Numerical investigation on the heat transfer characteristics of a liquid-metal pool subjected to a partial solidification process

The main purpose of this paper is to investigate numerically the effects of solidification on the heat transfer characteristics of the liquid metal layer, for use in accident analyses. The situation is very similar to an overlying liquid melt pool that could be fooned in the reactor lower head during the late phase of a severe nuclear accident. Based on a computational model, MPCOOL, the numerical predictions were then assessed through a comparison with the experimental data that was obtained with various boundary temperature conditions and geometrical aspect ratios, especially for the Ra-Nu relationship. For the cases with solidification, the results of the comparison show that(a) the computational model does show a good agreement with heat transter rates inferred from the experimental data, with a few exceptions at the Ra numbers which suggest a turbulent transport; and also (b) the computational model underpredicts the heat transfer rates by about 6% than that inferred from the experimental data when it is integrally evaluated with the Ra-Nu correlation. The foregoing results are mainly due to the currently limited applicability of the computational model up to the laminar-to-turbulence transition flows and its application to the turbulence flows because it is always subjected to a model uncertainty between the laminar and turbulence. Next, an additional comparison for the cases with and without solidification was made to examine the effects of the solidification on the energy partition within the liquid metal layer and its effects on the directional heat transfer rates. The results of the comparison show that the computational model for the case without solidification predicts higher heat transfer rates by about 15% than when solidification is included, but there isn't any experimental data that directly supports this trend.     

DEA based multi-period evaluation system for research in academia

Various types of incentive systems are widely used by many companies and organizations for better performances. However, despite the demand for the fair incentive systems, those systems in academia have not been well established and fairly operated. Using an example of a professor evaluation system, we examine two main problems of the existing incentive systems in academia – ignoring the input aspect and focusing only on the short-term performance. By applying the super-efficiency DEA and considering multi-period output, we show that the input factors and the time trend of outcomes need to be incorporated for the fair evaluation of professors and their research performance.     

Liquation behavior in the weld HAZ of high Si nodular iron

In the present study, liquation behavior including the constitutional liquation phenomena in the weld heataffected zone (HAZ) of nodular cast iron with 4% Si was investigated. The thermal cycle of the weld HAZ was simulated using a Gleeble simulator at various peak temperatures and a fast heating rate of 1000 °C/sec was selected to observe the constitutional liquation phenomenon. Differential thermal analysis has shown that the eutectic constituents of γ + Laves and γ + Fe₃C begin to melt at 1120 °C and 1140 °C, respectively. Meanwhile, constitutional liquation by γ/graphite reaction occurred above 1140 °C, due to carbon enrichment around the graphite nodules.     

Radiation-induced synthesis of solid alkaline exchange membranes with quaternized 1,4-diazabicyclo[2,2,2] octane pendant groups for fuel cell application

In this study, anion exchange membranes having various quaternary ammonium groups were prepared by using a radiation-induced graft polymerization method and followed by subsequent treatment of films with amines. FT-IR and SEM-EDX techniques were employed to monitor the reaction progress. The cross-sectional distribution of the anionic exchange functional groups through the membranes has also been investigated using SEM-EDX technique. The results reveal that the anion exchange groups were found to be evenly distributed throughout the membranes. It was also observed that the physico-chemical properties of the anion exchange membranes such as water uptake, ionic conductivity, thermal stability, chemical stability, mechanical properties, and dimensional stability are largely influenced by the chemical structure of the quaternary ammonium moiety which is attached to the graft chains. Among the prepared anion exchange membranes, the membrane having mono-quaternized 1,4-diazabicyclo[2,2,2]octane moiety was found to have both enhanced chemical and dimensional stabilities, while the others having quaternized trimethylamine or bis-quaternized 1,4-diazabicyclo[2,2,2]octane have the decreased dimensional and chemical stabilities.     

Effect of Compositional Variation in TiO<Subscript>2</Subscript>-Based Flux-Cored Arc Welding Fluxes on the Thermo-physical Properties and Mechanical Behavior of a Weld Zone

The effect of compositional variation in TiO₂-based flux-cored arc welding fluxes on viscosity, wettability, and electronegativity was studied. The thermo-physical properties of the retrieved fluxes and their relationship with the mechanical properties of the weld zone, including tensile strength and micro-Vickers hardness, after welding were identified. Microstructural observation under similar welding conditions revealed significant grain coarsening at a corrected optical basicity (Λ^corr) of 0.62, resulting in reduced strength and hardness due to greater heat transfer. Welding fluxes containing TiO₂-based simple structural units should result in greater heat transfer due to the deficiency in complex [AlO₄]^5?- and [SiO₄]^4?-based structural units, as identified through spectroscopic analyses using fourier transform infrared spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. The electronegativity of the retrieved fluxes was also evaluated since higher electronegativity results in greater absorption of electrons in the arc, resulting in arc condensation towards the center direction. Consequently, deeper penetration could be obtained, where the highest electronegativity was identified to be approximately 0.62 of the corrected optical basicity. Thus, both the thermal conductivity and electronegativity of the welding fluxes were identified to determine the heat transfer phenomenon during flux-cored arc welding.     

Gas-diffusion layer's structural anisotropy induced localized instability of nafion membrane in polymer electrolyte fuel cell

The design of robust polymer electrolyte fuel cell requires a thorough understanding of the materials' response of the cell components to the operational conditions such as temperature and hydration. As the electrolyte membrane's mechanical properties are temperature, hydration and rate dependent, its response under cyclic loading is of significant importance to predict the damage onset and thus the membrane lifetime. This article reports on the variation in stress levels in the membrane induced due to the gas-diffusion layer's (GDL) anisotropic mechanical properties while accurately capturing the membrane's mechanical response under time dependent hygrothermomechanical conditions. An observation is made on the evolution of negative strain in the membrane under the bipolar plate channel area, which is an indication of membrane thinning, and the magnitude of this strain found to depend upon the GDL's in-plane mechanical properties. In order to come up with a strategy that reduces the magnitude of tensile stresses evolved in the membrane during the hygrothermal unloading and to increase the membrane's lifetime, we numerically show that by employing a fast hygrothermal loading rate and unloading rate strategy, significant reduction (in this study, nearly 100%) in the magnitude of tensile stresses is achievable. The present study assists in understanding the relation between materials compatibility and durability of fuel cell components.     

Investigation of the Reduction/Oxidation Behavior of Cobalt Supported on Nano-ceria

The oxidation and reduction behavior of cobalt catalysts supported on nano-ceria (5–8 nm) was investigated under hydrogen, oxygen and water atmospheres. A novel quantitative isothermal reduction (QIR) technique was introduced to analyze the kinetics and activation barrier of Co reduction. CoO to Co reduction was found to be of first order in the 250–350 °C temperature range. Temperature programmed reduction and oxidation experiments were conducted and the Kissinger method was used to obtain apparent activation energies for reduction and oxidation with O₂. The apparent activation energy for CoO reduction was found to agree with that obtained from the QIR technique. Re-oxidation of Co metal was found to have a slightly lower activation energy barrier than the reduction. The reduction and oxidation behavior was further investigated with in situ XANES where the reaction order for reduction was observed to change at 450 °C. The pre-reduced samples were seen to re-oxidize under a water atmosphere, where the oxidation followed first order kinetics. Re-oxidation by water yielded a higher activation energy when compared to re-oxidation under oxygen.