Buoyancy induced flow in a nanofluid filled enclosure partially exposed to forced convection

A numerical study was performed on natural convection for water–CuO nanofluid filled enclosure where the top surface was partially exposed to convection. The cavity has a square cross-section and differentially heated. Except exposed convection part on the top, all sides are adiabatic on horizontal walls. Effects of Rayleigh number (10³ ? Ra ? 10⁵), Biot number (0 ? Bi ? ∞), length of partial convection (0.0 ? L ? 1.0) and volume fraction of nanoparticles (0.0 ? φ ? 0.1) on heat and fluid flow were investigated. The results showed that for the case of high Biot number that heat transfer along the heated was enhanced by increasing the Rayleigh number mainly at the upper portion of the heated wall. When the top wall was totally exposed to convection, the results prevail that the heat transfer was more effective at high Biot number especially at the upper portion of the heated wall. For the case of high Biot number, the results prevailed that the heat transfer at the upper portion of the heated wall increases considerably at high exposed length to convection (L); however, for L ? 0.75 the effect of L was less pronounced. Contour maps for percentage of heat transfer enhancement were presented and it was shown that the location of maximum enhancement in heat transfer was sensitive to Ra, φ and L.     

Mixed convection heat transfer of a nanofluid in a lid-driven enclosure with two adherent porous blocks

Journal of Thermal Analysis and Calorimetry - Convective flow and heat transfer in an alumina/water nanofluid enclosure having two centered adherent porous blocks under the effect of moving upper...     

pH-sensitive chitosan films for baker's yeast immobilization

Dried baker’s yeast cells were immobilized on a chitosan film, which is a natural polymer. Prepared chitosan films were treated with glutaraldehyde to facilitate the immobilization of the cells. The effects of the amount of glutaraldehyde, incubation time, pH, and temperature on immobilization were investigated. The amount of glutaraldehyde was chosen to be 0.01% (weight). The highest amount of yeast immobilization was obtained with 5 h incubation. It was determined that optimum temperature for immobilization is 25°C, and the optimum pH for immobilization is 6. Immobilized cells were allowed to stand for 3 d in distilled water and buffer solution (pH 6) to investigate the desorption, but no desorption was found. The maximum immobilization capacities were found to be 90 μg protein cm⁻² film in optimum conditions.     

The free energy of a reaction coordinate at multiple constraints: a concise formulation

The free energy as a function of the reaction coordinate (rc) is the key quantity for the computation of equilibrium and kinetic quantities. When it is considered as the potential of mean force, the problem is the calculation of the mean force for given values of the rc. We reinvestigate the PMCF (potential of mean constraint force) method which applies a constraint to the rc to compute the mean force as the mean negative constraint force and a metric tensor correction. The latter allows for the constraint imposed to the rc and possible artefacts due to multiple constraints of other variables which for practical reasons are often used in numerical simulations. Two main results are obtained that are of theoretical and practical interest. First, the correction term is given a very concise and simple shape which facilitates its interpretation and evaluation. Secondly, a theorem describes various rcs and possible combinations with constraints that can be used without introducing any correction to the constraint force. The results facilitate the computation of free energy by molecular dynamics simulations.     

Effect of d-psicose substitution on gelatin based soft candies: A TD-NMR study

Confectionary gels are considered as composite gel systems composed of high amount of sugar and gelling agent such as gelatin or starch. d -Psicose is classified as a type of rare sugar, which is a C-3 epimer of fructose and has 70% of the sweetness of sucrose with a caloric value of 0.39 kcal/g. Utilization of d -psicose in food products is gaining particular interest due to its low caloric value. In this study, gelatin-based soft candies were formulated, and the effect of d -psicose substitution was explored on the quality of the products. For characterization of the soft candies, moisture content, water activity, color, hardness, and glass transition temperature of samples were investigated. X-ray diffraction analysis was also performed to explain the crystallization tendency of jelly candies. Results showed that, the softest sample with the highest moisture content and the smallest crystallization tendency was the sample that included the highest amount of d -psicose. Time domain (TD) NMR relaxometry experiments were also conducted on gel samples, and three distinct proton populations were observed in the relaxation spectrum for all formulations. Spin–lattice relaxation times obtained through monoexponential fitting (T₁) were also obtained to explain some quality parameters.     

Influence of inclination angle on buoyancy-driven convection in triangular enclosure filled with a fluid-saturated porous medium

The present investigation deals with the numerical analysis of steady-state laminar buoyancy-driven convection in an inclined triangular enclosure filled with fluid saturated porous media using the Darcy law equation. One wall of the enclosure is isothermally heated and the other is cooled, while the remaining wall is adiabatic. The effect of inclination angle on natural convection is investigated by varying the angle of inclination (φ) between 0° and 360°. The governing transformed equations are solved numerically using a finite-difference method. Obtained results are shown in the form of streamlines, isotherms, mean Nusselt numbers and dimensionless stream function for different values of the Rayleigh number Ra in the range 100 ≤ Ra ≤ 1,000. It is found that the values of the maximum and minimum mean Nusselt number are reached for φ = 330° and φ = 210° , respectively. However, the lowest flow strength is formed at φ = 240° for all values of Ra.     

Fluid flow due to combined convection in lid-driven enclosure having a circular body

The present work is aimed to study mixed convection heat transfer characteristics for a lid-driven air flow within a square enclosure having a circular body. Flows are driven by the left lid, which slides in its own plane constant velocity. This wall is isothermal and it moves up or down in y direction while the other walls remain stationary. The horizontal walls are adiabatic. The cavity is differentially heated and the left wall is maintained at a higher temperature than the right wall. Three different temperature boundary conditions were applied for the inner cylinder as adiabatic, isothermal or conductive. The computation is carried out for wide ranges of Richardson numbers, diameter of inner cylinder and center and location of the inner cylinder. It was found that the most effective parameter on flow field and temperature distribution is the orientation of the moving lid. The circular body can be a control parameter for heat and fluid flow. An interesting obtained result that the thermal conductivity becomes insignificant for small values of diameter of the circular body.     

Experimental study and large Eddy simulation of a coaxial jet with perforated obstacles to control thermal mixing characteristics

Thermal mixing phenomena of a coaxial jet with perforated obstacles are analyzed both experimentally and numerically. Perforated obstacles are used in front of a coaxial jet that has different temperatures of the fluid to control the thermal mixing behavior. An experimental set-up was constructed to perform several cases and all cases are simulated by using large eddy simulation (LES) turbulence model. Results of the study presented that inserting perforated obstacle affects the mixing performance positively and the best mixing performance is obtained in the case that the obstacle has the highest permeability. Experimental and computational results were compared and a good agreement was obtained.     

Exploring the crystallinity of different powder sugars through solid echo and magic sandwich echo sequences

Time-domain nuclear magnetic resonance techniques are frequently used in polymer, pharmaceutical, and food industries as they offer rapid experimentation and generally do not require any considerable preliminary sample preparation. Detection of solid and liquid fractions in a sample is possible with the free induction decay (FID). However, for the classical FID sequence that consists of a single pulse followed by relaxation decay acquisition, the dead time of the probe (ring out of resonance circuitry) occurs and varies between 5 and 15 μs for standard 10-mm tubes. In such a case, there arises a risk that the signal from the solid fraction cannot be detected correctly. To obtain quantitative measurement on crystalline and more mobile amorphous fractions, alternative sequences to the classical FID in the solid-state nuclear magnetic resonance were developed. Solid echo and magic sandwich echo sequences perform the relaxation decay refocusing somehow excluding the dead time problem and allow detection of the signal from the solid fraction. In this study, knowledge of amorphous/crystal fraction, which is obtained through solid echo and magic sandwich echo, has been explored on powder sugar samples for the purpose of developing a groundwork for a reliable quality control method. Different sugars were examined for the utilization of the sequences. What is important to add and make this study unique is that the method proposed did not involve multiparameter fitting of the “bead” pattern FID signal that normally suffers from ambiguity; just the integration of the fast Fourier transform of the solid echo was needed to calculate the second moment, (M₂).     

Modification of a montmorillonite-illite clay using alkaline hydrothermal treatment and its application for the removal of aqueous Cs+ ions

A montmorillonite-illite clay was modified using alkaline hydrothermal treatment (reflux method) and applied to the removal of aqueous Cs+ ions. The alkaline solutions were prepared by dissolving NaOH in seawater and in distilled water, and the effect of the two alkaline media on the sorption capacities of the modified clay was discussed. The modified materials were characterized using XRD, SEM/EDS, and FTIR. As a result of the modification, the original mineral was partially transformed into a zeolitic material with spherical morphology. The results showed that the modification improved the Cs+ uptake capacity of the starting clay, with the clay modified in distilled water medium demonstrating higher sorption capacity. The sorption data were adequately described using the Freundlich and Dubinin-Radushkevich isotherm models.