/Na-MMT suspensions than increasing of nanoclay content. Shear viscosity and stress of the suspensions increased with increasing SPA concentration and decreased with increasing temperature from 50°C to 70°C.     

Preparation,Characterization, and Rheological Properties of Dibutyrylchitin

Dibutyrylchitin was prepared by an acylating reaction in a heterogeneous system with butyric anhydride as an acylating agent and perchloric acid as a catalyst. The effects of the preparation conditions, including chitin grain size, reaction temperature, and reaction time, on conversion ratio of chitin to dibutyrylchitin are discussed in detail. By adjusting reaction time and reaction temperature, dibutyrylchitin with an adequate intrinsic viscosity could be obtained. Steady-state rheological measurement was also performed and the results revealed that concentration, temperature, and shear rate had a great influence on the rheological properties of dibutyrylchitin/dimethylformamide solutions. Thus, a suitable spinning condition could be determined from the rheological analyses.     

基于基液连续假设的大体系Cu-H2O纳米流体输运特性的模拟研究

分子动力学模拟是研究纳米流体的输运特性的重要手段, 但计算量庞大. 为研究能体现流动传热过程的大体系纳米流体的输运特性, 本文对基液采用连续介质假设, 将基液的势能拟合在纳米团簇的势能中, 大幅度减小了计算量, 使得大体系输运特性的模拟成为可能, 且模拟结果与多组实验结果吻合较好. 采用此方法模拟研究了速度梯度剪切对Cu-H₂O纳米流体颗粒聚集过程和聚集特性的影响, 进而对Cu-H₂O纳米流体在流动传热过程中的热导率和黏度进行了模拟计算, 定量揭示了宏观流动传热过程中不同的速度梯度、速度、平均温度和温度梯度对于Cu-H₂O纳米流体热导率和黏度的影响.     

High Pressure High Temperature Dynamic Rheometry of Sulfonated Polyacrylamide Solutions in Electrolyte Media as Used for Oil Recovery Applications

Sulfonated polyacrylamide (SPAA) solutions were prepared and the effects of pressure, polymer concentration, and water temperature, pH and salinity on their rheological behavior were investigated using a concentric cylinder dynamic rheometer equipped with a high pressure cell. According to the rheological flow curves the shear stress of SPAA solutions increased less than in proportion to their shear rates; that is, a shear thinning effect occurred. For polymer solutions containing 15,000 ppm of SPAA, shear viscosity, and stress were nearly insensitive to pressure. However, the shear viscosity and stress of SPAA solutions were affected by temperature and this effect was more evident at lower pressure. The flow curves indicated the shear viscosity and stress of the samples increased with increasing SPAA concentration and pH of the water, but were decreased with increasing water salinity and temperature.     

High energy dissipation-based process to improve the rheological properties of bentonite drilling muds by reducing the particle size

Drilling mud is a multi-phase fluid that is used in the petroleum drilling process. Bentonite is the most important constituent of drilling mud; it endows the drilling mud with its rheological behaviors, such as viscosity, yield stress, and shear thinning. The process of manufacturing microscale bentonite at the nanoscale level is very promising for commercializing nano-based drilling mud. In contrast to the conventional method using the impeller, bentonite was manufactured in its nanoparticle state in the present work through ultrasonic and homogenizer processes in the solution state. In case of the ultrasonic process, the viscosity increase in the low shear rate region before and after processing of the 5 wt% bentonite-based mud and the rheological properties in the presence of polymer additive were compared. In case of the homogenizer process, the rheological properties of 3 wt% bentonite-based mud employed through the homogenizer process and 5 wt% mud prepared generally were compared. Both processes reported improvement of rheological properties, in which shear thinning behavior strongly occurred when particle size decreased through FE-SEM, TEM image analysis, and particle size analyzer. A regularized Herschel-Bulkley model suitable for rheological quantitative explanation of drilling mud including yield stress was selected. The homogenizer process has the potential to be applied in the petroleum drilling industry for large-scale production, and the mechanism was confirmed by numerical analyses. In conclusion, we presented a simple and easy-to-apply process to rapidly produce nano-based drilling mud.     

The effect of particle size on the thermal conductivity of alumina nanofluids

We present new data for the thermal conductivity enhancement in seven nanofluids containing 8–282 nm diameter alumina nanoparticles in water or ethylene glycol. Our results show that the thermal conductivity enhancement in these nanofluids decreases as the particle size decreases below about 50 nm. This finding is consistent with a decrease in the thermal conductivity of alumina nanoparticles with decreasing particle size, which can be attributed to phonon scattering at the solid–liquid interface. The limiting value of the enhancement for nanofluids containing large particles is greater than that predicted by the Maxwell equation, but is predicted well by the volume fraction weighted geometric mean of the bulk thermal conductivities of the solid and liquid. This observation was used to develop a simple relationship for the thermal conductivity of alumina nanofluids in both water and ethylene glycol.     

Brownian dynamic simulation for the prediction of effective thermal conductivity of nanofluid

Nanofluid is a colloidal solution of nanosized solid particles in liquids. Nanofluids show anomalously high thermal conductivity in comparison to the base fluid, a fact that has drawn the interest of lots of research groups. Thermal conductivity of nanofluids depends on factors such as the nature of base fluid and nanoparticle, particle concentration, temperature of the fluid and size of the particles. Also, the nanofluids show significant change in properties such as viscosity and specific heat in comparison to the base fluid. Hence, a theoretical model becomes important in order to optimize the nanofluid dispersion (with respect to particle size, volume fraction, temperature, etc.) for its performance. As molecular dynamic simulation is computationally expensive, here the technique of Brownian dynamic simulation coupled with the Green Kubo model has been used in order to compute the thermal conductivity of nanofluids. The simulations were performed for different concentration ranging from 0.5 to 3 vol%, particle size ranging from 15 to 150 nm and temperature ranging from 290 to 320 K. The results were compared with the available experimental data, and they were found to be in close agreement. The model also brings to light important physical aspect like the role of Brownian motion in the thermal conductivity enhancement of nanofluids.     

Experimental investigation into the pool boiling heat transfer of aqueous based γ-alumina nanofluids

This paper is concerned about pool boiling heat transfer using nanofluids, a subject of several investigations over the past few years. The work is motivated by the controversial results reported in the literature and the potential impact of nanofluids on heat transfer intensification. Systematic experiments are carried out to formulate stable aqueous based nanofluids containing γ-alumina nanoparticles (primary particle size 10–50 nm), and to investigate their heat transfer behaviour under nucleate pool boiling conditions. The results show that alumina nanofluids can significantly enhance boiling heat transfer. The enhancement increases with increasing particle concentration and reaches ∼ ∼40% at a particle loading of 1.25% by weight. Discussion of the results suggests that the reported controversies in the thermal performance of nanofluids under the nucleate pool boiling conditions be associated with the properties and behaviour of the nanofluids and boiling surface, as well as their interactions.     

Temperature Dependence of Thermal Conductivity of Nanofluids

Mechanism of thermal conductivity of nanofluids is analysed and calculated, including Brownian motion effects, particle agglomeration and viscosity, together influenced by temperature. The results show that only Brown- Jan motion as reported is not enough to describe the temperature dependence of the thermal conductivity of nanofluids. The change of particle agglomeration and viscosity with temperature are also important factors. As temperature increases, the reduction of the particle surface energy would decrease the agglomeration of nanopartieles, and the reduction of viscosity would improve the Brownish motion. The results egree well with the experimental data reported.     

Experimental investigation into the pool boiling heat transfer of aqueous based γ-alumina nanofluids

This paper is concerned about pool boiling heat transfer using nanofluids, a subject of several investigations over the past few years. The work is motivated by the controversial results reported in the literature and the potential impact of nanofluids on heat transfer intensification. Systematic experiments are carried out to formulate stable aqueous based nanofluids containing γ-alumina nanoparticles (primary particle size 10–50 nm), and to investigate their heat transfer behaviour under nucleate pool boiling conditions. The results show that alumina nanofluids can significantly enhance boiling heat transfer. The enhancement increases with increasing particle concentration and reaches ∼ ∼40% at a particle loading of 1.25% by weight. Discussion of the results suggests that the reported controversies in the thermal performance of nanofluids under the nucleate pool boiling conditions be associated with the properties and behaviour of the nanofluids and boiling surface, as well as their interactions.This revised version was published online in August 2005 with a corrected issue number.     

A New Derivation of the Quantum Navier–Stokes Equations in the Wigner–Fokker–Planck Approach

A quantum Navier–Stokes system for the particle, momentum, and energy densities is formally derived from the Wigner–Fokker–Planck equation using a moment method. The viscosity term depends on the particle density with a shear viscosity coefficient which equals the quantum diffusion coefficient of the Fokker–Planck collision operator. The main idea of the derivation is the use of a so-called osmotic momentum operator, which is the sum of the phase-space momentum and the gradient operator. In this way, a Chapman–Enskog expansion of the Wigner function, which typically leads to viscous approximations, is avoided. Moreover, we show that the osmotic momentum emerges from local gauge theory.     

A thermal conductivity model for nanofluids including effect of the temperature-dependent interfacial layer

The interfacial layer of nanoparticles has been recently shown to have an effect on the thermal conductivity of nanofluids. There is, however, still no thermal conductivity model that includes the effects of temperature and nanoparticle size variations on the thickness and consequently on the thermal conductivity of the interfacial layer. In the present work, the stationary model developed by Leong et al. (J Nanopart Res 8:245–254, 2006) is initially modified to include the thermal dispersion effect due to the Brownian motion of nanoparticles. This model is called the ‘Leong et al.’s dynamic model’. However, the Leong et al.’s dynamic model over-predicts the thermal conductivity of nanofluids in the case of the flowing fluid. This suggests that the enhancement in the thermal conductivity of the flowing nanofluids due to the increase in temperature does not come from the thermal dispersion effect. It is more likely that the enhancement in heat transfer of the flowing nanofluids comes from the temperature-dependent interfacial layer effect. Therefore, the Leong et al.’s stationary model is again modified to include the effect of temperature variation on the thermal conductivity of the interfacial layer for different sizes of nanoparticles. This present model is then evaluated and compared with the other thermal conductivity models for the turbulent convective heat transfer in nanofluids along a uniformly heated tube. The results show that the present model is more general than the other models in the sense that it can predict both the temperature and the volume fraction dependence of the thermal conductivity of nanofluids for both non-flowing and flowing fluids. Also, it is found to be more accurate than the other models due to the inclusion of the effect of the temperature-dependent interfacial layer. In conclusion, the present model can accurately predict the changes in thermal conductivity of nanofluids due to the changes in volume fraction and temperature for various nanoparticle sizes.