Stability and thermophysical properties of fly ash nanofluid for heat transfer applications

Improving the performance of heat transfer fluids is altogether significant. The best approach for improving the thermal conductivity is the addition of nanoparticles to the base fluid. In the present study, specific heat, dynamic viscosity, and thermal conductivity of water-based Indian coal fly ash stable nanofluid for 0.1% to 0.5% volume concentration in the temperature range of 30 to 60°C has been investigated. To evaluate an average particle diameter of 11.5 nm, the fly ash nanoparticles were characterized with scanning electron microscopy and dynamic light scattering. Using zeta potential, the stability of nanofluid in the presence of surfactant Triton X-100 was tested. Thermal conductivity and viscosity of fly ash nanofluid increased, while specific heat decreased as volume concentration increased. The effect of temperature on the fly ash nanofluid was directly proportional to its thermal conductivity and specific heat and inversely proportional to viscosity.     

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.     

Investigation of thermal conductivity and viscosity of Fe3O4 nanofluid for heat transfer applications

Experimental investigations and theoretical determination of effective thermal conductivity and viscosity of magnetic Fe₃O₄/water nanofluid are reported in this paper. The nanofluid was prepared by synthesizing Fe₃O₄ nanoparticles using the chemical precipitation method, and then dispersed in distilled water using a sonicator. Both experiments were conducted in the volume concentration range 0.0% to 2.0% and the temperature range 20 °C to 60 °C. The thermal conductivity and viscosity of the nanofluid were increased with an increase in the particle volume concentration. Viscosity enhancement was greater compared to thermal conductivity enhancement under at same volume concentration and temperature. Theoretical equations were developed to predict thermal conductivity and viscosity of nanofluids without resorting to the well established Maxwell and Einstein models, respectively. The proposed equations show reasonably good agreement with the experimental results.     

Experimental investigation and development of new correlation for thermal conductivity and viscosity of BioGlycol/water based SiO2 nanofluids

Nanofluids are a new class of engineered heat transfer fluids which exhibit superior thermophysical properties and have potential applications in numerous important fields. In this study, nanofluids have been prepared by dispersing SiO₂ nanoparticles in different base fluids such as 20:80% and 30:70% by volume of BioGlycol (BG)/water (W) mixtures. Thermal conductivity and viscosity experiments have been conducted in temperatures between 30 °C and 80 °C and in volume concentrations between 0.5% and 2.0%. Results show that thermal conductivity of nanofluids increases with increase of volume concentrations and temperatures. Similarly, viscosity of nanofluid increases with increase of volume concentrations but decreases with increase of temperatures. The maximum thermal conductivity enhancement among all the nanofluids was observed for 20:80% BG/W nanofluid about 7.2% in the volume concentration of 2.0% at a temperature of 70 °C. Correspondingly among all the nanofluids maximum viscosity enhancement was observed for 30:70% BG/W nanofluid about 1.38-times in the volume concentration of 2.0% at a temperature of 70 °C. The classical models and semi-empirical correlations failed to predict the thermal conductivity and viscosity of nanofluids with effect of volume concentration and temperatures. Therefore, nonlinear correlations have been proposed with 3% maximum deviation for the estimation of thermal conductivity and viscosity of nanofluids.     

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.     

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.     

Latest developments on the viscosity of nanofluids

The past decade has seen the rapid development of nanofluids science in many aspects. Number of research is conducted that is mostly focused on the thermal conductivity of these fluids. However, nanofluid viscosity also deserves the same attention as thermal conductivity. In this paper, different characteristics of viscosity of nanofluids including nanofluid preparation methods, temperature, particle size and shape, and volume fraction effects are thoroughly compiled and reviewed. Furthermore, a precise review on theoretical models/correlations of conventional models related to nanofluid viscosity is presented. The existing experimental results about the nanofluids viscosity show clearly that viscosity augmented accordingly with an increase of volume concentration and decreased with the temperature rise. However, there are some contradictory results on the effects of temperature on viscosity. Moreover, it is shown that particle size has some noteworthy effects over viscosity of nanofluids.     

Heat transfer and friction factor of water based TiO2 and SiO2 nanofluids under turbulent flow in a tube

The heat transfer coefficient and friction factor of TiO₂ and SiO₂ water based nanofluids flowing in a circular tube under turbulent flow are investigated experimentally under constant heat flux boundary condition. TiO₂ and SiO₂ nanofluids with an average particle size of 50 nm and 22 nm respectively are used in the working fluid for volume concentrations up to 3.0%. Experiments are conducted at a bulk temperature of 30 °C in the turbulent Reynolds number range of 5000 to 25,000. The enhancements in viscosity and thermal conductivity of TiO₂ are greater than SiO₂ nanofluid. However, a maximum enhancement of 26% in heat transfer coefficients is obtained with TiO₂ nanofluid at 1.0% concentration, while SiO₂ nanofluid gave 33% enhancement at 3.0% concentration. The heat transfer coefficients are lower at all other concentrations. The particle concentration at which the nanofluids give maximum heat transfer has been determined and validated with property enhancement ratio. It is observed that the pressure drop is directly proportional to the density of the nanoparticle.     

An experimental study on the thermal conductivity and dynamic viscosity of TiO2-SiO2 nanofluids in water: Ethylene glycol mixture

The hybrid nanofluid has been thriving among researchers due to its potential to improve heat transfer performance. Therefore, various studies on heat transfer properties need to be carried out to provide a better understanding on hybrid nanofluid performance. In this paper, the experimental work is focused on the thermal conductivity and dynamic viscosity of TiO₂-SiO₂ nanofluids in a mixture of water and ethylene glycol (EG) with volume ratio of 60:40. The stable suspension of TiO₂-SiO₂ prepared at volume concentrations of 0.5 to 3.0%. The measurements of thermal conductivity and dynamic viscosity were performed at a temperature range of 30 to 80 °C by using KD2 Pro Thermal Properties Analyser and Brookfield LVDV III Ultra Rheometer, respectively. The thermal conductivity of TiO₂-SiO₂ nanofluids was improved by increasing the volume concentration and temperature with 22.8% maximum enhancement. Besides, the viscosity of TiO₂-SiO₂ nanofluids showed evidence of being influenced by nanofluid concentration and temperature. Additionally, the TiO₂-SiO₂ nanofluids behaved as a Newtonian fluid for volume concentration up to 3.0%. The properties enhancement ratio suggested that TiO₂-SiO₂ nanofluids will aid in heat transfer for concentrations of more than 1.5% and within the range of the temperature studied. A new correlation for thermal conductivity and dynamic viscosity of TiO₂-SiO₂ nanofluids were developed and found to be precise.     

Experimental investigation of the thermal transport properties of graphene oxide/Co3O4 hybrid nanofluids

The in-situ growth and chemical co-precipitation method was used for the synthesis of uniform dispersion of Co₃O₄ nanoparticles on the graphene oxide (GO) nanosheet. The reductions of aqueous cobalt chloride in the presence of GO with sodium borohydrate result in the formation of hybrid GO/Co₃O₄ nanoparticles. The synthesized GO/Co₃O₄ nanoparticles were characterized using X-ray power diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). The hybrid nanofluids were prepared by dispersing synthesized GO/Co₃O₄ nanoparticles in water, ethylene glycol, and ethylene glycol/water mixtures. The properties such as thermal conductivity and viscosity were estimated experimentally at different volume concentrations and temperatures. The thermal conductivity enhancement of water-based nanofluid is 19.14% and ethylene glycol-based nanofluid is 11.85% at 0.2% volume concentration and at a temperature of 60 °C respectively compared to their respective base fluids. Similarly, the viscosity enhancement of water-based nanofluid is 1.70-times and ethylene glycol-based nanofluid is 1.42-times at 0.2% volume concentration and at a temperature of 60 °C respectively. The obtained thermal conductivity and viscosity data is compared with the literature values.     

Viscosity and thermal conductivity of dispersions of sub-micron TiO2 particles in water prepared by stirred bead milling and ultrasonication

Experiments were carried out on the preparation of dispersions of sub-micron TiO₂ particles in water by stirred bead milling, for potential use as coolants. The prepared dispersions were characterized through the measurement of particle size distribution, zeta potential, viscosity and thermal conductivity. The effects of particle concentration (0.27–1.39 vol%), ultrasonication time (0–7 h) on viscosity and thermal conductivity have been studied. The effect of temperature (29–55 °C) on viscosity has also been investigated. The results indicate that the ultrasonication can be utilized to tailor the transport properties of the sub-micron dispersions produced by stirred bead milling. The entire particle size distribution data has been utilized to develop correlations for prediction of relative viscosity and thermal conductivity ratio of these dispersions. These dispersions possess higher thermal conductivity than water and can also be utilized as coolants.     

Experimental investigation on the thermal conductivity and shear viscosity of viscoelastic-fluid-based nanofluids

Viscoelastic-fluid-based nanofluids with dispersion of copper (Cu) nanoparticles in viscoelastic surfactant solution (aqueous solution of cetyltrimethylammonium chloride/sodium salicylate) were prepared. A comparative study of thermal conductivity and viscosity between viscoelastic-fluid-based Cu nanofluids and distilled water based nanofluids was then performed experimentally. Different concentrations of viscoelastic base fluid and volume fraction of Cu nanoparticles were matched in order to check their influences on fluid’s thermal conductivity and viscosity. The experimental results show that the viscoelastic-fluid-based Cu nanofluids have a higher thermal conductivity than viscoelastic base fluid, and its thermal conductivity increases with increasing temperature and increasing particle volume fraction. Furthermore, the viscoelastic-fluid-based Cu nanofluid shows a non-Newtonian behavior in its viscosity, and the viscosity increases with the increase of Cu nanoparticle concentration and decrease of temperature.     

Optimal characteristics and heat transfer efficiency of SiO2/water nanofluid for application of energy devices: A comprehensive study

In a comprehensive study, the thermal conductivity, dynamic viscosity, and the rheological behavior of a SiO₂/water nanofluid are investigated experimentally at the temperatures, solid concentrations, and the shear rates of 25°C to 50°C, 0% to 1.5%, and 400 to 1400(s^?1), respectively. The Response Surface Methodology (RSM) is utilized to obtain regression models for the thermal conductivity and the dynamic viscosity. Subsequently, the sensitivity of the aforementioned models to 10% changes in the temperature, and the nanofluid concentration is analyzed. Afterward, Nondominated Sorting Genetic Algorithm II (NSGA‐II) is utilized to find the maximum thermal conductivity and the minimum viscosity. The nondominated optimal points are presented through a fitted correlation on a Pareto front to make the results more practical. The measurements of the investigated nanofluid could be summarized as a paper of a handbook. The workability of the investigated nanofluid is also examined in both laminar and turbulent flow regimes through analysis of the heat transfer merit graphs. To this end, the ratio of the dynamic viscosity enhancement to the thermal conductivity enhancement and the Mouromtseff number are chosen as two criteria of the laminar and turbulent flow regimes, respectively. Finally, the results are compared with those for SiO₂/glycerin and SiO₂/ethylene glycol nanofluids to check the workability in different base fluids. From a thermal‐efficiency point of view, the SiO₂/water nanofluid is not suggested for use in both laminar and turbulent pipe flows, except in temperatures higher than 30°C and volume concentrations lower than 1% for the case of laminar flow. This is because the favorable heat transfer enhancement of the nanofluid is more than the unfavorable increase of the pumping power. From the rheological point of view, though, a SiO₂/water nanofluid would be a good choice in lubricating moving surfaces for both laminar and turbulent flow regimes. It is found that in higher nanofluid concentrations, the thermal conductivity of a SiO₂/water nanofluid is highly influenced by temperature. Moreover, adding nanoparticles at temperatures of 35°C to 40°C would have the highest increasing effect on the thermal conductivity. It is also revealed that increasing the temperature does not significantly affect the viscosity when 1% SiO₂ nanoparticles are suspended within the water.     

Experimental thermal conductivity of ethylene glycol and water mixture based low volume concentration of Al2O3 and CuO nanofluids

Thermal conductivity of ethylene glycol and water mixture based Al₂O₃ and CuO nanofluids has been estimated experimentally at different volume concentrations and temperatures. The base fluid is a mixture of 50:50% (by weight) of ethylene glycol and water (EG/W). The particle concentration up to 0.8% and temperature range from 15 °C–50 °C were considered. Both the nanofluids are exhibiting higher thermal conductivity compared to base fluid. Under same volume concentration and temperature, CuO nanofluid thermal conductivity is more compared to Al₂O₃ nanofluid. A new correlation was developed based on the experimental data for the estimation of thermal conductivity of both the nanofluids.     

Experimental determination of thermal conductivity and dynamic viscosity of Ag–MgO/water hybrid nanofluid

The main goal of this experimental work is to investigate the effect of nanoparticle volume fraction on thermal conductivity and dynamic viscosity of Ag–MgO/water hybrid nanofluid with the particle diameter of 40(MgO) and 25(Ag) nm and nanoparticle volume fraction (50% Ag and 50% MgO by volume) range between 0% and 2% and presenting new correlations. Several existing theoretical and empirical correlations for thermal conductivity (four correlations) and dynamic viscosity (five correlations) of nanofluids have been examined for their accuracy in predicting the value of thermodynamics properties by comparing the predicted values with experimental data. The examined correlations were found to present inaccuracies (under predictions) in the range of nanoparticle volume fraction under study. Predictions of the new developed correlations by comparing the predicted values with experimental data showed that the new correlations are within a very good accuracy.     

An experimental study on thermal conductivity of MgO nanoparticles suspended in a binary mixture of water and ethylene glycol

In the present study, the effects of solid volume fraction and temperature on the thermal conductivity of MgO/water–EG (60:40) nanofluid are discussed. Samples of nanofluid are provided by two step method at different solid concentrations, including 0.1%, 0.2%, 0.5%, 0.75%, 1%, 1.5%, 2% and 3%. The experiments are performed for different temperatures ranging from 20 to 50 °C, using KD2 pro thermal analyzer which employed transient hot wire to measure thermal conductivity. The finding shows that thermal conductivity of nanofluid increases with increasing solid volume fraction or temperature. Based on the experimental data, new correlation for modeling the thermal conductivity of MgO/water–EG (60:40) for different solid volume fractions and temperatures was proposed.     

Numerical Investigations on the Advantage of Nanofluids under DDMC in a Lid‐Driven Cavity

Double diffusive mixed convection in a lid‐driven cavity filled with Cu–water nanofluid is studied in detail. Various numerical experiments are conducted under horizontal thermal and concentration gradients. Flow equations were solved in velocity vorticity form using Galerkin's weighted residual finite element method. The Maxwell‐Garnett model and Brinkman models are applied to predict the thermal conductivity and dynamic viscosity of the nanofluid, respectively. The effectiveness of a nanofluid on heat transfer enhancement with respect to change in Richardson number has been studied at different Reynolds numbers for variation in particle volume fraction from 0 to 0.05. Similarly, the effect of buoyancy ratio on heat and mass transfer is presented for buoyancy ratio in the range of ?25 to 25. Detailed contour plots comparing the streamlines, temperature, concentration with and without nanoparticles were presented for all the range of parameters considered. The role of particle concentration and change in type of nanofluid has been reported. The average Nusselt number has increased in all the cases where as the Sherwood number slightly decreased with an increase in particle volume fraction. The Ag–water nanofluid showed better improvement in heat transfer characteristics compared to other nanofluids for all Reynolds numbers and particle volume fractions.     

Viscosity and thermal conductivity comparative study for hybrid nanofluid in binary base fluids

Thermal conductivity and viscosity analysis of Al₂O ₃/CuO (50/50) hybrid nanofluid in various mass fractions of ethylene glycol (EG) and propylene glycol (PG) binary base fluid have been investigated in the present work. Hybrid nanofluid with vol. fraction range limited to 1.5% and within the higher temperature range of 50°C to 70°C is considered for thermal conductivity and viscosity analysis. Impact on viscosity and conductivity models with various shape nanoparticles, i.e, spherical, cylindrical, brick, platelets, and blades have been discussed and were compared in EG and PG binary base fluids. Also, the analysis extends to the prediction for the stability with zeta potential and synthesis of spherical shape Al₂O₃/CuO hybrid nanofluid with X‐ray diffraction (XRD) and scanning electron microscope (SEM). The theoretical analysis revealed that thermal conductivity of Al₂O₃/CuO hybrid nanofluid in EG binary base fluid is lower compared to in PG binary base fluid. The thermal conductivity is observed to be higher in spherical and cylindrical shape nanoparticle compared to bricks, blades, and platelets shape nanoparticles. Optimum viscosity of Al₂O₃/CuO hybrid nanofluid is observed at 50%EG and 30%PG of the binary base fluid. Hybrid nanofluid in 30% of PG as binary base fluid results 16.2% higher dynamic viscosity compared to pure PG base fluid for a volume concentration of 2%. Zeta potential measurement results in the stability of spherical Al₂O₃‐CuO/ (50/50) EG/W hybrid nanofluid, and it may be considered as a heat transfer fluid.     

Temperature-dependent aggregation and diffusion in nanofluids

While particle aggregates play a central role in recent models for nanofluid thermal conductivity, the effect of particle diffusion in a temperature field on the aggregation and transport has yet to be studied in depth. The present work separates the effects of particle aggregation and diffusion using parallel plate experiments, infrared microscopy, Monte Carlo simulations, and rate equations for particle and heat transport. The predicted thermal conductivity and viscosity enhancements are compared to determine the favorability of aggregating nanofluids. Experimental data show non-uniform temporal increases in thermal conductivity and are well described through simulation of the combination of particle aggregation and diffusion. The simulation shows concentration distributions due to thermal diffusion causing variations in aggregation, thermal conductivity and viscosity. The aggregation produces an unfavorable nanofluid. An optimum nanoparticle diameter is calculated to minimize settling, thermal diffusion and aggregation.     

The effect of alumina/water nanofluid particle size on thermal conductivity

This study examines the effect of particle size, temperature, and weight fraction on the thermal conductivity ratio of alumina(Al₂O₃)/water nanofluids. A Al₂O₃/water nanofluid produced by the direct synthesis method served as the experimental sample, and nanoparticles, each of a different nominal diameter (20, 50, and 100 nm), were dispersed into four different concentrations (0.5, 1.0, 1.5, and 2.0 wt%). This experiment measured the thermal conductivity of nanofluids with different particle sizes, weight fractions, and working temperatures (10, 30, 50 °C). The results showed a correlation between high thermal conductivity ratios and enhanced sensitivity, and small nanoparticle size and higher temperature. This research utilized experimental data to construct a new empirical equation, taking the nanoparticle size, temperature, and lower weight fraction of the nanofluid into consideration. Comparing the regression results with the experimental values, the margin of error was within ?3.5% to +2.7%. The proposed empirical equation showed reasonably good agreement with our experimental results.