排序方式: 共有114条查询结果,搜索用时 140 毫秒
1.
M. Akbari A. Behzadmehr F. Shahraki 《International Journal of Heat and Fluid Flow》2008,29(2):545-556
Fully developed laminar mixed convection of a nanofluid consists of water and Al2O3 in horizontal and inclined tubes has been studied numerically. Three-dimensional elliptic governing equations have been solved to investigate the flow behaviors over a wide range of the Grashof and Reynolds numbers. Comparisons with previously published experimental and numerical works on mixed convection in a horizontal and inclined tube are performed and good agreements between the results are observed. Effects of nanoparticles concentration and tube inclinations on the hydrodynamics and thermal parameters are presented and discussed. It is shown that the nanoparticles concentration does not have significant effects on the hydrodynamics parameters. Heat transfer coefficient increases by 15% at 4 Vol.% Al2O3. Skin friction coefficient continually increases with the tube inclination, but the heat transfer coefficient reaches a maximum at the inclination angle of 45°. 相似文献
2.
Biodegradable polyurethane elastomers with potential for applications in medical implants with tunable degradation rate and physical properties were synthesized from reaction of epoxy terminated polyurethanes (EUP) with 1,6-hexamethylene diamine (HMDA) as curing agent. Poly(ε-caprolactone) (PCL) and poly(ethylene glycol) (PEG) as well as 1,6-hexamethylene diisocyanate (HDI) were used for preparation of isocyanate terminated polyurethanes which were subsequently blocked with glycidol to prepare EUPs. All materials were characterized by conventional methods, and their properties were studied fully. Results showed that elastomers based on PEG exhibit superior degradation rate and inferior mechanical properties in comparison to elastomers based on PCL. Optimum degradation rate and mechanical properties were obtained from elastomers made from mixture of PCL and PEG base EUPs. 相似文献
3.
In this paper,an axial dispersion mathematical model is developed to simulate a three-phase slurry bubble column reactor for direct synthesis of dimethyl ether(DME) from syngas.This large-scale reactor is modeled using mass and energy balances,catalyst sedimentation andsingle-bubble as well as two-bubbles class flow hydrodynamics.A comparison between the two hydrodynamic models through pilot plantexperimental data from the literature shows that heterogeneous two-bubbles flow model is in better agreement with the experimental data thanhomogeneous single-bubble gas flow model.Also,by investigating the heterogeneous gas flow and axial dispersion model for small bubblesas well as the large bubbles and slurry(i.e.including paraffins and the catalyst) phase,the temperature profile along the reactor is obtained.Acomparison between isothermal and non-isothermal reactors reveals no obvious performance difference between them.The optimum values ofreactor diameter and height were obtained at 7 m and 50 m,respectively.The effects of operating variables on the axial catalyst distribution,DME productivity and CO conversion are also investigated in this research. 相似文献
4.
Mehdi Shahraki Sayyed Mostafa Habibi-Khorassani Ali Ebrahimi Malektaher Maghsoodlou Younes Ghalandarzehi 《Structural chemistry》2013,24(2):623-635
The chemoselectivity of geometrically ylide compounds is often hard to assign from experimental techniques, particular system with intramolecular hydrogen bonding (IHB) are even more challenging. Herein, theoretical calculations were performed to investigate whether theoretical results would provide consistent evidence for the existence of IHB to confirm experimental data and to evaluate strength of the N–H···O IHB from geometrical synthesized 2-substituted pyrrole stable phosphorus ylide (dimethyl 2-(1H-pyrrol-2-yl)-3-(triphenylphosphoranylidene) butanedioate in a single chemoselective compound. Topological parameters at the bond critical points (BCP) of intramolecular hydrogen bonds from Bader’s atoms in molecules (AIM) theory and Winhold’s natural bond orbital (NBO) calculations were analyzed at the B3LYP/6-311++g** level in details. A series of gage-including atomic orbital chemical shift (GIAO c.s.) calculations at the HF and DFT levels of theory were carried out to assign the 1H NMR chemical shifts. The best prediction of the experimental 1H NMR values was obtained at the mPW1PW91 levels using the 6-31G** basis set. Theoretical results, in agreement with the experimental data, were confirmed the N–H···O IHB was caused the deshielding of the proton to lower field. The barriers in double bond and single bond rotation were theoretically estimated in detailed and the AIM and NPA approaches were confirmed the loss of charge of the hydrogen atom involving in intramolecular N–H···O hydrogen bonding. The geometrical and topological parameters from AIM and NBO analyses were indicated the medium N–H···O IHB. 相似文献
5.
Ali Ebrahimi Mostafa Habibi‐Khorassani Asiyeh Shahraki 《Photochemistry and photobiology》2013,89(1):74-82
Radical cationic repair process of cis–syn thymine dimer has been investigated when (1) sugar‐phosphate backbones were substituted by hydrogen atoms, (2) phosphate group was substituted by two hydrogen atoms each on a sugar ring and (3) sugar‐phosphate backbone was taken into account. The effect of the interactions between N1 and N1′ lone pairs and the C6‐C6′ antibonding orbital are the most important evidences for the cleavage of the C6‐C6′ bond in the first step of radical cationic repair mechanism in the absence of the sugar‐phosphate backbone. The impact of the N1 and N1′ lone pairs on the C6‐C6′ bond cleavage decreases and the energy barrier of the cleavage of that bond significantly increases in the presence of the deoxynucleoside sugars and the sugar‐phosphate backbone. 相似文献
6.
Mir Mohammad Alavi Nikje Mohammad Amin Farahmand Nejad Keyvan Shabani Moslem Haghshenas 《Colloid and polymer science》2013,291(4):903-909
Super paramagnetic Fe3O4@SiO2 nanoparticle was incorporated into polyurethane rigid foams in order to prepare new corresponded magnetic nanocomposite foams via one-shot method. The core–shell-structured nanoparticles were prepared by sol–gel method and characterized by transmission electron microscopy, X-ray diffraction, as well as Fourier transform infrared spectroscopy techniques. Magnetic nanoparticles were used up to 3 % in the foam formulations and the samples prepared successfully. Thermal, mechanical, and magnetic properties of nanocomposites were studied and the results showed superior properties in comparison with pristine foams. 相似文献
7.
Elnaz Iranmanesh Moslem Jahani Azizollah Nezhadali Maliheh Mojarrab 《Journal of separation science》2020,43(6):1164-1172
This paper describes the synthesis of a molecularly imprinted polymer by chemical oxidation of pyrrole as the functional monomer, and at the presence of guaifenesin as the template. The prepared polymer was used as adsorbent in molecularly imprinted solid‐phase extraction followed by spectrophotometric determination. Different parameters in the solid‐phase extraction including sample pH, adsorbent weight, washing solution, and elution solvent were studied to determine optimum conditions for isolation and enrichment of guaifenesin. The results showed guaifenesin was quantitatively adsorbed on the molecularly imprinted polymer at pH 6.0 and completely eluted with an ethanol–water solution (50% v/v). An enrichment factor of four with satisfactory recoveries (87.0–95.0%) was obtained. The solid‐phase extraction columns could be used for up to six consecutive elution‐loading cycles without significant decreases in the analyte recoveries. The method had a dynamic range of 3.0 × 10?6–1.5 × 10?4 mol/L with a limit of detection and limit of quantification of 1.4×10?6 and 4.5×10?6 mol/L, respectively. The proposed procedure was used for the extraction and determination of guaifenesin in different pharmaceutical formulations, with satisfying results being achieved. 相似文献
8.
Nonlinear Dynamics - Different architectures and techniques have developed in the neuromorphic field to mimic and investigate the activity of biological neural networks. This paper presents a set... 相似文献
9.
M. Ghoranneviss A. Shokouhy M. M. Larijani S. H. Haji Hosseini M. Yari A. Anvari M. Gholipur Shahraki A. H. Sari M. R. Hantehzadeh 《Pramana》2007,68(1):135-140
This work presents the results of a low-energy nitrogen ion implantation of AISI 304 type stainless steel (SS) at a moderate
temperature of about 500°C. The nitrogen ions are extracted from a Kauffman-type ion source at an energy of 30 keV, and ion
current density of 100 μA cm−2. Nitrogen ion concentration of 6 × 1017, 8 × 1017 and 1018 ions cm−2, were selected for our study. The X-ray diffraction results show the formation of CrN polycrystalline phase after nitrogen
bombardment and a change of crystallinity due to the change in nitrogen ion concentration. The secondary ion mass spectrometry
(SIMS) results show the formation of CrN phases too. Corrosion test has shown that corrosion resistance is enhanced by increasing
nitrogen ion concentration.
相似文献
10.
S. K. El-Labany W. M. Moslem Kh. A. Shnishin S. A. El-Tantawy 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2011,61(2):409-420
By using the hydrodynamic equations of positive and two negative ions,
Boltzmann electron density distribution, and Poisson equation with immobile
positive/negative dust particles, a cylindrical Korteweg-de Vries (CKdV)
equation is derived for small but finite amplitude ion-acoustic waves. At the
critical total negative ion concentration and/or the critical density rate of
the second-negative ions, the pulses collapse at this limit as nonlinearity
fails to balance dispersion. Then the CKdV equation is not appropriate to
describe the system. Therefore, the modified CKdV (MCKdV) and extended CKdV
(ECKdV) equations are derived at the critical plasma compositions and in the
vicinity of the critical plasma compositions, respectively. The physical
parameters of two plasma environments (e.g., Xe+–F-–SF6-_{6}^{-} and
Ar+–F-–SF6-_{6}^{-} plasmas) are examined on the wave phase velocity and
the nonlinear localized pulse profile. The latter should satisfy necessary
condition to exist. The localized pulse of Ar+–F-–SF6-_{6}^{-} plasma is
much spiky than Xe+–F-–SF6-_{6}^{-} plasma. Thus, the mass ratio of the
negative-to-positive ions is focused upon and it emphasizes to play an
important role on the pulse profile. Dependence of the geometrical divergence
on the pulse profile is also investigated, which indicates that the localized
pulse damps with time. The implications of our results agrees with the
experimental observations. 相似文献