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The heat transfer performance of a closed-loop pulsating heat pipe (CLPHP) having 2.2 mm inner diameter is experimentally studied at different filling ratios (40%, 50%, 60% and 70%) in a heat load range of 50–300 W using distilled water as the PHP fluid in vertical orientation with evaporator at the bottom. This experimental study validate that the optimum charging ratio is 50% for CLPHP while 40% filling ratio is good at lower heat inputs. Optimum filling ratio gives an average reduction of 26.0% and 14.8% in thermal resistance compared to 70% and 60% fill ratios. The heat transfer performance of the CLPHP is also studied at various volume fractions of graphene–water nanofluid using 50% filling ratio. This study shows that the optimum volume fraction for this CLPHP is 0.045%. The volume fraction of 0.045% gives an average reduction of 27.3% in thermal resistance for all heat inputs and a maximum temperature reduction of 24 °C for the evaporator wall temperature at 275 W heat input. When optimum values of filling ratio and volume fraction are used, CLPHP gives the best heat transfer performance. A correlation is also developed for CLPHP by considering the influential parameters. [ABSTRACT FROM AUTHOR]

This study examined a room with a surface of 1.8 × 1.8 × 2.85 (m) and a well-insulated floor (adiabatic condition) and examined the heat exchange from the side surfaces and ceiling. In this closed room, the heat transfer effects with radiation were investigated while bringing them to comfort conditions ranging from 30 (°C) air temperature to 20–24 (°C). A computer with a power of 25 (W) as a source of heat, a person with an average metabolic activity of 50–70 (W) and a table were found in this closed room. In this study, the cooling of the room from the floor, ceiling and air conditioner was inquired while the computer was running, in a closed area under the specified heat transfer conditions. As a scenario, the air exchange coefficient was modeled via Ansys Fluent, fed with air of 15 (°C) with 1, 3, 5, 10, 15 air. In addition, the comfort values of the human wrist (the distance of 0.1 m) and the shoulder (the distance of 1.1 m) were researched according to ASHRAE-55. The obtained results were analyzed as a comparison of ACH results, and the comfort parameter values were analyzed by reading the sections taken from the ankle shoulder level and velocity, temperature, values according to ANSI/ASHRAE-55. The relative humidity was 50% in the room, while the metabolic activity is 1.2 (met). These parameters corresponded to the sitting position; the clothing effect was found to be 0.67 (clo). The novelty of this study encourages the production of the ideal CFD analysis on the thermal comfort conditions of a cooled room, the task of engineering. [ABSTRACT FROM AUTHOR]

We investigate, in this analysis, the entropy generation characteristics associated with the transport of nanofluid in a microfluidic channel under the combined influences of applied pressure gradient and electrical forcing. In our study, the nanofluid is subjected to an asymmetric cooling at the channel walls, while the conductive transport of heat through the channel walls is also taken into account. We show that the underlying thermo-electro-hydrodynamics of nanofluids in the channel lead to entropy generation, attributed to the irreversibilities associated with heat transfer, viscous dissipation, and Joule heating effects. We establish that a non-trivial interplay among these irreversibilities gives rise to an optimum value of the geometrical parameter, viz. the channel wall thickness (δ ), and the thermophysical parameters, viz. the thermal conductivity of the wall (γ) , Biot number (Bi), and the modified Peclet number ( Pe ¯ ), leading to a minimum entropy generation rate of the system. Also, we unveil through this study that changes in the electroosmotic parameter κ ¯ (representative of the EDL thickness) or the composition of the fluid (ϕ , the volume fraction of nanoparticles agglomerates) non trivially alter the optimum values of these parameters. Inferences drawn from this analysis may have consequences in the optimum design of thermal systems/devices, typically used for thermal management in micro-heat exchangers, micro-reactors, and micro heat pipes. [ABSTRACT FROM AUTHOR]

Thermal performance evaluation of TiO2-coated copper mesh wick in a cylindrical heat pipe with graphite nanofluid is experimentally analyzed at various inclinations, nanoparticle concentrations and power levels. Boiling heat transfer from the evaporator of a heat pipe depends on both thermal conductivity of the nanoparticle and suspension stability of the nanofluid. The lower the density of the nanoparticle, the better the suspension stability. Spherical graphite nanoparticles having lower density and good thermal conductivity quicken the heat transfer rate and hence the vaporization of base fluid. A hydrophilic coating of TiO2 is done on the copper wick structure to reduce the contact angle of graphite nanofluid. Results showed that the heat pipe worked well at 60° inclination compared to the other tested orientations. For this optimum condition, a reduction in 168.75% of thermal resistance is obtained compared with DI water with uncoated wick at horizontal position and also an improvement in thermal efficiency of 94.07% for 1.0 mass% particle loading. The enhancement in equivalent thermal conductivity is 90.87% for 1.0 mass% compared with DI water. Results from the repeatability test also confirm that the hydrophilic coating over the wick is stable, and results are repeatable. [ABSTRACT FROM AUTHOR]

Conical enclosures rely on the conical cavity and can be used as radiation concentrators. Two circular cross-section baffles were used to improve the heat transfer of this geometry. By changing the rigid fins to porous, it could improve the heat transfer. Al2O3/water nanofluid was also employed to enhance the heat transfer performance of the cavity. For this purpose, numerical analysis of three-dimensional natural convection heat transfer was performed in a conical cavity with two types of fins. The best combination of fins arrangement for the next step was selected using the differential evolutionary optimization method (D.E). In this case study, a new combination of laminar and turbulence methods was employed for the first time to increase the accuracy of the natural convection solution. This combination is based on the laminar solution by suppressing the perturbation parameter in the turbulence method which led to more accurate results. The analysis results showed that a conical cavity with optimized fin geometry can lead to a 23% increase in Nu. The best porosity for the inner fin was calculated 40% in the case of constant porosity. Ascending porosity along the fin, whose increase was more intense near the base and slower near the cone's tip, was the best variable porosity for the inner fin. [ABSTRACT FROM AUTHOR]

The natural heat convection within a square enclosure filled with water–Al2O3 nanofluid has been studied numerically in the presence of a magnetic field. The effect of heat source geometry attached to the bottom wall on the Nusselt number was investigated by changing its nondimensional width and height, and side walls thickness of the enclosure ranging from 0.1 to 0.5, 0.1 to 0.8 and 0.05 to 0.2, respectively. A regression model has been obtained along with conducting a sensitivity analysis seeking an optimal heat transfer. Results, reveal that Nusselt number increases by enlarging the fin, and reaching a peak point before it declines. Thus, interestingly, the ever-increasing heat transfer by means of fin size does not retain and there is an optimal point wherein the maximum heat transfer occurs. Moreover, the thermal performance of the system largely depends on the fin size rather than the relative side walls thickness. However, its effect intensifies as the fin width increases. Results of optimization show that the maximum heat transfer occurs at W = 0.4615 , H = 0.6467 and L b = 0.2 . [ABSTRACT FROM AUTHOR]

A numerical investigation is carried out to assess the hydrothermal performance of a water-based hybrid nanofluid containing both Fe3O4 (magnetite) nanoparticles and carbon nanotubes (CNTs) in a heated tube in the presence of a constant non-uniform magnetic field. The magnetic field is created by three pairs of permanent magnets. The effects of Reynolds number, magnetite, and CNT volume concentrations as well as magnetic field strength are investigated. The acquired data for the case of without magnetic field confirmed higher values of heat transfer and pressure drop as a result of utilizing nanofluid compared with water. Additionally, it was found that the Nusselt number and pressure drop of the studied nanofluid samples increase significantly under the magnetic field. Moreover, the influence of magnetic field increases with an increase in the nanoparticle concentrations and magnetic field strength and decrease in the Reynolds number. The maximum increments of 109.31% and 25.02% in comparison with the case of without field were obtained in the average Nusselt number and pressure drop for hybrid nanofluid containing 0.9% magnetite and 1.35% CNT at Reynolds number of 500. [ABSTRACT FROM AUTHOR]

This article studies buoyancy-driven natural convection of a nanofluid affected by a magnetic field within a square enclosure with an individual conductive pin fin. The effects of electromagnetic forces, thermal conductivity, and inclination angle of pin fin were investigated using non-dimensional parameters. An extensive sensitivity analysis was conducted seeking an optimal heat transfer setting. The novelty of this work lies in including different contributing factors in heat transfer analysis, rigorous analysis of design parameters, and comprehensive mathematical analysis of solution domain for optimization. Results showed that magnetic strength diminished the heat transfer efficacy, while higher relative thermal conductivity of pin fin improved it. Based on the problem settings, we also obtained the relative conductivity value in which the heat transfer is optimal. Higher sensitivity of heat transfer was, though, noticed for both magnetic strength and fin thermal conductivity in comparison to fin inclination angle. Further studies, specifically with realistic geometrical configurations and heat transfer settings, are urged to translate current findings to industrial applications. [ABSTRACT FROM AUTHOR]

In the present study, the effect of volume concentration (0.05, 0.1 and 0.15 %) and temperature (10-90 °C) on viscosity and surface tension of graphene-water nanofluid has been experimentally measured. The sodium dodecyl benzene sulfonate is used as the surfactant for stable suspension of graphene. The results showed that the viscosity of graphene-water nanofluid increases with an increase in the volume concentration of nanoparticles and decreases with an increase in temperature. An average enhancement of 47.12 % in viscosity has been noted for 0.15 % volume concentration of graphene at 50 °C. The enhancement of the viscosity of the nanofluid at higher volume concentration is due to the higher shear rate. In contrast, the surface tension of the graphene-water nanofluid decreases with an increase in both volume concentration and temperature. A decrement of 18.7 % in surface tension has been noted for the same volume concentration and temperature. The surface tension reduction in nanofluid at higher volume concentrations is due to the adsorption of nanoparticles at the liquid-gas interface because of hydrophobic nature of graphene; and at higher temperatures, is due to the weakening of molecular attractions between fluid molecules and nanoparticles. The viscosity and surface tension showed stronger dependency on volume concentration than temperature. Based on the calculated effectiveness of graphene-water nanofluids, it is suggested that the graphene-water nanofluid is preferable as the better coolant for the real-time heat transfer applications. [ABSTRACT FROM AUTHOR]

The addition of nanoparticles to a base fluid is one of the significant issues to enhance heat transfer. In this study, different nanofluids were developed by mixing a water base fluid with magnetic nanoparticles. Thermophysical properties such as thermal conductivity and viscosity of the obtained nanofluid were investigated. The effect of different nominal diameters of nanoparticles and concentrations of nanoparticles on the thermal conductivity and viscosity of nanofluids have been examined. Three different diameters of magnetic nanoparticles (about 37 nm, 71 nm, and 98 nm) have been tested in this experimental investigation. Experimental results indicate that thermal conductivity increases as volume fraction increases, and thermal conductivity of the nanofluid increases with a decrease of nanoparticle's size. Moreover, the nanofluid dynamics viscosity ratio increases with an increase in particle concentration and nanoparticle's diameter. This paper identifies several important issues that should be considered in future work. [ABSTRACT FROM AUTHOR]