Experimental study on the thermo-physical properties of car engine coolant (water/ethylene glycol mixture type) based SiC nanofluids

The engine coolant (water/ethylene glycol mixture type) becomes one of the most commonly used commercial fluids in cooling system of automobiles. However, the heat transfer coefficient of this kind of engine coolant is limited. The rapid developments of nanotechnology have led to emerging of a relatively new class of fluids called nanofluids, which could offer the enhanced thermal conductivity (TC) compared with the conventional coolants. The present study reports the new findings on the thermal conductivity and viscosity of car engine coolants based silicon carbide (SiC) nanofluids. The homogeneous and stable nanofluids with volume fraction up to 0.5 vol.% were prepared by the two-step method with the addition of surfactant (oleic acid). It was found that the thermal conductivity of nanofluids increased with the volume fraction and temperature (10–50 °C), and the highest thermal conductivity enhancement was found to be 53.81% for 0.5 vol.% nanofluid at 50 °C. In addition, the overall effectiveness of the current nanofluids (0.2 vol.%) was found to be ~ 1.6, which indicated that the car engine coolant-based SiC nanofluid prepared in this paper was better compared to the car engine coolant used as base liquid in this study.     

Enhancement of thermal conductivity with Cu for nanofluids using chemical reduction method

The enhancement of the thermal conductivity of water in the presence of copper (Cu) using the chemical reduction method is presented in this study. It is the first time that the chemical reduction method for synthesis of nanofluids containing Cu nanoparticles in water is reported. No surfactant is employed as the dispersant. The volume concentration of Cu–water nanofluids is below 0.2 vol.%. Without the addition of dispersant and surfactant, the thermal conductivity of the produced nanofluids reveals a time-dependent characteristic. The thermal conductivity is the largest at the starting point of measurement and decreases considerably with elapsed time. The results show that Cu–water nanofluids with low concentration of nanoparticles have noticeably higher thermal conductivities than the water base fluid without Cu. For Cu nanoparticles at a volume fraction of 0.001 (0.1 vol.%), thermal conductivity was enhanced by up to 23.8%.     

Simultaneous Measurement of Thermal Conductivity,Thermal Diffusivity,and Specific Heat of Nanofluids

Effective thermal conductivity, effective thermal diffusivity, and effective specific heat of nanofluids were simultaneously measured by using a transient double hot-wire technique. Several types of nanofluids were prepared by suspending different volume percentages (1 to 5%) of titanium dioxide (TiO₂), aluminum oxide (Al₂O₃), and aluminum (Al) nanoparticles in ethylene glycol and engine oil. While effective specific heats of these nanofluids decrease substantially with nanoparticle volume fraction, the enhanced effective thermal conductivity and effective thermal diffusivity were found to increase significantly with increasing volumetric loading of these nanoparticles. The increments of the effective thermal diffusivity of nanofluids were slightly larger than their effective thermal conductivity values. Predictions of the effective specific heats of nanofluids by the volume fraction mixture rule-based model showed fairly good agreement (within 7%) with the experimental results. Besides particle volume fraction, particle material, particle shape and the type of base fluid were identified to have influence on these properties of nanofluids. Both the calibration results of the base fluids (system accurate to ≤2.7%) and uncertainty analysis (uncertainty ≤2.1%) indicate high accuracy of using the double hot-wire method to simultaneously measure the effective thermal conductivity, effective thermal diffusivity, and specific heat of nanofluids.     

Thermal conductivity of ethylene glycol and water mixture based Fe3O4 nanofluid

Thermal conductivity of ethylene glycol and water mixture based Fe₃O₄ nanofluid has been investigated experimentally. Magnetic Fe₃O₄ nanoparticles were synthesized by chemical co-precipitation method and the nanofluids were prepared by dispersing nanoparticles into different base fluids like 20:80%, 40:60% and 60:40% by weight of the ethylene glycol and water mixture. Experiments were conducted in the temperature range from 20 °C to 60 °C and in the volume concentration range from 0.2% to 2.0%. Results indicate that the thermal conductivity increases with the increase of particle concentration and temperature. The thermal conductivity is enhanced by 46% at 2.0 vol.% of nanoparticles dispersed in 20:80% ethylene glycol and water mixture compared to other base fluids. The theoretical Hamilton–Crosser model failed to predict the thermal conductivity of the nanofluid with the effect of temperature. A new correlation is developed for the estimation of thermal conductivity of nanofluids based on the experimental data.     

Influence of CuO nanoparticles in enhancing the thermal conductivity of water and monoethylene glycol based nanofluids

In this paper the effect of CuO nanoparticles on the thermal conductivity of base fluids like mono ethylene glycol and water was studied. Both the base fluids showed enhancement in effective thermal conductivity. This enhancement was investigated with regard to various factors; concentration of nanoparticles, types of base fluids, sonication time and settlement time. For both the base fluids, an improvement in thermal conductivity was found as concentration of nanoparticles increased due to interaction between particles. It was also found that as the sonication time was increased, there was furthermore an improvement in the thermal conductivity of the base fluids. Effect of base fluids is the complex idea to understand. Lower base fluid's viscosities are supposed to contribute grater enchantment, but another factor of fluid nanoparticles surface interaction also more important. The experimentally measured thermal conductivities of base fluid's nanoparticles suspension were compared to a variety of models (Maxwell, Hamilton–Crosser and Bruggeman Model). It is observed that none of the mentioned models were found to predict accurately the thermal conductivities of nanofluids.     

Measurement and model on thermal conductivities of carbon nanotube nanorefrigerants

The objective of this study is to test thermal conductivity characteristics of CNT nanorefrigerants and to build a model for predicting the thermal conductivities of CNT nanorefrigerants. The influences of CNT diameters and CNT aspect ratios on nanorefrigerant's thermal conductivity were reflected in the experiments, and R113 was used as the host refrigerant for the convenience of the experiments. The experimental results show that the thermal conductivities of CNT nanorefrigerants are much higher than those of CNT–water nanofluids or spherical-nanoparticle-R113 nanorefrigerants. Experiments also show that the smaller the diameter of CNT is or the larger the aspect ratio of CNT is, the larger the thermal conductivity enhancement of CNT nanorefrigerant is. The existent models for predicting thermal conductivity of CNT nanofluid, including Hamilton–Crosser model, Yu–Choi model and Xue model, were verified by the experimental data of CNT nanorefrigerants' thermal conductivities. The verification shows that Yu–Choi model has the mean deviation of 15.1% and it is more accurate than the other two models. A modified Yu–Choi model was presented by improving the empirical constant of Yu–Choi model, and the mean deviation of the modified Yu–Choi model from the experimental results is 5.5%.     

Experimental investigation of the thermophysical properties of AL2O3-nanofluid and its effect on a flat plate solar collector

Experimental investigations have been carried out for obtaining the thermophysical properties of 60:40 (by mass) ethylene glycol/water mixture and water based alumina nanofluids. The effect of density and viscosity on the pumping power for flat plate solar collector has been investigated as well. Nanofluids of 0.05–0.1%v/v concentrations were prepared and characterized. Water based alumina nanofluids were found more preferable against sedimentation and aggregation than ethylene glycol/water mixture based nanofluids. The measured thermal conductivities of both types of the nanofluids increased almost linearly with concentration and are consistent in their overall trend with previous works done at lower concentrations by different researchers. In contrast to thermal conductivity, viscosity measurements showed that the viscosity of the Al₂O₃–water nanofluids exponentially decreases with increasing temperature. Furthermore, the measured viscosities of the Al₂O₃–water nanofluids showed a non-linear relation with concentration even in the low volume concentration except 0.05%v/v at below 40 °C. On the other hand, Al₂O₃–EG/water mixture exhibited Newtonian behavior. Existence of a critical temperature was observed beyond which the particle suspension properties altered drastically, which in turn triggered a hysteresis phenomenon. The hysteresis phenomenon on viscosity measurement, which is believed to be the first observed for EG/water-based nanofluids, has raised serious concerns regarding the use of nanofluids for heat transfer enhancement purposes. Results suggest that nanofluids can be used as a working medium with a negligible effect of enhanced viscosity and/or density. Results also show that the pressure drop and pumping power of the nanofluid flows are very close to that of the base liquid for low volume concentration.     

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.     

Synthesis,characterization, and thermal property measurement of nano-Al95Zn05 dispersed nanofluid prepared by a two-step process

Nanofluids are stable suspension of nanometer sized particles and exhibit extremely attractive thermal properties that make them a potential candidate for application in heat transfer devices ranging from microelectronic gadgets to thermal power plants. In the present study, we have synthesized Al-5wt%Zn nanoparticles by mechanical alloying, characterized these nanoparticles using X-ray diffraction and scanning and transmission electron microscopy. Subsequently, these nanoparticles are dispersed to the tune of 0.01–0.10 vol% in ethylene glycol (base fluid) following a careful mixing protocol. Thermal conductivity of the nanofluids and base fluid has been measured using the transient hot-wire method. It is observed that thermal conductivity of the nanofluids strongly depend on the concentration, particle size, fluid temperature and stability of dispersed nanoparticles in the base fluid. A maximum of 16% enhancement in thermal conductivity has been recorded at a nanoparticle loading of 0.10 vol%. Unlike data reported in some articles, thermal conductivity ratio of Al-5wt%Zn dispersed ethylene glycol based nanofluids is observed to decrease with the increase in crystallite/grain size of the particles.     

Influence of carbon nanotubes on heat transfer in MHD nanofluid flow over a stretchable rotating disk: A numerical study

The transfer of heat is an important phenomenon in the several areas due to its numerous applications in industries. Several fluids like water, ethylene glycol and oil, and so on have very‐low thermal conductivities due to which the transfer of heat in these fluids become very low. To enhance heat transfer rate, carbon nanotubes (CNTs) including single‐walled CNTs and multi‐walled CNTs are suspended into base fluids, this mixture is known as nanofluid. The aim of this study is to examine the heat transfer rate of nanofluid in the presence of CNTs over a stretchable rotating disk. The mathematical model, developed by Tiwari and Das, is used and solved numerically by using the shooting method. The impacts of governing constraints on the dimensionless velocities, temperature, skin friction, and Nusselt number are investigated. It is noted that heat transfer rate increases by enhancing the concentration of CNTs into base fluids. The numerical results show that the solid volume fraction of the CNTs augment heat transfer rate more in ethylene glycol as compared with water.     

Analysis of entropy generation using nanofluid flow through the circular microchannel and minichannel heat sink

In this paper, different types of entropy generations in the circular shaped microchannel and minichannel are discussed analytically using different types of nanoparticles and base fluids. In this analysis, Copper (Cu), alumina (Al₂O₃) as the nanoparticle and H₂O, ethylene glycol (EG) as the base fluids were used. The volume fractions of the nanoparticles were varied from 2% to 6%. In this paper, the irreversibility or entropy generation analysis as the function of entropy generation ratio, thermal entropy generation rate and fluid friction entropy generation rate for these types of nanofluids in turbulent flow condition have been analyzed using available correlations. Cu–H₂O nanofluid showed the highest decreasing entropy generation rate ratio (36%) compared to these nanofluids flow through the microchannel at 6 vol.%. The higher thermal conductivity of H₂O causes to generate much lower thermal entropy generation rate compared to the EG base fluid. The fluid friction entropy generation rate decreases fruitfully by the increasing of volume fraction of the nanoparticles. Cu–H₂O and Cu–EG nanofluid gave the maximum decreasing rates of the fluid friction entropy generation rate are 38% and 35% respectively at 6% volume fraction of the nanoparticles. Smaller diameter showed less entropy generation in case of all nanofluids.     

Enhancing thermal conductivity of palmitic acid based phase change materials with carbon nanotubes as fillers

Multi-walled carbon nanotubes (CNTs) as produced are usually entangled and not ready to be dispersed into organic matrix. CNTs were treated by mechano-chemical reaction with ball milling the mixture of potassium hydroxide and the pristine CNTs. Hydroxide radical functional groups have been introduced on the CNT surfaces, which enabled to make stable and homogeneous CNT composites. Treated CNTs were successfully dispersed into the palmitic acid matrix without any surfactant. Transient short-hot-wire apparatus was used to measure the thermal conductivities of these nanotube composites. Nanotube composites have substantially higher thermal conductivities than the base palmitic acid matrix, with the enhancement increasing with the mass fraction of CNTs in both liquid state and solid state. The enhancements of the thermal conductivity are about 30% higher than the reported corresponding values for palmitic acid based phase change nanocomposites containing 1 wt% CNTs treated by concentrated acid mixture.     

Investigation of Thermal Conductivity and Viscosity of Carbon Nanotubes–Ethylene Glycol Nanofluids

Conventional fluids used for heat transfer applications in automobiles limit the performance enhancement and compactness of the heat exchangers. These problems can be overcome by using the technology of nanofluids. The objectives of this work are to prepare nanofluids and to study their dynamic viscosity and thermal conductivity. Chemically treated carbon nanotubes (CNTs) were added with ethylene glycol (EG) and sonicated using a bath sonicator to have a homogeneous dispersion of CNTs in EG. In this study, the nanofluids were prepared with different concentrations of CNTs varying from 0.12 to 0.4 wt%. The dynamic viscosity of nanofluids was measured using a rheometer over a temperature range of 25°C to 60°C. It was observed that the viscosity of nanofluids decreases with an increase of temperature and enhances with CNT concentration. The nanofluid follows the characteristic behavior of Newtonian fluids. A linear rise in thermal conductivity of ethylene glycol was observed with an increase of CNT concentration. It is concluded that EG–CNT nanofluids are promising to meet the challenges required by automobile systems.     

Experimental studies on thermal conductivity of blends of ethylene glycol-water-based TiO2 nanofluids

A set of three nanofluids of different blends were prepared with ethylene glycol–water and TiO₂ nanoparticles and are characterized for thermal conductivity as a function of temperature and volume concentration of nanoparticles. The measurements were taken in the temperature range from 30 °C to 70 °C, which happens to be most widely used range of temperature for many cooling applications in heat transfer equipment. Nanofluids were prepared by dispersing the nanoparticles in base fluids such as (1) water, (2) ethylene glycol plus water in the ratio of 40%:60% and 3) ethylene glycol plus water in the ratio of 50%:50% by weight. Based on the experimental results, it is observed that the thermal conductivity of TiO₂ nanofluids, considered in the present investigation, increases with increase in percentage of volume concentration of TiO₂ and also with temperature. Current experimental investigation presents valuable data on the measured thermal conductivity of TiO₂ nanofluids for very low volume concentrations from 0.2% to 1.0% of nanoparticles in the temperature range of 30 °C–70 °C.     

Enhanced thermal conductivity of ethylene glycol with single-walled carbon nanotube inclusions

In the present work, we report measurements of the effective thermal conductivity of dispersions of single-walled carbon nanotube (SWNT) suspensions in ethylene glycol. The SWNTs were synthesized using the alcohol catalytic chemical vapour deposition method. Resonant Raman spectroscopy was employed to estimate the diameter distribution of the SWNTs based on the frequencies of the radial breathing mode peaks. The nanofluid was prepared by dispersing the nanotubes using a bile salt as the surfactant. Nanotube loading of up to 0.2 vol% was used. Thermal conductivity measurements were performed by the transient hot-wire technique. Good agreement, within an uncertainty of 2%, was found for published thermal conductivities of the pure fluids. The enhancement of thermal conductivity was found to increase with respect to nanotube loading. The maximum enhancement in thermal conductivity was found to be 14.8% at 0.2 vol% loading. The experimental results were compared with literature results in similar dispersion medium. Experimental results were compared with the Hamilton–Crosser model, the Lu–Lin model, Nan’s effective medium theory and the Hashin–Shtrikman model. Effective medium theory seems to predict the thermal conductivity enhancement reasonably well compared to rest of the models. Networking of nanotubes to form a tri-dimensional structure was considered to be the reason for the thermal conductivity enhancement.     

Viscosity and conductivity measurements for dilute dispersions of rodlike paraffin particles in silicone oil

In the present article, the rheological behavior and thermal conductivity for dispersions of paraffin rods in silicone oil were investigated experimentally by examining the effects of surfactant concentration, particle volume concentration and temperature. The test suspensions at 15°C showed a strong shear-thinning viscosity as the particle concentration increased, whereas the test emulsions at 58°C showed typical Newtonian viscosity behavior. Relative viscosities of the present results agree with the theoretical results for rodlike particle suspensions. Thermal conductivities of the paraffin, silicon oil and mixture were measured with varying temperatures, respectively. Thermal conductivities of paraffin dispersions correlated satisfactorily with the Brailsford-Major equation.     

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.     

Convective heat transfer of Titania nanofluids of different base fluids in cylindrical annulus with discrete heat source

In the current work, the hydrodynamic and thermal characteristics of Titania nanofluids filling a cylindrical annulus are numerically investigated. Ethylene glycol, engine oil, and water are used as base fluids. The Maxwell model for convective heat transfer in nanofluids is followed to account for the effects of nanoparticle volume fraction distribution on the continuity, momentum, and energy equations, in which a developed computer code is used. The latter is based upon the finite volume method coupled with the SIMPLER algorithm. Numerical results for the heat transfer are presented in the form of streamlines and isotherm profiles for a different value of Rayleigh number, base fluid, and nanoparticle volume fraction. The effects of these parameters on the local Nusselt number are analyzed.     

Molecular dynamics simulation of nanofluid’s effective thermal conductivity in high-shear-rate Couette flow

Effective thermal conductivity of Ar–Cu nanofluid in shear field is calculated by equilibrium molecular dynamics (EMD) simulation using Green–Kubo formula. The shear field is formed by imposing constant shear rate Couette flow with modified Lees–Edwards periodic boundary condition. The nanoparticle in the nanofluid in shear field rotates under the action of the velocity gradient. The rotation induces enhanced “microconvection” effect which is the main reason for the linear increase in the effective thermal conductivity of the shearing nanofluid with the shear rate increasing. The increase is more sharply with lower volume fraction of nanoparticle than with higher volume fraction, because the “microconvection” effect is weakened in the nanofluid with higher volume fraction of nanoparticle resulted by the slower nanoparticle rotation speed. The effective thermal conductivity obtained from the conventional correlation which is proposed for the flowing suspensions containing micro-sized particles are significantly lower than our numerical results. Moreover, the effect of nanoparticle volume fraction is more obvious in our numerical results. Therefore, the conventional correlation is not suitable when the sizes of the suspended particles are reduced to nanometers (nanofluid).     

Phase change characteristics of ethylene glycol solution‐based nanofluids for subzero thermal energy storage

Nanofluids, particularly water‐based nanofluids, have been extensively studied as liquid–solid phase change materials (PCMs) for thermal energy storage (TES). In this study, nanofluids with aqueous ethylene glycol (EG) solution as the base fluid are proposed as a novel PCM for cold thermal energy storage. Nanofluids were prepared by dispersing 0.1–0.4 wt% TiO₂ nanoparticles into 12, 22, and 34 vol.% EG solutions. The dispersion stability of the nanofluids was evaluated by Turbiscan Lab. The liquid–solid phase change characteristics of the nanofluids were also investigated. Phase change temperature (PCT), nucleation temperature, and half freezing time (HFT) were investigated in freezing experiments. Subcooling degree and HFT reduction were then calculated. Latent heat of solidification was measured using differential scanning calorimetry. Thermal conductivity was determined using the hot disk thermal constant analyzer. Experimental results show that the nanoparticles decreased the PCT of 34 vol.% EG solution but minimally influenced the PCT of 12 and 22 vol.% EG solutions. For all nanofluids, the nanoparticles decreased the subcooling degree, HFT, and latent heat but increased the thermal conductivity of the EG solutions. The mechanism of the improvement of the phase change characteristics and decrease in latent heat by the nanoparticles was discussed. The nanoparticles simultaneously served as nucleating agent that induced crystal nucleation and as impurities that disturbed the growth of water crystals in EG solution‐based nanofluids. Copyright © 2016 John Wiley & Sons, Ltd.