Thesis (Ph.D.) - University of KwaZulu-Natal, Pietermaritzburg, 2010. / Full text also available online. Scroll down for electronic link.

Diss. (sammanfattning) Uppsala : Sveriges lantbruksuniv. / Härtill 4 uppsatser.

RBCCPS / Evaporation is one of the inherent processes of the earth’s ecosystem. Water bodies, earth’s surface and vegetation all contribute significantly towards the total evaporation which eventually leads to the formation of clouds. The factors which affect the total evaporation (evaporation & transpiration) are the surface temperature, ambient temperature, relative humidity, external wind speed, pressure, surface area and geometry. This thesis deals with the contributors of total evaporation individually viz. open water bodies; soil-like surfaces; and leaf-like surfaces. A ceramic infrared heater has been used to mimic the heating due to sun’s radiation in all the experiments which were conducted in the quiescent atmosphere. This thesis has been broadly categorized into two parts: - (a) evaporation from bare water surface; and (b) evaporation from a porous media. In part (a), we present experimental results on the evaporation from a bare water surface heated either from above using the infrared radiations or from below using immersed heaters. Heating from below leads to unstable stratification and convection while infrared heating from above leads to stable stratification. The effect of water-side convection on the evaporation from a bare water surface has been investigated and all the experimental results have been combined to obtain a power law relation between Sherwood number (Sh) and Rayleigh number (Ra). Part (b) of the thesis has been further split into three major categories: - (1) evaporation from spheres based conventional porous media; (2) evaporation from unconventional porous media containing rods, capillaries, and plates; and (3) evaporation from leaf-like surfaces. In all the experiments, a precision weighing balance was used to measure the evaporation rate. A thermal camera was used to get the surface temperature fields, and fluorescein dye mixed with water gave insightful results on the evaporation process. In particular the red deposits of fluorescein particles revealed the evaporation sites. In most of the experiments, the infrared heating was of the order of 1000W/m2. Different sized glass and acrylic containers were used in this thesis. Mono-disperse glass beads (closest to mimic the natural soils), stainless steel balls, sieved natural sand and hydrophobic Ball Grid Array balls have been used to create the spheres-based conventional porous media. Evaporation was found to undergo three stages which depended on the spheres size and the heat flux supplied. In the 1st stage of evaporation capillary film(s) pulls water from beneath the porous media to the top surface and the evaporation rate remained high, close to that obtained from a water surface. Capillary break-up occurs in the transition regime which is followed by the 2nd stage of evaporation where a new vaporization plane is formed within the porous media. In the 2nd stage, heat is conducted through the top dried layer to the water below where evaporation takes place and the evaporation rate drops drastically. Transition to 2nd stage happened earlier for coarser spheres at constant heat flux. Along with the wetting properties, the spheres size has been found to effect capillary break-up length (a measure of capillary film strength) and hence the duration of the stages of evaporation drastically. Surface images captured using the thermal camera clearly showed the presence of water till the capillary break-up. The capillary break-up length was also found to be affected significantly by the heat flux. Apart from the experimental findings of mono-disperse spheres, two layers of different sized glass spheres have also been investigated. The presence of complicated network of textural layering in the earth’s surface is a well-known fact. Preferential evaporation was clearly seen in the experiments with texturally layered porous media independent of the orientation viz. vertical or horizontal layering. The stacking positions are found to be critical in determining the overall evaporation characteristics. The geometry of a pore between three spheres in mutual contact is very complicated. Simpler pore geometry would be between two rods/plates in contact or three rods in mutual contact or stacks of either of these two. We call these types of the porous media as “Unconventional porous media” as they possess many unique features not shown by a conventional porous media. The evaporation characteristics of vertically stacked rods was found to be dominated by the corner films present in the near-zero radii contacts. Unlike the conventional porous media, the capillary break-up length was found not to depend on the rod diameter. The capillary break-up length for vertically stacked rods was larger than for the spheres case and was also found to be independent of the heat flux, for the range investigated in this thesis. A mathematical model has been developed for understanding the evaporation from the vertically stacked rods. Experiments with horizontally stacked pencil leads showed early capillary break-up while with horizontally stacked glass rods, capillary break-up was not observed. Experimental investigations of porous media containing vertically stacked plates have also been studied. Water trapped between two consecutive plates are treated as 2D source of evaporation. Plants regulate their O2-CO2 content via tiny holes present on the leaves called “Stomata”. The average size of a stoma is nearly 20μm and the total area covered by stomata is close to 5% of the leaf area. However the higher transpiration rates (60-70 % compared to a bare water source) sustained by a plant has remained a mystery for the phytologists. In view of this we mimic the leaf-type using regularly spaced holes on the silicon wafers from which water evaporates. The leaf-mimics had different hole-diameter but open area ratio was kept constant. In all the cases the evaporation ratio (ratio of the evaporation rate from the leaf mimic to that of the evaporation rate of a bare water surface at the same surface temperature) was found to increase at lower heat fluxes. With increasing the hole-size evaporation rate was found to decrease. The leaf-mimic with the smallest hole-size had the highest evaporation rate and the evaporation ratio increased from 0.46 at 800W/m2 to 0.64 at 400W/m2. The 3D nature of diffusion near these tiny holes enhances the evaporative flux which explains the high evaporation rates even for low open area ratios.

Die Verdunstung ist die Klimaanlage der Erde. Sie verbindet den globalen Wasserkreislauf mit dem Energiekreislauf. Die Komponenten des Wasser- und Energiekreislaufs stehen für jeden Standort in einem dynamischen Gleichgewicht. Mit der Ausführung von Bauvorhaben wird in das Gleichgewicht eingegriffen. Entscheidend für die Beurteilung der Folgen für die Umwelt sind die langfristigen Auswirkungen. Diese können durch den Vergleich langjähriger mittlerer Jahresbilanzen vor und nach der Bebauung aufgezeigt werden. Bei der Genehmigung neuer Baugebiete müssen diese Auswirkungen ein Entscheidungskriterium werden, wenn der Eingriff in den Naturhaushalt so gering wie möglich gehalten werden soll. Nur die Betrachtung von einzelnen Starkregenereignissen ist nicht ausreichen. Von der Versiegelung der Oberflächen ist die Verdunstung in der Jahresbilanz stärker als die anderen Komponenten des Wasserkreislaufs betroffen. Trotzdem werden bisher bei der Planung neuer Baugebiete hauptsächlich der Oberflächenabfluss und in zunehmendem Maße die Versickerung untersucht. Die Reduzierung der Verdunstung wird zumeist vernachlässigt. Ursache für diese Reduzierung ist die fehlende Zwischenspeicherung des Wassers. Das wirkt sich direkt auf den Energiekreislauf aus, da die nicht für den Verdunstungsprozess benötigte Energie in den bodennahen Schichten bleibt. Im ersten Teil werden die Einflussfaktoren auf die Verdunstung erläutert und ein Überblick über die Berechnungsmethoden gegeben. Im zweiten Teil werden die Oberflächen unbebauter und bebauter Gebiete systematisiert und in Landnutzungsarten unterteilt. Für diese werden die hydrologischen und energetischen Eigenschaften und deren Auswirkungen auf den Wasser- und Energiehaushalt erläutert und die mittleren Jahresbilanzen berechnet. Die tatsächliche Verdunstung wird auf der Basis der Gras-Referenzverdunstung und der Landnutzungsart ermittelt. Ausgangswerte sind langjährige meteorologische Jahresmittelwerte. Die Verdunstung von Wasserflächen wird mit dem Temperaturgleichgewichtsverfahren berechnet. Mit den vorgestellten Verfahren können Einzugsgebiete von Bebauungsplangröße untersucht werden. Es werden Lösungen zur Beibehaltung eines möglichst hohen Verdunstungsanteils in bebauten Gebieten vorgeschlagen. Ansatzpunkt ist dabei stets die Zwi-schenspeicherung des Regenwassers. Am wirkungsvollsten sind dabei Dachbegrünungen, Wasserflächen und Bäume. Das Verfahren wird an zwei Beispielen angewandt - die Erschließung eines Industriegebietes auf einer vorher land- und forstwirtschaftlich genutzten Fläche in Treuen im Vogtland und der Neubau einer Untergrundstation im Zentrum der schwedischen Großstadt Malmö. / Evapotranspiration could be called the air-conditioner of the earth. It is connecting the water and the energy cycle. The components of the water and energy cycle are related to each other in a dynamic system. Urban development is interfering with this system. Changes of the water and energy balance resulting from construction can be calculated on the basis of long-standing annual average balances and compared with the balance in the catchment area before construction. Before granting building permission, the impacts on the water and energy balance should be evaluated in order to minimize interference with nature. Causing long-term impacts must be considered beforehand in planning. Coping only with design storm events does not suffice. Evaporation is more intensely affected by the paving of streets and squares and by constructing buildings then the other components of the water cycle. However, up to now, in the process of design and planning permission of new development areas, the focus is on runoff and, increasingly, on infiltration of rainwater. The large reduction of evaporation is mostly neglected. The reason for the reduction is the lack of buffer storage for water. Thus directly affects the energy cycle. Energy which is not used for evaporation remains in the near-ground layers. In the first part, the factors influencing evaporation are explained and an overview over the methods of calculation is given. In the second part all surfaces of urban and natural areas are systematized and subdivided into types of land use. The hydrological and energy properties as well as their effects on the water and energy balance are elucidated for this types of land use and their average annual balances are calculated. Solutions are presented for retaining in urban areas an evaporation rate as high as possible. Starting point hereby is always the buffer storage of rainwater. Most effective measures are the installation of rooftop greening, open water surfaces and trees. The calculations are performed on the basis of the FAO reference evaporation and the types of land use. Starting values are long-stand average annual meteorologic values. The evaporation of water surfaces is calculated with the temperature balance model. The method is applied to two examples showing the impacts of land use change on water and energy balance: the development of agricultural and forest land in Saxony into an industrial development site, and the impact of the construction of an underground station in the centre of the City Malmö, Sweden.

Due to failure of the continuum hypothesis for higher Knudsen numbers, rarefied gases and microflows of gases are particularly difficult to model. Macroscopic transport equations compete with particle methods, such as the direct simulation Monte Carlo method (DSMC) to find accurate solutions in the rarefied gas regime. Due to growing interest in micro flow applications, such as micro fuel cells, it is important to model and understand evaporation in this flow regime. To gain a better understanding of evaporation physics, a non-steady simulation for slow evaporation in a microscopic system, based on the Navier-Stokes-Fourier equations, is conducted. The one-dimensional problem consists of a liquid and vapor layer (both pure water) with respective heights of 0.1mm and a corresponding Knudsen number of Kn=0.01, where vapor is pumped out. The simulation allows for calculation of the evaporation rate within both the transient process and in steady state. The main contribution of this work is the derivation of new evaporation boundary conditions for the R13 equations, which are macroscopic transport equations with proven applicability in the transition regime. The approach for deriving the boundary conditions is based on an entropy balance, which is integrated around the liquid-vapor interface. The new equations utilize Onsager relations, linear relations between thermodynamic fluxes and forces, with constant coefficients that need to be determined. For this, the boundary conditions are fitted to DSMC data and compared to other R13 boundary conditions from kinetic theory and Navier-Stokes-Fourier solutions for two steady-state, one-dimensional problems. Overall, the suggested fittings of the new phenomenological boundary conditions show better agreement to DSMC than the alternative kinetic theory evaporation boundary conditions for R13. Furthermore, the new evaporation boundary conditions for R13 are implemented in a code for the numerical solution of complex, two-dimensional geometries and compared to Navier-Stokes-Fourier (NSF) solutions. Different flow patterns between R13 and NSF for higher Knudsen numbers are observed which suggest continuation of this work. / Graduate