Thermosolutal convection in a horizontal porous layer heated from below in the presence of a horizontal through flow

In this paper, we study the effect of a homogeneous longitudinal through flow on the onset of convection in a horizontal porous layer saturated by a binary fluid and heated from below or above. The layer boundaries are subjected to a constant heat flux. The investigation is made by taking the Soret effect into account. It is found that in the case of positive separation ratio when the denser component moves toward the cooler wall, through flow has no effect on the stability threshold but exerts an orientating effect on the convective patterns. For negative separation ratio, a strong destabilization occurs of the spatially homogeneous state with respect to long-wave disturbances. The stability range for long-wavelength convective rolls is defined

Modelling and experimental validation of a fluidized bed reactor freeboard region: application to natural gas combustion

A theoretical and experimental study of natural gas-air mixture combustion in a fluidized bed of sand particles is presented. The operating temperatures are lower than a critical temperature of 800 °C above which the combustion occurs in the vicinity of the fluidized bed. Our study focusses on the freeboard zone where most of the methane combustion takes place at such temperatures. Experimental results show the essential role of the projection zone in determining the global thermal efficiency of the reactor. The dense bed temperature, the fluidizing velocity and the mean particle diameter significantly affect the thermal behaviours. A model for natural gas-air mixture combustion in fluidized beds is proposed, counting for interactions between dense and dilute regions of the reactor [Pré et al. (1998)] supplemented with the freeboard region modelling of Kunii-Levenspiel (1990). Thermal exchanges due to the convection between gas and particles, and due to the conduction and radiation phenomena between the gas-particle suspension and the reactor walls are counted. The kinetic scheme for the methane conversion is that proposed by Dryer and Glassman (1973). Model predictions are in good agreement with the measurements

GEANT4 : a simulation toolkit

Abstract Geant4 is a toolkit for simulating the passage of particles through matter. It includes a complete range of functionality including tracking, geometry, physics models and hits. The physics processes offered cover a comprehensive range, including electromagnetic, hadronic and optical processes, a large set of long-lived particles, materials and elements, over a wide energy range starting, in some cases, from 250 eV and extending in others to the TeV energy range. It has been designed and constructed to expose the physics models utilised, to handle complex geometries, and to enable its easy adaptation for optimal use in different sets of applications. The toolkit is the result of a worldwide collaboration of physicists and software engineers. It has been created exploiting software engineering and object-oriented technology and implemented in the C++ programming language. It has been used in applications in particle physics, nuclear physics, accelerator design, space engineering and medical physics. PACS: 07.05.Tp; 13; 2

GEANT4--a simulation toolkikt

Geant4 is a toolkit for simulating the passage of particles through matter. It includes a complete range of functionality including tracking, geometry, physics models and hits. The physics processes offered cover a comprehensive range, including electromagnetic, hadronic and optical processes, a large set of long-lived particles, materials and elements, over a wide energy range starting, in some cases, from 250 eV and extending in others to the TeV energy range. It has been designed and constructed to expose the physics models utilised, to handle complex geometries, and to enable its easy adaptation for optimal use in different sets of applications. The toolkit is the result of a worldwide collaboration of physicists and software engineers. It has been created exploiting software engineering and object-oriented technology and implemented in the C++ programming language. It has been used in applications in particle physics, nuclear physics, accelerator design, space engineering and medical physics

Light Response and Switching Behavior of Graphene Oxide Membranes Modified with Azobenzene Compounds

Here, we report on the fabrication of light-switchable and light-responsive membranes based on graphene oxide (GO) modified with azobenzene compounds. Azobenzene and para-aminoazobenzene were grafted onto graphene oxide layers by covalent attachment/condensation reaction prior to the membranes’ assembly. The modification of GO was proven by the UV-vis, IR, Raman and photoelectron spectroscopy. The membrane’s light-responsive properties were investigated in relation to the permeation of permanent gases and water vapors under UV and IR irradiation. Light irradiation does not influence the permeance of permanent gases, while it strongly affected that of water vapors. Both switching and irradiation-induced water permeance variation is described, and they were attributed to over 20% of the initial permeance. According to in situ diffraction studies, the effect is ascribed to the change to the interlayer distance between the graphene oxide nanoflakes, which increases under UV irradiation to ~1.5 nm while it decreases under IR irradiation to ~0.9 nm at 100% RH. The last part occurs due to the isomerization of grafted azobenzene under UV irradiation, pushing apart the GO layers, as confirmed by semi-empirical modelling

Onset of Soret-induced convection in a horizontal layer of ternary fluid with fixed vertical heat flux at the boundaries

The paper deals with the investigation of the onset and weakly nonlinear regimes of the Soret-driven convection of ternary liquid mixture in a horizontal layer with rigid impermeable boundaries subjected to the prescribed constant vertical heat flux. It is found that there are monotonous and oscillatory longwave instability modes. The boundary of the monotonous longwave instability in the parameter plane Rayleigh number Ra - net separation ratio $\Psi$ at fixed separation ratio of one of solutes consists of two branches of hyperbolic type. One of the branches is located at ${\rm Ra} >0$, the other one at ${\rm Ra} 0$ only for the heating from below and at $\Psi 0$ and the other at ${\rm Ra} <0$. Corrections to the Rayleigh number obtained in the higher order of the expansion show that the longwave perturbations can be most dangerous at any values of $\Psi$. The numerical solution of the linear stability problem for small perturbations with finite wave numbers confirms this conclusion. The weakly nonlinear analysis shows that all steady solutions are unstable to the modes of larger wavelength and stable to the modes of smaller wavelength, i.e. the solution with maximal possible wavelength is realized

Light Response and Switching Behavior of Graphene Oxide Membranes Modified with Azobenzene Compounds

Here, we report on the fabrication of light-switchable and light-responsive membranes based on graphene oxide (GO) modified with azobenzene compounds. Azobenzene and para-aminoazobenzene were grafted onto graphene oxide layers by covalent attachment/condensation reaction prior to the membranes’ assembly. The modification of GO was proven by the UV-vis, IR, Raman and photoelectron spectroscopy. The membrane’s light-responsive properties were investigated in relation to the permeation of permanent gases and water vapors under UV and IR irradiation. Light irradiation does not influence the permeance of permanent gases, while it strongly affected that of water vapors. Both switching and irradiation-induced water permeance variation is described, and they were attributed to over 20% of the initial permeance. According to in situ diffraction studies, the effect is ascribed to the change to the interlayer distance between the graphene oxide nanoflakes, which increases under UV irradiation to ~1.5 nm while it decreases under IR irradiation to ~0.9 nm at 100% RH. The last part occurs due to the isomerization of grafted azobenzene under UV irradiation, pushing apart the GO layers, as confirmed by semi-empirical modelling

The origin for hydrocarbons fast transport and photoswitching permeation behavior in grafted laminar CdTe membranes

Thin laminar CdTe nanoflake membranes grafted by oleic acid were investigated for sorption characteristics and permeation of permanent gases and hydrocarbons depending on the process temperature. A huge influence of temperature was revealed, enabling enhancement of the ideal selectivity for butane-methane separation from 28 to over 100 with the change of process conditions from 25 °C to 10 °C. The separation factor attains 26 in mixed-gas CH$_4$/C$_4$H$_{10}$ separation experiments. Both sorption enthalpy and activation energy of diffusion were evaluated for penetrants, exhibiting the role of low activation barriers in fast transport of condensable hydrocarbons. The experimental results were proved by a semi-empirical calculation, illustrating swelling of grafted layer with heavy hydrocarbons. The effect was proved experimentally with in situ diffraction experiment. The low activation energy for the transport of condensable hydrocarbons, strong temperature dependence of permeance, and huge extinction coefficient of CdTe nanoflakes enable modulating the membrane permeance with laser irradiation, demonstrated with permeance photoswitching

The role of oxidation level in mass-transport properties and dehumidification performance of graphene oxide membranes

Here we report on gas and vapor transport properties of ultra-thin graphene oxide (GO) membranes, with various C:O ratios. Graphene oxide nanosheets with an average lateral size of 800 nm and C:O ratio ranging from 2.11 to 1.81 have been obtained using improved Hummers’ method by variation of graphite:KMnO4 ratio. Thin-film selective layers based on the obtained graphene oxide have been spin-coated onto porous substrates. To extend the C:O range to 2.60, thermal reduction of GO membranes was applied. A decrease in C:O ratio leads to significant water vapor permeance growth to over 60 m$^3$(STP)·m$^{−2}$·bar$^{−1}$·h$^{−1}$ while the permeance towards permanent gases reduces slightly. According to the permeation and sorption measurements, a decisive role of H$_2$O diffusivity has been established, while the water sorption capacity of the graphene oxide stays nearly independent of C:O ratio in GO. The result is supported by semi-empirical modeling which reveals diminution of H$_2$O jump activation barriers with both increasing GO interlayer spacing and its oxidation degree. The height of the activation barriers was found to vary up to an order of magnitude within the entire range of relative humidity (0–100% RH), lowering significantly for strongly oxidized GO. Our results evidence the necessity of attaining maximum GO oxidation degree for improving water transport in GO, especially at low partial pressures