Sharif Digital Repository

In the present paper, several experimental and theoretical studies conducted on the thermal conductivity of nanofluids are represented and investigated. Based on the reviewed studies, various factors affect thermal conductivity of nanofluids such as temperature, the shape of nanoparticles, concentration and etc. Results indicated the increase in temperature and concentration of nanoparticles usually leads to the higher thermal conductivity of nanofluids. In addition, it is concluded that there are some novel approaches in order to obtain nanofluids with more appropriate thermal properties including using binary fluids as the base fluid or utilizing hybrid nanofluids. © 2018 Elsevier B.V  

Nanofluids containing Fe3O4 and carbon nanotubes nanoparticles emulsified and dispersed using gum arabic (GA) and tetramethylammonium hydroxide (TMAH) were made and characterized for potential use as heat transfer fluids. Due to the interaction between the TMAH and GA molecules, the magnetic nanoparticles and CNTs were physically adsorbed. This paper reports an experimental work on the effect of ultrasonication on thermal conductivity of this aqueous suspension. The characterization and surface morphology of the dried samples were studied by using XRD and TEM measurements. Experiments were conducted in the magnetic nanoparticles mass concentration range 0.494-2.428%,... 

In the present study, nanoscale iron oxide was synthesized using a hydrothermal method; XRD analysis revealed that all the produced crystals are iron oxide. FESEM microscopic imaging showed that particles are on the scale of nano and their morphology is cloud fractal. To study the laboratory properties of thermal conductivity, viscosity, and electrical conductivity of the nanoparticles, they were dispersed in ethylene glycol-based fluid and the nanofluid was in a two-step synthesis during this process. The experiments were carried out with a weight fraction between 0 and 2 % at temperatures between 25 and 45 °C. According to the results of the experiments, increasing the density of... 

Just a few investigations have been conducted to study the mixed nanofluids(MNs), which contain more than one type of nanoparticles, despite considerable advances in the field of nanofluids thermal conductivity. In present research, by combining different volume fractions of various nanoparticles, the variation of mixed nanofluids thermal conductivity was considered. The mentioned nanofluids have different fabrication cost. First, the effect of each specific nanoparticle presence in the base fluid on the thermal conductivity of nanofluid was surveyed both experimentally and theoretically. Then, the thermal conductivities of two MNs, one consisted of a metallic nanoparticle (high thermal... 

In this study, a hybrid of silica nanosphere/multiwall carbon nanotube (MWCNT) has been synthesized by wet chemical method at room temperature. The effect of MWCNTs, silica nanospheres and hybrid nanostructures (80% silica nanosphere/20% MWCNT and 50% silica nanosphere/50% MWCNT) on the thermal conductivity of distilled water has been investigated. SDBS was used as the dispersant to stabilize nanomaterials in the aqueous suspension and its concentration was 1.5 times of the concentration of nanomaterials. As results show, by increasing the concentration of nanomaterials, effective thermal conductivity of nanofluids increased. The most and the least enhancement in the effective thermal... 

In this paper, the thermal conductivity of water-based hematite Fe2O3 and magnetite Fe3O4 nanofluids have been investigated in the absence and presence of a uniform magnetic field. The experiments have been performed in the volume concentration range of 0 % to 4.8 % and the temperature range of 20∘C to 60∘C. The effects of the particle volume fraction, temperature, and magnetic field strength on the thermal conductivity have been analyzed. Results show that the thermal conductivity of iron oxide nanofluids has a direct relation with the particle volume fraction and temperature, without the presence of a magnetic field. But surprisingly, when the magnetic field is applied, it is observed that... 

Experimental investigations are performed in order to determine the thermal conductivity of NiFe2O4 nanoparticles dispersed in deionized water. The magnetic nanoparticles are synthesized using a microemulsion method. The X-ray diffraction (XRD), transmission electronic microscopy (TEM), and vibration sample magnetometer (VSM) are used to characterize the structure, the size and the magnetic properties of the nanoparticles. The VSM results disclose that the NiFe2O4 nanoparticles are ferromagnetic at room temperature. Experimental measurements on thermal conductivity of the prepared nanofluids are conducted at different volume concentrations between 0% and 2% and in the temperature range of... 

In this work, two-dimensional laminar flow and heat transfer across a heated square cylinder, covered by a porous layer in a plane channel have been numerically investigated. The flow and thermal fields inside the porous layer were simulated using BrinkmaneForchmeyer extended Darcy model. Simulations were performed in different Reynolds numbers (Re = 60, 120, 160, and 200), porosities (ω = 0.7, 0.87, and 0.96), solid to fluid thermal conductivity ratios (λR = 10, 200, and 2000) and blockage ratios (BR = 0.5, 0.25 and 0.125). The effects of the mentioned parameters on pressure drop and heat transfer rate were investigated in detail. Also, the contribution of each side of the central squared... 

The present work examines experimentally the effect of magnetic field on the viscosity and thermal conductivity of a hybrid nanofluid containing tetramethylammonium hydroxide (TMAH) coated Fe3O4 nanoparticles and Gum arabic (GA) coated carbon nanotubes (CNTs). The hybrid nanofluid was prepared by using ultrasonic dispersion method. Magnetic field was created by a pair of spaced apart magnet plates. The effect of temperature on the time variation of thermal conductivity under applied magnetic field was also investigated. According to the results of this study, viscosity of the hybrid nanofluid increases with the strength of magnetic field, while it decreases with the increase of temperature.... 

Thermal conductivity of nanofluids plays key rol in heat transfer capacity of fluids. adding nanoparticles to a base fluid can lead to enhancement in thermal conductivty ratio. CuO/Ethyle Glycol (EG) is one of the most applicable nanofluids for heat transfer purposes. In the present study, thermal conductivty ratio of CuO/EG nanofluid is modeled by applying Group Method of Data Hnadling and Least Square Support Vector Machine – Gentic Algorithm approaches. Results indicated that the utilized model are very accurate in predicting thermal conductivty ratio of the nanofluid. The R-squared values for the proposed model are equal to 0.994 and 0.991 by applying Group Method of Data Handling and... 

Pulsating Heat Pipes (PHPs) are cooling devices that are compact in size and have an ability to transfer heat in low temperature differences. Working fluids strongly affect the thermal performance of PHPs. In this paper, effects of some thermophysical parameters relating to working fluids, such as boiling point, latent heat of vaporization, surface tension, thermal conductivity and dynamic viscosity, are presented based on experimental and numerical studies done in recent years. Addition of nanoparticles to fluids, or making nanofuild, is a new method of improving thermophysical properties of fluids. Recently, many studies are carried out on thermophysical properties of nano-fuild. Results... 

Supercritical water was used for simultaneous fragmentation and reduction of graphene oxide (GO) sheets into water-dispersible graphene quantum dots (GQDs) with tunable sizes. Transmission electron microscopy (TEM) demonstrated that by increasing the temperature above the critical point of water, the average size and thickness of the GQDs were decreased and the size uniformity and production yield were increased. The results of thermal conductivity measurement of GQD nanofluids with different weight fractions indicated that the GQDs prepared at supercritical condition could enhance the thermal conductivity of water by 65% as compared to 35% for the GQDs synthesized at sub-critical... 

In this research, it is aimed to enhance the heat transfer properties of the carbon nanotubes through nitrogen doping. To this end, nitrogen-doped multiwall carbon nanotubes (N-CNTs) were synthesized via chemical vapor deposition method. For supplying carbon and nitrogen during the synthesis of N-CNTs, camphor and urea were used, respectively, at 1000 °C over Co–Mo/MgO nanocatalyst in a hydrogen atmosphere. N-CNTs with three different nitrogen loadings of 0.56, 0.98, and 1.38 mass% were synthesized, after which, water/N-CNT nanofluids of these three samples with concentrations of 0.1, 0.2, and 0.5 mass% were prepared. To obtain a stable nanofluid, N-CNTs were functionalized by nitric acid... 

Heat pipes are useful devices in heat transfer and particularly, in cooling systems. Given the high demand for cooling systems in various applications, an improvement in the performance of heat pipes has gained much attraction in recent years. In this study, the effects of utilizing working fluids with different thermal properties on the performance of pulsating heat pipes (PHP) are experimentally studied. Hence, nano-encapsulated phase change material (NPCM), reduced graphene oxide nanosheets, and their mixture, as a novel hybrid nanofluid, are prepared and dispersed in water as a working fluid. NPCM at 3 concentrations of 5, 10, and 20 g/L, as well as nanosheets at three concentrations of...