Reality oriented simulation models of power plants for restoration studies

With the deregulation of the European Electricity Network a reduction of the stability of the electrical power system has to be expected. In this context the availability of simulation models of power plants becomes more and more important. These models are necessary for pre-fault-scenarios concerning the estimation of stability limits in normal operation and for post-fault-scenarios concerning the restoration process after blackouts. In this paper, the development of a simulation model of a high pressure hydro power plant in the Swiss Alps will be presented. This power plant is a very important one in the restoration plan of the Swiss grid.     

A cascade control strategy for a space nuclear reactor system

This work presents a linear feedback control for the space nuclear reactor power system TOPAZ II. The point-kinetics approximation with six-delayed-neutron-group is used to represent the neutron field dynamics. A favorable choice of input control variables is demostrated, which leads to a cascade control configuration with two classical either PI or P controllers. The strategy is based on linearizing-like feedback control endowed with a modeling error estimator via a reduced order-observer. The effectiveness of the control law to the tracking of a given thermal power profile in the start-up regime and the tracking of a given electric power profile in the operation regime are illustrated via numerical simulations.     

Investigation of seasonal metallurgical shift at inco's clarabelle mill

The seasonal metallurgical shift at Inco's Clarabelle mill, Copper Cliff, Ontario represents a significant variation in pentlandite and precious metals recovery from summer to winter. The decrease in metals recovery in winter has been a subject of research and debate in the past decades. To understand this seasonal phenomenon, on-site testwork using a portable batch flotation column was conducted in two winter seasons and two summer seasons. On-site batch tests on the main flotation feed were used to obtain flotation rate and selectivity data. Other data such as dissolved ions and thiosalts in process and feed waters that could affect flotation was also collected.One of the most important findings from the present investigation was that both sulfide and gangue minerals were floated significantly faster in winter than in summer. The pentlandite /pyrrhotite and pentlandite / rock selectivity was, however, considerably decreased in winter. The process and feed water analysis indicated that the high concentration of dissolved nickel ions in the winter process water could have activated all minerals, leading to the higher flotation rates. The increase in pyrrhotite and rock flotation rates was, however, larger than the increase in pentlandite flotation rate. This resulted in the reduced pentlandite /pyrrhotite and pentlandite / rock selectivity in winter, which was the main cause for the seasonal metallurgical shift.     

Nonlinear control of U-tube steam generators via H∞ control

The objective of this paper is to design, analyze and evaluate a controller for the water level of U-tube steam generators. The control objective is to closely regulate the water level during extreme load variations and despite drifts in process parameters. The controller design is based on the H_∞-norm. Gain-scheduling is used to obtain a global water-level controller. The controller performance is satisfactory at all power levels. In view of a comparative analysis of the H_∞ controller with an LQG/LTR controller, implementation and use of the gain-scheduled H_∞ controller over the equivalent LQG/LTR controller is recommended.     

Bulk amorphous FC20 (Fe–C–Si) alloys with small amounts of B and their crystallized structure and mechanical properties

The addition of a small amount (0.4 mass%) of B to a commercial FC20 cast iron was found to cause the formation of an amorphous phase in melt-spun ribbon and cast cylinders with a diameter of up to 0.5 mm. The structure of a melt-spun B-free FC20 alloy consisted of α-Fe, γ-Fe and Fe₃C. The effectiveness of additional B is presumably due to the generation of attractive bonding nature among the constituent elements. The amorphous alloy ribbon exhibits a high tensile strength of 3480 MPa and good bending ductility. The annealing causes the formation of an amorphous phase containing α-Fe particles with a size of about 30 nm. The mixed phase alloy exhibits an improved tensile strength of 3800 MPa without detriment to good ductility. With further increasing temperature, the mixed amorphous and α-Fe structure changes to α-Fe+Fe₃C+graphite through the metastable structure of α-Fe+Fe₃C. The structure after annealing for 900 s at 1200 K has fine grain sizes of about 0.5 μm for α-Fe, 0.3 μm for Fe₃C and 1 μm for graphite. The graphite-containing alloy exhibits high tensile strength of 1200–2000 MPa and large elongation of 5–13%. The high tensile strength and good ductility were also obtained for the 0.5 mm cylinder annealed at 1200 K. The good mechanical properties are due to the combination of fine subdivision of crack initiation sites by the homogeneous dispersion of small graphite particles and the dispersion strengthening of Fe₃C particles against the deformation of the α-Fe phase. The synthesis of the finely mixed α-Fe+Fe₃C+graphite alloys having good mechanical properties by crystallization of the new amorphous alloy in the melt-spun ribbon and cast cylinder forms is encouraging for the future development of a new Fe-based high-strength and high-ductility material.     

Growth behavior of fine grains formed by diffusion induced recrystallization in the Cu(Zn) system

The growth behavior of fine grains produced due to diffusion induced recrystallization (DIR) was experimentally studied for the Cu(Zn) system using single crystal Cu specimens. The Cu specimens were annealed at 693 K for various times together with a Zn source alloy by a capsule zincification technique and then DIR was observed to take place on the surfaces of the specimens. The DIR region consists of an almost single layer of fine grains. Although the average height l and the average diameter s of the fine grains increase with increasing annealing time, the aspect ratio s/l is kept to be nearly unity during annealing. The Zn concentration on the specimen surface is smaller than that in the Zn source alloy at early stages of the reaction. Such lower Zn concentrations could be explained using the maximum driving force model proposed by Kajihara and Gust (Acta metall. mater., 1991, 39, 2565). The composition on the specimen surface gradually approaches the equilibrium value with increasing annealing time. This means that the amount of the Zn source alloy is large enough to maintain the supply of Zn atoms to the specimen in each zincification capsule. The kinetic model by Li and Hillert (Acta metall., 1981, 29, 1949) and the extended model by Kawanami et al. (ISIJ Int., 1997, 37, 921) could rather well reproduce the annealing time dependence of the growth rate of the DIR region. The contribution of volume diffusion to the transfer of Zn atoms from the surface to the moving boundaries in the DIR region was estimated to be negligible. Thus, the chemical driving force is considered to be the most important driving force for the grain boundary migration and the boundary diffusion along the grain boundaries in the DIR region is recognized to be the rate controlling process of DIR.     

Domain morphology evolution associated with the relaxor–normal ferroelectric transition in the Bi- and Zn-modified Pb(Ni1/3Nb2/3)O3–PbZrO3–PbTiO3 system

To understand the dielectric behavior from a viewpoint of domain configuration, the domain morphology evolution in (Pb_0.985Bi_0.01)(Ni_1/4Zn_1/12Nb_2/3)_0.2(Zr_1-σTi_σ)_0.8O₃ ceramics (0.30⩽σ⩽0.60) has been investigated by transmission electron microscopy and high resolution electron microscopy. The results indicated that the domain morphology evolved from the normal micron-sized domains to herringbone domain patterns, and finally to the polar nanodoamains approximately 3∼6 nm in size when the PT content was decreased from 60 to 30 mol%. The normal twin-related 90° macrodomains are closely correlated with the normal dielectric response of the composition with higher PT content, whereas the relaxor response of the composition with lower PT content is directly attributable to nanometer domains that contain 1:1 short-range ordering on the B-site sub-lattice. A model is proposed to describe the effect of the PbTiO₃ content on the ferroelectric domain morphology evolution in the system.     

A new mechano-optical technique to measure local velocities in opaque fluids

A novel technique has been developed to measure the local velocities in opaque liquid flows such as liquid metals. The measuring principle is based on the separation of a direct mechanical interaction between flow and sensor tip and the optical acquisition and processing of the signal. In principle, this fact allows the extension of the range of applicability to higher temperatures. Furthermore, the insensitivity of the system to electrical noise and external magnetic fields can be considered as an important advantage. Until now, the sensor has been tested in metallic melts up to temperatures of about 350°C. We present measurements of the local velocity obtained in an eutectic InGaSn melt driven by a rotating magnetic field.