The drying of small drops of particulate slurries

The literature on the evaporation of drops of pure liquids, drops containing solids and droplet sprays has been critically reviewed. An experimental study was undertaken on the drying of suspended drops of pure water and aqueous sodium sulphate decahydrate with concentrations varying from 5 to 54. 1 wt. %. Individual drops were suspended from a glass filament balance in a 26 mm I.D. vertical wind tunnel, designed and constructed to supply hot de-humidified air, to simulate conditions encountered in commercial spray driers. A novel thin film thermocouple was developed to facilitate the simultaneous measurement of drop weight and core temperature. The heat conduction through the thermocouple was reduced because of its unique design; using essentially a single 50μ diameter nickel wire. For pure water drops, the Nusselt number was found to be a function of the Reynolds, Prandtl and Transfer numbers for a temperature range between 19 to 79°C.                  Nu = 2 + 0.19 (1/B)0.24 Re0.5 Pr0.33 Two distinct periods were observed during the drying of aqueous sodium sulphate decahydrate. The first period was characterised by the evaporation from a free liquid surface, whilst drying in the second period was controlled by the crust resistance. Fracturing of the crust occurred randomly but was more frequent at higher concentrations and temperatures. A model was proposed for the drying of slurry drops, based on a receding evaporation interface. The model was solved numerically for the variation of core temperature, drop weight and crust thickness as a function of time. Experimental results were in excellent agreement with the model predictions although at higher temperatures modifications to the model had to be made to accommodate the unusual behaviour of sodium sulphate slurries, i.e. the formation of hydrates.

Mechanisms of heat and mass transfer to and from single drops freely-suspended in an air stream

A specially-designed vertical wind tunnel was used to freely suspend individual liquid drops of 5 mm initial diameter to investigate drop dynamics, terminal velocity and heat and mass transfer rates. Droplets of distilled, de-ionised water, n-propanol, iso-butanol, monoethanolamine and heptane were studied over a temperature range of 50oC to 82oC. The effects of substances that may provide drop surface rigidity (e.g. surface active agents, binders and polymers) on mass transfer rates were investigated by doping distilled de-ionised water drops with sodium di-octyl sulfo-succinate surfactant. Mass transfer rates decreased with reduced drop oscillation as a result of surfactant addition, confirming the importance of droplet surface instability. Rigid naphthalene spheres and drops which formed a skin were also studied; the results confirmed the reduced transfer rates in the absence of drop fluidity. Following consideration of fundamental drop dynamics in air and experimental results from this study, a novel dimensionless group, the Oteng-Attakora, (OT), number was included in the mass transfer equation to account for droplet surface behaviour and for prediction of heat and mass transfer rates from single drops which exhibit surface instability at Re>=500. The OT number and the modified mass transfer equation are respectively: OT=(ava2/d).de1.5(d/) Sh = 2 + 0.02OT0.15Re0.88Sc0.33 Under all conditions drop terminal velocity increased linearly with the square root of drop diameter and the drag coefficient was 1. The data were correlated with a modified equation by Finlay as follows: CD=0.237.((Re/P0.13)1.55(1/We.P0.13) The relevance of the new model to practical evaporative spray processes is discussed.

A study of the drying of single droplets in free-flight

The literature relating to evaporation from single droplets of pure liquids and the drying of solution and slurry droplets, and of droplet sprays has been reviewed. The heat and mass transfer rates for individual droplets suspended in free-flight, were investigated using a specially-designed vertical wind tunnel, to simulate conditions in a spray drier. The technique represented a unique alternative method for investigating evaporation from unrestrained single droplets with variable residence times. The experiments covered droplets of pure liquid allowbreak (water, isopropanol) allowbreak and of significantly different solutions (sucrose, potassium sulphate) over a range of temperatures of 37oC to 97oC, initial concentrations of 5 to 40wt/wt% , and initial drop sizes of 2.8 to 4.6mm. Drop behaviour was recorded photographically and dried particles were examined by Scanning Electron Microscopy. Correlations were developed for mass transfer coefficients for pure water droplets in free-flight; (i) experiencing oscillations, rotation and deformation, Sh = -105 + 3.9 [Ta - Td/Tamb]0.18Re0.5Sc033 for Re approx. > 1380 (ii) when these movements had ceased or diminished, Sh = 2.0 + 0.71 [Ta - Td/Tamb]0.18Re0.5Sc033 for Re approx. < 1060. Data for isopropanol drops were correlated resonably well by these equations. The heat transfer data showed a similar transition range. The drying rate curves for drops of sucrose and potassium sulphate solution exhibited three distinct stages; an initial increase in the drying rate as drop temperature reduced to the wet-bulb temperature, a short constant-rate period and a falling-rate period characterised by formation of a crust which controlled the mass transfer rate. Due to drop perturbation the rates in the high Re number region were up to 5 times greater than predicted from theory for spherical droplets. In the case of sucrose solution a `skin' formed over the drop surface prior to crust formation. This provided an additional resistance to mass transfer and resulted in extended drying times and a smooth crust of low porosity. The relevance of the results to practical spray drying operations is discussed.

Mechanisms of drying of particulate slurries

The literature on the evaporation of pure liquid drops and the drying of drops of solutions and slurries has been reviewed with particular reference to spray drying. A 0.1-0.2 mm glass filament-thermocouple was constructed and used to study simultaneously, heat and mass transfer from a single suspended drop placed in a humidity and temperature controlled, 28 mm OD vertical wind tunnel. Heat conduction through the filament was minimised eg at 100¦C it accounted for only 9.3% of the total heat transferred to a drop. Evaporation of single water drops was also studied in a 101 mm OD vertical wind tunnel. The Nusselt number was found to be a function of the Reynolds, Prandtl and Transfer number over an air temperature range of 17¦C to 107¦C. The proposed correlation is: Nu = 2+(-12.96B+0.76)Re¦-5Pr0-33 Experimental drying studies were carried out on single suspended 1 to 2.5 mm diameter drops of aqueous sodium sulphate decahydrate, sodium chloride, potassium sulphate, copper sulphate and sodium acetate solutions and slurries at temperatures of 20¦C to 124¦C. Dried crusts were examined by Scanning Electron Microscopy. The drying history of any material depended upon the nature of the crust formed. Sodium acetate formed a non-rigid skin prior to the formation of a rigid crust. A modified receding evaporation interface model was proposed for the drying of solutions and slurries. This covered both the constant rate period prior to crust formation and the subsequent falling rate period. The model was solved numerically for the variation in core temperature, drop weight and crust thickness. Good agreement was obtained between model predictions and experimental results for materials forming rigid crusts i.e. sodium sulphate decahydrate, sodium chloride, potassium sulphate and copper sulphate. However, the drying histories of drops of 10-20% weight initial concentration sodium acetate were unpredictable since formation of a non-rigid skin deviated from the model assumption of a rigid outer surface. At higher initial concentrations (40% weight) where a rigid crust was formed for sodium acetate, good agreement was obtained between experimental results and model predictions. Single suspended drop studies are concluded to provide a valuable insight into the drying mechanisms of specific solutions and slurries.

Alteração climática dos índices de estabilidade na Peninsula Iberica

Este trabalho tem como objetivo a comparação da intensidade, frequência e distribuição de um conjunto de índices de estabilidade atmosférica simulados entre o clima histórico (1986-2005) e um cenário climático (2081-2100) na Península Ibérica. Considerou-se o cenário de emissão de gases RCP8.5. Estes índices avaliam a instabilidade atmosférica que é um elemento fundamental e percursor no desenvolvimento de tempestades. Através dos seus valores limite, é possível estimar alterações na probabilidade de ocorrência de eventos extremos que se poderão desenvolver no clima futuro, relativamente ao histórico. Primeiro, utilizou-se um conjunto de simulações do WRF com dois forçamentos: reanálises do ERA-Interim e um modelo do Max Planck Institute. De seguida, foram calculados diferentes índices de estabilidade. A validação do modelo consistiu no cálculo das médias sazonais, da sua diferença e das respetivas PDFs dos índices simulados pelo WRF-MPI e WRF-ERA. Verifica-se uma sobrestimação do CAPE, SHR6km (vento de corte) e SWEAT simulados pelo WRF-MPI. No entanto, nos campos dos índices simulados pelos dois forçamentos para o período histórico, verifica-se que os padrões espaciais são semelhantes apesar das diferenças na intensidade. Como as alterações climáticas dos índices são avaliadas através de diferenças, estas discrepâncias não invalidam a utilização do modelo no futuro. Posteriormente foram estudadas as alterações climáticas dos índices através da comparação entre o clima histórico e futuro. Estima-se um aumento da intensidade do CAPE e uma diminuição (aumento) da frequência de eventos com intensidade reduzida (elevada). Estas alterações são robustas no verão e outono. Também é esperado um aumento da intensidade do SHR6km na primavera e inverno tal como da frequência de SHR6km elevado nestas estações e uma redução da intensidade e da frequência de eventos com SHR6km elevado nas restantes. Haverá um possível aumento robusto da intensidade do SWEAT no verão e outono, bem como da frequência destes valores. Concluindo, será provável um aumento da frequência dos ambientes favoráveis ao desenvolvimento de tempestades, devido a uma maior intensidade e probabilidade de ocorrência de valores extremos do CAPE e do SWEAT. No entanto, a redução do SHR6km, poderá diminuir a organização das tempestades e o seu tempo de vida.

Aerodynamics of vertical-axis wind turbines in full-scale and laboratory-scale experiments

Within a wind farm, multiple turbine wakes can interact and have a substantial effect on the overall power production. This makes an understanding of the wake recovery process critically important to optimizing wind farm efficiency. Vertical-axis wind turbines (VAWTs) exhibit features that are amenable to dramatically improving this efficiency. However, the physics of the flow around VAWTs is not well understood, especially as it pertains to wake interactions, and it is the goal of this thesis to partially fill this void. This objective is approached from two broadly different perspectives: a low-order view of wind farm aerodynamics, and a detailed experimental analysis of the VAWT wake.

One of the contributions of this thesis is the development of a semi-empirical model of wind farm aerodynamics, known as the LRB model, that is able to predict turbine array configurations to leading order accuracy. Another contribution is the characterization of the VAWT wake as a function of turbine solidity. It was found that three distinct regions of flow exist in the VAWT wake: (1) the near wake, where periodic blade shedding of vorticity dominates; (2) a transition region, where growth of a shear-layer instability occurs; (3) the far wake, where bluff-body oscillations dominate. The wake transition can be predicted using a new parameter, the dynamic solidity, which establishes a quantitative connection between the wake of a VAWT and that of a circular cylinder. The results provide insight into the mechanism of the VAWT wake recovery and the potential means to control it.

Studies on the Vertical Structure of Horizontal Wind Variability in the Surface Boundary Layer Over Sriharikota

The present study on the vertical structure of horizontal wind variability in the surface boundary layer over Sriharikota. Based on clock wind speed and direction measuring meteorological tower facility from seven levels in the 100 m layer. The study on wind variability and elliptical approximation of wind hodographs investigated for this tropical coastal station established that Sriharikota is of meso-scale weather entity. Wind variability ratio increases from lower levels to upper levels. In South West monsoon months the station is of high ratio values and it gets affected with meso-scale weather features like thunderstorms. Average total shears are observed greater values than scalar shears. Scalar shears are high in the lowest shear levels compared to upper levels. Semi diurnal types of oscillation in average total shears are found in south west monsoon months. During cyclonic storm passage it is observed that there can be significant difference in mean wind speed from 10 m to 100 m level, but it is not so for peak wind speeds. The variations in wind variability ratio in different months is clearly depicted its strong link to define meso-scale or synoptic –scale forcing domination for this station. Meso-scale forcing is characterized by diurnal wind variability and synoptic- scale forcing by interdiurnal wind variability.

Dynamic stall for a Vertical Axis Wind Turbine in a two-dimensional study

The last few years have proved that Vertical Axis Wind Turbines (VAWTs) are more suitable for urban areas than Horizontal Axis Wind Turbines (HAWTs). To date, very little has been published in this area to assess good performance and lifetime of VAWTs either in open or urban areas. At low tip speed ratios (TSRs<5), VAWTs are subjected to a phenomenon called 'dynamic stall'. This can really affect the fatigue life of a VAWT if it is not well understood. The purpose of this paper is to investigate how CFD is able to simulate the dynamic stall for 2-D flow around VAWT blades. During the numerical simulations different turbulence models were used and compared with the data available on the subject. In this numerical analysis the Shear Stress Transport (SST) turbulence model seems to predict the dynamic stall better than the other turbulence models available. The limitations of the study are that the simulations are based on a 2-D case with constant wind and rotational speeds instead of considering a 3-D case with variable wind speeds. This approach was necessary for having a numerical analysis at low computational cost and time. Consequently, in the future it is strongly suggested to develop a more sophisticated model that is a more realistic simulation of a dynamic stall in a three-dimensional VAWT.

An analytical investigation into shear wall and slab interconnections between reinforced concrete modules for high -rise buildings utilising modular construction under extreme seismic and wind loading

This research investigates a new structural system utilising modular construction. Five-sided boxes are cast on-site and stacked together to form a building. An analytical model was created of a typical building in each of two different analysis programs utilising the finite element method (Robot Millennium and ETABS). The pros and cons of both Robot Millennium and ETABS are listed at several key stages in the development of an analytical model utilising this structural system. Robot Millennium was initially utilised but created an analytical model too large to be successfully run. The computation requirements were too large for conventional computers. Therefore Robot Millennium was abandoned in favour of ETABS, whose more simplistic algorithms and assumptions permitted running this large computation model. Tips are provided as well as pitfalls signalled throughout the process of modelling such complex buildings of this type. ^ The building under high seismic loading required a new horizontal shear mechanism. This dissertation has proposed to create a secondary floor that ties to the modular box through the use of gunwales, and roughened surfaces with epoxy coatings. In addition, vertical connections necessitated a new type of shear wall. These shear walls consisted of waffled external walls tied through both reinforcement and a secondary concrete pour. ^ This structural system has generated a new building which was found to be very rigid compared to a conventional structure. The proposed modular building exhibited a period of 1.27 seconds, which is about one-fifth of a conventional building. The maximum lateral drift occurs under seismic loading with a magnitude of 6.14 inches which is one-quarter of a conventional building's drift. The deflected shape and pattern of the interstorey drifts are consistent with those of a coupled shear wall building. In conclusion, the computer analysis indicate that this new structure exceeds current code requirements for both hurricane winds and high seismic loads, and concomitantly provides a shortened construction time with reduced funding. ^

Effect of spacing of reinforcement on the behaviour of partially grouted masonry shear walls

Partially Grouted Reinforced Masonry (PGRM) shear walls perform well in places where the cyclonic wind pressure dominates the design. Their out-of-plane flexural performance is better understood than their inplane shear behaviour; in particular, it is not clear whether the PGRM shear walls act as unreinforced masonry (URM) walls embedded with discrete reinforced grouted cores or as integral systems of reinforced masonry (RM) with wider spacing of reinforcement. With a view to understanding the inplane response of PGRM shear walls, ten full scale single leaf, clay block walls were constructed and tested under monotonic and cyclic inplane loading cases. It has been shown that where the spacing of the vertical reinforcement is less than 2000mm, the walls behave as an integral system of RM; for spacing greater than 2000mm, the walls behave similar to URM with no significant benefit from the reinforced cores based on the displacement ductility and stiffness degradation factors derived from the complete lateral load – lateral displacement curves.