Pool boiling of water-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and water-Cu nanofluids on horizontal smooth tubes

Experimental investigation of heat transfer during pool boiling of two nanofluids, i.e., water-Al₂O₃ and water-Cu has been carried out. Nanoparticles were tested at the concentration of 0.01%, 0.1%, and 1% by weight. The horizontal smooth copper and stainless steel tubes having 10 mm OD and 0.6 mm wall thickness formed test heater. The experiments have been performed to establish the influence of nanofluids concentration as well as tube surface material on heat transfer characteristics at atmospheric pressure. The results indicate that independent of concentration nanoparticle material (Al₂O₃ and Cu) has almost no influence on heat transfer coefficient while boiling of water-Al₂O₃ or water-Cu nanofluids on smooth copper tube. It seems that heater material did not affect the boiling heat transfer in 0.1 wt.% water-Cu nanofluid, nevertheless independent of concentration, distinctly higher heat transfer coefficient was recorded for stainless steel tube than for copper tube for the same heat flux density.     

CO2 Absorption Using Nanofluids in a Wetted‐Wall Column with External Magnetic Field

A new method for enhancing the mass transfer coefficient in the gas absorption process is reported. CO₂ absorption experiments were carried out in a wetted‐wall column using different aqueous nanofluids as the solvent. The mass transfer characteristics were found to increase by applying Al₂O₃/water nanofluid. The mass transfer coefficient decreased with TiO₂/water nanofluid. In the case of Fe₃O₄/water nanofluid, the mass transfer rate was enhanced by increasing the nanoparticle volume fraction, but the mass transfer coefficient was lower than that obtained with water for all experimental conditions studied. Finally, applying a downward magnetic field resulted in higher mass flux and mass transfer coefficient in comparison with experiments without a magnetic field.     

Lattice Boltzmann simulation of alumina-water nanofluid in a square cavity

A lattice Boltzmann model is developed by coupling the density (D2Q9) and the temperature distribution functions with 9-speed to simulate the convection heat transfer utilizing Al₂O₃-water nanofluids in a square cavity. This model is validated by comparing numerical simulation and experimental results over a wide range of Rayleigh numbers. Numerical results show a satisfactory agreement between them. The effects of Rayleigh number and nanoparticle volume fraction on natural convection heat transfer of nanofluid are investigated in this study. Numerical results indicate that the flow and heat transfer characteristics of Al₂O₃-water nanofluid in the square cavity are more sensitive to viscosity than to thermal conductivity.     

脉冲加热下微加热器在Al2O3纳米流体中的沸腾换热

以Al₂O₃-H₂O纳米流体中的微加热器为研究对象,通过实验方法对脉冲加热条件下微加热器的温度响应曲线和气泡动力学行为进行了详细的研究。比较了在纯水及浓度为0.1%和0.2%的Al₂O₃-H₂O纳米流体中微加热器的温度变化和气泡动力学行为。发现在脉冲加热条件下,微加热器在不同浓度的纳米流体中将出现不同的温度响应曲线,加热膜表面的气泡动力学行为也不相同。实验表明,在脉冲加热条件下,微加热器在Al₂O₃-H₂O纳米流体中的换热效果要明显高于纯水,纳米粒子的浓度对于加热膜表面的气泡动力学行为有明显影响,对微加热器换热的影响也很大。最后根据实验结果以及纳米粒子对气液固三相线的影响,对实验中Al₂O₃纳米流体的换热情况进行了合理的解释。     

Nanoparticles for convective heat transfer enhancement: heat transfer coefficient and the effects of particle size and zeta potential

This work presents a study of using the Wilson Plot method to determine the convective heat transfer coefficient (CHTC) of the following nanoparticles in water as the base fluid: SiO₂, TiO₂, and Al₂O₃. The experiments were carried out in a double layer concentric glass tube in which the hot fluid and nanofluids exchange heat in a counter current fashion without direct contact. Attention was also given to the volumetric concentration, flow rate, and the size of nanoparticles to investigate their effects on CHTC. From the experiments, it was found that by adding nanoparticles, the CHTC of water can generally be enhanced and a 45% increase has been achieved with a 0.5?vol% concentration of Al₂O₃ nanoparticles at an intermediate Reynolds number around 4100. Moreover, simply reducing nanoparticle size and increasing the nanofluid flow rate do not necessarily lead to the CHTC enhancement, rather, they have adverse effects. It is concluded that the enhancement depends on the stability of the dispersed nanoparticles that can be characterized by their overall mean size and zeta potential as useful measures.     

Numerical simulation of natural convection in a square enclosure filled with nanofluid using the two-phase Lattice Boltzmann method

Considering interaction forces (gravity and buoyancy force, drag force, interaction potential force, and Brownian force) between nanoparticles and a base fluid, a two-phase Lattice Boltzmann model for natural convection of nanofluid is developed in this work. It is applied to investigate the natural convection in a square enclosure (the left wall is kept at a high constant temperature (T_H), and the top wall is kept at a low constant temperature (T_C)) filled with Al₂O₃/H₂O nanofluid. This model is validated by comparing numerical results with published results, and a satisfactory agreement is shown between them. The effects of different nanoparticle fractions and Rayleigh numbers on natural convection heat transfer of nanofluid are investigated. It is found that the average Nusselt number of the enclosure increases with increasing nanoparticle volume fraction and increases more rapidly at a high Rayleigh number. Also, the effects of forces on nanoparticle volume fraction distribution in the square enclosure are studied in this paper. It is found that the driving force of the temperature difference has the biggest effect on nanoparticle volume fraction distribution. In addition, the effects of interaction forces on flow and heat transfer are investigated. It is found that Brownian force, interaction potential force, and gravity-buoyancy force have positive effects on the enhancement of natural convective heat transfer, while drag force has a negative effect.     

Numerical investigation of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/water nanofluid laminar convective heat transfer through triangular ducts

In this article, laminar flow-forced convective heat transfer of Al₂O₃/water nanofluid in a triangular duct under constant wall temperature condition is investigated numerically. In this investigation, the effects of parameters, such as nanoparticles diameter, concentration, and Reynolds number on the enhancement of nanofluids heat transfer is studied. Besides, the comparison between nanofluid and pure fluid heat transfer is achieved in this article. Sometimes, because of pressure drop limitations, the need for non-circular ducts arises in many heat transfer applications. The low heat transfer rate of non-circular ducts is one the limitations of these systems, and utilization of nanofluid instead of pure fluid because of its potential to increase heat transfer of system can compensate this problem. In this article, for considering the presence of nanoparticl: es, the dispersion model is used. Numerical results represent an enhancement of heat transfer of fluid associated with changing to the suspension of nanometer-sized particles in the triangular duct. The results of the present model indicate that the nanofluid Nusselt number increases with increasing concentration of nanoparticles and decreasing diameter. Also, the enhancement of the fluid heat transfer becomes better at high Re in laminar flow with the addition of nanoparticles.     

Laminar heat transfer of non-Newtonian nanofluids in a circular tube

Forced convection heat transfer behavior of three different types of nanofluids flowing through a uniformly heated horizontal tube under laminar regime has been investigated experimentally. Nanofluids were made by dispersion of γ-Al₂O₃, CuO, and TiO₂ nanoparticles in an aqueous solution of carboxymethyl cellulose (CMC). All nanofluids as well as the base fluid exhibit shear-thinning behavior. Results of heat transfer experiments indicate that both average and the local heat transfer coefficients of nanofluids are larger than that of the base fluid. The enhancement of heat transfer coefficient increases by increasing nanoparticle loading. At a given Peclet number and nanoparticle concentration the local heat transfer coefficient decreases by axial distance from the test section inlet. It seems that the thermal entry length of nanofluids is greater than the base fluid and becomes longer as nanoparticle concentration increases.     

EXPERIMENTAL INVESTIGATION OF PARAMETERS AFFECTING NANOFLUID EFFECTIVE THERMAL CONDUCTIVITY

The present research reports nanofluid effective thermal conductivity enhancements (ETCE) using an accurate transient short hot wire method system. Preparation of nanofluids was carried out through a two-step method with highly powered pulses similar to that for nanoparticle dispersion in base fluids. Parameters affecting nanofluid heat conductivity such as concentration, sizes, and material of nanoparticle? type of base fluid, temperature, ultrasonic mixing time, and elapsed time after preparation were studied. In the present study, nanoparticles of Al, Al₂O₃, CuO, SnO₂, TiO₂, and SiO₂ with base fluids of water and ethylene glycol were used. Parameters like concentration, size, temperature, and the type of base fluid showed more noticeable effect on the effective thermal conductivity than the others, and mixing time had the least effect. The results showed that any increase in concentration and temperature, and also any decrease in size of nanoparticles and time elapsed after nanofluid preparation, leads to the ETCE of the nanofluid. However, the effects of nanoparticle material, base fluid, and mixing time on thermal conductivity of the nanofluid showed varying trends. Last, a number of mathematical models for prediction of thermal conductivity of nanofluids were applied.     

Intensification of convective heat transfer in water/ethylene glycol based nanofluids containing TiO2 nanoparticles

This paper presents a study of heat transfer performance of water, ethylene glycol (EG) and their mixtures of varying compositions and comparison thereof. The present work demonstrates the enhancement in convective heat transfer in nanofluids. The nanofluids were prepared by adding TiO₂ nanoparticles (having a particle size below 100 nm) in a base fluid. A binary mixture of EG (40%) and water (60%) was used as a base fluid. Nanofluids with varied volume fraction between 0 and 0.5 (volume fraction of TiO₂ nanoparticles) were considered in the present study. The experimental setup used was consisting of a test section that includes 750 mm long copper pipe with 8 mm inner diameter and a heater. The test section was covered with an insulation layer to minimize the heat losses. Temperature measurement was done with thermocouples. The experiments were conducted to study the effects of solid volume fraction, nanofluid flow rate and the inlet temperature on the heat transfer performance of the nanofluids. The results show an enhancement in heat transfer coefficient with increased volume fraction of TiO₂ nanoparticles. The maximum enhancement of 105% in heat transfer coefficient was observed for the nanofluid with solid volume fraction of 0.5.     

Experimental results of nanofluids flow effects on metal surfaces

The results of an experimental study on the effects of the flow of nanofluids (water-based suspensions of nanometer-sized solid particles) on metal surfaces are presented. Either different nanofluids (containing TiO₂, Al₂O₃, ZrO₂, and SiC, respectively) and different target materials (aluminum, copper, stainless steel) have been investigated, under similar operating conditions. Different behaviors were observed depending on the specific combination nanofluid-target material, which in some cases led to severe damaging of the tested target, thus highlighting the need for an adequate preliminary investigation of the possible interactions between the selected nanofluid and the apparatus materials, before its adoption as heat transfer fluid.     

Application of different CFD multiphase models to investigate effects of baffles and nanoparticles on heat transfer enhancement

In this work, the effect of baffles in a pipe on heat transfer enhancement was studied using computational fluid dynamics (CFD) in the presence of Al₂O₃ nanoparticles which are dispersed into water. Fluid flow through the horizontal tube with uniform heat flux was simulated numerically and three dimensional governing partial differential equations were solved. To find an accurate model for CFD simulations, the results obtained by the single phase were compared with those obtained by three different multiphase models including Eulerian, mixture and volume of fluid (VOF) at Reynolds numbers in range of 600 to 3000, and two different nanoparticle concentrations (1% and 1.6%). It was found that multiphase models could better predict the heat transfer in nanofluids. The effect of baffles on heat transfer of nanofluid flow was also investigated through a baffled geometry. The numerical results show that at Reynolds numbers in the range of 600 to 2100, the heat transfer of nanofluid flowing in the geometry without baffle is greater than that of water flowing through a tube with baffle, whereas the difference between these effects (nanofluid and baffle) decreases with increasing the Reynolds number. At higher Reynolds numbers (2100–3000) the baffle has a greater effect on heat transfer enhancement than the nanofluid.     

Investigations of effect of radial flow impeller type swirl generator fitted in an electronic heat sink and Al2O3/water nanofluid on heat transfer enhancement

In this paper experimental investigations on convective heat transfer and pressure drop characteristics in a novel electronic heat sink fitted with dynamic mixers using water and Al₂O₃/water nanofluid are presented. For this, Al₂O₃ nanoparticles of average size 47 nm are synthesized using microwave assisted chemical precipitation method and characterized using XRD and SEM. Heat transfer and pressure drop characteristics of water and nanofluid are studied in the electronic heat sink of base size 60 mm × 60 mm made of copper with and without the dynamic mixer. The enhancements in the heat transfer and pressure drop results are compared with water and the heat sink without any mixer. The results reveal that there is significant increase in the convective heat transfer when the dynamic mixer is used for enhancing the heat transfer and further increase in heat transfer is found for the nanofluid. The rise in pressure drop compared with the increase of heat transfer is less.     

Experimental investigation on low concentration nanofluids with Fresnel lens and evacuated tubes for solar applications

Optical and thermo-physical properties of the nanofluids play a major role in the absorption of solar energy. In the present study, photo-thermal energy conversion of low concentration Al₂O₃/Deionised water (DI) water and CuO/DI water nanofluids in solar thermal collector is experimentally investigated. Properties of 50,100,150, and 200 ppm concentrations of nanofluids are reported. The absorbance results in the visible range indicate that CuO nanofluid of 200 ppm concentration is nearly three times higher compared to Al₂O₃ nanofluid. The extinction coefficient, optical energy band gap, and photoluminescence obtained from the absorbance data are also reported. Surfactant free nanofluids are used, and the thermal conductivity measurements show a negligible enhancement for both the nanofluids. Maximum receiver temperatures of 89 and 72°C are found with CuO and Al₂O₃ nanofluids, respectively, for the maximum concentration. A maximum receiver efficiency of 34.89% is obtained for CuO nanofluids.     

Heat transfer and flow behaviour of aqueous suspensions of titanate nanotubes (nanofluids)

Titanate nanotubes of an aspect ratio of ~ 10 are synthesized, characterised and dispersed in water to form stable nanofluids containing 0.5, 1.0 and 2.5 wt.% of the nanotubes. Experiments are then carried out to investigate the effective thermal conductivity, rheological behaviour and forced convective heat transfer of the nanofluids. The results show a small thermal conductivity enhancement of ~ 3% at 25 °C and ~ 5% at 40 °C for the 2.5 wt.% nanofluid. The nanofluids are found to be non-Newtonian with obvious shear thinning behaviour with the shear viscosity decreasing with increasing shear rate at low shear rates. The shear viscosity approaches constant at a shear rate higher than ~ 100-1000 s^− 1 depending nanoparticle concentration. The high shear viscosity is found to be much higher than that predicted by the conventional viscosity models for dilute suspensions. Despite the small thermal conduction enhancement, an excellent enhancement is observed on the convective heat transfer coefficient, which is much higher than that of the thermal conductivity enhancement. In comparison with nanofluids containing spherical titania nanoparticles under similar conditions, the enhancement of both thermal conductivity and convective heat transfer coefficient of the titanate nanotube nanofluids is considerably higher indicating the important role of particle shape in the heat transfer enhancement. Possible mechanisms are also proposed for the observed enhancement of the convective heat transfer coefficient.     

Experimental and theoretical investigation of thermal conductivity of some water-based nanofluids

Modified transient plane source method has been applied for thermal conductivity measurements of three water-based nanofluids containing Al₂O₃, TiO₂, and graphene nanoparticles. Experiments were conducted at different temperatures and concentrations. The effects of sort of nanoparticles, concentration, and diameter of nanoparticles as well as temperature were studied by comparing the experimental results with the predictions of ten preceding models. The overall performances of these models were compared in terms of percent error. Percent errors were observed in the current study ranging from vicinity of zero up to nearly 110% that belonged to Bruggeman model in predicting the thermal conductivity ratio of graphene/water nanofluids. All ten models performed acceptably in calculating thermal conductivity ratio of Al₂O₃ nanofluids with the maximum percent error of 2.16%. Four correlations are proposed based on the experimental results of this work three of which are special to each nanofluid and the fourth one is overall. These models succeeded to predict the thermal conductivity ratio of the studied nanofluids with considerably lower percent errors which was maximum 5.19% observed in predicting the thermal conductivity ratio of graphene/water nanofluid.     

Al2O3-based nanofluids: a review

Ultrahigh performance cooling is one of the important needs of many industries. However, low thermal conductivity is a primary limitation in developing energy-efficient heat transfer fluids that are required for cooling purposes. Nanofluids are engineered by suspending nanoparticles with average sizes below 100 nm in heat transfer fluids such as water, oil, diesel, ethylene glycol, etc. Innovative heat transfer fluids are produced by suspending metallic or nonmetallic nanometer-sized solid particles. Experiments have shown that nanofluids have substantial higher thermal conductivities compared to the base fluids. These suspended nanoparticles can change the transport and thermal properties of the base fluid. As can be seen from the literature, extensive research has been carried out in alumina-water and CuO-water systems besides few reports in Cu-water-, TiO₂-, zirconia-, diamond-, SiC-, Fe₃O₄-, Ag-, Au-, and CNT-based systems. The aim of this review is to summarize recent developments in research on the stability of nanofluids, enhancement of thermal conductivities, viscosity, and heat transfer characteristics of alumina (Al₂O₃)-based nanofluids. The Al₂O₃ nanoparticles varied in the range of 13 to 302 nm to prepare nanofluids, and the observed enhancement in the thermal conductivity is 2% to 36%.     

纳米流体在芯片微通道中的流动与换热特性

对去离子水及体积分数分别为0.15%和0.26%的水基γ-Al₂O₃纳米流体在当量直径为194.5 μm的硅基梯形芯片微通道内的层流流动和换热特性进行了实验研究。考察了Reynolds数、Prandtl数以及体积分数对流动换热的影响。结果发现,使用纳米流体后,压降无明显增加,纳米流体的流动阻力特性与去离子水基本相同;对流换热Nusselt数较去离子水有明显提高,且随着体积分数的增加而增加;相同泵功下换热热阻显著下降。实验还发现纳米流体的强化传热效果在较高温度时更加明显。根据实验数据得到了梯形硅微通道内低浓度纳米流体的层流对流换热关联式。研究结果对于集成高效芯片散热系统设计具有重要意义。     

Conjugate heat transfer of laminar mixed convection of a nanofluid through an inclined tube with circumferentially non-uniform heating

Laminar mixed convection of a nanofluid consisting of water and Al₂O₃ in an inclined tube with heating at the top half surface of a copper tube has been studied numerically. The bottom half of the tube wall is assumed to be adiabatic (presenting a tube of a solar collector). Heat conduction mechanism through the tube wall is considered. Three-dimensional governing equations with using two-phase mixture model have been solved to investigate hydrodynamic and thermal behaviours of the nanofluid over wide range of nanoparticle volume fractions. For a given nanoparticle mean diameter the effects of nanoparticle volume fractions on the hydrodynamics and thermal parameters are presented and discussed at different Richardson numbers and different tube inclinations. Significant augmentation on the heat transfer coefficient as well as on the wall shear stress is seen.     

A review on boiling heat transfer enhancement with nanofluids

There has been increasing interest of late in nanofluid boiling and its use in heat transfer enhancement. This article covers recent advances in the last decade by researchers in both pool boiling and convective boiling applications, with nanofluids as the working fluid. The available data in the literature is reviewed in terms of enhancements, and degradations in the nucleate boiling heat transfer and critical heat flux. Conflicting data have been presented in the literature on the effect that nanofluids have on the boiling heat-transfer coefficient; however, almost all researchers have noted an enhancement in the critical heat flux during nanofluid boiling. Several researchers have observed nanoparticle deposition at the heater surface, which they have related back to the critical heat flux enhancement.