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The current paper reports the impact of variation of ribs' pitch distance in the case of laminar flow and forced convection heat transfer upon flow characteristics in a rectangular channel experimentally and numerically. The thermal behavior of the ribbed channel with water as a working fluid was measured experimentally and compared to numerical simulations performed using Galerkin finite element method under the same operating and boundary conditions. Numerical analysis has been extended to investigate the entire ribbed channel with the presence of nano-particles in the base fluid to predict the performance of the heat transfer process with interaction with extended geometries. The analysis was done keeping heat flux constant at the bottom boundary condition. The impacts of the rib pitch-to-height ratio (p/e = 2.5, 3.125, 3.75, 4.375, and 5), nano-particle volume concentration (0% ≤ φ ≤ 4%), also Reynolds numbers (427.44 to 1282. 9), were all examined in the present study. The results show a good coincidence between the numerical and experimental study with a maximum deviation was nearly 3%. The main findings show that when the nano-fluid volume fraction increases from 0% to 4%, the average Nusselt number for all aspect ratios increases too. Meanwhile, the average skin friction decreases as the Reynolds number increase for different values of AR. Furthermore, the results revealed that the water – aluminum oxide nano-fluid with the concentration φ = 0.04 aid to improve the coefficient of heat transfer by 5% higher than the case of φ = 0.02, and up to 9.4% compared to pure water (φ = 0) due to an increase in viscosity. Finally, the current work shows that these extended surfaces with the use of nano-fluids can remarkably improve heat transfer properties with barely increased friction, making them suitable candidates for the development of efficient heat exchanger devices. [ABSTRACT FROM AUTHOR]

Global warming is increasing as CO 2 emissions from the use of conventional fossil fuels continue to rise. The development of geothermal energy is gradually becoming the key to the sustainable development of human society in the future. Enhancing the heat transfer performance of circulating heat transfer fluids is one of the effective means to efficiently exploit deep geothermal resources. In this study, a self-developed circulating heat exchange experimental setup was established. The convection heat transfer characteristics of nanofluids in high-temperature rocks were experimentally investigated. Al 2 O 3 nanofluid and CuO nanofluid were used as the working medium for heat transfer in high temperature granite samples. The effects of the Reynolds number (1000–6000) and parameters such as nanoparticle type (CuO & Al 2 O 3), size (20 nm & 40 nm) and mass fraction (0 wt%-2wt%) on the heat transfer intensity were analyzed. The effect of nanoparticle accumulation on the rock surface on the local heat transfer strength was also investigated. The results indicate that the increase of Reynolds number enhances the heat transfer strength of nanofluids in rocks. Nanofluid containing 20 nm Al 2 O 3 nanoparticles has better heat transfer performance in rock than nanofluid containing 20 nm CuO nanoparticles. Meanwhile, the heat transfer performance of nanofluid containing 20 nm Al 2 O 3 nanoparticles in rock is better than that of nanofluid containing 40 nm Al 2 O 3 nanoparticles. The heat transfer intensity increases and then decreases as the nanoparticle mass fraction increases. The heat transfer coefficient at Reynolds number 6000 is 52.2 % higher than that of water. For nanofluids and water, the local heat transfer intensity decreases with increasing distance from the inlet. Meanwhile, accumulation of nanoparticles on rock surfaces during convective heat transfer enhances heat transfer intensity. The results show that nanofluids have the potential to be applied to deep geothermal resource extraction. This paper provides data and technical support for the application of nanomaterials in medium and deep geothermal systems to enhance heat transfer efficiency. [ABSTRACT FROM AUTHOR]

The convection heat transfer in a countercurrent double-tube heat exchanger with various fins in a turbulent flow is investigated. The suitable heating or cooling process of fluids is the effective use of the double-pipe heat exchanger. We use water-aluminum oxide nanofluid and water-titanium dioxide at four concentrations (0.4%, 2%, 4%, 6%) as the cold fluid in the inner tube and water as the hot fluid in the annular space. The single-phase model for nanofluid modeling and the standard k-ε model with scalable wall function for simulating the turbulent flow is utilized. To better examine this novel geometry, its performance is compared with simple and rectangular-finned geometries. The results show that the water aluminum oxide nanofluid has a better convection heat transfer coefficient than water titanium dioxide and pure water. Raising the nanofluid concentration from 0.4% to 6% increases the convection heat transfer coefficient by 12%. Heat exchangers with a rectangular and curved fin have 81% and 85% better efficiency than the heat exchanger without a fin. The novel geometry causes a smaller pressure drop despite its higher convection heat transfer coefficient. Also, it is shown that with raising the Reynolds number and nanofluid concentration, the pressure drop increases.

In the current article, the effect of nanofluid blast on the forced convection heat transfer in the intra-channel turbulent flow with a sudden expansion is investigated. The different cases of Newtonian, non-Newtonian, and incompressible fluids have been studied. The effects of non-Newtonian fluid index, magnetic field, nanoparticle volume fraction and nanofluid blowing frequency parameters on the pressure drop and heat transfer in the intra-channel flow have been reviewed. The simulation was performed using finite volume method as the solution strategy. The results revealed that higher values of Re and nanoparticles volume fraction resulted in the Nusselt number increasement. Also, increasing the magnetic field and decreasing the non-Newtonian fluid index increased the forced heat transfer coefficient and subsequently the Nusselt number. The results showed that the sinus velocity caused to increase the rate of the heat transfer and slightly lower friction coefficient. However, higher frequencies caused increasing of heat transfer and friction coefficient. Finally, it was founded that higher values of the Re had the most influence on the performance of the system and the magnetic field and sinus velocity were in the next step. [ABSTRACT FROM AUTHOR]

Fluid flow, heat transfer and entropy generation inside triangular enclosure equipped with rotational obstacle filled with nanofluid were analyzed numerically by finite difference method and SIMPELR algorithm. Five cases for apex angle, swirling circle radios and spine velocity, swirling circle radios and positions and five cases for wavy wall domain and amplitude were simulated. Although α = 20 has higher average Nusslet number, α = 30 has chosen as optimum apex angle due to meet to geometry problem in next step of optimization process. Optimum wavy wall according to average Nusselt number is constructed with A = 0.05 and D = 1.45π. Radius equal to 0.1 and position equal to 3 has been obtained as optimum geometry between multiple choices. Maximum Nusselt number and entropy generation deal with blades shapes and followed by bricks, platelets and cylinder shapes for nanoparticle volume fraction of between 0.5 and 5 and Rayleigh numbers from 103 and 106. [ABSTRACT FROM AUTHOR]

The aim of the present study is to experimentally investigate the thermal performance and the pressure drop in copper tubes partially filled with open-cell metal foams using MgO nanofluid. The tubes are produced by casting liquid metal around space holder material with the porosity of 70% and bounded by constant heat flux. MgO nanoparticles are added to the base fluid (water and ethylene glycol) to make the working fluid at the volumetric concentrations of 0.0625%, 0.25%, and 0.5%. The experiments are performed in a pipe with steady-state laminar flow. The obtained experimental results are compared with the correlation outputs, where a proper agreement between the results was observed. It is perceived that both pressure drop and Nusselt number are highly dependent on the Reynolds number. The data further shows that using nanofluid and porous media inside the tubes increases the pressure drop and convection heat transfer coefficient, significantly. Unlabelled Image • Experiments of MgO/water-EG flow through tubes containing metal foam were conducted. • Effects of MgO concentration and porous medium on heat transfer were analyzed. • Convection heat transfer and pressure loss were directly dependent on Re number. • Copper foams affected pressure drop and thermal performance substantially. [ABSTRACT FROM AUTHOR]

• Computational modeling of convective thermal transport and liquid circulation within a rotating enclosure loaded with water-based nanoliquid is performed. • Mathematical model formulated using the dimensionless non-primitive parameters has been worked out by the finite difference technique. • Investigation has been performed in a wide range of the Rayleigh number, Taylor number and nanoparticles volume fraction. • Addition of nanoparticles in the case of rotationless and low Taylor numbers diminishes the average Nusselt number. Convective energy transport within rotating cavities is a very important topic taking into account different rotating systems in electronics, chemical industry and thermal engineering. This study is consecrated to computational modeling of convective thermal transport and liquid circulation within a rotating enclosure loaded with water-based nanoliquid. Mathematical model formulated using the dimensionless non-primitive parameters has been worked out by the finite difference technique. The developed computational code has been verified using experimental and theoretical data of other researchers. Investigation has been performed in a broad range of the Rayleigh number, Taylor number and nanoparticles volume fraction. Calculated data have indicated the energy transport enhancement with nanoparticles concentration for high Taylor numbers. Image, graphical abstract [ABSTRACT FROM AUTHOR]

In this paper, effect of the nanoparticles diameter on the forced convection heat transfer of turbulent flow of Al2O3-water nanofluids in a circular tube under constant heat flux on the wall using two phase mixture model is numerically investigated and statistically analysed. The Volume fraction of nanoparticles are %1, %3 and %5, the Reynolds numbers are 25×103, 5×104 and 75×103 and the variation of diameter of nanoparticles is assumed in the range of 20- 100nm. In the statistical analysis, from the continuous probability distribution functions such as Gamma, Normal, Lognormal, Gumbel, Weibull and Frechet are used. After reviewing the results, it was found that by increasing the diameter of nanoparticles, the Nusselt number is reduced and due to changes in the nanoparticles diameter has followed the Frechet probability distribution function. [ABSTRACT FROM AUTHOR]

The present paper is a numerical study on heat transfer and pressure drop of a nanofluid including water as base fluid with Al2O3 nanoparticles inside a square channel having an inner cylinder, with and without fin under constant heat flux condition using two-phase Euler-Lagrange approach. Numerical investigation has been carried out for various combinations of base fluid, nanoparticle size and concentration through a straight cylinder. Simulation has been performed in a laminar flow regime using finite volume method. Besides, the thermal boundary condition of constant uniform heat flux on the channel wall was applied. The results show that the increase in Reynolds number and nanoparticle volume concentration have considerable effects on heat transfer coefficient enhancement. The heat transfer coefficient decreases when nanoparticles diameter increases. The passive way used in this study, leads to higher pressure drops. For all fluids under consideration, pressure drop escalates with Reynolds number. Adding nanoparticles to the base fluid leads to rise in pressure drop, and this effect is more intensive for higher concentrations. [ABSTRACT FROM AUTHOR]

Existence of magnetic field causes heat transfer to reduce in free convection but in several engineering uses for example: electronic application; enhancing heat transfer is a purpose. Thus, nanofluid should be selected as working fluid. Nanofluid is dispersion of very small metal particles in the base fluid. Two phase and single phase are two ways for estimating the behavior of nanofluid. At first model, nanofluid suppose as homogenous fluid without any slip mechanism. But in second method, slip velocities are included. Brownian motion and Thermophoresis impacts are taken into consideration in second approach. In this paper, previous publications about nanofluid hydrothermal treatment in existence of magnetic field are reviewed. Rely of variable and constant magnetic fields, Ferrohydrodynamic (FHD) and Magnetohydrodynamic (MHD) can take role in simulations. Numerical and analytical methods are considered by authors. Results proved that temperature gradient augments with augment of solid particle concentration and buoyancy forces, while it decreases with augment of magnetic field. [ABSTRACT FROM AUTHOR]

Heat sink has been widely used as a cooling device for electronic systems in recent years. Because of the considerable heat generated by electronic devices and low efficiency of conventional heat sinks, a large number of researches have been focused on the performance enhancement of these systems. Innovative heat sink in the presence of Al 2 O 3 -water nanofluid is experimentally investigated in this study. The effects of adding various volume fractions of nanoparticles on the convective heat transfer coefficient, pressure drop, thermal resistance and heat sink base temperature are investigated in different flow rates from 6.14 to 32.2 (cm 3 /s). The obtained results reveal that the convective heat transfer enhances 16% and thermal resistance reduces more than 15%. The innovative thermal energy absorption system in present research is a tangential heat sink with impinging liquid jet and elliptical cross section which is expected to be able to achieve significant enhancement in thermal energy removal as a cooling device, which is proposed as the novelty here. Also comparison between cylindrical and elliptical cross sections in this type of heat sink is carried out. The results disclose that using tangential heat sink with liquid jet impingement achieves convective heat transfer coefficient 2.26 times greater than conventional heat sink in the presence of pure water. [ABSTRACT FROM AUTHOR]

Nanofluid has great potentials in heat transfer enhancement and entropy generation decrease as an effective cooling medium. Effects of Al2O3-water nanofluid flow on entropy generation and heat transfer performance in a rectangular conventional channel are numerically investigated in this study. Four different volume fractions are considered and the boundary condition with a constant heat flux is adopted. The flow Reynolds number covers laminar flow, transitional flow and turbulent flow. The influences of the flow regime and nanofluid volume fraction are examined. Furthermore, dimples and protrusions are employed, and the impacts on heat transfer characteristic and entropy generation are acquired. It is found that the average heat transfer entropy generation rate descends and the average friction entropy generation rate rises with an increasing nanofluid volume fraction. The effect of nanofluid on average heat transfer entropy generation rate declines when Reynolds number ascends, which is inverse for average friction entropy generation rate. The average wall temperature and temperature uniformity both drop accompanied with increasing pumping power with the growth in nanofluid volume fraction. The employment of dimples and protrusions significantly decreases the average entropy generation rate and improve the heat transfer performance. The effect of dimple-case shows great difference with that of protrusion-case. [ABSTRACT FROM AUTHOR]

We carry out experimental studies of turbulent convective heat transfer of several n-decane-in-water nanoemulsions and micelles-in-water fluids. We characterize the viscosity and thermal properties of the nanofluids, and the nanoparticle distributions and shapes with various techniques, including transmission electron microscopy. We find that the thermal conductivity of the nanofluids is lower than that of the base fluid, and the conductivity shows no anomalous behavior. Despite this we find that the average Nusselt numbers and convective heat transfer coefficients are enhanced for low volume fractions when the Reynolds number Re > 7000, in agreement with solid particle nanofluids. However, when the pressure losses are taken into account, we find that the nanofluids studied have a practical efficiency equal to or less than that of the base fluid. The highest heat transfer enhancements were obtained with volume fractions considerably smaller than those for the solid particle nanofluids. Our results indicate that the improved heat transfer is due to enhancement of turbulence in the large Re regime. [ABSTRACT FROM AUTHOR]

Nanofluid has great potentials in heat transfer enhancement and entropy generation decrease as an effective cooling medium. Effects of Al2O3-water nanofluid flow on entropy generation and heat transfer performance in a rectangular conventional channel are numerically investigated in this study. Four different volume fractions are considered and the boundary condition with a constant heat flux is adopted. The flow Reynolds number covers laminar flow, transitional flow and turbulent flow. The influences of the flow regime and nanofluid volume fraction are examined. Furthermore, dimples and protrusions are employed, and the impacts on heat transfer characteristic and entropy generation are acquired. It is found that the average heat transfer entropy generation rate descends and the average friction entropy generation rate rises with an increasing nanofluid volume fraction. The effect of nanofluid on average heat transfer entropy generation rate declines when Reynolds number ascends, which is inverse for average friction entropy generation rate. The average wall temperature and temperature uniformity both drop accompanied with increasing pumping power with the growth in nanofluid volume fraction. The employment of dimples and protrusions significantly decreases the average entropy generation rate and improve the heat transfer performance. The effect of dimple-case shows great difference with that of protrusion-case.

The fluid flow and heat transfer in a three-dimensional microchannel filled with Al2O3- water nanofluid is numerically investigated. The hybrid scheme is used to discretize the convection terms and SIMPLER algorithm is adopted to couple the velocity and pressure field in the momentum equations. The thermal and flow fields were analyzed using different volume fractions of nanoparticles and different Reynolds numbers. The temperature fields, the average Nusselt number on the bottom surface, the thermal resistance and the pressure drop were obtained from the simulations. Results indicated enhanced performance with the usage of nanofluids, and slight penalty in pressure drop. The increase in Reynolds number caused increase in the heat transfer rate as well as pressure drop and decrease in the thermal resistance. [ABSTRACT FROM AUTHOR]

Due to their superior thermophysical properties, there is a growing body of work on nanofluids in the field of thermal systems. However, there is no specific review of the role of the nanoparticle shape, which has been found crucial to their performance adjustment. A comprehensive literature review of the effect of nanoparticle shape on the hydrothermal performance of thermal systems utilizing nanofluids was compiled. The review covered the forced, mixed, and natural convection regimes and included heat exchangers, boundary layer flows, channel flows, peristaltic flows, impinging jets, cavity flows, and flows of hybrid nanofluids. It indicated that the control of nanoparticle shape is a promising technique for the optimization of heat exchange and the required pumping power. However, no uniform conclusion was reached for the role of nanoparticle shape on the hydrothermal performance of thermal systems. In most of the previous studies in the natural and forced convection regimes, the platelet–like nanoparticle acquired the highest heat transfer rate. However, most of the works in the mixed convection regime reported the best heat transfer performance for the blade–like nanoparticle. More research studies are required in future to determine the role of nanoparticle shape for thermal management of energy systems. • Nanoparticle shape is crucial to the performance control of various nanofluids. • The effects of nanoparticle shape on thermal system performance were reviewed. • Nanoparticle shape control is a promising technique for heat transfer optimization. • More research is required for the role of nanoparticle shape in thermal systems. [ABSTRACT FROM AUTHOR]

To improve the heat transfer characteristics of lubricant, graphene-based lubricants were prepared by adding graphene particles, due to its advantages of excellent thermal conductivity and two-dimensional sheet structure. In the present study, its physical properties were measured. A flow heat transfer experiment platform was built to study the flow and heat transfer characteristics of the graphene lubricating oil in a horizontal circular tube. The results show that the graphene lubricant prepared using a two-step approach had good stability, and the dispersibility was good without the agglomeration phenomenon, according to measurements undertaken using an electron microscope and centrifuge. The thermal conductivity and viscosity of graphene lubricant increased with the increase of the graphene concentration, and the thermal conductivity of graphene lubricant with the same concentration decreased with the increase of temperature. When the concentration was equal, the convective heat transfer Nusselt number (Nu) of graphene lubricant increased with the increase of Reynolds number (Re). When Re was equal, the convective heat transfer Nu increased with the increase of graphene particle concentration, and the maximum Nu increased by 40%. [ABSTRACT FROM AUTHOR]