217 research outputs found
Numerical simulation of ion transport membrane reactors: Oxygen permeation and transport and fuel conversion
Ion transport membrane (ITM) based reactors have been suggested as a novel technology for several applications including fuel reforming and oxy-fuel combustion, which integrates air separation and fuel conversion while reducing complexity and the associated energy penalty. To utilize this technology more effectively, it is necessary to develop a better understanding of the fundamental processes of oxygen transport and fuel conversion in the immediate vicinity of the membrane. In this paper, a numerical model that spatially resolves the gas flow, transport and reactions is presented. The model incorporates detailed gas phase chemistry and transport. The model is used to express the oxygen permeation flux in terms of the oxygen concentrations at the membrane surface given data on the bulk concentration, which is necessary for cases when mass transfer limitations on the permeate side are important and for reactive flow modeling. The simulation results show the dependence of oxygen transport and fuel conversion on the geometry and flow parameters including the membrane temperature, feed and sweep gas flow, oxygen concentration in the feed and fuel concentration in the sweep gas.King Fahd University of Petroleum and MineralsKing Abdullah University of Science and Technology (KAUST) (grant number KSU-I1-010-01
Vapor Flow Patterns During a Start-Up Transient in Heat Pipes
The vapor flow patterns in heat pipes are examined during the start-up transient phase. The vapor core is modelled as a channel flow using a two dimensional compressible flow model. A nonlinear filtering technique is used as a post process to eliminate the non-physical oscillations of the flow variables. For high-input heat flux, multiple shock reflections are observed in the evaporation region. The reflections cause a reverse flow in the evaporation and circulations in the adiabatic region. Furthermore, each shock reflection causes a significant increase in the local pressure and a large pressure drop along the heat pipe
Multiphysics Optimization for First Wall Design Enhancement in Water-Cooled Breeding Blankets
The commercial feasibility of the first fusion power plant generation adopting D-T plasma is strongly dependent upon the self-sustainability in terms of tritium fuelling. Within such a kind of reactor, the component selected to house the tritium breeding reactions is the breeding blanket, which is further assigned to heat power removal and radiation shielding functions. As a consequence of both its role and position, the breeding blanket is heavily exposed to both surface and volumetric heat loads and, hence, its design requires a typical multiphysics approach, from the neutronics to the thermo-mechanics. During last years, a great deal of effort has been put in the optimization of the breeding blanket design, with the aim of maximizing the tritium breeding and heat removal performances without undermining its structural integrity. In this paper, a derivative-free optimization method named “Complex method” is applied for the design optimization of the European DEMO Water-Cooled Lithium Lead breeding blanket concept. To this purpose, a potential performances-based objective function, focusing on the maximization of the tritium breeding, is defined and a multiphysics numerical model of the blanket is developed in order to solve the coupled thermo-mechanical problem, while the optimization algorithm leads the design towards a minimum optimum point compliant with the prescribed requirements. Once the optimized design is obtained, its nuclear and thermo-structural performances are assessed by means of specific neutron transport and multiphysics simulations, respectively. Finally, the structural integrity is verified by means of the application of the RCC-MRx design criteria
Point defect dynamics in bcc metals
We present an analysis of the time evolution of self-interstitial atom and
vacancy (point defect) populations in pure bcc metals under constant
irradiation flux conditions. Mean-field rate equations are developed in
parallel to a kinetic Monte Carlo (kMC) model. When only considering the
elementary processes of defect production, defect migration, recombination and
absorption at sinks, the kMC model and rate equations are shown to be
equivalent and the time evolution of the point defect populations is analyzed
using simple scaling arguments. We show that the typically large mismatch of
the rates of interstitial and vacancy migration in bcc metals can lead to a
vacancy population that grows as the square root of time. The vacancy cluster
size distribution under both irreversible and reversible attachment can be
described by a simple exponential function. We also consider the effect of
highly mobile interstitial clusters and apply the model with parameters
appropriate for vanadium and iron.Comment: to appear in Phys. Rev.
Helical vortex core dynamics and flame interaction in turbulent premixed swirl combustion:A combined experimental and large eddy simulation investigation
Postirradiation fiber debonding and pull-out in Sic-Sic composites *
Abstract The toughness of ceramic matrix composites is contributed by crack bridging, matrix crack deflection, fiber debonding and pull-out and other minor effects. Crack bridging relies on fibers being intact close to the crack tip, while pull-out toughening relies on the debonding and frictional characteristics of the fiber-matrix interface. The interface friction depends on the interface pressure (i.e., on misfit strains) and interface roughness. In this paper, a calculational model for postirradiation fiber debonding and pull-out toughness in Sic-Sic composites is presented. It is shown that fiber debonding and pull-out toughness in Sic-Sic composites vary significantly with neutron fluence and irradiation tem~rature, which is a direct wnsequen~ of the dependence of the misfit strain on these irradiation variables. I~t~uction Sic-Sic composites have been proposed for structural applications in fusion reactor first walls and blankets. The fracture toughness of these composites can be measured from work-of-fracture experiments, and can be theoreticafly determined by investigating the mechanisms of energy dissipation during composite failure. In addition to matrix toughness and matrix crack deflection, two other contributions are considered important in toughening SIC-SIC materials. First is the crack-tip bridging by intact fibers, which contributes a closure traction and lowers the stress intensity at the crack tip. This contribution is important in case of small cracks. The second contribution is caused by fiber debonding, fiber fracture and pull-out, which occurs at significant crack openings, thus involving * This material is based upon work supported by the US Department of Energy under award number DE-FGO3-91ER.54115. energy dissipation by interface friction. Fiber bridging, debonding and pull-out depend on the composite mismatch stresses, i.e., on misfit strains. Neutron irradiation alters the composite behavior in a complex fashion. In addition to basic property changes under irradiation, irradiation-induced swelling and creep change the mismatch stress state, which has a direct influence on the fracture strength and toughness of Sic-Sic composites. Detailed calculations of the time-evolution of mismatch stresses in Sic-Sic composites under high-temperature neutron irradiation are performed [l]. General inelastic wnstitutive equations for Sic fibers and SiC matrix, which are developed by the present authors 121 are used for that purpose. It is found that misfit strains change significantly during early irradiation, and that long-term changes depend on helium swelling and creep only, regardless of the initial thermal mismatch state. Accordingly, fiber debonding and pull-out behavior are expected to depend on the neutron fluence, In the present work, we calculate the postirradiation pull-out toughness and fiber debonding in Sic-Sic composites as functions of neutron fluence and irradiation temperature. 0022-3115/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDZ 0022-3115(94)00058-
L-Drawings of Directed Graphs
We introduce L-drawings, a novel paradigm for representing directed graphs
aiming at combining the readability features of orthogonal drawings with the
expressive power of matrix representations. In an L-drawing, vertices have
exclusive - and -coordinates and edges consist of two segments, one
exiting the source vertically and one entering the destination horizontally.
We study the problem of computing L-drawings using minimum ink. We prove its
NP-completeness and provide a heuristics based on a polynomial-time algorithm
that adds a vertex to a drawing using the minimum additional ink. We performed
an experimental analysis of the heuristics which confirms its effectiveness.Comment: 11 pages, 7 figure
Evidence for Irradiation Triggered Nonuniform Defect Distribution In Multiharmonic Magnetic Susceptibility of Neutron Irradiated YBa2Cu3O7-x
Multiharmonic ac-magnetic susceptibility \ch11,\chi2,chi3, of neutron
irradiated Li-doped YBa2Cu3O7-x has revealed a nonmonotonic dependence of all
harmonics on the neutron fluence. The irradiation has a strongly depressive
influence on the intergrain connection suggesting an increase of the effective
thickness of the intergranular Josephson junction at aneutron fluence of
0.98x10 cm. Less damaged are the intragrain properties. A
spectacular enhancement of the superconducting intragranular properties
reflected in the characteristics of all harmonics was observed at highest
fluence \Phi = 9.98x10 cm. We assume that this effect results
from the development of a space inhomogeneous distribution with alternating
defectless and defect rich regions.Comment: 24 pages, 9 figures, accepted to J. Supercon
Revisited experimental comparison of node-link and matrix representations
Visualizing network data is applicable in domains such as biology, engineering, and social sciences. We report the results of a study comparing the effectiveness of the two primary techniques for showing network data: node-link diagrams and adjacency matrices. Specifically, an evaluation with a large number of online participants revealed statistically significant differences between the two visualizations. Our work adds to existing research in several ways. First, we explore a broad spectrum of network tasks, many of which had not been previously evaluated. Second, our study uses a large dataset, typical of many real-life networks not explored by previous studies. Third, we leverage crowdsourcing to evaluate many tasks with many participants
On the interaction of vortices with mixing layers
We describe the perturbations introduced by two counter-rotating vortices - in a two-dimensional configuration - or by a vortex ring - in an axisymmetric configuration - to the mixing layer between two counterflowing gaseous fuel and air streams of the same density. The analysis is confined to the near stagnation point region, where the strain rate of the unperturbed velocity field, A0, is uniform. We restrict our attention to cases where the typical distance 2r0 between the vortices - or the characteristic vortex ring radius r0 - is large compared to both the thickness, δv, of the vorticity core and the thickness, δm∼(ν/A0)1/2, of the mixing layer. In addition, we consider that the ratio, Γ/ν, of the vortex circulation, Γ, to the kinematic viscosity, ν, is large compared to unity. Then, during the interaction time, A0,-1, the viscous and diffusion effects are confined to the thin vorticity core and the thin mixing layer, which, when seen with the scale r0, appears as a passive interface between the two counterflowing streams when they have the same density. In this case, the analysis provides a simple procedure to describe the displacement and distortion of the interface, as well as the time evolution of the strain rate imposed on the mixing layer, which are needed to calculate the inner structure of the reacting mixing layer as well as the conditions for diffusion flame extinction and edge-flame propagation along the mixing layer. Although in the reacting case variable density effects due to heat release play an important role inside the mixing layer, in this paper the analysis of the inner structure is carried out using the constant density model, which provides good qualitative understanding of the mixing layer response
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