An efficient algorithm for nucleolus and prekernel computation in some classes of TU-games

We consider classes of TU-games. We show that we can efficiently compute an allocation in the intersection of the prekernel and the least core of the game if we can efficiently compute the minimum excess for any given allocation. In the case where the prekernel of the game contains exactly one core vector, our algorithm computes the nucleolus of the game. This generalizes both a recent result by Kuipers on the computation of the nucleolus for convex games and a classical result by Megiddo on the nucleolus of standard tree games to classes of more general minimum cost spanning tree games. Our algorithm is based on the ellipsoid method and Maschler's scheme for approximating the prekernel. \u

Excitation of surface plasmons at a SiO2/Ag interface by silicon quantum dots: Experiment and theory

The excitation of surface plasmons (SPs) by optically excited silicon quantum dots (QDs) located near a Ag interface is studied both experimentally and theoretically for different QD-interface separations. The Si QDs are formed in the near-surface region of an SiO2 substrate by Si ion implantation and thermal annealing. Photoluminescence decay-rate distributions, as derived from an inverse Laplace transform of the measured decay trace, are determined for samples with and without a Ag cover layer. For the smallest, investigated Si-QDs-to-interface distance of 44 nm the average decay rate at lambda=750 nm is enhanced by 80% due to the proximity of the Ag-glass interface, with respect to an air-glass interface. Calculations based on a classical dipole oscillator model show that the observed decay rate enhancement is mainly due to the excitation of surface plasmons that are on the SiO2/Ag interface. By comparing the model calculations to the experimental data, it is determined that Si QDs have a very high internal emission quantum efficiency of (77±17)%. At this distance they can excite surface plasmons at a rate of (1.1±0.2)×104 s¿1. From the model it is also predicted that by using thin metal films the excitation of surface plasmons by Si QDs can be further enhanced. Si QDs are found to preferentially excite symmetric thin-film surface plasmons

A Role of the Bile Salt Receptor FXR in Atherosclerosis

This study reviews current insights into the role of bile salts and bile salt receptors on the progression and regression of atherosclerosis. Bile salts have emerged as important modifiers of lipid and energy metabolism. At the molecular level, bile salts regulate lipid and energy homeostasis mainly via the bile salt receptors FXR and TGR5. Activation of FXR has been shown to improve plasma lipid profiles, whereas Fxr(-/-) mice have increased plasma triglyceride and very-low-density lipoprotein levels. Nevertheless, high-density lipoprotein cholesterol levels are increased in these mice, suggesting that FXR has both anti-and proatherosclerotic properties. Interestingly, there is increasing evidence for a role of FXR in "nonclassical" bile salt target tissues, eg, vasculature and macrophages. In these tissues, FXR has been shown to influence vascular tension and regulate the unloading of cholesterol from foam cells, respectively. Recent publications have provided insight into the antiinflammatory properties of FXR in atherosclerosis. Bile salt signaling via TGR5 might regulate energy homeostasis, which could serve as an attractive target to increase energy expenditure and weight loss. Interventions aiming to increase cholesterol turnover (eg, by bile salt sequestration) significantly improve plasma lipid profiles and diminish atherosclerosis in animal models. Bile salt metabolism and bile salt signaling pathways represent attractive therapeutic targets for the treatment of atherosclerosi

Modelling of a reverse flow catalytic membrane reactor for the partial oxidation of methane

Gas-To-Liquid (GTL) processes have great potential as alternative to conventional oil and coal processing for the production of liquid fuels. In GTL-processes the partial oxidation of methane (POM) is combined with the Fischer-Tropsch reaction. An important part of the investment costs of a conventional GTL-plant is related to cryogenic air separation. These costs could be substantially reduced by separating air with recently developed oxygen perm-selective perovskite membranes, which operate at similar temperatures as a POM reactor. Integration of these membranes in the POM reactor seems very attractive because oxygen reacts at the membrane surface resulting in a high driving force over the membrane increasing the oxygen permeation

Diffusion effects in rotating rotary kilns

A novel approach to the modeling of mass transfer in rotary kilns has been described (Heydenrych et al, 2001). It considers the mass transfer to occur by the inclusion of gas in the interparticle voids in between the particles that move concentrically with the kiln. By doing so, the rate of mass transfer was found to be dependent on bed fill and the ratio of reaction rate constant to angular velocity (k/ ). The model was found to be valid at slow to medium fast reactions. For fast reactions it under-predicted mass transfer. Therefore in this paper, the model will be extended to include diffusion effects. An additional dimensionless number is necessary then to describe the system. This can either be a Peclet number (R2/De) or a Thiele modulus (kR2/De)1/2. The solution of the 2-dimensional partial differential equations that describe the extended model gives a handle on the effect of scale-up in rotary kilns. For industrial-scale kilns, the Peclet number is large, which means that diffusion within the lower (passive) layer of the bed is unimportant for slower rates. With high reaction rates, iso-concentration lines are closely stacked near the surface of the bed, implying that it is important to model the active layer rather than the bed as a whole in these circumstances. However, the stiff differential equations are not easily solved then, and other methods of solution are advisable

Optical control of solar sails using distributed reflectivity

The dynamics of solar sails with a variable surface reflectivity distribution are investigated. When changing the reflectivity across the sail film, which can be achieved using electro-chromic coatings, the solar radiation pressure forces and torques across the sail film can be controlled without changing the attitude of the spacecraft relative to the Sun and without using mechanical systems. The paper presents two approaches. First, a continuous reflectivity distribution is presented to control the sail attitude under the influence of, for example, gravity gradient torques in Earth orbit. The second approach assumes discrete on/o reflectivity regions across the surface. Both concepts of `optical reconfiguration' of solar sails enable a more flexible steering of the spacecraft and minimize actuation effort

Gas dispersion and bubble-to-emulsion phase mass exchange in a gas-solid bubbling fluidized bed: a computational and experimental study

Knowledge of gas dispersion and mass exchange between the bubble and the emulsion phases is essential for a correct prediction of the performance of fluidized beds, particularly when catalytic reactions take place. Test cases of single rising bubble and a bubbling fluidized bed operated with a jet without a chemical reaction were studied in order to obtain fundamental insights in the prevailing mass transfer phenomena. Numerical simulations were carried out to predict the dispersion of tracer gas using a two-fluid model based on Kinetic Theory of Granular Flow (KTGF). The simulations of a single-bubble rising through an incipiently fluidized bed revealed that the assumptions often made in phenomenological models in the derivation of correlations for the mass transfer coefficient, mainly that the bubble diameter remains constant and that the tracer concentration is uniform in the bubble, are not valid. The predicted bubble-to-emulsion phase mass transfer coefficient showed good agreement with the estimated values from the literature correlations assuming additive convection-diffusion transport for different bubble sizes and different particle sizes, indicating the importance of the convective distribution even for relatively small particles. Experiments were carried out to measure the steady state concentration profiles of a tracer gas in a pseudo two-dimensional bubbling fluidized bed operated with a jet. The simulated steady state concentration profiles of the tracer gas agreed well the experimental measurements. The radial convection of the gas is significantly influenced by the bubble `throughflow¿ and therefore depends upon the particle and bubble size. The experimental comparison of theoretical results was extended to study the influence of the jet velocity and the particle diameter on the radial dispersion of the tracer gas in the bed

Adiabatic reactor simulations of the reverse flow catalytic membrane reactor concept with Perovskite membranes

Abstract only

A reverse flow catalytic membrane reactor for the production of syngas: an experimental study

In this paper experimental results are presented for a demonstration unit of a recently proposed novel integrated reactor concept (Smit et. al., 2005) for the partial oxidation of natural gas to syngas (POM), namely a Reverse Flow Catalytic Membrane Reactor (RFCMR). Natural gas has great potential as a feedstock for the production of liquid fuels via the Gas-To-Liquid (GTL) process, but this process has not found widespread application yet, mainly due to the large costs associated with cryogenic air separation and complex heat integratio