Experimental investigation of turbulent flow and convective heat transfer characteristics of alumina water nanofluids in fully developed flow regime

In this paper the convective heat transfer and friction factor of the nanofluids in a circular tube with constant wall temperature under turbulent flow conditions were investigated experimentally. Al₂O₃ nanoparticles with diameters of 40 nm dispersed in distilled water with volume concentrations of 0.1–2 vol.% were used as the test fluid. All physical properties of the Al₂O₃–water nanofluids needed to calculate the pressure drop and the convective heat transfer coefficient were measured. The results show that the heat transfer coefficient of nanofluid is higher than that of the base fluid and increased with increasing the particle concentrations. Moreover, the Reynolds number has a little effect on heat transfer enhancement. The experimental data were compared with traditional convective heat transfer and viscous pressure drop correlations for fully developed turbulent flow. It was found that if the measured thermal conductivities and viscosities of the nanofluids were used in calculating the Reynolds, Prandtl, and Nusselt numbers, the existing correlations perfectly predict the convective heat transfer and viscous pressure drop in tubes.     

Estimation of heat transfer coefficient and friction factor in the transition flow with low volume concentration of Al2O3 nanofluid flowing in a circular tube and with twisted tape insert

Experiments to evaluate heat transfer coefficient and friction factor for flow in a tube and with twisted tape inserts in the transition range of flow with Al₂O₃ nanofluid are conducted. The results showed considerable enhancement of convective heat transfer with Al₂O₃ nanofluids compared to flow with water. It is observed that the equation of Gleninski applicable in transitional flow range for single-phase fluids showed considerable deviation when compared with values obtained with nanofluid. The heat transfer coefficient of nanofluid flowing in a tube with 0.1% volume concentration is 23.7% higher when compared with water at number of 9000. Heat transfer coefficient and pressure drop with nanofluid has been experimentally determined with tapes of different twist ratios and found to deviate with values obtained from equations developed for single-phase flow. A regression equation is developed to estimate the Nusselt number valid for both water and nanofluid flowing in the transition flow Reynolds number range in circular plain tube and with tape inserts. The maximum friction factor with twisted tape at 0.1% nanofluid volume concentration is 1.21 times that of water flowing in a plain tube.     

The effects of nanofluid on thermophysical properties and heat transfer characteristics of a plate heat exchanger

Improving heat exchanger's performance by increasing the overall heat transfer as well as minimising pressure drop is one of the promising fields of research to focus on. Nanofluids with higher thermal conductivity and better thermophysical properties can be applied in heat exchanger to increase the heat transfer rate. In the present study SiO₂, TiO₂ and Al₂O₃ are applied in a plate heat exchanger and the effects on thermophysical properties and heat transfer characteristics are compared with the base fluid. Since it is desired to minimize the pressure drop, the influence of nanofluid application on pressure drop and entropy generation is investigated. It is concluded that the thermal conductivity, heat transfer coefficient and heat transfer rate of the fluid increase by adding the nanoparticles and TiO₂ and Al₂O₃ result in higher thermophysical properties in comparison with SiO₂. The highest overall heat transfer coefficient was achieved by Al₂O₃ nanofluid, which was 308.69 W/m².K in 0.2% nanoparticle concentration. The related heat transfer rate was improved around 30% compared to SiO₂ nanofluid. In terms of pressure drop, SiO₂ shows the lowest pressure drop, and it was around 50% smaller than the pressure drop in case of using TiO₂ and Al₂O₃.     

Heat transfer and flow characteristics of AL2O3–water nanofluid in a double tube heat exchanger

An experimental study was carried out in order to find out the effects of Al₂O₃ nanofluid with a mean diameter of 20 nm on heat transfer, pressure drop and thermal performance of a double tubes heat exchanger. The effective viscosity of nanofluid was measured in various temperatures ranging from 27 °C to 55 °C. Experiments were carried out at different Reynolds numbers ranging from 5000 to 20,000, approximately, and in various nanoparticles concentration up to 1% by volume. Results indicate that there is a good potential in promoting the thermal performance of heat exchanger by adding nanoparticles in the investigated ranges where there is not a severe pressure drop penalty. The empirical correlation was created for Nusselt number variation based on the Reynolds number and nanoparticles concentration.     

Three-Dimensional Numerical Study on Thermal-Hydraulic Performance of Twisted Mini-Channel Using Al2O3-H2O Nanofluid

Abstract

Effects of design parameters of the twisted mini-channel, including twist pitch, width, height, and length, are investigated numerically at the Reynolds numbers between 300 and 1500. The Al₂O₃-H₂O nanofluid is used as working fluid, and it is simulated using the mixture two-phase model. The volume fraction of Al₂O₃ nanoparticles is 1, 2, 3, and 4%. The results show that decreasing of twist pitch and channel length leads to higher values of the Nusselt number. As the Reynolds number is increased, the influence of these factors enhances. The increase of the channel width and height decreases the convective thermal resistance and thus improves the Nusselt number along with a certain penalty in the pressure drops. For all models, by increasing the volume fraction of nanoparticles, the convective thermal resistance is reduced leading to the thermal performance improvement. Finally, two correlations are proposed for the Nusselt number and friction factor, which are useful as a tool to predict the heat transfer and pressure drop characteristics of the twisted mini-channel.     

Numerical investigation of thermal and hydraulic performance in novel oblique geometry using nanofluid

To capitalize the advantage of oblique fin heat sink (OFHS) with Al₂O₃–water nanofluids of different volumetric concentration (1, 2, and 4%), a comprehensive computational analysis has been performed for OFHS with nanofluid through the single-phase modeling. The present investigation focuses on the full domain simulation because the conventional periodic computational model approach is unable to investigate the flow migration effect and predicts higher value of Nusselt number. Apart from the disruption of boundary layer, vortices are observed in the secondary oblique channel due to flow separation that promotes an additional heat transfer enhancement. Higher Severity of the flow migration and hence more non-uniformity of nanofluid flow rate through the primary and secondary channels was observed at higher Reynolds numbers. The increment observed in the average Nusselt number (Nu_avg) at Re = 750 for OFHS is about 90% and 115% for water and 4% volumetric concentration of nanofluid respectively compared to conventional SCHS. Also, Al₂O₃–water nanofluid exhibits about 30% higher enhancement at 4% volumetric concentration at Re = 750 in the OFHS with compared to water. The increase in heat transfer exceeded the pressure drop penalty at all the Reynolds numbers.     

Cooling performance investigation of electronics cooling system using Al2O3–H2O nanofluid

In this study, the cooling performance of Al₂O₃–H₂O nanofluid was experimentally investigated as a much better developed alternative for the conventional coolant. For this purpose the nanofluid was passed through the custom-made copper minichannel heat sink which is normally attached with the electronic heat source. The thermal performance of the Al₂O₃–H₂O nanofluid was evaluated at different volume fraction of the nanoparticle as well as at different volume flow rate of the nanofluid. The volume fraction of the nanoparticle varied from 0.05 vol.% to 0.2 vol.% whereas the volume flow rate was increased from 0.50 L/min to 1.25 L/min. The experimental results showed that the nanofluid successfully has minimized the heat sink temperature compared to the conventional coolant. It was noticed also that the thermal entropy generation rate was reduced via using nanofluid instead of the normal water. Among the other functions of the nanofluid are to increase the frictional entropy generation rate and to drop the pressure which are insignificant compared to the normal coolant. Given the improved performance of the nanofluid, especially for high heat transportation capacity and low thermal entropy generation rate, it could be used as a better alternative coolant for the electronic cooling system instead of conventional pure water.     

Three‐Dimensional Numerical Investigation of Nanofluids Flow in Microtube with Different Values of Heat Flux

Forced convective laminar flow of different types of nanofluids such as Al₂O₃, CuO, SiO₂, and ZnO, with different nanoparticle size 25, 45, 65, and 80 nm, and different volume fractions which ranged from 1% to 4% using ethylene glycol as base fluids were used. A three‐dimensional microtube (MT) with 0.05 cm diameter and 10 cm in length with different values of heat fluxes at the wall is numerically investigated. This investigation covers Reynolds number (Re) in the range of 80 to 160. The results have shown that SiO₂‐EG nanofluid has the highest Nusselt number (Nu), followed by ZnO‐EG, CuO‐EG, Al₂O₃‐EG, and finally pure EG. The Nu for all cases increases with the volume fraction but it decreases with the rise in the diameter of nanoparticles. In all configurations, the Nu increases with Re. In addition, no effect of heat flux values on the Nu was found.     

Forced convective heat transfer of nanofluids in microchannels

Convective heat transfer coefficient and friction factor of nanofluids in rectangular microchannels were measured. An integrated microsystem consisting of a single microchannel on one side, and two localized heaters and five polysilicon temperature sensors along the channel on the other side were fabricated. Aluminum dioxide (Al₂O₃) with diameter of 170 nm nanofluids with various particle volume fractions were used in experiments to investigate the effect of the volume fraction of the nanoparticles to the convective heat transfer and fluid flow in microchannels. The convective heat transfer coefficient of the Al₂O₃ nanofluid in laminar flow regime was measured to be increased up to 32% compared to the distilled water at a volume fraction of 1.8 volume percent without major friction loss. The Nusselt number measured increases with increasing the Reynolds number in laminar flow regime. The measured Nusselt number which turned out to be less than 0.5 was successfully correlated with Reynolds number and Prandtl number based on the thermal conductivity of nanofluids.     

Modeling of convective heat transfer of a nanofluid in the developing region of tube flow with computational fluid dynamics

In this article, convective heat transfer effect on the nanofluid flow in the developing region of a tube with constant heat flux was investigated using computational fluid dynamics (CFD). For this purpose, nanofluid containing Al₂O₃ and water as a liquid single phase with two average particle sizes of 45 and 150 nm and four particle concentrations of 1, 2, 4 and 6 wt.% were used. Effect of particle size on convective heat transfer coefficient was investigated in different Reynolds numbers (500 < Re < 2500) for various axial locations of tube. According to the modeling results, an equation was obtained for Nusselt number prediction using the dimensionless numbers. The results showed that the predicted data were in very good agreement with experimental data obtained from the literature. The maximum error was around 10%.     

Thermal performance augmentation using water based Al2O3-gamma nanofluid in a horizontal shell and tube heat exchanger under forced circulation

Shell and tube heat exchanger is one of the most prevalent heat exchangers with a wide variety of industrial applications, i.e., power plants, chemical processes, marine industries, HVAC systems, cooling of hydraulic fluid and engine oil in heavy duty diesel engines and the like specifically where a need to heat or cool a large fluid volume exist and also higher-pressure use. In the present study, the effect of using Al₂O₃-water nanofluid on thermal performance of a commercial shell and tube heat exchanger with segmental baffles is assessed experimentally. For this purpose, Al₂O₃-gamma nanoparticles with 15 nm mean diameter (99.5% purity) and Sodium Dodecyl Benzene Sulphonate (SDBS) as surfactant are used to make aqueous Al₂O₃ nanofluid at three various volume fractions of nanoparticles (φ = 0.03, 0.14 and 0.3%). Indeed, in this paper the effect of some parameters of hot working fluid such as Reynolds number and volume concentration of nanoparticles on heat transfer characteristics, friction factor and thermal performance factor of a shell and tube heat exchanger under laminar flow regime is investigated. The results indicate a substantial increment in Nusselt number as well as the overall heat transfer coefficient of heat exchanger by enhancement of Reynolds number and it can be seen that, at a certain Reynolds number, heat transfer characteristics of heat exchanger increase as the nanoparticles volume concentration increases. Outcomes of the heat transfer evaluation demonstrate that applying nanofluids instead of base fluid lead to increment of Nusselt number up to 9.7, 20.9 and 29.8% at 0.03, 0.14 and 0.3 vol%, respectively. Likewise it is seen that at mentioned nanoparticles volume fractions, overall heat transfer coefficient of heat exchanger enhances around 5.4, 10.3 and 19.1%, respectively. In term of pressure drop, a little penalty is found by using nanofluid in the test section. Eventually a thermal performance assessment on the heat exchanger was conducted. According to the analysis results, utilizing nanofluid at minimum and maximum nanoparticles volume fractions (φ = 0.03 and 0.3%) results in average augmentation of around 6.5% and 18.9% in thermal performance factor (η) of the heat exchanger compared to the base liquid, respectively.     

Numerical investigation of effective parameters in convective heat transfer of nanofluids flowing under a laminar flow regime

This article presents a numerical investigation on heat transfer performance and pressure drop of nanofluids flows through a straight circular pipe in a laminar flow regime and constant heat flux boundary condition. Al₂O₃, CuO, carbon nanotube (CNT) and titanate nanotube (TNT) nanoparticles dispersed in water and ethylene glycol/water with particle concentrations ranging between 0 and 6 vol.% were used as working fluids for simulating the heat transfer and flow behaviours of nanofluids. The proposed model has been validated with the available experimental data and correlations. The effects of particle concentrations, particle diameter, particles Brownian motions, Reynolds number, type of the nanoparticles and base fluid on the heat transfer coefficient and pressure drop of nanofluids were determined and discussed in details. The results indicated that the particle volume concentration, Brownian motion and aspect ratio of nanoparticles similar to flow Reynolds number increase the heat transfer coefficient, while the nanoparticle diameter has an opposite effect on the heat transfer coefficient. Finally, the present study provides some considerations for the appropriate choice of the nanofluids for practical applications.     

Nanofluids transport through a novel concave/convex convergent pipe

The thermal and hydraulic performance of Al₂O₃-water nanofluid forced convective heat transfer through a concave/convex convergent pipe has been investigated in this work. The investigation is performed for a wide variety of concavity/convexity in the pipe wall profile, volume fraction of nanoparticles, and Reynolds number. An excellent agreement has been confirmed between the results of our numerical model and the available data from the literature. The findings of the current work reveal that as the pipe wall tends toward the concavity, the average heat transfer coefficient and the pressure drop along the pipe increase. Further, the concave wall profile shows a prominent enhancement in heat transfer up to 41%; while, the convex wall profile provides a sustainable and superior performance factor up to 1.223 compared to the straight one, respectively. Moreover, at any fixed wall profile, a modest rise in heat transfer and pressure drop has been observed when the nanoparticles volume fraction increases. According to the information provided in this study, the addressed configuration improvements play a crucial role in augmenting heat transfer more than employing nanofluids.     

Analysis of entropy generation and convective heat transfer of Al2O3 nanofluid flow in a tangential micro heat sink

Effect of using Al₂O₃–water nanofluids with different volume fractions and particle diameters on generated entropy, hydrodynamic performance and heat transfer characteristics of a tangential micro-heat sink (TMHS) was numerically investigated in this research. Results indicated that considerable heat transfer enhancement is possible when using Al₂O₃–water nanofluids as coolant and clearly the enhancement improves with increasing particles concentration and decreasing particles size. However, using nanofluid has also induced drastic effects on the pumping power that increases with particles volume fraction and Reynolds number. Finally, it was found that generated total entropy decreases with increasing volume fraction and Reynolds number and decreasing particles size.     

Heat transfer and nanofluid flow characteristics through a circular tube fitted with helical tape inserts

Numerical investigations are performed using finite volume method to study laminar convective heat transfer and nanofluids flows through a circular tube fitted with helical tape insert. The wall of tube was subjected to a uniform heat flux boundary condition. The continuity, momentum and energy equations are discretized and the SIMPLE algorithm scheme is applied to link the pressure and velocity fields inside the domain for plain tube. Four different twist ratios of 1.95–4.89, two different types of nanoparticles, Al₂O₃ and SiO₂ with different nanoparticle shapes of spherical, cylindrical and platelets, and 0.5–2.0% volume fraction in base fluid (water) and nanoparticle diameter in the range of 20–50 nm were used to identify their effect on the heat transfer and fluid flow characteristics through a circular tube fitted with helical tape insert geometries. The results indicate that the four types of nanofluid achieved higher Nusselt number than pure water. Nanofluid with Al₂O₃ particle achieved the highest Nusselt number. For all the cases studied, the Nusselt number increased with the increase of Reynolds number and with the decrease of twist ratio of helical tape insert.     

An experimental study of forced convective heat transfer for fully developed Al 2 O 3 –water nanofluid in an annulus tube

Nanofluids have been known as practical materials to ameliorate heat transfer within diverse industrial systems. The current work presents an empirical study on forced convection effects of Al₂O₃–water nanofluid within an annulus tube. A laminar flow regime has been considered to perform the experiment in high Reynolds number range using several concentrations of nanofluid. Also, the boundary conditions include a constant uniform heat flux applied on the outer shell and an adiabatic condition to the inner tube. Nanofluid particle is visualized with transmission electron microscopy to figure out the nanofluid particles. Additionally, the pressure drop is obtained by measuring the inlet and outlet pressure with respect to the ambient condition. The experimental results showed that adding nanoparticles to the base fluid will increase the heat transfer coefficient (HTC) and average Nusselt number. In addition, by increasing viscosity effects at maximum Reynolds number of 1140 and increasing nanofluid concentration from 1% to 4% (maximum performance at 4%), HTC increases by 18%.     

Comparative study of Euler and mixture models for turbulent flow of Al2O3 nanofluid inside a horizontal tube

In this paper, turbulent forced convective flow of water Al₂O₃ nanofluid, with particle diameter equal to 40 nm in a horizontal circular tube, exposed to convection with saturated steam at the wall, is numerically analyzed. Two different approaches are taken into consideration: Euler and mixture models. It is comprehended that convective heat transfer coefficient enhances with increasing the particle volume concentration and Reynolds number. The two models almost showed the same results. However, mixture model was in a better agreement with experimental results for the estimation of average Nusselt number.     

Numerical Investigation of Forced Convection Flow of Nanofluids in Rotating U-Shaped Smooth and Ribbed Channels

The impact of the nanoparticles and ribs on the thermal performance of the rotating U-type cooling channel are investigated for turbulent forced convection flow of nanofluids. The nanofluids are provided by the inclusion of the nanoparticles of TiO₂ and Al₂O₃ in water as the base fluid, namely, water/Al₂O₃ and water/TiO₂ nanofluids mixtures. The simulations are performed for three-dimensional turbulent flow and heat transfer using an RNG k-? turbulence model for Reynolds number range of 5000 to 20,000. To show the effectiveness of the ribs and nanofluids, three criteria are employed: heat transfer enhancement, pressure drop or power consumed, and the thermal performance factor. It is found that the contribution of turbulence promotion in heat transfer enhancement of the ribbed channel is more effective than that of enlarging the heat surface area. The results show that using ribs at the lowest Reynolds number and utilizing nanofluids at the highest one provide high heat transfer rate and thermal performance. At the middle Reynolds numbers, the effects of these two methods on heat transfer enhancement are relatively close to each other. In this case, if the pumping power is the main concern, using nanofluids is recommended due to providing a smaller pressure drop penalty.     

Thermal Enhancement of Triangular Ducts Using Compound of Vortex Generators and Nanofluids

Laminar flow and heat transfer of three different types of nanofluids; Al₂O₃, CuO, and SiO₂ suspended in ethylene glycol, in a triangular duct using delta-winglet pair of vortex generator are numerically simulated in three dimensions. The governing equations of mass, momentum and energy are solved using the finite volume method. The effects of types, concentrations, and diameter of solid nanoparticles and Reynolds number on thermal and hydraulic performance of triangular duct are examined. The range of Reynolds number, volume fraction and nanoparticles diameters is 100–1200, 1–4%, and 25–85 nm, respectively. The results indicate that the average Nusselt number increases with the particles volume fraction and Reynolds number associated with an increase in the pressure drop. The heat transfer enhancement and pressure drop penalty reduce with increasing the particles diameters. However, a reduction in the pumping power required is observed to force the nanofluids when the volume fraction increases, assuming the heat transfer coefficient remains constant.     

MHD heat transfer and entropy production of an Al2O3-water nanofluid in a horizontal cylinder

This paper deals with the effect of magnetic fields (B_r, B_θ, B_z) applied in r-, θ-, z-directions, respectively, on entropy production and heat transfer and in a horizontal cylinder filled with an Al₂O₃-water nanofluid. The results are verified using literature data. For different Richardson, Ri, and Hartmann numbers, Ha, the nanoparticles (NP) ϕ, and magnetic field orientation combined effect provide a better understanding of heat transfer and entropy optimization. The results indicate that entropy production and heat transfer and rates depend on magnetic field intensity and direction. Also, increasing Ri and NP increases entropy generation and heat transfer. Finally, applying a radial magnetic field promotes a better convective heat transfer and minimizes entropy production.