I.M. Mahbubul

The green reduction of graphene oxide

Graphene is an ultra-thin material, which has received broad interest in many areas of science and technology because of its unique physical, chemical, mechanical and thermal properties. Synthesis of high quality graphene in an inexpensive and eco-friendly manner is a big challenge. Among various methods, chemical synthesis is considered the best because it is easy, scalable, facile, and inexpensive. Different kinds of chemical reducers have been used to produce graphene sheets. However, some chemicals are toxic, corrosive, and hazardous. For this reason, researchers have been using different environmentally friendly substances (termed green reducers) to produce functional graphene sheets. This paper presents an overview and discussion of the green reduction of graphene oxide (GO) to graphene. It also reviews the characterization of GO and its oxide reduction through the analysis of different spectroscopic and microscopic techniques such as Raman spectroscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and atomic force microscopy.

Effectiveness Study of a Shell and Tube Heat Exchanger Operated with Nanofluids at Different Mass Flow Rates

Several challenging issues, such as global warming, greenhouse effect, fuel security, and the high price of energy, motivate people to think about energy savings. Energy can be saved by effectively using available materials and facilities. Heat exchangers play a significant part in the field of energy conservation, conversion, and recovery. Nanofluids can be used in the heat exchangers to reduce global energy losses. Thermal performance of a shell and tube heat exchanger operated with nanofluids has been analytically investigated at different mass flow rates and compared with water as the base fluid. Suspensions of ZnO, CuO, Fe3O4, TiO2, and Al2O3 nanoparticles in water (W) at 0.03 volumetric fractions have been considered. It is found that, for a certain mass flow rate (50 kg/min) of tube side and shell side fluid, the highest heat transfer coefficient (h) belongs to Al2O3-Wnanofluid and the lowest to CuO-W nanofluid. However, maximum energy effectiveness (ϵ) improvement took place by 43% for ZnO-W nanofluid and minimum ϵ improvement that was around 31% happened for Al2O3-W nanofluid. Furthermore, this energy effectiveness also improved with the decrease of the mass flow rates of nanofluids and increasing the mass flow rates of the base fluid. However, changing the base fluids mass flow rate has a limited effect on this improvement of energy effectiveness. For example, a maximum overall heat transfer coefficient was found to vary from 18.31 to 18.35 W/m² · K for Al2O3-W nanofluid when changing the mass flow rate of nanofluid from 50 to 70 kg/min. On the other hand, with the same changes of hot water mass flow rates, a maximum overall heat transfer coefficient was obtained to shift from 18.31 to 22 W/m².K for the same nanofluid. Viscosity and specific heat of nanofluids are responsible for these phenomena. However, energy effectiveness of the shell and tube heat exchanger can be increased by using metal oxide nanofluids, and better performance can be achieved by maintaining higher mass flow rates of shell side fluid and lower mass flow rates for tube side fluid.

Heat Transfer Performance of Different Nanofluids Flows in a Helically Coiled Heat Exchanger

Helically coiled heat exchangers are globally used in various industrial applications for their high heat transfer performance and compact size. Nanofluids can provide excellent thermal performance of this type of heat exchangers. In the present study, the effect of different nanofluids on the heat transfer performance in a helically coiled heat exchanger is examined. Four different types of nanofluids CuO/water, Al2O3/water, SiO2/water, and ZnO/water with volume fractions 1 vol.% to 4 vol.% was used throughout this analysis and volume flow rate was remained constant at 3 LPM. Results show that the heat transfer coefficient is high for higher particle volume concentration of CuO/water, Al2O3/water and ZnO/water nanofluids, while the values of the friction factor and pressure drop significantly increase with the increase of nanoparticle volume concentration. On the contrary, low heat transfer coefficient was found in higher concentration of SiO2/water nanofluids. The highest enhancement of heat transfer coefficient and lowest friction factor occurred for CuO/water nanofluids among the four nanofluids. However, highest friction factor and lowest heat transfer coefficient were found for SiO2/water nanofluids. The results reveal that, CuO/water nanofluids indicate significant heat transfer performance for helically coiled heat exchanger systems though this nanofluids exhibits higher pressure drop.

Nanofluids for Thermal Performance Improvement in Cooling of Electronic Device

Nanofluid is a promising coolant for high-heat dissipation electronics device or system. The effect of nanofluids as thermal performances on a rectangular shape microchannel heat sink (MCHS) is analytically studied. Al2O3, SiC, and CuO nanoparticles dispersing in water were considered for analysis. A steady, laminar, and incompressible flow with constant heat flux was assumed in the channel. Nanofluids with concentrations of 0.5 to 4.0 vol. % were analyzed at two different inlet velocities of 0.5 m/s and 3.0 m/s. The results showed that highest thermal conductivity enhancement was 12.45% by using SiC-water nanofluids. In the case of Al2O3-water and CuO-water nanofluids maximum improvement were 11.98% and 11.36%, respectively for 4.0 vol. % of nanoparticle concentration. Furthermore, nanofluids as a coolant instead of water showed a highest improve of heat flux 8.51% for water-CuO, and 6.44% and 5.60% increase for Al2O3-water and SiC-water, respectively. The maximum pumping power found 0.33 W at 3 m/s and 0.0091 W at 0.5 m/s for the same concentration of 4.0 vol. % for all of these nanofluids.

Performance Investigation of a Plate Heat Exchanger Using Nanofluid with Different Chevron Angle

Plate heat exchanger with chevron angle has higher heat transfer area than flat type and increases the level of turbulent due to its corrugated channel. In this study, both water and nanofluid were used to determine the heat transfer coefficient and rate, pumping power, and pressure drop. A commercial plate heat exchanger with two different symmetric (300/300, 600/600) and one mixed (300/600) chevron angle plates were considered for analysis. Al2O3 and SiO2 nanoparticles with 0-1 vol. % concentration were used with water. From the analysis it was found that, convective heat transfer coefficient, heat transfer rate, pressure drop and pumping power increases with the increase of volume concentration. Moreover, the above parameters were found to be higher for 600/600 chevron angle plates. A correlation of Nusselt number as a function of Reynolds number and Prandtl number for different chevron angles needs to be obtained based on experimental and analytical work. Nomenclature

Global Effects of MWCNT-W Nanofluid in a Shell & Tube Heat Exchanger

Global warming and other problems can be reduced by effectively using the available materials and facilities. Heat exchangers play an important part of the field of energy conservation, conversion and recovery. Shell & tube heat exchangers are widely using in industrial processes and power plants. Suspension of small amounts of nanoparticles into the base fluid called nanofluid can reduce the global energy losses. Thermal conductivity of Multi Walled Carbon Nanotube (MWCNT) is highest among the different nano materials [1]. Therefore, in this paper, the overall performance of a shell & tube heat exchanger has been analytically investigated by using MWCNT-W nanofluid with 0.02-0.1 vol. fractions of MWCNT and compared with water. Mathematical formula, specifications of heat exchanger and nanofluid properties were taken from the literatures to analyze the energy performance and other effects within the system. It is found that for certain mass flow rates of nanofluid and base fluid, the convective heat transfer coefficient increased around 4% to 17% compared to pure water, respectively for 0.02-0.1 vol. fractions of MWCNT in water. However, for constant vol. fractions of MWCNT, convective heat transfer coefficient of the above nanofluid negligibly changed for different mass flow rates. Furthermore, energy effectiveness of the heat exchanger also improved approximately by 3% to 14%, respectively. This energy effectiveness again improved with the decrease of the mass flow rates of nanofluids (tube side) and increase of the mass flow rates of base fluid (shell side). As energy effectiveness is increased by using MWCNT-W nanofluid, therefore, a significant amount of heat losses will be reduced. As a result, with the reduced heat emissions, global warming and greenhouse effects can be reduced by using MWCNT-W nanofluid as working fluid in shell & tube heat exchanger system.

Influence of Nanoparticle Type, Size and Weight on Migration Properties of Nanorefrigerant

Refrigerant-based nanofluids are termed as nanorefrigerants, which are capable of improving the performance of refrigeration systems. Refrigerants act as coolants due to their low boiling temperature. Therefore, the condition of nanoparticles during this phase change needs to be clarified. In this paper the migration properties of nanoparticles during pool boiling of a nanorefrigerant have been experimentally studied. The effects of nanoparticle type, size and weight on the migration of nano-sized particles have been investigated. Al2O3 and TiO2 particles, each with two different average diameters, were used with R141b refrigerant as the base fluid. Experimental results show that migrated mass of nanoparticles increases with the increase of initial mass of nanoparticles and sizes of nanoparticles as well. However, migration of nanoparticles decreases with the increase of the density of nanoparticles. Hence, migration properties of nanoparticles have a notable relationship with the distribution of nano-sized particles.