Bubble-based EMMS mixture model applied to turbulent fluidization Powder Technology

Turbulent fluidization is now widely recognized as a distinct flow regime and is commonly utilized in industrial fluidized-bed reactors. However, relatively fewer attempts have been made to rigorously model these systems in comparison to bubbling and circulating fluidized beds. In this work, we have rewritten the original bubble based EMMS model in form of a mixture to apply it to turbulent fluidization. At microscale this mixture is composed of gas and particles whereas voids and gas-particle suspension make up this mixture at mesoscale level. Subsequently, all the system properties are then calculated in terms of mixture rather than individual phases. With the minimization of the objective function for the bubbling mixture, the set of equations is then solved numerically. The objective function, used to close the system of equations, is composed of the energy consumption rates required to suspend gas-particle suspension and the energy consumed due to interaction between suspension and voids. The model is then applied to simulate gas-solid turbulent fluidized beds. Simulation results are encouraging as the model is able to predict the dense bottom and dilute top zones along the height of the bed. Comparison of results with experimental data and homogeneous drag model has been made for validation purposes

Space capsule Patent

Manned space capsule configuration for orbital flight and atmospheric reentr

Damage identification in a concrete beam using curvature difference ratio

Previous studies utilising changes in mode shape or curvature to locate damage rely on the fact that the greatest change occurs around the defect. However, in concrete beams this fact is undermined due to the nature of the defect as distributed multi-site cracks. In addition, differences in mode shape and curvature as ways to locate the damage is unstable because of occurrence of modal nodes and inflection points. In this paper, one interesting solution to this problem is being tested by establishing a new non-dimensional expression designated the 'Curvature Difference Ratio (CDR)'. This parameter exploits the ratio of differences in curvature of a specific mode shape for a damaged stage and another reference stage. The expression CDR is reasonably used to locate the damage and estimate the dynamic bending stiffness in a successively loaded 6m concrete beam. Results obtained by the proposed technique are tested and validated with a case study results done by Ren and De Roeck [1] also by Maeck and De Roeck [2]. Another contribution of this work is that relating changes in vibration properties to the design bending moment at beam sections as defined in Eurocode 2 specifications [3]. Linking between a beam section condition and the change in vibration data will help to give a better comprehension on the beam condition than the applied load

Mechanisms and Mitigation of Agglomeration during Fluidized Bed Combustion of Biomass: A Review

A key issue associated with fluidized bed combustion of biomass is agglomeration. The presence of high quantities of alkali metal species in biomass ash leads to the formation of sticky alkali silicate liquid phases during combustion, and consequently the adhesion and agglomeration of bed material. This review examines probable mechanisms of agglomeration and the effects of operational variables in reducing its severity. Additionally, an overview of monitoring and prediction of agglomerate formation is given. Two key mechanisms of agglomeration are apparent in literature, and both may occur concurrently dependending on fuel composition. Coating-induced agglomeration is defined by the interaction of alkali metals in fuel ash with silica in the bed material to form an alkali silicate melt. Melt-induced agglomeration is defined by the presence of sufficient amounts of alkali metals and silica in the fuel ash which together form a eutectic melt. Physical mechanisms, such as tumble agglomeration and sintering, may further enhance either of the coating-induced or melt-induced mechanisms. Of the operational variables examined in this review, temperature, fluidizing gas velocity, fuel, bed material and additives have been shown to have the greatest effect on agglomeration severity. Prediction of agglomeration propensity may be attempted with mathematical correlations or lab-scale fuel testing before use in the boiler, or with in-situ methods, which are typically focused on temperature or pressure analysis. The review of the literature has highlighted the need for further research in several areas, including: mechanisms when using alternate bed materials, use of dual-fuel biomass blends, technical and economic optimisation of the use of alternative bed materials and additives, and further modelling of coating growth behaviours

Eutectic Colony Formation: A Stability Analysis

Experiments have widely shown that a steady-state lamellar eutectic solidification front is destabilized on a scale much larger than the lamellar spacing by the rejection of a dilute ternary impurity and forms two-phase cells commonly referred to as `eutectic colonies'. We extend the stability analysis of Datye and Langer for a binary eutectic to include the effect of a ternary impurity. We find that the expressions for the critical onset velocity and morphological instability wavelength are analogous to those for the classic Mullins-Sekerka instability of a monophase planar interface, albeit with an effective surface tension that depends on the geometry of the lamellar interface and, non-trivially, on interlamellar diffusion. A qualitatively new aspect of this instability is the occurence of oscillatory modes due to the interplay between the destabilizing effect of the ternary impurity and the dynamical feedback of the local change in lamellar spacing on the front motion. In a transient regime, these modes lead to the formation of large scale oscillatory microstructures for which there is recent experimental evidence in a transparent organic system. Moreover, it is shown that the eutectic front dynamics on a scale larger than the lamellar spacing can be formulated as an effective monophase interface free boundary problem with a modified Gibbs-Thomson condition that is coupled to a slow evolution equation for the lamellar spacing. This formulation provides additional physical insights into the nature of the instability and a simple means to calculate an approximate stability spectrum. Finally, we investigate the influence of the ternary impurity on a short wavelength oscillatory instability that is already present at off-eutectic compositions in binary eutectics.Comment: 26 pages RevTex, 14 figures (28 EPS files); some minor changes; references adde

A comparative assessment of biomass ash preparation methods using X-ray fluorescence and wet chemical analysis

X-ray fluorescence (XRF) spectroscopy is a rapid method used to determine the composition of biomass ash, but the accuracy of the method is sensitive to various factors including ash preparation methods. In this study different types of biomass ash were examined by using wet chemical analysis (WCA) and compared with the respective XRF results. The biomass ash was initially prepared in accordance with the European Standard method at 550 °C. At this low combustion temperature the amount of residual unburned carbon is significant. To eliminate this, the ashes were heated at higher temperatures: a batch of twenty biomass ashes were heated at 850 °C and a batch of five heated to 815 °C. At these higher temperatures there may be loss of inorganic components by vaporisation. Variation in these effects may lead to unreliable results. The relationship between XRF and WCA results are given by regression equations. The ashes processed at 815 °C show better agreement between the two analysis methods

Measurement of corrosion content of archaeological lead artifacts by their Meissner response in the superconducting state; a new dating method

Meissner fraction in the superconducting state of lead archaeological artifacts is used to evaluate the mass of the uncorroded metal in the sample. Knowing the total mass of the sample the mass of all corrosion products is established. It is shown that this mass correlates with the archaeological age of the lead artifacts over a time span of ~2500 years. Well-dated untreated lead samples from Tel-Dor, the Persian period, Caesarea, the Byzantine and the Crusader periods as well as contemporary data were used to establish the dating correlation. This new chemical dating method is apparently applicable to lead artifacts buried in soils with the pH>6.5. In such soils the corrosion process is very slow and the corrosion products, mainly PbO and PbCO3, accumulate over hundreds of years. The method presented is in principle non-destructive. (corresponding author: )Comment: File ARCH_4.pdf 14 pages including 1 table and 5 figure