Pool boiling heat transfer of non-Newtonian nanofluids

This paper reports on the investigation of pool boiling heat transfer of γ-Al₂O₃/CMC non-Newtonian nanofluids. To prepare nanofluids, γ-Al₂O₃ nanoparticles were dispersed in CMC solution (carboxy methyl cellulose in water) using ultrasonic mixing and mechanical mixer. Different concentrations of CMC non-Newtonian fluids and γ-Al₂O₃/CMC non-Newtonian nanofluids were tested under nucleate pool boiling heat transfer conditions. Experiments were carried out at atmospheric pressure. Results show that the pool boiling heat transfer coefficient of CMC solutions is lower than water. The decrease in boiling heat transfer is more pronounced at higher CMC concentrations and, as a result, higher solution viscosity. Adding nanoparticles to CMC non-Newtonian solutions results in an improved boiling heat transfer performance. The enhancement in the boiling heat transfer coefficient increases with the nanoparticle concentration; at a concentration of 1.4 wt.%, the boiling heat transfer coefficient increases by about 25% when compared to the base fluid.     

Effect of Volume Fraction of γ‐Al2O3 Nanofluid on Heat Transfer Enhancement in a Concentric Tube Heat Exchanger

In this work, γ-Al₂O₃ nanoparticles with mean diameter of 10 nm are dispersed in deionized water with four nanoparticle volume concentrations of 0.25, 0.5, 0.75, and 1%. The effect of γ-Al₂O₃/water nanofluids on the heat transfer enhancement of heat exchangers is investigated under turbulent regime for four different volumetric flow rates of 150, 200, 250, and 300 L/h. The experimental results showed that the convective heat transfer is increased by increasing particles volume fraction as well as flow rate. The maximum enhancement obtained in Nusselt number and heat transfer coefficient was 20 and 22.8%, respectively, at Reynolds number of 6026 and particle volume fraction of 1%. The experimental Nusselt numbers of nanofluids showed good agreement with the available empirical correlation at particle volume fractions of 0.25 and 0.5%. An empirical correlation is obtained to estimate the Nusselt number of nanofluid under the conditions of this work.     

Free convection heat transfer of non Newtonian nanofluids under constant heat flux condition

Two different kinds of non-Newtonian nanofluids were prepared by dispersion of Al₂O₃ and TiO₂ nanoparticles in a 0.5 wt.% aqueous solution of carboxymethyl cellulose (CMC). Natural convection heat transfer of non-Newtonian nanofluids in a vertical cylinder uniformly heated from below and cooled from top was investigated experimentally. Results show that the heat transfer performance of nanofluids is significantly enhanced at low particle concentrations. Increasing nanoparticle concentration has a contrary effect on the heat transfer of nanofluids, so at concentrations greater than 1 vol.% of nanoparticles the heat transfer coefficient of nanofluids is less than that of the base fluid. Indeed it seems that for both nanofluids there exists an optimum nanoparticle concentration that heat transfer coefficient passes through a maximum. The optimum concentrations of Al₂O₃ and TiO₂ nanofluids are about 0.2 and 0.1 vol.%, respectively. It is also observed that the heat transfer enhancement of TiO₂ nanofluids is higher than that of the Al₂O₃ nanofluids. The effect of enclosure aspect ratio was also investigated. As expected, the heat transfer coefficient of nanofluids as well as the base fluid increases by increasing the aspect ratio.     

Experimental Investigation of Al2O3/Water Nanofluid Through Equilateral Triangular Duct With Constant Wall Heat Flux in Laminar Flow

This paper experimentally investigates the heat transfer of an equilateral triangular duct by employing an Al₂O₃/water nanofluid in laminar flow and under constant heat flux conditions to improve the heat transfer performance of this type of duct. The Nusselt numbers were obtained for different nanoparticle concentrations of the nanofluid at various Peclet numbers. The results show that the experimental heat transfer coefficient of Al₂O₃/water nanofluid is higher than that of distillated water. Also, the experimental heat transfer coefficient of Al₂O₃/water nanofluid is higher than the theoretical one. The experimental results also indicate that the heat transfer enhancement increases with increases in the nanofluid volume concentration and Peclet number.     

Experimental Study on Laminar Forced Convection of Al2O3/Water and Multiwall Carbon Nanotubes/Water Nanofluid of Varied Particle Concentration with Helical Twisted Tape Inserts in Pipe Flow

In this study an experimental investigation has been carried out to analyze the laminar forced convection of Al₂O₃/water and multiwall carbon nanotubes (MWCNT)/water nanofluids through uniformly heated horizontal circular pipe with helical twisted tape inserts. Tests were conducted for varied range of nanoparticle volume concentration (0.15%, 0.45%, 0.60%, and 1%) and helical tape inserts of twist ratios of 1.5, 2.5, and 3. The heat transfer enhancement and the increase of friction factor of nanofluids with helical inserts are compared with that of pure water results with plain tube without inserts. The Nusselt number is found to increase with the increase in Peclet number and nanofluid concentration. The MWCNT/water nanofluids with helical screw tape inserts exhibits higher thermal performance compared to Al₂O₃/water nanofluid. The maximum thermal performance factor was found to be 1.79 and 1.99 for Al₂O₃/water and MWCNT/water nanofluids with helical twisted tape inserts, respectively. The pressure drop for Al₂O₃ nanofluid is found to be higher compared to the MWCNT nanofluid for all the twist ratio of helical screw tape inserts.     

Unconfined laminar nanofluid flow and heat transfer around a square cylinder

The momentum and forced convection heat transfer for a laminar and steady free stream flow of nanofluids past an isolated square cylinder have been studied numerically. Different nanofluids consisting of Al₂O₃ and CuO with base fluids of water and a 60:40 (by mass) ethylene glycol and water mixture were selected to evaluate their superiority over conventional fluids. Recent correlations for the thermal conductivity and viscosity of nanofluids, which are functions of particle volumetric concentration as well as temperature, have been employed in this paper. The simulations have been conducted for Pe = 25, 50, 100 and 200, with nanoparticle diameters of 30 and 100 nm and particle volumetric concentrations ranging from 0% to 4%. The results of heat transfer characteristics of nanofluid flow over a square cylinder showed marked improvement comparing with the base fluids. This improvement is more evident in flows with higher Peclet numbers and higher particle volume concentration, while the particle diameter imposes an adverse effect on the heat transfer characteristics. In addition, it was shown that for any given particle diameter there is an optimum value of particle concentration that results in the highest heat transfer coefficient.     

The impact of various nanofluid types on triangular microchannels heat sink cooling performance

This paper discusses the impact of using various types of nanofluids on heat transfer and fluid flow characteristics in triangular shaped microchannel heat sink (MCHS). In this study, an aluminum MCHS performance is examined using water as a base fluid with different types of nanofluids such as Al₂O₃, Ag, CuO, diamond, SiO₂, and TiO₂ as the coolants with nanoparticle volume fraction of 2%. The three-dimensional steady, laminar flow and heat transfer governing equations are solved using the finite volume method. It is inferred that diamond-H₂O nanofluid has the lowest temperature and the highest heat transfer coefficient, while Al₂O₃-H₂O nanofluid has the highest temperature and the lowest heat transfer coefficient. SiO₂-H₂O nanofluid has the highest pressure drop and wall shear stress while Ag-H₂O nanofluid has the lowest pressure drop and wall shear stress among other nanofluid types. Based on the presented results, diamond-H₂O and Ag-H₂O nanofluids are recommended to achieve overall heat transfer enhancement and low pressure drop, respectively, compared with pure water.     

Experimental heat transfer of nanofluid through an annular duct

This paper is related to heat transfer performance of Al₂O₃/H₂O and TiO₂/H₂O nonofluids through an annular channel with constant wall temperature boundary condition under turbulent flow regime. The constant temperature is applied on the outer wall of the channel. Experimental investigation was done for a wide range of Al₂O₃ and TiO₂ nanoparticle concentrations and Reynolds number. Based on the experimental results, for specific Peclet number, Nusselt number of nanofluids is higher than that of the base fluid. The enhancement increases with increase of nanparticle concentration for both employed nanofluids. Based on the results of this investigation there is no significant difference on the heat transfer enhancement associated with two employed 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.     

Efficiency and effectiveness enhancement of an intercooler of two‐stage air compressor by low‐cost Al2O3/water nanofluids

The present study was aimed to utilize low‐cost alumina (Al₂O₃) nanoparticles for improving the heat transfer behavior in an intercooler of two‐stage air compressor. Experimental investigation was carried out with three different volume concentrations of 0.5%, 0.75%, and 1.0% Al₂O₃/water nanofluids to assess the performance of the intercooler, that is, counterflow heat exchanger at different loads. Thermal properties such as thermal conductivity and overall heat transfer coefficient of nanofluid increased substantially with increasing concentration of Al₂O₃ nanoparticles. Specific heat capacity of nanofluids were lower than base water. The intercooler performance parameters such as effectiveness and efficiency improved appreciably with the employment of nanofluid. The efficiency increased by about 6.1% with maximum concentration of nanofluid, that is, 1% at 3‐bar compressor load. It is concluded from the study that high concentration of Al₂O₃ nanoparticles dispersion in water would offer better heat transfer performance of the intercooler.     

Comparative Numerical Study of Nanofluid Heat Transfer through an Annular Channel

In this article, turbulent convective heat transfer of Al₂O₃–water and TiO₂–water nanofluids through an annular channel have been studied numerically. A comparison was made between single-phase and two-phase models (mixture and Eulerian) to find out which one predicts experimental data more accurately. Effects of some important parameters such as nanoparticle type, nanoparticles concentration, and Reynolds number on heat transfer rate of nanofluids have been investigated. Results reveal that the Nusselt number improves with raising the nanoparticle concentration and Reynolds number. Also it has been observed that in all nanoparticle concentrations, two-phase models give closer results to experimental data comparing to single-phase model.     

Rheological characteristics of non-Newtonian nanofluids: Experimental investigation

γ-Al₂O₃, TiO₂ and CuO nanoparticles were dispersed in a 0.5 wt.%. aqueous solution of carboxymethyl cellulose (CMC) to prepare three types of non-Newtonian nanofluids. Rheological characteristics of the base fluid and nanofluids with various nanoparticle concentrations at different temperatures were measured. Results show that all nanofluids as well as the base fluid exhibit pseudoplastic (shear thinning) behavior. The rheological characteristics of nanofluids and those of the base fluid are functions of temperature and particle concentrations.     

Numerical and Experimental Investigation of Heat Transfer of α-Al2O3/Water Nanofluid in Double Pipe and Shell and Tube Heat Exchangers

This research presents an experimental and numerical study on the heat transfer of α-Al₂O₃/water nanofluid flowing through the double pipe and shell and tube heat exchangers, under laminar flow conditions. Effects of important parameters such as hot and cold volume flow rates, nanofluid temperature, and nanoparticles concentration on the heat transfer characteristics are investigated. The results indicated that the heat transfer performance of both double pipe and shell and tube heat exchangers increases with increasing the hot and cold volume flow rates, as well as the particle concentrations and nanofluid inlet temperature. Compared with pure water, the results indicated that the heat transfer coefficients of nanofluid in the double pipe and shell and tube heat exchangers are higher than those of water by 13.2% and 21.3%, respectively. Also, the heat transfer performance of nanofluid in a shell and tube heat exchanger is 26.2% higher than the double pipe heat exchanger. A computational fluid dynamics (CFD) technique was used for heat transfer simulation in the previously mentioned heat exchangers. Computed overall heat transfer coefficients of the nanofluids are in good agreement with the experimental data.     

A comparison of experimental heat transfer characteristics for Al2O3/water and CuO/water nanofluids in square cross-section duct

The present paper is a comparison between heat transfer characteristics of Al₂O₃/water and CuO/water nanofluids through a square cross-section cupric duct in laminar flow under uniform heat flux. Sometimes because of pressure drop limitations the need for noncircular ducts arises in many heat transfer applications, and a testing facility has been constructed for this purpose and experimental studies were performed on both nanofluids under different nanoparticles concentrations in distilled water as a base fluid. The results indicate that a considerable heat transfer enhancement has been achieved by both nanofluids compared with base fluid. However, CuO/water nanofluid shows better heat transfer augmentation compared with Al₂O₃/water nanofluid through square cross-section duct.     

Performance Analysis of a Louvered Fin Automotive Radiator Using Hybrid Nanofluid as Coolant

This study deals with the theoretical enhancement of thermal performance using water‐based (50/50) volume fraction of Fe₂O₃, CuO, TiO₂, Ag, Cu in Al₂O₃ hybrid nanofluids as coolants for a louvered fin automobile radiator. The effects on thermophysical properties and various performance parameters, i.e., heat transfer, effectiveness, and pumping power of hybrid nanofluids have been compared with water. Among all studied hybrid nanofluids, Al₂O₃‐Ag/water hybrid nanofluid has higher effectiveness, heat transfer rate, pumping power, and pressure drop of 0.8%, 3%, 6%, and 5.6%, respectively, as compared to water and is followed by (50/50) volume fraction of Cu, CuO, Fe₂O₃, TiO₂ hybrid nanofluids as radiator coolant. For the same radiator size and heat transfer rate, coolant flow rate and pumping work decreases by 3%, 4%, respectively, for Al₂O₃‐Ag/water hybrid nanofluid and for the same coolant flow rate and heat transfer rate the radiator size decreases by 3% and pumping power increases by 3.4% for Al₂O₃‐Ag/water hybrid nanofluid as compared to water. Reduction in radiator size may lead to a reduction in radiator cost, engine fuel consumption, and environmental benefit.     

Impact of volume fraction and fluid layer thickness on heat transfer effectiveness of water-based nanofluids

The heat transfer effectiveness of nanofluids is adversely affected by the delay in convection onset. The lesser effectiveness, when compared to that of base fluid, is observed in a range of nanofluid layer thickness. The heat transfer coefficient of water–Al₂O₃ nanofluid can be enhanced by sustaining the equilibrium between Rayleigh number, temperature, particle volume fraction, and enclosure aspect ratio. In this paper, the specific correlation of fluid layer thickness and the onset of convection, which can significantly dominate the heat transfer characteristics of nanofluids are investigated using the concept of critical Rayleigh number. The water layer thickness for convection onset is first experimentally assessed for different real-life heat flux densities. It is then performed for Al₂O₃–water nanofluid for varying volume fractions. With the increase in volume fraction even though thermal conductivity increases, the overall heat transfer enhancement of the nanofluid is reduced. Temperature involved (heat flux density), the volume fraction of the nanofluid used, nanofluid layer thickness (space availability for the cooling system), and mass of the nanoparticle influence heat transfer enhancement. A higher volume fraction may not always result in enhancement of heat transfer as far as nanofluids are concerned.     

Comparison between single-phase and two-phases CFD modeling of laminar forced convection flow of nanofluids in a circular tube under constant heat flux

In this article, forced convection heat transfer with laminar and developed flow for water-Al₂O₃ nanofluid inside a circular tube under constant heat flux from the wall was numerically investigated using computational fluid dynamics method. Both single and two-phase models are accomplished for either constant or temperature dependent properties. For this study nanofluids with size particles equal to 100 nm and particle concentrations of 1 and 4 wt% were used. It is observed that the nanoparticles when dispersed in base fluid such as water enhance the convective heat transfer coefficient. The Nusselt number and heat transfer coefficient of nanofluids were obtained for different nanoparticle concentrations and various Reynolds numbers. Heat transfer was enhanced by increasing the concentration of nanoparticles in nanofluid and Reynolds number. Also, a correlation based on the dimensionless numbers was obtained for the prediction the Nusselt number. The modeling results showed that the predicted values were in very good agreement with reference experimental data.     

Thermal conductivity of non-Newtonian nanofluids: Experimental data and modeling using neural network

Three different types of nanofluids were prepared by dispersing γ-Al₂O₃, TiO₂ and CuO nanoparticles in a 0.5 wt% of carboxymethyl cellulose (CMC) aqueous solution. Thermal conductivity of the base fluid and nanofluids with various nanoparticle loadings at different temperatures were measured experimentally. Results show that the thermal conductivity of nanofluids is higher than the one of the base fluid and the increase in the thermal conductivity varies exponentially with the nanoparticle concentration. In addition to increase with the nanoparticle concentration, the thermal conductivity of nanofluids increases with the temperature. Neural network models were proposed to represent the thermal conductivity as a function of the temperature, nanoparticle concentration and the thermal conductivity of the nanoparticles. These models were in good agreement with the experimental data. On the other hand, the Hamilton Crosser model was only satisfactory for low nanoparticle concentrations.     

Conjugate heat transfer enhancement of laminar slot jets with various nanofluids on an array of protruding hot sources using MPM approach

Conjugate heat transfer in laminar slot jets impinging on multiple protruding hot sources using various nanofluids has been investigated numerically by employing (i) a mass-based modeling and an (ii) Eulerian-based multi-phase modeling (MPM). Various parameters such as streamline contours, isotherm profiles, local Nusselt number (Nu), average Nusselt number (Nuavg) are evaluated for different nanofluids (Ag–water, Al₂O₃–water, CuO–water and TiO₂–water), various range of Reynolds number (Re), particle volume fraction (?), diameter of the nanoparticle (d) and thermal conductivity ratio (kr). The steady, laminar, incompressible and two-dimensional flows are considered for the analysis. Finite-volume method with SIMPLE algorithm is used to solve continuity, momentum and energy equations along with boundary conditions. The highest heat transfer rate is achieved at ??=?0.05 for any protruding blocks and Reynolds number. Conjugate heat transfer rate of nanofluids increases with decreasing the diameter of nanoparticles. Here, Al₂O₃–water nanofluid is found to exhibit highest average Nusselt number compared to other nanofluids. The mixture based MPM approach with considering slip velocity yields higher heat transfer rate compared to the results obtained by single phase modeling approach.     

Experimental Investigation of Overall Heat Transfer Coefficient of Al2O3/Water–Mono Ethylene Glycol Nanofluids in an Automotive Radiator

In this research, the overall heat transfer coefficient of Al₂O₃/water–mono ethylene glycol (MEG) nanofluids is investigated experimentally in a car radiator under laminar flow conditions. The experimental rig developed is similar to the automotive cooling system. The stable nanofluid used is prepared by a two‐step method. Ultrasonication is done for proper dispersion of 20 nm Al₂O₃ nanoparticle in carrier fluid water and MEG mixture with 50:50 proportions by volume. The experimental study showed that use of a nanofluid enhances the overall heat transfer coefficient as compared to the base fluid. In this study as the nanoparticle volume fraction increases from 0% to 0.8%, the overall heat transfer coefficient also increases. It was observed that as the nanofluid inlet temperature increased from 65 °C to 85 °C, the overall heat transfer coefficient decreased. It was found that using a 0.2% volume fraction Al₂O₃/water–MEG nanofluid can enable a 36.69 % reduction in surface area of the radiator.