Selecting risk levels in chance-constrained reservoir operation modeling: A fuzzy set approach

Many modifications, extensions, discussions, and evaluations of chance-constrained reservoir operating models have been reported in the technical literature. Lack of economic data and the fact that the establishment of acceptable risk levels in these types of models involves a human factor with all its vagueness of perception, subjectivity, and attitudes may not permit proper application of either reliability or multiobjective programming approaches. This paper presents a unique methodology for handling a practical problem of selecting risk levels in chance-constrained reservoir operation modeling. The proposed methodology is based on fuzzy set theory. Two types of fuzzy sets are used in the formulation of the reservoir long-term planning model, one for constraints and one for the objective function. An iterative solution algorithm for deriving an optimal decision using fuzzy set operations and the chance-constrained approach is developed and presented. A practical application of the approach demonstrates the feasibility and efficiency of both the proposed approach and its iterative search procedure for selecting risk levels in chance-constrained reservoir modeling.     

Properties of thermomechanically rolled Nb/Ti microalloyed steel sheets

Structure and mechanical properties of a Nb/Ti microalloyed steel, thermomechanically rolled on a strip-production facility, are evaluated. Small (0.02%) titanium addition to niobium-containing steel is found to increase precipitation while reducing grain-refining potential of niobium. This effect increases with increasing finish-rolling temperature, and is also reflected in an increased charpy energy transition temperature. A reduced tendency for precipitation in a nitrogen-free austenite together with the short interpass times in the finishing train, is assumed to reduce the tendency for pancaking, which is prerequisite for an effective grain refining.     

Effect of rapid solidification and alloying addition on lattice distortion and atomic ordering in L10 tial alloys and their ternary alloys

Binary and ternary L1₀ type titanium alloys were rapidly quenched into thin foils by the hammer-anvil quenching technique. The nominal alloy compositions include Ti_1-xAl_x and (Ti_1-xAl_x)_{100-@#@ y}M_y, wherex = 0.5 to 0.6; y= 1 to 4; and M is V, Cr, Mn, Fe, Co, Ni, and Ge. Lattice parameters of Ll₀ phases in these alloys were studied by X-ray diffractometer using the foils and their powders. Simultaneously, long-range order parameters of these Ll₀ phases in the as-quenched foils and the annealed powders were determined from the superlattice intensity measured by X-ray diffractometer. Rapid quenching suppresses long-range ordering (S) during solidification and, thereby, reduces lattice distortion (c/a) in Ll₀ titanium alloys. A small amount of alloying element has a large effect on thec/a ratio. The atomic size effect and probably substitutional site preference play an important role in such ac/a reduction. From the density measurement, it is found that antisite defects are a predominant mode in Al-rich off-stoichiometric TiAl compound alloys. Determined long-range order value is significantly low in as-quenched binary TiAl due to rapid quenching, and the highest ordering occurs at as-quenched Ti₄₅Al₅₅ alloy. The relationship betweenc/a andS in Ll₀ Ti₄₅Al₅₅ may be written asc/a = 1 + 0.025 ·S.     

The Value of Considering Autocorrelation Between Inflows in the Stochastic Planning of Water Resource Systems

Reliability programming formulations offer a family ofexplicit stochastic models for planning the operationof complex water resources systems. These models usecumulative probability distributions of the sum ofinflows to characterize their variability in theplanning period. Applicability of these models for avariety of problems has been limited, mainly due tothe assumption of independence between inflows indifferent time periods that leads to the derivation ofconservative operating policies. This paper presentsthree new approaches to overcome this limitation. Theperformance of the proposed approaches is demonstratedthrough comparison of the operating policies derivedfrom these approaches and the Independent Approach. Operational planning of the Manitoba Hydro energygeneration system, a predominantly hydro-based utilitycompany in Manitoba (Canada), is used as the case study.     

Simulation of Transcritical Flow in Pipe/Channel Networks

Using finite difference methods in conjunction with the reduced momentum equation and applying boundary condition structure inherent to subcritical flow to all regimes, is an approach that enables efficient numerical simulation of supercritical and transcritical flows in pipe/channel systems. However, as well as certain errors within a single channel due to incomplete equations, this technique also may introduce unwanted effects propagating across a network in both upstream and downstream directions. These may include: unrealistic backwater effects due to improper boundary conditions, nonamplifying oscillations due to jerky jump movement, and other computational instabilities. Practical implications of these are analyzed in detail and are illustrated using a set of examples. Sensitivity analyzes and comparisons with analytical solutions and laboratory experiments are made. The measures to reduce the inaccuracies inevitable in simulation of transcritical flows are discussed.     

Synthesis of Spherical β-Silicon Carbide Particles by Ultrasonic Spray Pyrolysis

Fine agglomerate-free spherical β-SiC powder was synthesized from a dispersion of colloidal silica, saccharose, and boric acid, by means of an ultrasonic spray pyrolysis method. Droplets of 2.2 μm were formed with an aerosol generator, operated at 2.5 MHz, and carried into a reaction furnace at 900°C with argon. Spherical X-ray amorphous gel particles of 1.1 μm were obtained. β-SiC particles with a mean diameter of 0.79 μm and spherical shape resulted when the SiC gel precursor particles were heated at 1500°C in argon.     

Extracting information from the molten salt database

Molten salt technology is a catchall phrase that includes some very diverse technologies; electrochemistry, heat transfer, chemical oxidation/reduction baths, and nuclear reactors. All of these technologies are linked by the general characteristics of molten salts that can function as solvents, have good heat-transfer characteristics, function like a fluid, can attain very high temperatures, can conduct electricity, and also may have chemical catalytic properties. The Janz molten salt database is the most comprehensive compilation of property data about molten salts available today and is widely used for both fundamental and applied purposes. Databases are traditionally viewed as “static” documents that are used in a “search and retrieval” mode. These static data can be transformed by informatics and data mining tools into a dynamic dataset for analysis of the properties of the, materials and for making predictions. While this approch has been successful in the chemical and biochemical sciences in searching for and establishing structure-property relationships, it is not widely used in the materials science community. Because the design of the original molten salt database was not oriented toward this informatics goal, it was essential to evaluate this dataset in terms of data mining standards. Two techniques were used—a projection (principal components analysis (PCA)) and a predictive method (partial least squares (PLS))—in conjunction with fundamental knowledge acquired from the long-term practice of molten salt chemistry. This article is based on a presentation made in the symposium entitled “Fourth International Alloy Conference,” which occurred in Kos, Greece, from June 26 to July 1, 2005, and was Sponsored by Engineering Conferences International (ECI) and co-sponsored by Lawrence Livermore National Laboratory and Naval Research Laboratory, United Kingdom.     

Adsorption of n-Alkanes on Silicon Nitride Nanopowder

Adsorption properties of silicon nitride beads were investigated by the inverse gas chromatography (IGC) method. Silicon nitride beads were obtained from a commercial nanopowder. Retentions of n -pentane, n -hexane, n -heptane, and n -octane were measured in the temperature range from 343 to 383 K, at zero surface coverage. Surface partition coefficients and the related thermodynamic functions (standard free energy changes, standard state enthalpy, and entropy changes) of the adsorption process were determined. The dispersive component of the surface free energy of silicon nitride, at investigated temperatures, was also calculated.     

Calculation of excess molar volumes with different cubic equations of state and different mixing rules

The effects of the simple composition-dependent combining rules (conventional, Adachi-Sugie and van Laar-Stryjek-Vera-type) and Huron-Vidal-NRTL mixing rules and different cubic equations of state (Redlich-Kwong, Soave-Redlich-Kwong, Peng-Robinson and Peng-Robinson-Stryjek-Vera) on excess molar volume correlation are examined. The results obtained for some asymmetric non-ideal mixtures indicate very good agreement between correlated and experimental data in all cases when Huron-Vidal-NRTL mixing rules are coupled with the Peng-Robinson-Stryjek-Vera equation of state.