1,721,196 research outputs found
GENERATION OF SINGLET OXYGEN BY 2,2'/5',2''-TERTHIOPHENE AND SOME OF ITS DERIVATIVES
No full textThe kinetics of oxygen quenching of the lowest triplet states of 2,2':5',2#-terthiophene and several of its substituted derivatives in 95% ethanol were analysed as part of an investigation of the chemical and physicochemical basis of the phototoxicity of these compounds. Overall bimolecular rate constants and singlet oxygen (O-2((1) Delta(g))) formation quantum yields were measured by laser flash photolysis and time-resolved phosphorescence detection. With the exception of one compound, both parameters proved to be almost independent of the chemical nature, position and number of substituents, thus failing to indicate an explanation for the very large differences in phototoxicity observed between the various derivatives during in vivo experiments. On the basis of the measured kinetic parameters, a schematic description of the mechanism of interaction with oxygen of these compounds in their lowest triplet states is proposed
Structural and spectroscopic characterisation of 2-(2'-hydroxybenzoyl)pyrrole and its O-methyl derivative
No full textDensity functional theory calculations on the tautomeric and conformational equilibria of 2-(2''-hydroxybenzoyl)pyrrole (HBP) and 2-(2''- methoxybenzoyl)pyrrole (MBP) were performed. Moreover, the experimental IR and UV spectra of the same compounds were recorded and compared with the theoretical data. The presence of an intramolecular hydrogen bond in HBP can be related to the biological activities of some of its derivatives
Fully developed laminar flow and heat transfer in serpentine pipes
No full textA serpentine pipe is a sequence of parallel straight pipe segments connected by U-bends. Its geometry is fully characterized by pipe radius, a, bend curvature radius, c and length of the straight segments, l. The repeated curvature inversion forces the recirculation (secondary flow) pattern to switch between two specular configurations, which may enhance mixing and heat or mass transfer with respect to a constant-curvature pipe at the cost of an increase in pressure drop. In the present work, fully developed laminar flow and heat transfer in serpentine pipes were investigated by numerical simulation. The curvature d = a/c was made to vary between 0.1 and 0.5 while the parameter g = l/c was made to vary between 0 and 8; for each geometry, the friction velocity Reynolds number Ret = uta/n was made to vary between a very low value (5), yielding almost creeping flow, and the highest value Re∗t still yielding steady laminar flow (~35e40 in most cases). For Ret Re∗t results were obtained for values of the Prandtl number between 1 and 100; predicted values of the friction coefficient and of the Nusselt number were compared with experimental results and correlations proposed in the literature. For Ret Re∗t convergence to steady flow was not achieved and an oscillatory behaviour of the solution was observed, indicating a transition to unsteady regimes which deserves a more focused study
Natural Convection Cooling of a Hot Vertical Wall Wet by a Falling Liquid Film
Full text linkThe system studied is a plane channel in which one of the two vertical walls is kept at an arbitrary temperature profile and may be partially or completely wet by a falling liquid film, while the opposite wall is adiabatic. Air from the environment flows along the channel with a mass flow rate which depends on the balance between hydraulic resistances and buoyancy forces. These latter, in their turn, depend on the distribution of temperature and humidity (hence, density) along the channel and eventually on the heat and mass transferred from wall and film to the humid air.
A simplified computational model of the above system was developed and applied to the prediction of relevant quantities, such as the total energy subtracted to the hot wall, as functions of the geometrical and physical quantities that characterize the problem (channel height and thickness, localized hydraulic resistance, hot wall temperature and its distribution, film flow rate, ambient air temperature and humidity.). Conclusions were also drawn on the cooling strategy to be adopted in the case when only a limited amount of coolant is available
On the influence of gravitational and centrifugal buoyancy on laminar flow and heat transfer in curved pipes and coils
No full textThe effects of gravitational and centrifugal buoyancy on laminar flow and heat transfer in curved and helical pipes were investigated by numerical simulation. Six dimensionless numbers characterizing the problem were identified, and an analysis was conducted on the possible combinations of signs of the gravitational and centrifugal buoyancy effects. Two distinct Richardson numbers were introduced in order to quantify the importance of the two types of buoyancy, and it was shown that, in the case of heating from the wall, a maximum realizable value of the centrifugal Richardson number exists which is a linear function of the curvature δ (ratio of pipe radius a to curvature radius c). Detailed results were obtained for δ = 0.9, torsion λ (ratio of reduced pitch H/(2π) to curvature radius c) = 0 (toroidal pipe) or 0.4 (helical pipe), Re = 100, Pr = 1 and gravitational and centrifugal Richardson numbers Rig, Ric each varying from −0.1 to +0.1. A complex interaction between the two forms of buoyancy was found to exist. In the helical geometry, provided |Rig| ≈ |Ric| they exhibited effects of the same order. The lowest values of both the friction coefficient and the mean Nusselt number were obtained in the presence of positive gravitational and centrifugal buoyancy, while the highest values were obtained when both forms of buoyancy were negative; the reason for this behavior was identified in the different degree of coupling between the distributions of axial velocity and temperature. In the toroidal geometry, a simpler behavior was predicted due to the presence of top–bottom symmetry; both the friction coefficient and the mean Nusselt number were found to decrease with the intensity of centrifugal buoyancy and to be little affected by gravitational buoyancy in the range of Rig investigated
Direct numerical simulation of turbulent heat transfer in curved pipes
No full textFully developed turbulent convective heat transfer in curved pipes was investigated by Direct Numerical Simulation for a friction velocity Reynolds number of 500, yielding bulk Reynolds numbers between 12 630 and ~17 350 according to the curvature (pipe radius/curvature radius). Three different curvatures were compared, i.e. 0 (straight pipe), 0.1 and 0.3. The Prandtl number was 0.86. The computational domain was a tract of pipe 5 diameters in length. A finite volume method was used, with multiblock structured grids of ~5.3x10E6 hexahedral volumes. Simulations were typically protracted for 20 LETOT’s starting from coarse-grid results. Results were post-processed to compute first and second order time statistics, including rms fluctuating temperature and turbulent heat fluxes, on a cross section of the pipe. In curved pipes, time-mean results exhibited Dean circulation and a strong velocity and temperature stratification in the radial direction. Turbulence and heat transfer were strongly asymmetric, with higher values near the outer pipe bend. Overall turbulence levels were lower than in a straight pip
Selective and efficient synthesis of phototoxic 2,2':5',2"-terthiophene derivatives bearing a functional substituent in the 3'- or 5-position
No full textStraight fiber bundles with non-uniform porosity: Shell-side hydrodynamics and mass transfer in cross flow
Full text linkThis study explores fully developed shell-side hydrodynamics and mass transfer past straight fiber bundles with non-uniform porosity in cross-flow. Simplified geometries made up by checkerboard arrays of alternately high and low porosity regions are considered. Simulations are performed for two domain sizes: a small geometry (26 fibers) and a large geometry (104 fibers). In the small geometry, the Darcy friction coefficient (fT) exhibits hydraulic isotropy at low transverse flow Reynolds numbers (ReT) but becomes dependent on the flow attack angle (θ) at higher ReT. In the large geometry, this dependency is observed at lower ReT. A non-uniform porosity reduces fT at almost all ReT and θ in the small geometry, with the large geometry exhibiting a more complex behavior. Regarding mass transfer, up to ReT≈1-10 (depending on θ), a non-uniform porosity leads to lower Sherwood numbers compared to regular square arrays. However, at higher ReT, it enhances mass transfer
Synthesis and characterization of 2,2':5',2"-terthiophene derivatives of possible therapeutic use
No full textSeveral derivatives of 2,2':5',2''-terthiophene, 1a, and a structural analogue of this natural phototoxin, i.e. 5-[4-(1,2-methylenedioxy)phenyl]-2,2'-bithiophene, 2, have been conveniently synthesized on a medium scale using a general approach which involves palladium-catalyzed carbon-carbon bond forming reactions between (hetero)aryl halides and heteroaryl organometallics such as Grignard reagents, organozinc and organotin compounds. These 2,2':5',2''-terthiophene derivatives, which include some naturally-occurring compounds, have been characterized by EIMS spectrometry, H-1 and C-13 NMR spectroscopy as well as by absorption and fluorescence measurements. Four of these phototoxins exhibit considerable antibiotic and antiviral properties in the presence of UVA light and are cytotoxic against the P 815 mouse mastocytoma cell line
Turbulent heat transfer in spacer-filled channels: Experimental and computational study and selection of turbulence models
Full text linkHeat transfer in spacer-filled channels of the kind used in Membrane Distillation was studied in the Reynolds number range 100–2000, encompassing both steady laminar and early-turbulent flow conditions. Experimental data, including distributions of the local heat transfer coefficient h, were obtained by Liquid Crystal Thermography and Digital Image Processing. Alternative turbulence models, both of first order (k-ε, RNG k-ε, k-ω, BSL k-ω, SST k-ω) and of second order (LRR RS, SSG RS, ω RS, BSL RS), were tested for their ability to predict measured distributions and mean values of h. The best agreement with the experimental results was provided by first-order ω-based models able to resolve the viscous/conductive sublayer, while all other models, and particularly ε-based models using wall functions, yielded disappointing predictions
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