Continuous and discrete variable-structure controls for parallel three-phase boost rectifier

We describe three nonlinear control schemes for a parallel three-phase boost rectifier consisting of two modules. The basic idea, however, can be extended to a system with N modules. All of the control schemes are developed in a synchronous frame. Moreover, each of the closed-loop power-converter modules operates asynchronously without any communication with the other module. Based on the dynamical equations of the parallel converter, we find that independent control of both of the modules on the DQ axes is not necessary and possible. Consequently, we develop control schemes that stabilize the dq axes and limit the zero-axis disturbance by preventing the flow of the pure zero-sequence current. One of the control schemes is developed purely in the discrete domain. It combines the space-vector modulation scheme with a variable-structure control, thereby keeping the switching frequency constant and achieving satisfactory dynamic performance. The performances of the other control schemes are also satisfactory.     

9(R)‐[6(R)‐Hydroxy­methyl‐1‐oxa‐4‐thia­cyclo­hexan‐2‐yl]hypoxanthine–water (4/3), a nucleoside analogue

The title compound, 9(R)‐[6(R)‐hydroxy­methyl‐1‐oxa‐4‐thia­cyclo­hexan‐2‐yl]‐1,9‐di­hydro‐6H‐purin‐6‐one–water (4/3), C₁₀H₁₂N₄O₃S·0.75H₂O, crystallizes in the triclinic space group P1 with four mol­ecules in the asymmetric unit and 0.75 waters of hydration per mol­ecule. The structure was refined to an R value of 0.072 for 3382 observed reflections. The four crystallographically independent mol­ecules are designated A, B, C and D. All four oxa­thia­ne rings adopt the chair conformation and the purine bases are in an anti orientation with respect to the sugar moieties. Molecules A and D and mol­ecules C and B are base paired by a single hydrogen bond of the type N—H?N. These base pairs are again hydrogen bonded to their translated pairs in the direction of a cell diagonal.     

Transport of reactive solutes in unsteady annular flow subject to wall reactions

The present paper concerns with the dispersion process in steady and oscillatory flows through an annular pipe in presence of reversible and irreversible reactions at the wall. Method of homogenization, a multiple-scale method of averaging, is adopted for deriving the effective transport equations. The main objective is to look into the effect of aspect ratio of the annular pipe on the dispersion coefficient due to the combined effect of axial convection and radial diffusion in steady and oscillatory flows along the annulus, subject to the kinetic reversible phase exchange and irreversible absorption at the outer wall. Results demonstrate that upto a certain critical value of aspect ratio, dispersion coefficient increases with increase of aspect ratio when the wall is retentive, though the wall inertness may lead to decrease of dispersion coefficient with increase of aspect ratio. The results would be useful to the medical practitioners working in the domain of catheterized artery.     

Laboratory Experiments on Environmental Friendly Means to Improve Coalbed Methane Production by Carbon Dioxide/Flue Gas Injection

Scaled in situ laboratory core flooding experiments with CO₂, N₂ and flue gas were carried out on coal in an experimental high P,T device. These experiments will be able to give an insight into the design of the injection system, management, control of the operations and the efficiency of an ECBM project. Although the experience gained by the oil industry represents a valuable starting point, several problems are still to be studied and solved before CO₂ improved deep coalbed methane production may be operationally feasible. These are all related to the heterogeneous nature of the pore structure of coal, and in particular to the presence of fractures. More specifically, a number of questions need to be addressed, e.g. what are the conditions under which the fluid in the micro pores of the coal is displaced by the CO₂ in the presence of competitive adsorption; what is the role of compositional heterogeneity and fracture anisotropy of coal for the injection design and the efficiency of the sequestration in relation to the swelling and shrinkage characteristics of coal; how does the mobile and the immobile water in the coal affect the exchange process. These questions can be answered by means of downscaled laboratory experiments that are capable of accurately describing the coupled process of multiphase flow, competitive adsorption and geo-mechanics. The laboratory conditions have been simulated to match pressure and temperature at depths of 800 to 1,000 m. Under those conditions the injected CO₂ remains supercritical. Upto now, the results show that dewatering will be an essential step for successful ECBM combined with a CO₂ sequestration process.     

Time-domain modeling of high-speed interconnects by modified methodof characteristics

In this paper, a new model of lossy transmission lines is presented for the time-domain simulation of high-speed interconnects. This model is based on the modified method of characteristics (MMC). The characteristic functions are first approximated by applying lower order Taylor series in the frequency domain, and then a set of simple recursive formulas are obtained in the time domain. The formulas, which involve tracking performances between two ends of a transmission line, are similar to those derived by the method of characteristics for lossless and undistorted lossy transmission lines. The algorithm, based on the proposed MMC model, can efficiently evaluate transient responses of high-speed interconnects. It only uses the quantities at two ends of the lines, requiring less computation time and less memory space than required by other methods. Examples indicate that the new method has high accuracy and is very efficient for the time-domain simulation of interconnects in high-speed integrated circuits     

Characteristics of Fe2O3 Nanoparticles Prepared by Heat Treatment of a Nonaqueous Powder Precipitate

Nanosized particles of Fe₂O₃ with a narrow size distribution (5–20-nm) were prepared by heat treatment of powder precipitates obtained from a nonaqueous homogeneous solution of stearic acid and iron(III) nitrate. Differential scanning calorimetric studies and infrared spectroscopic studies on the as prepared powder precipitates revealed the presence of Fe–O-stearate intermediate/complex, which subsequently transformed to -Fe₂O₃ when heat treated at 200°C for 30-min. The crystallographic -Fe₂O₃ transformation and the particle size distribution of the nanosized particles were studied with the help of X-ray diffraction and small angle X-ray scattering technique. Crystallographic transformation of nanosized Fe₂O₃ has been found to occur at 200°C, which is well below the temperature reported elsewhere.     

Solutions of LRS Bianchi I space-time filled with a perfect fluid

LRS Bianchi I space-time filled with a perfect fluid is considered and it is shown that the field equations are solvable for any arbitrary cosmic scale function. Solutions for a particular form of cosmic sclae functions are presented and all solutions, except for some cases, are shown to represent an empty universe for large time.     

A new thermal assembly design for the directional solidification of transparent alloys

A modified design of the thermal assembly is presented for the directional solidification of transparent alloys that eliminates the radial temperature gradient and minimizes the curvature of the interface. An additional booster heater is designed, and the position of the heater is shown to be critical in obtaining a flat interface. A full-scale numerical calculation, carried out for succinonitrile-0.5 wt% Salol, shows that the interface concavity can be reduced gradually by placing the booster heater just above the cold end and by adjusting the temperature of the booster heater while keeping the hot and cold zone temperatures fixed. Experimental measurements of temperatures at the wall and at the center have been carried out systematically by using two calibrated thermocouples, and the observed thermal profiles have been shown to strongly support the numerical prediction. When a macroscopically flat interface is obtained, it is shown that columnar growth away from the ampoule wall can be observed and photographed. The effects of thermal gradient and the temperature setting of the booster heater on the planarity of the interface are discussed.     

9[1-(2-Hydroxy ethoxy)-3-hydroxy propyl] guanine: a nucleoside analog

The compound 9[1-(2-hydroxy ethoxy)-3-hydroxy propyl] guanine crystalizes in the tetragonal system, space group P4₁2₁2 with a = 11.106 (1), c = 20.558 (2) Å, and Z = 8. The acyclic chain C1¯O1¯C4¯C5¯O5 is in the extended configuration and the glycosidic torsion angle (C4¯N9¯C1¯O1) is 125.1 (8)°. The molecules are held together by Van der Waal's forces.     

1-(Ethoxymethyl)-6-(phenylselenyl)-5-ethyl uracil: a nucleoside analog

The compound, 1-(ethoxymethyl)-6-(phenylselenyl)-5-ethyl uracil, crystallizes in the monoclinic space group P2₁/n with unit cell parameters a = 5.304(1), b = 21.261(4), c = 13.996(4) Å, = 94.30(2)°, and Z = 4. The acyclic chain C1, O4, C4, C5 is in fully extended form and nearly perpendicular to the uracil base. The molecules are held together by van der Waal's forces.