Temperature Solid Phase Research Articles

Double diffusive convection in a porous layer saturated with viscoelastic fluid using a thermal non-equilibrium model

In this paper, a study of double diffusive convection in an anisotropic porous layer, saturated with viscoelastic fluid, heated form below, and cooled from above, has been performed; the fluid and solid phases are not in thermal equilibrium. Extended Darcy model, which includes the time derivative term in the momentum equation, has been used. For the fluid and solid phase temperature fields, a two-field model has been used separately for energy equation. Linear stability analysis is performed, using normal mode technique, and the expression for Rayleigh number has been obtained. It is found that small inter-phase heat coefficient has substantial effect on the stability of the system. The criterion for both stationary and oscillatory convection is derived analytically. The effects of various parameters on the stability of the system have been investigated. A weak nonlinear stability analysis based on the truncated representation of Fourier series is performed to find Nusselt number and Sherwood number. Further, we studied the transient behavior of the Nusselt number and Sherwood number by solving the finite amplitude equations using a numerical method. The results obtained during the analysis have been presented graphically. A study of streamlines, isotherms, and isohalines has been also made for fluid and solid phases.

Solid to Gas Temperature Gradient in a Microwave Fluidized Bed

Convective heat transfer from solid to gas was investigated in a microwave heated fluidized bed and compared to an identical conventionally heated bed. Fluidizing gas was fed at room temperature and heated in contact with the bed solid phase. Solid phase temperature was varied in range of 300-600°C. Temperature difference of the two phases and heat loss by gas stream were measured in both heating types and discussed regarding chemical reaction engineering and energy efficiency. Advantages of microwave heating in such systems were elaborated with an outlook to perspective applications of the process in field of materials science and engineering.

A downdraft high temperature steam-only solar gasifier of biomass char: A modelling study

A numerical model of a solar downdraft gasifier of biomass char (biochar) with steam based on the systems kinetics is developed. The model calculates the dynamic and steady state profiles, predicting the temperature and concentration profiles of gas and solid phases, based on the mass and heat balances. The Rosseland equation is used to calculate the radiative transfer within the bed. The char reactivity factor (CFR) is taken into account with an exponential variation. The bed heating dynamics as well as the steam velocity effects are tested. The model results are compared with different experimental results from a solar packed bed gasifier, and the temperature profile is compared to an experimental downdraft gasifier. Hydrogen is the principal product followed by carbon monoxide, the carbon dioxide production is small and the methane production is negligible, indicating a high quality syngas production. By applying the temperature gradient theory in the steam-only gasification process for a solar gasifier design, a solar downdraft gasifier improves the energy conversion efficiency by over 20% when compared to a solar packed bed gasifier. The model predictions are in good agreement with the experimental results found in the literature.

EFFECT OF NONUNIFORM TEMPERATURE GRADIENTS ON THERMOGRAVITATIONAL CONVECTION IN A POROUS LAYER USING A THERMAL NONEQUILIBRIUM MODEL

The effect of various forms of nonuniform basic temperature gradients on the onset of convection in a Newtonian fluidsaturated isotropic porous layer is investigated when the fluid and solid phases are not in local thermal equilibrium. A two-field model that represents the fluid and solid phase temperature fields separately is used for energy equation and the Forchheimer-extended Darcy model is used to describe the flow. The eigenvalue problem is solved numerically using the Galerkin technique. Comparisons are also made of the critical stability parameters between the present results and published ones for the linear basic temperature profile case, and the agreement is found to be good. The possibility of delaying or hastening the onset of convection by the basic state temperature gradients along with the influence of parameters representing the local thermal non-equilibrium effect is analyzed in detail. When compared with the nonuniform temperature gradients, it is found that the linear temperature profile indicates a reinforcement of stability. In addition, the role of thermal depth on the critical conditions is assessed in the case of piecewise linear temperature profiles. © 2011 by Begell House, Inc.

Ionic liquid co-lyophilized enzyme for biocatalysis in organic solvent: Remarkably enhanced activity and enantioselectivity

The room temperature solid-phase ionic liquid (RTSPIL) co-lyophilized enzyme exhibited markedly enhanced activity in organic solvent. The enzyme co-lyophilized with a dodecyl-imidazolium salt was 660-fold more active compared to its RTSPIL-free counterpart. The activity enhancement by RTSPILs was mainly attributable to the reduced particle sizes and improved dispersion of enzymes suspended in organic solvent. Also, the RTSPIL co-lyophilized enzyme displayed significantly enhanced enantioselectivity. Its enantioselectivity was 2.5-fold higher than that of its RTSPIL-free counterpart.

Astrocatalysis—Abiogenic synthesis and chemical evolution at pregeological stages of the Earth’s formation

The Earth’s biosphere appeared in a self-organization process along with the appearance of the Solar System. It is shown, based on the methods of self-organization examination and existing knowledge, that major stages of the chemical evolution in the early development of the biosphere include the “cold prehistory of life” in dense molecular nebulae, “pre-planetary chemocoenosis,” “RNA-world” in a circumsolar nebula, and primary biocoenoses of protocells (life) on the planetary bodies. Estimates for carbon in the primordial biosphere on the young Earth’s surface give 2.4 × 1019 kg. The decay of the primordial Earth’s biomass and biogeochemical cycles in 2.5 Myr led to the “planet of bacteria” with 2.0 × 1015 kg of biota in the Proterozoic (at the time of an oxygenated atmosphere). The main parameters (pressure, temperature, and state of catalytic solid phase) are estimated for these stages of the early evolution of life. It is shown that the abiogenic synthesis of the primordial matter was preformed in the Solar System on a grand scale with practically every atom in nanoparticles-catalysts participating. Selection among catalytically active nanoparticles worked towards the ability to synthesize high molecular compounds in a protoplanetary disk. Autocatalysts participated in the preplanetary chemical evolution, beginning from such simple substances as ethylene or glycolaldehyde. Primary synthesis of autocatalysts depended on external sources of energy, e.g., on ultraviolet radiation.

Systematic characterization of a PBI/H3PO4 sol–gel membrane—Modeling and simulation

This work presents a three-dimensional, steady-state, non-isothermal model of a high-temperature polymer-electrolyte-membrane fuel cell (HTPEMFC) using a phosphoric acid-doped polybenzimidazole (PBI/H3PO4) sol–gel membrane. The model accounts for the gold-plated copper current collector plates, the bipolar plates, all gas flow channels (flow-field), the gas diffusion layers, the reaction layers, and the membrane. Electrochemical reactions are modeled using an agglomerate approach and include the gas diffusivity and the gas solubility. The conductivity of the membrane is modeled using the Arrhenius equation to describe the temperature dependence. Finite elements are used to discretize all computational subdomains, and a commercially available code is used to solve the problem. The predicted values are compared to typical operating conditions, and a good agreement is found. The current density, the solid- and fluid-(gas)-phase temperatures and other quantities are analyzed throughout the computational subdomains. It was observed that the Arrhenius approach is valid in a certain temperature range and may overpredict the PBI/H3PO4 sol–gel membrane conductivity at higher solid-phase temperatures. Moreover, it is shown how the fluid-(gas)-phase temperature influences the solid-phase temperature and the current density distribution. Concrete values are deduced from the simulations and discussed according to experimental test.

ChemInform Abstract: New Thermal Routes to ortho‐Benzyne

AbstractReview: 80 refs.

Calculation of premixed combustion within inert porous media with model parametric uncertainty quantification

The present study focuses on uncertainties existing in porous media parameters and in the inlet reactant mixture conditions of solid oxide fuel cell off-gas combustion. Propagation of uncertainty from the model input parameters to the output stochastic variables is quantified using a non-intrusive spectral projection method based on polynomial chaos expansion. The non-intrusive nature of this method allows the solution of the stochastic problem to be obtained directly from the deterministic model without requiring modification of the governing equations. Quantification of uncertainty is investigated in a one-dimensional model for premixed combustion within inert porous media. The model includes detailed chemistry and solves the gas- and solid-phase energy balances coupled by convective heat exchange, including radiative heat transfer in the solid-phase. The results denote that the uncertainties in the porous media heat transfer parameters are relevant and originate a relatively high error bar on the CO emission and burning velocity. When the inlet reactant mixture uncertain conditions is also accounted for, it overcomes the influence of the other uncertain parameters on the gas- and solid-phase temperatures error bar. Both types of parametric uncertainty sources (inlet conditions and porous media parameters) are important in order to establish the error bar on the CO emission and burning velocity predictions.

Gas and solid phase temperature measurements of porous media combustion

This work introduces a novel method to determine gas and solid phase temperature profiles in a packed bed porous media combustor. Bare and coated thermocouple junctions were installed in the centerline of the porous combustor and the temperatures of solid phase and bare junctions were recorded simultaneously. Gas phase temperature was obtained considering the influence of flow velocity around the hot junction and the average solid temperature of the hot junction surroundings using the empirical formula for the Nusselt number based on the energy balance for the hot junction. Results show that the junction placement has a lot of influence on hot junction temperature measurement, which leads to a large uncertainty of gas temperature correction. The preheating procedure is used to reduce the effect of the junction placement. It is found that the flow velocity over junction, average solid temperature of the junction surroundings and empirical formula for the Nu number have limited influence on the gas phase temperature correction due to opposite compensating effects of these factors. The temperature profiles measured by thermocouples provide a complete temperature distribution in porous combustor, while a time-based method offers detail gas and solid temperature distributions near the reaction zone. Temperature profiles recorded for solid and gas phases vary for combustion waves propagating upstream and downstream. For all recorded regimes, the temperature gap between gas and solid has maximum and minimum points in the reaction and preheating zones, respectively.

Study into melting-crystallization of silver thiogallate by the statistical thermal analysis method

The influence of heat treatment of a liquid phase on “melting-crystallization” processes of silver thiogallate (AgGaS2) having chalcopyrite structure I \( \bar{4}2{\text{d}} \), has been studied by the method of statistical thermal analysis (STA). It is shown that the melting temperature of solid phase (Tm) increases non-monotonic from 970 °C due to rise in the preliminary melt overheating, and Tm reaches asymptotically 1010 °C. The equilibrium melting-crystallization temperature (\( T_{\text{m}}^{\text{o}} \)) has been defined as 989.2 °C. It is also found an extreme dependence of the melt supercooling on its overheating. The two curves of irregular dependence of nucleation rate on melt supercooling have been plotted at different melt overheating.

Effect of rotation on the onset of thermal convection in a sparsely packed porous layer using a thermal non-equilibrium model

Linear and nonlinear stability of a rotating fluid-saturated sparsely packed porous layer heated from below and cooled from above is studied when the fluid and solid phases are not in local thermal equilibrium. The extended Darcy–Brinkman model that includes the time derivative and Coriolis terms is employed as a momentum equation. A two-field model that represents the fluid and solid phase temperature fields separately is used for energy equation. The onset criterion for both stationary and oscillatory convection is derived analytically. It is found that small inter-phase heat transfer coefficient has significant effect on the stability of the system. There is a competition between the processes of rotation and thermal diffusion that causes the convection to set in through oscillatory mode rather than stationary. The rotation inhibits the onset of convection in both stationary and oscillatory mode. The Darcy number stabilizes the system towards the oscillatory mode, while it has dual effect on stationary convection. Besides, the effect of porosity modified conductivity ratio, Darcy–Prandtl number and the ratio of diffusivities on the stability of the system is investigated. The nonlinear theory is based on the truncated representation of Fourier series method. The effect of thermal non-equilibrium on heat transfer is brought out. The transient behavior of the Nusselt number is investigated by using the Runge–Kutta method. Some of the convection systems previously reported in the literature is shown to be special cases of the system presented in this study.

Spectroscopy and structural state of V 4+ ions in lithium aluminosilicate glass and glass–ceramics

The optical property of LAS glass and the structural states of V 4+ ions were studied by use of kinds of spectra (UV–visible, IR, Raman and EPR) in LAS glass and glass–ceramics containing V 2O 5. It was found that the V 4+ ions formed by reduction of V 5+ ions, and existed as the VO 2+ complex in octahedral coordination with a tetragonal compression with C 4v symmetry in LAS glass. Additionally, the V 4+ ions partially replaced Li + and occupied tetrahedral sites in beta-quartz solid solution crystal phase. The color of the glasses drastically changed from light yellow-green to claret during crystallization, and the optical absorption in visible range attribute to the transitions of 2B 2g → 2B 1g and 2B 2g → 2E g of V 4+ ions in high crystal field, which was affected by the above structural variation of V 4+. Moreover, the addition of V 2O 5 in LAS glass decreased crystallization temperature of main beta-quartz solid solution crystal phase.

New Thermal Routes to ortho-Benzyne

There is experimental evidence that intermediate ortho-benzynes can be made by intramolecular [2 + 4] cycloaddition of a 1,3-diyne with an alkyne. Computations by several groups support a concerted mechanism for the cycloaddition of butadiyne with acetylene. High temperature benzyne cycloreversion has also been demonstrated experimentally; this may in fact be a common reaction in hydrocarbon fuel combustion. Following leads from earlier pyrolysis experiments, herein we predict that cycloaddition of benzyne with butadiyne can proceed by a stepwise mechanism to 2,3-naphthyne. However, a slightly lower energy path leads to a benzocyclobutadiene. ortho-Benzyne can be generated by solution-phase and solid-phase reaction in a microwave reactor. We have developed the method of microwave flash pyrolysis (MFP) for high temperature solid-phase microwave reactions. MFP reaction of phthalic anhydride, a classic benzyne precursor, results in a typical suite of products expected from a relatively high concentration of benzyne.

EFFECTS OF VARIABLE VISCOSITY AND DENSITY MAXIMUM ON THE ONSET OF DARCY-BENARD CONVECTION USING A THERMAL NONEQUILIBRIUM MODEL

The effects of the temperature-dependent viscosity and density maximum on the criterion for the onset of convection in a horizontal porous layer are investigated when the solid and fluid phases are not in local thermal equilibrium. A two-field model that represents the fluid-and solid-phase temperature fields separately is used for the energy equation. The Galerkin method is used to obtain the eigenvalue equation, which is then solved numerically. It is noted that higher values of the viscosity number and the parameter representing the maximum-density property have the effect of delaying the onset of convection and reducing the size of convection cells. In addition, the influence of parameters representing the thermal nonequilibrium effects on convective instability is discussed in detail. © 2010 by Begell House, Inc.

Supercritical carbon dioxide-assisted solid-phase free radical grafting of butyl acrylate onto PP

AbstractFree radical grafting copolymerization of butyl acrylate (BA) onto polypropylene (PP) matrix was investigated using supercritical CO2 as a swelling agent and carrier solvent. The monomer BA and initiator AIBN were first dissolved in supercritical CO2, and then these small molecules were diffused into the grafting zones of PP matrix. As grafting reaction temperature (about 80 °C) was far below the melting point, PP was modified in the solid phase. The effects of different experimental conditions such as soaking time and temperature, supercritical CO2 pressure; monomer and initiator concentration; PP particle diameter; solid phase grafting reaction time and temperature on grafting percentage were studied. Fourier transform infrared spectroscopy (FTIR) results and gel content experiment prove that the monomers were indeed grafted onto PP chains. Scanning electron microscope (SEM) analysis of the grafted sample indicates that the grafting branches PBA were homogeneously distributed onto the surfaces and micropores of PP particles. Thermal gravimetric analysis (TGA) results show that the grafted samples had a higher onset thermal degradation temperature and a lower weight loss at a particular temperature than pure PP. The water contact angle of PP-g-BA decreased from 98° to 72°, indicating the polarity and hydrophilicity of PP were improved effectively. The melt flow rate (MFR) and mechanical analysis results show that the grafting sample remained its versatile physical properties though introducing the grafting BA branch chain. This is prominent when it was compared with normal solid grafting product.

Linear and nonlinear double diffusive convection in a rotating sparsely packed porous layer using a thermal non-equilibrium model

Double diffusive convection in a fluid-saturated rotating porous layer is studied when the fluid and solid phases are not in local thermal equilibrium, using both linear and nonlinear stability analyses. The Brinkman model that includes the Coriolis term is employed as the momentum equation. A two-field model that represents the fluid and solid phase temperature fields separately is used for the energy equation. The onset criterion for stationary, oscillatory, and finite amplitude convection is derived analytically. It is found that small inter-phase heat transfer coefficient has significant effect on the stability of the system. There is a competition between the processes of thermal diffusion, solute diffusion, and rotation that causes the convection to set in through either oscillatory or finite amplitude mode rather than stationary. The effect of solute Rayleigh number, porosity modified conductivity ratio, Lewis number, diffusivity ratio, Vadasz number, and Taylor number on the stability of the system is investigated. The nonlinear theory based on the truncated representation of Fourier series method predicts the occurrence of subcritical instability in the form of finite amplitude motions. The effect of thermal non-equilibrium on heat and mass transfer is also brought out. Some of the convection systems previously reported in the literature is shown to be special cases of the system presented in this study.

Low temperature SPME device: A convenient and effective tool for investigating photodegradation of volatile analytes

Hexachlorobenzene (HCB), a model volatile compound, was exposed to UV irradiation (16W, 254nm) after being sorbed in an internally cooled or low temperature solid-phase microextraction (LT-SPME) fibre. Photolysis took place directly on the polydimethylsiloxane coating of the LT-SPME fibre, yielding an “in situ” generation of photoproducts. Maintaining the temperature of the cold fibre at 0°C eliminated, for the first time, problems of analyte losses due to volatilisation, inherent to the conventional room temperature photo-SPME studies. During the present studies, nearly complete photoremoval of HCB could be achieved within 20min of irradiation. Photoreduction through photodechlorination was shown to be the main decay pathway in which lesser chlorinated congeners were sequentially formed as intermediates. Accordingly, initial generation of pentachlorobenzene was followed in order from 1,2,3,5-tetrachlorobenzene, 1,2,4,5-tetrachlorobenzene and 1,3,5-trichlorobenzene. The present findings were in agreement with previously reported results. Overall, the use of the LT-SPME device as a photoreaction support not only eliminated analyte losses but also greatly facilitated photochemical investigations of volatile compounds in general.

The onset of double diffusive convection in a sparsely packed porous layer using a thermal non-equilibrium model

We examine the effect of local thermal non-equilibrium on double diffusive convection in a fluid-saturated sparsely packed porous layer heated from below and cooled from above, using both linear and nonlinear stability analyses. The Brinkman model is employed as the momentum equation. A two-field model that represents the fluid and solid phase temperature fields separately is used for the energy equation. The onset criterion for stationary, oscillatory and finite amplitude convection is derived analytically. It is found that a small inter-phase heat transfer coefficient has significant effect on the stability of the system. There is a competition between the processes of thermal and solute diffusion that causes the convection to set in through either oscillatory or finite amplitude mode rather than stationary. The effect of solute Rayleigh number, porosity modified conductivity ratio, Lewis number, ratio of diffusivities, Vadasz number and Darcy number on the stability of the system is investigated. The nonlinear theory based on the truncated representation of Fourier series method predicts the occurrence of subcritical instability in the form of finite amplitude motions. The effect of thermal non-equilibrium on heat and mass transfer is also brought out.

Numerical and experimental investigation for heat transfer in triplex concentric tube with phase change material for thermal energy storage

This paper addresses a numerical and experimental investigation of a thermal energy storage unit involving phase change process dominated by heat conduction. The thermal energy storage unit involves a triplex concentric tube with phase change material (PCM) filling in the middle channel, with hot heat transfer fluid (HHTF) flowing outer channel during charging process and cold heat transfer fluid (CHTF) flowing inner channel during discharging process. A simple numerical method according to conversation of energy, called temperature & thermal resistance iteration method has been developed for the analysis of PCM solidification and melting in the triplex concentric tube. To test the physical validity of the numerical results, an experimental apparatus has been designed and built by which the effect of the inlet temperature and the flow rate of heat transfer fluid (HTF, including HHTF and CHTF) on the thermal energy storage has been studied. Comparison between the numerical predictions and the experimental data shows good agreement. Graphical results including fluid temperature and interface of solid and liquid phase of PCM versus time and axial position, time-wise variation of energy stored/released by the system were presented and discussed.