Bi doped 2CaO-Al.sub.2O.sub.3-SiO.sub.2 (gehlenite) glasses were prepared by conventional melting. The concentration of Bi.sup.3+ was 0, 0.25, 0.50 and 1.50 mol. %. The prepared samples were X-ray ...amorphous except of the undoped sample and the sample with the highest content of Bi.sup.3+, for which XRD records revealed traces of crystalline gehlenite.
In this paper we derived an expression that allows the determination of the thermo-optic coefficient of weakly-guiding germanium-doped silica fibers, based on the thermal behavior of optical fiber ...devices, such as, fiber Bragg gratings (FBGs). The calculations rely on the full knowledge of the fiber parameters and on the temperature sensitivity of FBGs. In order to validate the results, we estimated the thermo-optic coefficient of bulk GeOsub.2 glass at 293 K and 1.55 μm to be 18.3 × 10sup.−6 Ksup.−1. The determination of this value required to calculate a correction factor which is based on the knowledge of the thermal expansion coefficient of the fiber core, the Pockels’ coefficients (p sub.11 = 0.125, p sub.12 = 0.258 and p sub.44 = −0.0662) and the Poisson ratio (ν = 0.161) of the SMF-28 fiber. To achieve that goal, we estimated the temperature dependence of the thermal expansion coefficient of GeOsub.2 and we discussed the dispersion and temperature dependence of Pockels’ coefficients. We have presented expressions for the dependence of the longitudinal and transverse acoustic velocities on the GeOsub.2 concentration used to calculate the Poisson ratio. We have also discussed the dispersion of the photoelastic constant. An estimate for the temperature dependence of the thermo-optic coefficient of bulk GeOsub.2 glass is presented for the 200–300 K temperature range.
The thermal expansion coefficient (TEC) of single-layer graphene is estimated with temperature-dependent Raman spectroscopy in the temperature range between 200 and 400 K. It is found to be strongly ...dependent on temperature but remains negative in the whole temperature range with a room temperature value of (−8.0 ± 0.7) × 10–6 K–1. The strain caused by the TEC mismatch between graphene and the substrate plays a crucial role in determining the physical properties of graphene, and hence its effect must be accounted for in the interpretation of experimental data taken at cryogenic or elevated temperatures.
A new engineering medium, called nanofluid attracted a wide range of researches on many cooling processes in engineering applications, which are prepared by dispersing nanoparticles or nanotubes in a ...host fluid. In this paper, the stability of nanofluids is discussed as it has a major role in heat transfer enhancement for further possible applications. It also represents general stabilization methods as well as various types of instruments for stability inspection. Characterization, analytical models and measurement techniques of nanofluids after preparation by a single step or two-step method are studied.
Experimental investigations have been carried out for determining the thermal conductivity of three nanofluids containing aluminum oxide, copper oxide and zinc oxide nanoparticles dispersed in a base ...fluid of 60:40 (by mass) ethylene glycol and water mixture. Particle volumetric concentration tested was up to 10% and the temperature range of the experiments was from 298 to 363
K. The results show an increase in the thermal conductivity of nanofluids compared to the base fluids with an increasing volumetric concentration of nanoparticles. The thermal conductivity also increases substantially with an increase in temperature. Several existing models for thermal conductivity were compared with the experimental data obtained from these nanofluids, and they do not exhibit good agreement. Therefore, a model was developed, which is a refinement of an existing model, which incorporates the classical Maxwell model and the Brownian motion effect to account for the thermal conductivity of nanofluids as a function of temperature, particle volumetric concentration, the properties of nanoparticles, and the base fluid, which agrees well with the experimental data.
A critical synthesis of the variants within the thermophysical properties of nanofluids is presented in this work. The experimental results for the effective thermal conductivity and viscosity ...reported by several authors are in disagreement. Theoretical and experimental studies are essential to clarify the discrepancies in the results and in proper understanding of heat transfer enhancement characteristics of nanofluids. At room temperature, it is illustrated that the results of the effective thermal conductivity and viscosity of nanofluids can be estimated using the classical equations at low volume fractions. However, the classical models fail to estimate the effective thermal conductivity and viscosity of nanofluids at various temperatures. This study shows that it is not clear which analytical model should be used to describe the thermal conductivity of nanofluids. Additional theoretical and experimental research studies are required to clarify the mechanisms responsible for heat transfer enhancement in nanofluids. Correlations for effective thermal conductivity and viscosity are synthesized and developed in this study in terms of pertinent physical parameters based on the reported experimental data.
This study investigates the impact of incorporating TiOsub.2 nanoparticles into two types of oils at different temperatures and with varying volume fractions: transformer oil (NYTRO LIBRA) and virgin ...coconut oil (manufactured by Govi Aruna Pvt. Ltd., Gampaha, Sri Lanka). The nanofluids were prepared using a two-step method by adding CTAB (cetyltrimethylammonium bromide) surfactant. To minimize nanoparticle agglomeration, this study employed relatively low-volume fractions. Thermal properties by means of thermal conductivity, thermal diffusivity, and volumetric heat capacity were measured in accordance with ASTM (American Society for Testing and Materials) standard methods using a multifunctional thermal conductivity meter (LAMBDA thermal conductivity meter). The measured thermal conductivity values were compared with theoretical models and previous research findings. It was confirmed that the modification of thermal properties was enhanced by doping TiOsub.2 nanoparticles with different volume fractions.
We report in this paper the successful production of stable high-concentration graphene dispersions in low-boiling-point, low-polarity conventional organic solvents (chloroform and THF) by ...liquid-phase noncovalent exfoliation of graphite assisted with a hyperbranched polyethylene (HBPE) as the stabilizer. In the exfoliation process, HBPE adsorbs onto the surface of exfoliated graphene flakes, providing steric stabilization against their restacking. A systematic investigation on the effects of exfoliation conditions, including the solvent and the amounts of graphite and HBPE, has been conducted. Graphene dispersions with the concentration up to 0.18 mg/mL in chloroform and 0.045 mg/mL in THF have been obtained. It is also demonstrated that the dispersions can be further concentrated by solvent evaporation to give highly concentrated stable dispersions at 3.4 mg/mL. Through their characterizations with transmission electron microscopy, atomic force microscopy, and Raman spectroscopy, the majority of the graphene products is found to be high-quality, defect-free, few-layer graphene flakes with the layer number between 2 and 4 and the lateral dimension in the range of 0.2–0.5 μm. The dispersions can be fabricated into flexible conductive free-standing graphene films and be used to prepare graphene/ethylene copolymer composites through solution blending, which show significant enhancements in both thermal and mechanical properties.
In this research, the effects of using nanofluid as a coolant on the thermal and electrical efficiencies of a PV/T (photovoltaic thermal unit) are experimentally studied. Coolant fluids in the ...experiments are pure water and silica (SiO2)/water nanofluid 1% and 3% by weight (wt%). A brief uncertainty analysis is performed which shows that the measurements are sufficiently accurate. By converting the output electrical energy of the PV/T system into an equivalent thermal energy, it is found that the overall energy efficiency for the case with a silica/water nanofluid of 1 wt% is increased by 3.6% compared to the case with pure water. When using the silica/water nanofluid of 3 wt%, however, the increase is 7.9%. The thermal efficiency of the PV/T collector for the two cases of 1 wt% and 3 wt% of silica/water nanofluids are increased by 7.6% and 12.8%, respectively. The total exergy of the PV/T system, with and without nanofluids, is also compared with that of the PV system with no collector. It is observed that by adding a thermal collector to a PV system, the total exergy for the three cases with pure water, 1 wt% silica/water nanofluid, and 3 wt% silica/water nanofluid is increased by 19.36%, 22.61% and 24.31%, respectively.
•The effects of adding nanofluids as a coolant on a PV/T system are investigated.•Experiments are performed on SiO2/water nanofluids (1 wt % and 3 wt %).•The system is analyzed from both energetic and exergetic viewpoints.•Adding nanofluids to the PV/T system increased overall efficiency up to 7.9%.•Total exergy of the PV/T system with nanofluids was increased up to 24.31%.
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