Heat transfer between a fluidized bed and an immersed vertical U-tube

The design details of a 0.254 m stainless steel cylindrical fluidized bed pilot plant facility, whose fabrication, installation and testing have been recently completed, are described. It primarily consists of a fluidized-bed reactor, fluidizing air-supply system, solids feeder, off-gas cleaning and exhaust system, and cooling water-supply system for heat-transfer tubes provided in the bed and in the freeboard sections. The plant is operated at ambient pressure in the temperature range 300–600 K, both in the batch and continuous modes for solids feed. Bed-pressure drop measurements as a function of fluidizing velocity for two different bed heights reveal that the quality of fluidization is good. Similar experiments have been conducted with the continuous solids feed. The heat-transfer coefficient between the bed and an immersed stainless steel U-tube is measured as a function of fluidizing air velocity at five different temperatures. The effects on bed-to-tube heat-transfer rate of solids feed rate, bed height, air-flow rate, and bed temperature are examined. All of these observed variations are interpreted in terms of the solids mixing and bubble mechanics in the bed.     

Buffer Behavior for Mixed Arrivals and Single Server with Random Interruptions

A queueing model with finite buffer size, mixed input traffic (Poisson and burst Poisson arrivals), synchronous transmission and server interruptions through a Bernoulli sequence of independent random variables is studied. Using average burst length, traffic intensity and input traffic mixture ratio as parameters, the relationships among buffer size, overflow probability and expected message queueing delay are obtained. An integrated digital voice-data system with synchronous time division multiplexing (STDM) for a large number of voice sources and mixed arrival process for data messages is considered as an application for this model. The results of this study are portrayed on graphs and may be used as guidelines in buffer design problems in digital voice-data systems. The queueing model developed is quite general in a sense that it covers pure Poisson and burst Poisson arrival processes and the mixture of the two as well.     

Unusual thickness dependence of the dielectric constant of erbium oxide films

The thickness dependence of the dielectric constant of Er₂O₃ films was studied. It was found that for films less than 700 Å thick and greater than 1300 Å the dielectric constant ε shows the usual behaviour of increasing with increasing thickness and then assuming the limiting bulk value. However, for films of intermediate thicknesses (700–1300 Å) the dielectric constant first decreases, attains a minimum value and then increases to obtain the bulk value. It is suggested, based on electron microscope observations, that this curious thickness dependence of ε arises because of the transformations from (i) amorphous to crystalline and (ii) f.c.c. type to b.c.c. type crystalline phases in the films.     

Photoconductivity storage in Ga1−xAlxAs alloys at low temperatures

Intentionally undoped n-type and high purity Ga_1?xAl_xAs alloys with compositions in the range 0.19≤x≤0.78 are found to show a long life time photoconductivity effect at low temperatures (T < 80 K) when irradiated with white light filtered through a Ge filter and also when the light source is removed after photoexcitation. For the direct gap materials (0≤x≤ 0.43), it is shown that the deep level in the alloys, which controls the electrical properties of the crystals, captures and emits electrons via the first higher energy subsidiary conduction band inima L although the Γ minimum is the lowest in energy. These indirect electron transitions by the deep level, via the L minima, which is found to have an acceptor like nature, provide a natural explanation of the photoconductivity storage at low temperatures. For indirect gap materials (x > 0.43), when X minima are the lowest energy subsidiary minima, the photoconductivity storage at low temperatures is due to the double acceptor nature of the deep level.     

Mechanisms of heat transfer between a surface and a gas-fluidized bed for combustor application

There is currently considerable interest in the development of fluidized-bed boilers for efficient and environmentally clean combustion of coal. Fluidized beds have the advantages of high heat transfer rates and intimate mixing of additives such as limestone or dolomite for sulfur dioxide absorption produced as a result of combustion of coal containing sulfur. However, design for optimum heat transfer remains uncertain and essentially empirical. The mechanisms of heat transfer are complicated because of the many variables in a commercial combustion operation such as particle size distribution, particle shape, particle and gas thermal properties, reactor geometry and boiler tube design.An understanding of the mechanisms of bed to tube heat transfer is essential to sound design and interpretation of empirically derived correlations. Here we will review and criticize the major mechanisms of heat transfer that have been proposed. These mechanisms are proposed and developed from two schools of thought: (a) The principal resistance to heat transfer is a fluid film, and the moving fluidized particles scour the film to reduce the resistance to heat transfer; (b) Heat is absorbed by the fluidized particles and the rate of heat transfer depends on the rate of heat absorption.Radiant heat transfer is also discussed in this review in detail. Heat transfer by radiation is an important consideration in combustors but has received limited attention. The results of theoretical calculations are given which have been recently reported on the basis of the alternate-slab model of Gabor.The review will predominantly deal with the mechanistic models of heat transfer and various correlations developed over years will not be covered as this topic is dealt with in another review article by Saxena, Grewal, Gabor, Zabrodsky and Galershtein.     

Generalised plane strain problem for a circular inclusion in anisotropic elasticity

This paper is concerned with a generalised plane deformation problem in the linear theory of anisotropic elasticity. As is well known, the generalised plane deformation is the deformation of a body of infinite length bounded by a cylindrical surface, when all the stress and strain components exist but they are functions of two co-ordinates x₁, and x₂ only. It may be shown that if u₃ = 0, it is impossible to satisfy all the three equations of equilibrium of anisotropic elastic body. One has to choose u₃ as a non-zero function of x₁, x₂ for satisfying equations of equilibrium. In isotropic elasticity, u₃ = 0, makes the third equation of equilibrium identically equal to zero.The problem in this paper concerns an elastic circular cylindrical inclusion embedded in a matrix of different anisotropic material. The matrix and the inclusion are perfectly bonded at the interface. Each of the two materials possesses anisotropy of a general form with all the 21 elastic constants. The matrix is subjected to a uniform stress at infinity. The equations of elasticity theory demand that the rotation component ω₃ must also be prescribed at infinity. The complex variable technique is used and exact analytical expressions are derived for the elastic field in both the regions.     

Virus Matryoshka: A Bacteriophage Particle—Guided Molecular Assembly Approach to a Monodisperse Model of the Immature Human Immunodeficiency Virus

Immature human immunodeficiency virus type 1 (HIV‐1) is approximately spherical, but is constructed from a hexagonal lattice of the Gag protein. As a hexagonal lattice is necessarily flat, the local symmetry cannot be maintained throughout the structure. This geometrical frustration presumably results in bending stress. In natural particles, the stress is relieved by incorporation of packing defects, but the magnitude of this stress and its significance for the particles is not known. In order to control this stress, we have now assembled the Gag protein on a quasi‐spherical template derived from bacteriophage P22. This template is monodisperse in size and electron‐transparent, enabling the use of cryo‐electron microscopy in structural studies. These templated assemblies are far less polydisperse than any previously described virus‐like particles (and, while constructed according to the same lattice as natural particles, contain almost no packing defects). This system gives us the ability to study the relationship between packing defects, curvature and elastic energy, and thermodynamic stability. As Gag is bound to the P22 template by single‐stranded DNA, treatment of the particles with DNase enabled us to determine the intrinsic radius of curvature of a Gag lattice, unconstrained by DNA or a template. We found that this intrinsic radius is far larger than that of a virion or P22‐templated particle. We conclude that Gag is under elastic strain in a particle; this has important implications for the kinetics of shell growth, the stability of the shell, and the type of defects it will assume as it grows.     

Reconnaissance mapping and management of salt-affected soils using satellite images

The Indo-Gangetic alluvial plain is subjected to large scale soil alkalization. In order to map and characterize salt-affected soils, with the aim of applying management techniques, Etah district in Uttar Pradesh, located between 26o^o45' to 28^o02' N and 78^o15' to 79^o20' E was selected. Multidate, high resolution, IRS-LISS II, geocoded FCC images on 1550 000 scale were used. Integrating visual image interpretation, physiographic analysis, ground data and laboratory analysis of soil samples, a legend for mapping salt-affected soils (SAS) was formulated. Based on variations in physicochemical properties: nature, intensity and depth wise distribution of salts, five categories of SAS requiring specific reclamation measures were identified. Soil categories S2, S3 and S4 have a gypsum requirement (GR) of 20, 12 and 4t/ha^-1 respectively. Reclamation of medium to heavy textured highly alkali soils requires the addition of amendments and a rice-wheat rotation for the initial 3-4 years. Under resource constraints, pit planting of Prosopis juliflora can bring about slow but effective reclamation. The soil category S5 is slightly alkali in the substratum, needing only biological reclamation by growing salt tolerant varieties of rice and wheat crops. Soils of category S1 are saline and need management by hydrological treatments. Incorporation of village boundaries on a map showing SAS would facilitate decision taking in planning reclamation projects and accelarate management operations directly at village level.     

Effect of long-term stress and temperature exposure on the fracture toughness of Ti-62222 alloy

Fracture toughness tests were conducted on a Ti-62222 (titanium alloy) sheet being considered for use in high temperature aircraft applications in the as received condition and after exposing the pre-cracked specimens to a sustained stress intensity, K, level between 55 and 60.5 MPa for 200 h at 350°C. It was concluded that the fracture toughness does not degrade as a result of exposure to high temperature and the K levels in this material. The tensile strength in the exposed condition also remained the same as in the as received condition.