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This research paper focuses on CFD analysis for comparative evaluation of different hybrid nanofluids (NFs): MWCNT-Al 2 O 3 /H 2 O, MWCNT-MgO/H 2 O, Al 2 O 3 -CuO/H 2 O performance as working fluid (WF) inside parabolic trough solar collector (PTSC). The performance of the collector is evaluated using different performance indicators, including Nusselt number (Nu), friction factor (f), and Performance Evaluation Criterion (PEC) for different Reynolds number (Re) (10000-50000). Among the investigated hybrid nanoparticles (NPs), MWCNT-Al 2 O 3 emerges as the most promising choice for significantly enhancing the system's thermal performance with the highest PEC value of 1.9. This outcome signifies the superior efficiency of MWCNT-Al₂O₃/H₂O hybrid NF in enhancing convective heat transfer, thus underlining the paramount significance of adopting them in thermal systems operating within the specified Re range. [ABSTRACT FROM AUTHOR]

The present paper, for the first time, examines the influences of utilizing oil-ZnO nanofluid with different volume fractions φ = 0.5 %, 1 %, and 2 % in alternating flattened tubes (AFTs) with different alternating pitch angles of 30°, 45°, 60°, and 90° on the performance of a dual-tube heat exchanger (DTHE). This work is conducted experimentally and numerically for the Reynolds number (Re) range of 300 < Re < 1900 for oil-ZnO nanofluid and Re = 2000 for water. Based on the experimental results, the optimal case is selected for the numerical simulations of AFTs. The performance evaluation criterion (PEC) is defined for the simultaneous evaluation of pressure drop (Δp) and heat transfer coefficient (HTC). The results demonstrate that the overall heat transfer coefficient (U) and Δp are augmented with the inlet flow rate and the alternating angle between the pitches. Therefore, the maximum heat transfer (HT) and Δp correspond to the AFTs with the angle of 90° (AF4) at Re = 1900. The PEC amount of AF4 shows a 56 % enhancement compared to the circular tube. It is also observed that using copper oxide nanoparticles inside the oil improves the HT rate and Δp in the heat exchanger. Besides, an increment in φ increases U and Δp; however, the values of PEC show that the positive effects of the nanofluid are larger than their negative impacts in such a way that the PEC is improved by 64 % when the nanofluid with φ = 2 % is utilized in AFTs compared to the circular tube. [ABSTRACT FROM AUTHOR]

Active and passive cooling are the two possible methods for removing heat. An active cooling system is the one that involves the use of energy as opposed to passive cooling that uses no energy. Passive cooling methods are cost effective and more reliable than active cooling due to the absence of moving parts. Microchannel heat sink is one of high-tech devices that have widely considered passive cooling methods especially for electronics cooling. In this paper, the use of passive cooling methods in microchannel heat sink is comprehensively discussed. This paper also present the effects of some important parameters such as the type of channel types, surface roughness, fluid additives, and Reynolds number on the rate of heat transfer in microchannel heat sink. Finally, the conclusions and important summaries were presented according to the data collected. [ABSTRACT FROM AUTHOR]

In this article, turbulent flow characteristic of nanofluids is thoroughly reviewed. Turbulent flows have unique characteristics and are preferred in many industrial applications. Therefore, this paper reviews different techniques used to enhance heat transfer using nanofluids within turbulent regime. This paper also presents the effects of some important parameters such as nanoparticle type, nanoparticles concentration, and Reynolds number on heat transfer rate. Studies on numerical techniques are also discussed. Finally, the conclusions and important summaries are presented according to the data collected. [ABSTRACT FROM AUTHOR]

The study of the forced convection in a channel has many practical applications. In this paper, the forced convection heat transfer from surface mounted blocks attached to the bottom wall of a horizontal channel with nanofluid is numerically studied by the second-order lattice Boltzmann method (LBM). The effects of Reynolds numbers and geometrical parameters of the blocks in different aspect ratios on the flow field and temperature distribution for various volume fractions of nanofluid (φ = 0, 0.01, 0.03 and 0.05) are analyzed. Also, the influence of these parameters is investigated on the local and average Nusselt numbers. It is concluded that heat transfer in channels can be enhanced by using the block on the walls and adding nanoparticles. There is a maximum value of 39.04% increase in average heat transfer coefficient for all the examined cases compared to the base fluid (i.e., water). [ABSTRACT FROM AUTHOR]

The present paper deals with entropy generation study of CuO/water nanofluid flow over a spherical heat source that is installed inside a cylindrical channel with sudden expansion and contraction. To estimate the entropy generation rate; first, the governing equations are solved numerically to find the velocity and temperature fields in the domain where the flow is steady and laminar. Next, using the simulation data, the entropy generation number is calculated theoretically. The effects of nanofluid concentration, nanoparticle size, blockage ratio, and Reynolds number on the entropy generation have been investigated. The outcomes of this research elucidate that entropy generation number rises with an increase in the nanoparticle volume fraction and surface temperature of the heat source. On the other hand, it was found that the entropy generation is diminished by implementing finer nanoparticles and lowering the blockage ratio. In this study, a new correlation has also been presented to calculate the Nusselt number that is a function of Reynolds number, Prandtl number, and blockage ratio. [ABSTRACT FROM AUTHOR]

Presented research paper is an investigation of water, paraffin and ethylene glycol based Al 2 O 3 nanofluid. Heat transfer coefficient is measured using shell and tube heat exchanger under the conditions of fully developed laminar and turbulent flow. Concentration of nanoparticles in the respective base fluids was varied from 0.01 vol% to 0.08 vol% in the step of 0.1%. Nanofluids were used as the working fluid and sent to tubular side. The effects of Reynold's number and concentration of nanoparticles in nanofluid on heat transfer characteristics were studied. A significant enhancement in heat transfer coefficient and thermal conductivity of nanofluid over base fluid is found in the study. As the concentration of nanoparticles was increased, heat transfer coefficient was also improved marginally. At greater turbulence, the improvement in the heat transfer coefficient of nanofluid over base fluid was found to be more than that at lower turbulence. This is due to homogeneous distribution of nanoparticles in a base fluid at higher temperatures was achieved. The various factors that were considered in the characterization of nanofluid heat transfer coefficients are: sonication time, temperature, pressure difference, base fluid (water, paraffin and EG). Pressure drop in case of nanofluid was higher than base fluid in the turbulent regime; however, no significant change was observed in the laminar region. [ABSTRACT FROM AUTHOR]

In the present work, the effect of channel cross section on the heat transfer performance of an oblique finned micro-channel heat sink was investigated. Water and Al 2 O 3 /water nanofluid of volume fraction 0.25% were used as a coolant. The oblique finned microchannels are designed with three channel cross-sections namely square, semicircle and trapezoidal. The primary work of this paper is to study the heat transfer and hydrodynamic characteristics in the oblique finned microchannel. The experimental setup and procedure are validated using water as coolant in a micro-channel heat sink. Heat transfer and flow characteristics are examined for three cross-sections of varying mass flux. The trapezoidal channel cross-section increases the considerable heat transfer rate improvement for both water and nanofluid by 3.133% and 5.878% compared to square and semicircle cross section. Also, the pressure drop is higher in the trapezoidal cross-section over the square and semicircle cross section. This is due to increase in friction loss of trapezoidal cross section. The results indicate that trapezoidal cross-section oblique finned micro-channel is more suitable for heat transfer in the electronic cooling application. [ABSTRACT FROM AUTHOR]

In this paper, nanofluid forced convection heat transfer is investigated in existence of magnetic field. Three dimensional simulations are presented by means of Lattice Boltzmann Method. Koo-Kleinstreuer-Li (KKL) model is considered to estimate the properties of nanofluid. Roles of Hartmann number, Reynolds number, Al 2 O 3 volume fraction are illustrated graphically. Outputs are depicted in forms of velocity, isokinetic energy, streamlines, isotherms contours and Nusselt number. Results demonstrate that velocity of nanofluid augments with rise of Reynolds number and Al 2 O 3 volume fraction but it reduces with increase of Hartmann number. Convection mode reduces with enhance of Lorentz forces. Temperature gradient over the moving wall augments with augment of hot surface velocity and Al 2 O 3 volume fraction. [ABSTRACT FROM AUTHOR]

In this paper, the performance of ethylene glycol and water based TiO 2 nanofluids as an automobile radiator coolant is determined experimentally. Forced convective heat transfer coefficient of TiO 2 nanofluids has been measured and compared with the data of base fluid in an automobile radiator. Nanofluids were prepared taking 40% ethylene glycol and 60% water with volume concentrations of 0.1%, 0.3% and 0.5% of TiO 2 nano powder. All the experiments were conducted in the range of Reynolds numbers from 4000 to 15,000. In all the experiments the nanofluids made to flow through the radiator tubes with elliptical cross section and airflows with constant speed in the crosswise direction in between the tubes of a tube bundle. Results demonstrate that increasing the fluid circulation rate can improve the heat transfer performance while the fluid inlet temperature to the radiator has little or no effect. Nanofluids investigated in the present work with low concentrations enhanced the heat transfer rate up to 37% in comparison with base fluid. The results of the investigation are presented graphically in terms of non-dimensional heat transfer coefficient as a function of volume concentration, temperature and flow rate. [ABSTRACT FROM AUTHOR]

Heat transfer characteristics of Fe 2 O 3 /water and Fe 2 O 3 /EG nanofluids were measured in a shell and tube heat exchanger under laminar to turbulent flow condition. In the shell and tube heat exchanger, water and ethylene glycol-based Fe 2 O 3 nanofluids with 0.02%, 0.04%, 0.06% and 0.08% volume fractions were used as working fluids for different flow rates of nanofluids. The effects of Reynold's number, volume concentration of suspended nanoparticles and different base fluids on the heat transfer characteristics were investigated. Based on the results, adding nanoparticles to the base fluid causes a significant enhancement of the heat transfer characteristics and thermal conductivity. This enhancement was investigated with regard to various factors; concentration of nanoparticles, types of base fluids, sonication time and temperature of fluids. In this paper, the effect of Fe 2 O 3 nanoparticles on the thermal conductivity of base fluids like ethylene glycol and water was studied. The thermal conductivity measurement was made for different concentrations and temperatures. As the concentration of the nanoparticles increased, there was a significant enhancement in thermal conductivity and overall heat transfer due to more interaction between particles. It was also observed that there was an improvement in the thermal conductivity of the base fluid as the temperature increased. The measurements also showed that the pressure drop of nanofluid was higher than that of the base fluid in a turbulent flow regime. However, there was no significant increase in pressure drop at laminar flow. [ABSTRACT FROM AUTHOR]

The paper reports an experimental investigation into the thermal performance and pressure drop characteristics of nanofluid-cooled heat sinks and then compares the results with the data for water-cooled heat sinks. Heat sinks with miniature circular-fin (MCFHS) and square-fin (MSFHS) structures are used and made from aluminum material with dimensions of 28 × 33 mm. Similarly, SiO 2 nanoparticles dispersed in deionized water with particle concentrations of 0.2%, 0.4%, and 0.6% volume are used as working fluids. The effects of pin fin configuration, particle concentration, and flow rate on the heat transfer performance and flow behaviors are presented. Reynolds numbers based on the hydraulic diameter of each flow channel ranging between 700 and 3700, fluid temperature of 15 °C, and heat flux ranging from 2 and 5 W/cm 2 are tested. Hydraulic diameters based on each flow channel are equally designed at 1.2 mm for both heat sinks. The experimental results indicate that the heat transfer coefficient increased with increasing Reynolds numbers and particle concentrations. The MCFHS gave greater heat transfer performance than that of the MSFHS by about 6–9%. For pressure drop data, the measured data showed that the pin fin configuration and particle concentration had small effects on the pressure drop and pumping power. [ABSTRACT FROM AUTHOR]

This paper investigates experimentally the effects of laminar nanofluid flow over the microscale backward-facing step (MBFS) and forward-facing step (MFFS) on the heat transfer characteristics. The experiments were implemented on MBFS and MFFS with a step height of 600 μm. Both MBFS and MFFS have the upstream and downstream lengths of 0.1 m and 0.15 m respectively. The Reynolds number ranged of 280–480. The concentrations of SiO 2 nanoparticle valued at 0.005 and 0.01 with a diameter of 30 nm were immersed in a distilled water. The experimental results revealed that the concentration of 0.01 water–SiO 2 nanofluid recorded the highest Nusselt number. The comparison between MBFS and MFFS revealed that the highest Nusselt number is obtained through the use of the MFFS, which is approximately twice that of MBFS. However, the friction factor recorded a higher value for MFFS. The experimental results were in a good agreement with the numerical published results. [ABSTRACT FROM AUTHOR]

• An optimization methodology is proposed for heat transfer enhancement studies. • MOO and MCDM are combined to find optimum design parameters. • Optimization of nanofluid flows in pipes with inserts are examined. • Effect of design parameters on thermal-hydraulic performance are investigated. A hybrid approach that combines Multi-objective Optimization (MOO) and Multiple-criteria Decision-making (MCDM) techniques has been introduced in the present paper for heat transfer augmentation in pipes. The combination of two passive heat transfer enhancement techniques namely the nanofluid and insert employment have been applied in the representative problem. In this context, the performance of water-based C u O , F e 3 O 4 , and C N T - F e 3 O 4 nanofluids and two different inserts as twisted tape (TT) and wire coil (WC) were evaluated and their optimum design and flow parameters were determined. The methodology introduced here has two major parts. Firstly, a MOO study was conducted employing the experimentally obtained correlations for the Nusselt number (Nu) and the friction factor (f) as objective functions. The optimum values of the selected design parameters (the Reynolds number (Re), nanoparticle volume fraction ((ϕ)), twist ratio (h/d), wire coil pitch ratio (p/d), and the Prandtl number (Pr)) that maximizes N u while minimizing f were found as Pareto front. Secondly, the Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) method was used to help decision-makers making an optimal choice among the Pareto optimal solutions that meet their requirements. In the MCDM problems, two criteria (Nu and f) and fifteen alternatives were evaluated for each case. A sensitivity analysis was also conducted by changing the weights of criteria. It was found that the combination of TT and C N T - F e 3 O 4 with the values of design parameters (R e ; ϕ ; h / d ; P r)=(20,000; 0.27%; 10; 5.50) exhibited the best thermal-hydraulic performance for the equal weights of criteria. [ABSTRACT FROM AUTHOR]

In this paper, 3D analysis of forced convection of Newtonian and non-Newtonian nanofluids through a horizontal pipe using single- and two-phase mathematical models is performed. The models include the Newtonian single-phase model, non-Newtonian single-phase model, Newtonian two-phase mixture model and finally non-Newtonian two-phase mixture model. The geometry of the problem is a horizontal pipe that is filled with Al 2 O 3 /water nanofluid. The effects of particle volume fraction and the Reynolds number on the heat transfer and hydrodynamic characteristics of the flow are examined. Results indicate that for the volume fraction of 0.01, the non-Newtonian mixture model gives more accurate results in comparison with the other models. By increasing the solid volume fraction, the mixture models give a higher heat transfer coefficient in comparison with that of the corresponding single-phase model; however, by comparing the results of Newtonian and the corresponding non-Newtonian models, no meaningful relationship can be obtained. Furthermore, for all the investigated models, the Nusselt number is increased with increasing the Reynolds number. [ABSTRACT FROM AUTHOR]

• Investigating forced convection of a nanofluid through a wavy-sinusoidal minitube using two-phase model. • Evaluation of the effect of geometrical and thermal parameters to achieve the most advantageous geometry. • Wavy amplitude, wavelength and phase shift have serious impact on wavy-wall minitube PEC. • PEC amount for optimum tube is about 1.670. In present study heat transfer and forced convection flow of non-Newtonian two-phase ethylene-glycol/water mixture based nanofluid through a wavy-sinusoidal minitube has been investigated. The 2D governing system of equations is solved numerically in the computational field using the Control-Volume-Approach founded on the SIMPLE technique. Flow velocities or Reynolds numbers are varied in the turbulent regime range from Re =6000 to Re =15,000 and the tube walls boundary condition is uniform temperature. The optimization was conducted considering five different constant and variable α(x) amplitudes, five various constant and variable λ(x) wavelengths and five various phase shifts in order to achieve the most advantageous geometry in view of the highest thermal-hydraulic performance evaluation criterion index. From obtained results in present paper, it has been realized that the wavy-wall minitube thermal-hydraulic performance is seriously affected with varying the wavy amplitude, wavelength and phase shift. The highest thermal-hydraulic PEC has been obtained for the amplitude of α=0.5 mm with increasing wavelength and phase shift of θ=90° at Re =6000 Reynolds number. After opting of the optimum geometry, Silicon-Carbide (SiC) nanoparticles were added to base fluid in volume fraction of 4% and sizes of 16 to 90 nm. Based on results, the PEC of EG/water based nanofluid with nanoparticle diameter of d np =90 nm has been conducted to be the most efficient among all models during investigated range of flow velocities. PEC amount for optimum tube using EG/water based SiC nanofluid with volume fraction of ϕ=4% and nanoparticle diameter of d np =90 nm at Re =15,000 is about 1.670. [ABSTRACT FROM AUTHOR]

The experimental system: (a) Schematic diagram. (b) Physical diagram. • Effects of round hole diameter on the flow and heat transfer are studied. • Effects of pitch-row on the flow and heat transfer are studied. • Thermal and exergy efficiency for thermo-hydraulic performances are studied. For investigating the thermal and hydraulic characteristics of water-based SiO 2 nanofluids in a triangular tube with different turbulators, an experimental system has been designed and verified in this paper. The effects of different round hole diameters (d = 3 mm, 4 mm, 5 mm) and round hole pitch-rows (l = 5 cm, 10 cm, 15 cm) of perforated turbulators on the thermo-hydraulic characteristics are researched. Meanwhile, the influences of Reynolds numbers (Re = 400–8000) and nanoparticles mass fractions (D-I water, ω = 0.1%, 0.3%, 0.5%) are also studied. These experimental results show that, under the same circumstance, the nanofluids in the triangular tube with ω = 0.5% have the largest positive influence on the heat transfer enhancement ratio which is up to 16.73%. For a comprehensive study of the flow and heat transfer, thermal efficiency (comprehensive performance index) and exergy efficiency are adopted. It can be found that the larger the diameter and the smaller the pitch-row of the holes is, the greater the comprehensive evaluation index can be. In addition, all working conditions exhibit the superior exergy efficiency. The highest exergy efficiency can be got when Re = 6000 and ω = 0.5%. [ABSTRACT FROM AUTHOR]