A study is made of the simultaneous heat and mass transfer processes which occur within a finite wood cylinder of circular cross-section when it is convectively dried. Governing transport equations are developed allowing for the functional dependence of both thermal and moisture diffusivities on moisture content and temperature. Derivative type boundary conditions are included in the analysis. The equations are formulated using a cylindrical coordinate system because it is well suited to modelling wood's anisotropy. In the case considered, the axial coordinate direction is aligned with the wood-grain. Due to the coupling and non-linearity present in the transport and boundary equations, an implicit finite-difference solution scheme is formulated. The three-time-level scheme uses an equation splitting technique to simplify its solution on the computer. A mathematical wood-model, as available in the literature, is refined and used to determine moisture and thermal diffusivities, and mass transfer boundary conditions. Results from this wood-model apply to softwoods below the fiber-saturation moisture content. Combined diffusivities of liquid and vapour are calculated for the radial and axial directions and results for the radial direction are compared to those found from diffusion experiments. Desorptional isotherms are used in the mass transfer boundary condition equations to relate surface humidity and moisture content. The wood-model is used in the mass transfer equation to determine wood drying behaviour under isothermal conditions. Initial moisture contents are uniform and equal to the fiber-saturation value. Solutions are presented to give local and average moisture content as a function of time as well as moisture content profiles. The effect of drying temperature was investigated for one case of wood density and shrinkage. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

With the increasing pressure of providing reliable potable water in a sustainable way, it is important to understand water phase change phenomena (condensation and evaporation) as the water phase change is involved in many processes such as membrane distillation and solar still which can be a feasible choice of supplementing the present potable water access. In the present thesis, we first elucidate the role of wettability of water condensation substrate by combining the droplet growth dynamics and droplet population evolution. The results show that wettability has a negligible effect on water condensation rate in an atmospheric environment. After confirming the role of substrate wettability, we provide a quantitative analysis of the effect of substrate geometry on water condensation in the atmospheric environment. The analysis can help to predict the efficiency of water condensation rate with a given substrate of a certain geometry with the aid of computational simulation tools. The results show that water condensation can be increased by 40% by rationally designing the geometry of the condensation surface. However, the condensation rate in the atmospheric environment is relatively slow due to the presence of non-condensable gas. In order to increase the condensation rate, a relatively pure vapor environment is desired, in which condensed water will be the major heat transfer barrier. Coalescence induced jumping of condensed droplets on superhydrophobic surfaces is an interesting phenomenon to help faster removal of condensed droplets. However, it is still not clear how to optimize the overall heat transfer efficiency by condensation on such surfaces. We observed an interesting phenomenon on a superhydrophobic nano-cones array, on which water preferentially condenses within larger cavities among the nanocones. Droplets growing form larger cavities have larger growth rate. This finding can possibly provide a solution to optimizing heat transfer efficiency. Finally, a nylon-carbon black composite is prepared by electrospinning to enhance water evaporation under solar radiation. The composite shows an interesting light absorption property. In a wet state, the composite can absorb around 94% of the incident sunlight. The composite also shows strong mechanical and chemical stability. Thus, the composite is considered to be a practical candidate to be applied in the solar distillation process.

Carbon dioxide serves as co-substrate in the production of succinic acid by Actinobacillus succinogenes. The transient concentration of dissolved CO2 in the broth (CCO2) controls the uptake of CO2 in the cell. Based on CCO2 , three distinct regimes could be identified in which the behaviour of the organism differed with CCO2 availability. When CCO2 was higher than 8.4 mM (44.4% saturated at an atmospheric pressure of 86 kPa), there was no evidence of CO2 limiting succinic acid productivity and flux to succinic acid remained constant. When CCO2 decreased below 8.4 mM a decrease in the succinic acid production and glucose consumption rates was observed to 28.01% and 19.89% of their original value respectively, at the lowest CCO2 value investigated. Below a CCO2 of 4 mM (21.16% saturated at an atmospheric pressure of 86 kPa), the productivity continued to decrease along with a shift in the total carbon flux from the succinic acid-producing pathway (C4-pathway) to the by-product-producing pathway (C3-pathway). The fraction of total carbon flux directed to the C4-pathway decreased from 0.48 to 0.33 at the lowest CCO2 value investigated. Although the by-product acetic acid concentration decreased to 88% of the original value, formic acid remained relatively stable and the ethanol concentration increased from an average of 0.26 g.L-1 to 1 g.L-1. The organism starts producing ethanol in order to satisfy the redox balance when the C4-pathway becomes less active. It was calculated that the flux shift to the C3-pathway does not favour ATP production. The organism is, however, still viable at the very low ATP production rates found at very low values of CCO2. Since succinic acid production is not limited at relatively low values of CCO2 (44.4% saturation), adequate CO2 supply to the fermenter can be achieved without major CO2 sparging which is beneficial from an industrial processing perspective. / Dissertation (MEng)--University of Pretoria, 2016. / Chemical Engineering / MEng / Unrestricted

Casting is a substantial part of modern manufacturing and production, typically used in the production of aluminum alloys. The high pressure die casting process is extremely suitable for mass production. Due to the high volume, wasted time and resources during the production cycle become more significant. Aluminum die castings require the die to be at elevated temperatures to produce acceptable castings. When the inner surfaces of a die are cold, the outer shell of the casting will cool too rapidly, and solidification of the outer shell occurs before the aluminum has time to uniformly fill the cavities. Therefore, without the die being within the proper temperature range, the castings produced will have significant issues in porosity and casting incompleteness. Furthermore, stresses are introduced to the casting surfaces when warm-up shots are used to raise the temperature prior to production. In the present work, research is conducted on designing a heating method for a casting die used in the manufacturing of an automotive transmission intermediate plate. An electric, short wave infrared heating system is simple and effective for the purpose. By utilizing an electric infrared heater in combination with a flat mirror reflector, the aluminum high pressure die casting die was heated to 300 ◦C surface temperature within 30 minutes. Further research can be done to optimize heat flux distribution and minimize energy consumption.

Please read the abstract in the dissertation / Dissertation (MEng)--University of Pretoria, 2012. / gm2014 / Chemical Engineering / unrestricted

Mass transfer in a porous solid, partially saturated with asingle solvent and multicomponent liquid mixtures, has beenexperimentally and theoretically studied. A porous materialcontaining single liquids and mixtures of organic solvents wasisothermally dried. Experiments were performed using a jacketedwind tunnel, through which a humidity andtemperature-controlled air stream flowed. The wetted porousmaterial was placed in a cylindrical vessel, whose top isexposed to the air stream until the material became dried to acertain extent. Drying experiments with the single solventswater, methanol, ethanol and 2-propanol, were performed atdifferent temperatures and transient liquid content profileswere determined. In isothermal drying experiments with liquidmixtures,the transient concentration profiles of thecomponents along the cylindrical sample as well as the totalliquid content were determined. The liquid mixtures examinedwere water-methanolethanol and isopropanol-methanol-ethanol.Two different temperatures and initial compositions were usedin the experiments. Mathematical models that describe nonsteadystate isothermal drying of a solid containing single liquidsand multicomponent liquid mixtures were developed. In the solidwetted with a single liquid, capillary movement of the liquidwas the main mechanism responsible for mass transfer. In thesolid containing liquid mixtures, interactive diffusion inliquid phase was superimposed to the capillary movement of theliquid mixture. In addition, interactive diffusion of thevapours in empty pores was considered. The parameters todescribe the retention properties of the solid and thecapillary movement of the liquid were determined by comparingtheoretical and experimental liquid content profiles obtainedduring drying of the solid wetted with single liquids. Tosimulate the transport of the liquid mixtures these parameterswhere weighed according to liquid composition. A fairly goodagreement between theoretical and experimental liquidcomposition profiles was obtained if axial dispersion isincluded in the model when the moisture consists of amixture. Keywords:Internal mass transfer, capillary flow,multicomponent, diffusion, solvent mixtures

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A finite difference model simulating a liquid desiccant dehumidification tower with lithium chloride as the desiccant solution has been developed. The model determines the packing height needed for a condensation rate. Comparisons with experimental data illustrates that the model produces valid results. Air and desiccant solution temperatures within the dehumidification tower show that a temperature increase is experienced for both the air and desiccant solution from their respective entrances and exits from the tower. Increasing the air mass velocity or the amount of moisture removed from the air supply causes an increase in packing height. Increasing the desiccant mass velocity decreases the packing height.