Effect of Stefan number on thermoelastic instabilities in unidirectional solidification

During the casting process, thermoelastic distortion of the partially solidified material affects the contact pressure at the solid/mold interface, which in turn can affect the thermal contact resistance, thus coupling the heat transfer and thermomechanical problems. This coupled system has the potential for instability. In this paper, the effect of Stefan number on the stability of unidirectional solidification is investigated, under the simplifying assumption that the solidified material is linear elastic. The Stefan number is a measure of the influence of thermal capacity on the solution and previous analyses have generally been restricted to the case of zero Stefan number, corresponding to a solidifying material of zero thermal capacity.This generalization necessitates a numerical solution, which is here implemented using the finite difference method. However, since the growth of the perturbation is linear, the two-dimensional stability problem is reduced to two one-dimensional numerical problems which can be solved sequentially.The results show that, in all cases, an initial sinusoidal perturbation grows to a maximum amplitude in the solidification front and then decays, the maximum being reached when the mean solidified layer thickness is about half the wavelength of the perturbation.In general, increasing Stefan number has a stabilizing effect on the process. This effect is most noticeable in cases where the zero Stefan number approximation predicts substantial growth of an initial perturbation, e.g. where the thermal contact resistance is very sensitive to pressure.     

Simulation of solar-powered absorption cooling system

With developing technology and the rapid increase in world population, the demand for energy is ever increasing. Conventional energy will not be enough to meet the continuously increasing need for energy in the future. In this case, renewable energy sources will become important. Solar energy is a very important energy source because of its advantages. Instead of a compressor system, which uses electricity, an absorption cooling system, using renewable energy and kinds of waste heat energy, may be used for cooling. In this study, a solar-powered, single stage, absorption cooling system, using a water–lithium bromide solution, is simulated. A modular computer program has been developed for the absorption system to simulate various cycle configurations and solar energy parameters for Antalya, Turkey. So, the effects of hot water inlet temperatures on the coefficient of performance (COP) and the surface area of the absorption cooling components are studied. In addition, reference temperatures which are the minimum allowable hot water inlet temperatures are determined and their effect on the fraction of the total load met by non–purchased energy (FNP) and the coefficient of performance are researched. Also, the effects of the collector type and storage tank mass are investigated in detail.     

Role of the Mold Properties on Gap Nucleation in Solidification

The role of the mold properties on gap nucleation in pure metal solidification is investigated. The mold is assumed to be finite and deformable, and has a sinusoidal surface micro-geometry. Unlike previous models, the model developed herein assumes that the mold material has a non-negligible thermal capacitance. Of particular interest are the roles played by the mold thickness and mold thermal capacitance on the existence of critical mold surface wavelength that corresponds to the situation where both contact pressure and its time derivative simultaneously fall to zero. The present work also assumes that the thermal and mechanical problems in the mold-shell interface are uncoupled. It is shown that the inclusion of the thermal capacitance of the mold material, together with thermal capacitance of the shell and the mold distortion, may be sufficient to predict a critical wavelength beyond which no gap nucleation occurs at the troughs. The role of the mold properties is examined through qualitative comparisons of the present and previous models. Gap nucleation times, associated mean shell thicknesses, and critical wavelengths are calculated for pure copper and pure iron molds under identical process conditions. It is found that a copper mold leads to faster gap nucleation compared to an iron mold. The associated critical wavelengths of iron molds are shown to be larger than those of copper. An optimum mean mold thickness corresponding to the longest gap nucleation time for a given set of process parameters is determined. The effect of the mean pressure on the optimum mold thickness is also investigated.     

Sparse-then-dense alignment-based 3D map reconstruction method for endoscopic capsule robots

Despite significant progress achieved in the last decade to convert passive capsule endoscopes to actively controllable robots, robotic capsule endoscopy still has some challenges. In particular, a fully dense three-dimensional (3D) map reconstruction of the explored organ remains an unsolved problem. Such a dense map would help doctors detect the locations and sizes of the diseased areas more reliably, resulting in more accurate diagnoses. In this study, we propose a comprehensive medical 3D reconstruction method for endoscopic capsule robots, which is built in a modular fashion including preprocessing, keyframe selection, sparse-then-dense alignment-based pose estimation, bundle fusion, and shading-based 3D reconstruction. A detailed quantitative analysis is performed using a non-rigid esophagus gastroduodenoscopy simulator, four different endoscopic cameras, a magnetically activated soft capsule robot, a sub-millimeter precise optical motion tracker, and a fine-scale 3D optical scanner, whereas qualitative ex-vivo experiments are performed on a porcine pig stomach. To the best of our knowledge, this study is the first complete endoscopic 3D map reconstruction approach containing all of the necessary functionalities for a therapeutically relevant 3D map reconstruction.     

Effect of monolithic zirconia on the degree of conversion of two resin cements analyzed by FT-IR/ATR spectroscopy

Objective: This study aimed to evaluate the degree of conversion (DC) of two different resin cements polymerized under the monolithic zirconia specimens in different thicknesses and colors.

Material and methods: Partially stabilized monolithic zirconia blocks (inCoris TZI) were cut into three different thicknesses (0.5, 1.0, and 2.0 mm) and the specimens were divided into four color groups (A1, A2, A3, and A4). The light transmittance of each specimen was measured. Panavia F 2.0 or Variolink N resin cement was applied into teflon mold and irradiated using the light emitting diode curing unit for 20 s under monolithic zirconia specimen (n = 10). The resin cement specimens were stored at room temperature under dry conditions. The DC of each specimen was measured by Fourier transform infrared-attenuated total reflection (FT-IR/ATR) spectroscopy after the 1st and 10th day. Data were analyzed with two-way analysis of variance (ANOVA), two-way repeated measures ANOVA, three-way repeated measures ANOVA, and the Tukey least significant difference (LSD) tests (α = 0.05).

Results: The light-cure resin cement groups showed higher DC than the dual-cure resin cement groups (p < 0.05). The DC of both resin cements reduced with an increase in the thickness and darkening of the color of monolithic zirconia specimens. There was a statistically meaningful increase in the 10th-day values for dual-cure resin cement (p < 0.05), whereas there were no significant differences between the 1st- and 10th-day values for light-cure resin cement (p > 0.05).

Conclusion: The use of light-cure resin cement can be suggested for the luting of monolithic zirconia restorations.     

Optimizing modular product design for reconfigurable manufacturing

The problem of optimizing modular products in a reconfigurable manufacturing system is addressed. The problem is first posed as a generalized subset selection problem where the best subsets of module instances of unknown sizes are determined by minimizing an objective function that represents a trade-off between the quality loss due to modularization and the cost of reconfiguration while satisfying the problem constraints. The problem is then formulated and solved as an integer nonlinear programming problem with binary variables. The proposed method is applied to the production of a modular drive system composed of a DC motor and a ball screw. The study is a first attempt toward developing a systematic methodology for manufacturing modular products in a reconfigurable manufacturing system.     

Al-Pillared clay for cottonseed oil bleaching: An optimization study

In this study, Al-pillared white bentonite (Ordu-Unye, Turkey) was used for cottonseed oil bleaching. Pillaring process parameters were optimized in terms of bleaching efficiency as the bleaching capacity of cottonseed oil. The initial cation concentration, hydrolyzing agent ratio, and thermal treatment temperature were chosen as major process parameters. Pillared clays were characterized by FTIR and differential thermal analysis. The bleaching efficiency of bentonite increased from 11.8 to 17.5% by acid activation and to 33.5% by further Al pillaring. The optimal pillaring process conditions for cottonseed bleaching were an initial concentration of AICl₃, of 0.5 M, a OH⁻/Al³⁺ molar ratio of 0.3, and a thermal treatment temperature of 700°C.