سال:1392 | دوره: | شماره: |تعداد مقالات:8

A nanofluid is a dilute liquid suspension of particles with at least one critical dimension smaller than ~100 nm. Researches so far suggest that nanofluids offer excellent heat transfer enhancement over conventional base fluids. The enhancement depends on several factors such as particle shape, particle size distribution, volume fraction of nanoparticles, temperature, pH, and thermal conductivities of nanoparticles and base fluids.This paper presents an updated review on nanofluids with the emphasis on heat transfer enhancement including formulation, physical properties, biological and non-biological applications, stability, possible mechanisms for the enhancement of heat conduction, and numerical modelling of nanofluids. Based on the research findings, a number of challenges are emphasized in order to understand the underlying physics for future industrial take-up of the nanofluids technology. Further computational studies are also required in order to understand all of the factors affecting on the enhancement of thermal conductivity of nanaofluids.

In this article, the second quantization method has been used to investigate some thermodynamic properties of spin-polarized metallic nanowire with radius 5 and 10 nm and infinite length, in the presence of magnetic field at zero temperature. The spin-polarization parameter corresponding to the equilibrium state of the system has been plotted as a function of density for different values of the magnetic field. We have concluded that for the magnetic fields less than 100T at high densities, the system nearly becomes unpolarized. However, for higher magnetic fields, the system has a substantial spin-polarization, even at high densities. Finally, we have calculated the pressure and incompressibility vs. the density for different values of the magnetic field.

The structural and energetic characteristics of O3-H2O complexes have been investigated by means of B3LYP, MP2, MP4 (SDTQ), CCSD (T) and QCISD (T) methods in conjunction with AUG-cc-pVDZ and AUG-cc-pVTZ basis sets. Six conformers were found for the O3- H2O complex. Two different intermolecular interactions were expected to participate in the formation of complexes, namely conventional O¼H hydrogen bonding and O¼O interaction. The most stable structure is non-hydrogen bonded one with double O¼O interactions. The binding energies of the most stable complex, corrected with BSSE and ZPE, range from -5.99 to -12.20 kJ mol-1 at CCSD (T) /AUG-cc-pVTZ, QCISD (T) /AUG-cc-pVTZ and MP4 (SDTQ) /AUG-cc-pVTZ high levels of theory. The equilibrium distance between centers of monomers (O3¼OH2) in the most stable complex at the CCSD (T) /AUG-cc-pVDZ and CCSD (T) /AUG-cc-pVTZ levels is 2.9451 and 2.9448 Ao, respectively, in good agreement with the experimental value of 2.957 Ao. The AIM calculations predict that the O¼O and O¼H interactions in O3-H2O complexes are electrostatic in nature.

This study demonstrates the spectral data for two triarylmethane dyes, malachite green and crystal violet, at room temperature. Effects of micellar environment and crown ether addition on the spectral behaviour of these dyes are studied using absorption spectroscopic method. The dye-anionic surfactant interactions in micellar media are investigated. The chosen surfactants with different alkyl chain length are SOS, STS, SHS and SDS. Effect of a crown-ether (i.e. aza-15-crown-5) addition on the absorption spectra and decolorization of the dyes is also studied at room temperature. Moreover, effect of crown ether addition on the micellar properties of the media is studied and compared. Spectroscopic analysis shows that the dye decolorization can be accelerated in presence of the crown ether. Conversely, the decolorization process can be inhibited in the micellar media. The nature of decolorization process is interpreted in terms of direct and indirect mechanisms of crown-ether and the dye interactions.

Molecular dynamics simulation has been performed to analyze the effect of the presence of gold nanoparticle on dynamics of Kob-Anderson binary Lennard-Jones mixture upon supercooling within the framework of the mode coupling theory of the dynamic glass transition. The presence of gold nanoparticle has a direct effect on the liquid structure and causes the peaks of the radial distribution functions to become shorter with respect to the bulk binary Lennard-Jones liquid. It is found that the dynamics of the liquid at a given density is consistent with the mode coupling theory (MCT) predictions. In accordance with the idealized MCT, the diffusion constants D(T) show power-law behavior at low temperatures for both types of binary Lennard-Jones (BLJ) particles as well as the 13 gold atoms comprising the nanoparticle. The mode coupling crossover temperature Tc is the same for all particle types, however, Tc=0.4 is reduced with respect to that of the bulk BLJ liquid and the exponent is found to depend on the particle type. The existence of the nanoparticle causes the short-time b-relaxation regime to shorten and the range of validity of the MCT shrink with respect to the bulk BLJ. It is also found that the behavior of intermediate scattering function (ISF) is in agreement with MCT prediction and in spite of the presence of gold nanoparticle the time temperature superposition principle at intermediate and low temperatures is still valid and the curves of ISF vs. t/t(T) fall onto a master curve.

The kinetic of Fischer-Tropsch synthesis over a co-precipitated Fe-Ce catalyst was investigated in a fixed bed micro reactor. Experimental conditions were varied as follow: reaction pressure 1-15 bar, H2/CO feed ratio of 1-3 and space velocity of 3600-5400 h-1 at the temperature range of 270-310oC. Four models according to the Langmuir-Hinshelwood-Hougen-Watson (LHHW) type rate equation were derived on the basis of detailed set of possible reaction mechanisms. The reaction rate of this study is fitted fairly well by one kinetic expression based on LHHW mechanism. The kinetic parameters were estimatedvia non-linear regression method and the MARR% values for all proposed models were obtained. The activation energy was 72 kJ mol-1 for optimum kinetic model. This model had the lower MARR% value than those of the other proposed models.

The effect of interactions of various Ia and IIa cations with two positions of the adenine-thymine (A-T) and guanine-cytosine (G-C) base pairs on the geometries and individual hydrogen bond (HB) energies have been investigated by using the atoms in molecules (AIM) method at the B3LYP/6-311++G (d, p) level of theory. The cations that possess higher charge/radius (q/rad) ratio make higher changes on the individual HB energies and the total binding energies ofG-C base pair, while no meaningful changes are observed for the A-T base pair. The effect of coordination with cations on the H-bond energies ofA-T type complexes, especially in the X’- A-T derivatives, is slight and different from the GC complexes. The changes of total binding energies are most important in the presence of bivalent cations. The estimated individual EHBs are in good agreement with the results of natural bond orbital (NBO) analysis and also with the structural parameters of HBs.

The structural and electrostatic properties of the single-walled two representative (8, 0) zigzag and (4, 4) armchair models of pristine and GaAs-doped on boron phosphide nanotubes (BPNTs) were investigated by calculating the nuclear magnetic resonance tensors and with performing the density function theory. The geometrical structures of all representative pristine and GaAs-doped models of BPNTs have been allowed to relax by optimization and then the isotropic and anisotropic chemical shielding (CS) parameters (CSI and CSA) of 11B and 31P have been calculated. The results reveal that with doping gallium and arsenic atoms in spite of boron and phosphorus atoms, the geometrical structure, the band gape energy between HOMO and LUMO orbitals and NMR parameters of the boron and phosphorus sites change. Comparisons of results reveal that the variation NMR parameters and band gape energy of zigzag model are more than those of armchair model. The NMR properties of boron atoms show only slight changes but those of phosphorous atoms are more notable.