A second order turbulence model based on a Reynolds stress approach for two-phase boiling flow. Part 1: Application to the ASU-annular channel case

High-thermal performance PWR (pressurized water reactor) spacer grids require both low pressure loss and high critical heat flux (CHF) properties. Numerical investigations on the effect of angles and position of mixing vanes and to understand in more details the main physical phenomena (wall boiling, entrainment of bubbles in the wakes, recondensation) are required.In the field of fuel assembly analysis or design by means of CFD codes, the overwhelming majority of the studies are carried out using two-equation eddy viscosity models (EVM), especially the standard K-? model, while the use of Reynolds Stress Transport Models (RSTM) remains exceptional.But extensive testing and application over the past three decades have revealed a number of shortcomings and deficiencies in eddy viscosity models. In fact, the K-? model is totally blind to rotation effects and the swirling flows can be regarded as a special case of fluid rotation. This aspect is crucial for the simulation of a hot channel in a fuel assembly. In fact, the mixing vanes of the spacer grids generate a swirl in the coolant water, to enhance the heat transfer from the rods to the coolant in the hot channels and to limit boiling.First, we started to evaluate computational fluid dynamics results against the AGATE-mixing experiment: single-phase liquid water tests, with Laser-Doppler liquid velocity measurements upstream and downstream of mixing blades. The comparison of computed and experimental azimuthal (circular component in a horizontal plane) liquid velocity downstream of a mixing vane for the AGATE-mixing test shows that the rotating flow is qualitatively well reproduced by CFD calculations but azimuthal liquid velocity is underestimated with the K-? model.Before comparing performance of EVM and RSTM models on fuel assembly geometry, we performed calculations with a simpler geometry, the ASU-annular channel case. A wall function model dedicated to boiling flows is also proposed.     

A second order turbulence model based on a Reynolds stress approach for two-phase boiling flow and application to fuel assembly analysis

High-thermal performance PWR (pressurized water reactor) spacer grids require both low-pressure loss and high critical heat flux (CHF) properties. Numerical investigations on the effect of angles and position of mixing vanes and to understand in more details the main physical phenomena (wall boiling, entrainment of bubbles in the wakes, recondensation) are required.In the field of fuel assembly analysis or design by means of CFD codes, the overwhelming majority of the studies are carried out using two-equation Eddy Viscosity Models (EVM), especially the standard K-? model, while the use of Reynolds Stress Transport Models (RSTM) remains exceptional.The simulation of swirling flow generated by the mixing vanes plays an important role for the prediction of the CHF for the fuel assemblies. For this reason, according to [14] and [Mimouni et al., 2009b], rotation effects and RSTM model are more specifically addressed in the paper.Before comparing performance of EVM and RSTM models on fuel assembly geometry, we performed calculations with simpler geometries, the DEBORA case and the ASU-annular channel case. ASU-annular channel case has already been addressed in [14] and [Mimouni et al., 2009b].Then, a geometry closer to actual fuel assemblies is considered. It consists of a rectangular test section in which a 2 × 2 rod bundle equipped with a simple spacer grid with mixing vanes is inserted. The influence of the turbulence model on target variables linked to CHF limitation will be discussed. Moreover, the sensitivity to the mesh refinement will be particularly examined. The study of this case is a further step towards the modelling of the two-phase boiling flow in real-life grids and rod bundles.     

Discharge Redistribution as a Key Process for Heuristic Optimization of Energy Production with Pumps as Turbines in a Water Distribution Network

Water Resources Management - Water distribution networks often exhibit excess pressure that could lead to extensive leakage and infrastructure damages. While this problem can be mitigated with...     

Flow patterns and sediment deposition in rectangular shallow reservoirs

This work involves the experimental investigation of flow patterns, preferential regions of deposition and trapping efficiency in rectangular shallow reservoirs. The main flow patterns that can be encountered in rectangular shallow reservoirs are described: symmetrical flows without any reattachment point (S0), asymmetrical flows with one reattachment point (A1) and asymmetrical flows with two reattachment points (A2). The influence of geometrical and hydraulic parameters on reattachment lengths is intensively investigated. A shape parameter is introduced to classify symmetrical and asymmetrical flows. For each flow pattern, the preferential regions of deposition are studied. To conclude, a number of practical recommendations are given. Reservoirs with a shape parameter lower than 6.2 limit sediment deposition. Reservoirs with a shape parameter greater than 6.8 are favourable for sediment deposition. Finally, perspectives for maximizing and minimizing deposition are given, respectively by exploiting the great trapping potential of the flow pattern A1 and the poor trapping potential of the flow pattern S0.     

Tumor necrosis factor-alpha enhances antitumor effects of radiation against glioma xenografts

Benzodiazepine discontinuation is characterized by a syndrome of increased activity and reduced seizure threshold that is similar to effects mediated by the glutamatergic system. To elucidate the involvement of the glutamatergic system in benzodiazepine tolerance and discontinuation, we administered lorazepam, the NMDA antagonist CPP, and the combination of these compounds either concomitantly or consecutively to mice via osmotic pumps and evaluated pentylenetetrazole-induced seizure threshold, open-field activity, and benzodiazepine receptor binding during and after chronic administration. Animals receiving lorazepam alone developed partial tolerance at 7 days and complete tolerance at 14 days to the anticonvulsant effects of lorazepam. This effect was partly attenuated by CPP coadministration with lorazepam. This combination produced only partial tolerance. A reduction in seizure threshold was observed 4 days after discontinuation of lorazepam alone. This effect was abolished by coadministration of CPP with lorazepam and by CPP administration during the withdrawal period. Benzodiazepine binding in most structures examined was significantly reduced at 14 days during chronic lorazepam administration (versus 1 day), and coadministration of CPP did not alter this decrement. After lorazepam discontinuation, binding was increased at 4 and 7 days versus chronically treated animals and versus vehicle within the cerebral cortex. This effect was abolished by coadministration of CPP as well as by CPP administration during the lorazepam withdrawal period. These data support the involvement of the glutamatergic system in benzodiazepine tolerance and discontinuation.     

Interpersonal violence and mental health outcomes among Asian American and Native Hawaiian/other Pacific Islander college students.

In a cross-sectional survey of college students (N = 614), we studied interpersonal violence victimization, perpetration, and mental health outcomes in an ethnoracially diverse rural-based sample of Asian Americans (27%) and Native Hawaiian/Other Pacific Islanders (25%), two groups vastly underrepresented in trauma research. High rates of interpersonal violence (34%), violence perpetration (13%), and probable psychiatric diagnoses (77%), including posttraumatic stress disorder, were found. Exposure to physical violence, sexual violence, and life stress all were predictive of psychopathology. Female participants were associated with higher likelihood of sexual violence victimization compared to male participants, and Asian American status (especially among males) was associated with lower likelihood of physical and sexual violence compared with European Americans. These data enhance our understanding of interpersonal violence and mental health outcomes among previously understudied minority groups. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

A Comparison of Variational and Markov Chain Monte Carlo Methods for Inference in Partially Observed Stochastic Dynamic Systems

In recent work we have developed a novel variational inference method for partially observed systems governed by stochastic differential equations. In this paper we provide a comparison of the Variational Gaussian Process Smoother with an exact solution computed using a Hybrid Monte Carlo approach to path sampling, applied to a stochastic double well potential model. It is demonstrated that the variational smoother provides us a very accurate estimate of mean path while conditional variance is slightly underestimated. We conclude with some remarks as to the advantages and disadvantages of the variational smoother.     

Deposition of titanium oxide films by reactive High Power Impulse Magnetron Sputtering (HiPIMS): Influence of the peak current value on the transition from metallic to poisoned regimes

In this study, reactive High Power Impulse Magnetron Sputtering (HiPIMS) experiments were carried out to synthesize titanium oxide films, using a 45 × 15 cm² titanium target in Ar/O₂ gas mixtures. The deposition process was studied as a function of the peak current (i_peak) at constant voltage during the pulse (1 kV) and constant average power (P_av). As the oxygen flow was increased, i_peak was kept constant (160, 300 or 400A) by adjusting the pulse duration and the average power (2 or 4 kW) by adjusting the pulse repetition frequency. For all experimental conditions, an abrupt transition from metallic towards poisoned regimes was observed. The transition curves exhibit hysteresis. As i_peak is increased from 160 A to 450 A, for P_av = 4 kW, the oxygen content (Ω) in the Ar/O₂ mixture needed to poison the target surface was reduced from Ω = 11.5% to Ω = 8.5%. These values are much smaller than those recorded for DC magnetron sputtering (DCMS) (Ω = 42%) and pulsed DCMS (Ω = 36%) experiments carried out at the same power. These results are explained by the enhancement of the ionization and dissociation rates of oxygen molecules with the increase of i_peak.     

Organic solar cells with an ultra thin organized hole transport layer

Discotic liquid crystals (LC) are promising materials to manufacture devices for organic photovoltaic conversion. These molecules possess a mesophase permitting the fabrication of columnar films having particularly desirable properties of charge transport and exciton diffusion along the axes of the columns. After the deposition of an organic layer on the Indium Tin Oxide (ITO) substrate, a thermal annealing, up to the clarification temperature of the LC, is generally necessary to reach a uniform orientation of the columns. It generally leads to a pronounced droplet formation on the substrate if no surface treatment had been performed on the ITO prior to the film deposition. This paper deals with the study of how this partial dewetting can contribute to good solar cell performances with the formation of an ultra thin organized layer.     

LaPO4 Mineral Liquid Crystalline Suspensions with Outstanding Colloidal Stability for Electro‐Optical Applications

Mineral liquid crystals are materials in which mineral's intrinsic properties are combined with the self‐organization behavior of colloids. However, the use of such a system for practical application, such as optical switching, has rarely been demonstrated due to the fundamental drawbacks of colloidal systems such as limited dispersion stability. Studying colloidal suspensions of LaPO₄ nanorods, it is found that drastic improvement of colloidal stability can be obtained through a transfer of particles from water towards ethylene glycol, thus enabling the investigation of liquid crystalline properties of these concentrated suspensions. Using polarization microscopy and small‐angle x‐ray scattering (SAXS), self‐organization into nematic and columnar mesophases is observed enabling the determination of the whole phase diagram as a function of ionic strength and rod volume fraction. When an external alternative electric field is applied, a very efficient orientation of the nanorods in the liquid‐crystalline suspension is obtained, which is associated with a significant optical birefringence. These properties, combined with the high colloidal stability, are promising for the use of such high transparent and athermal material in electro‐optical devices.