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.
| Identifer | oai:union.ndltd.org:IISc/oai:etd.iisc.ernet.in:2005/3575 |
| Date | January 2016 |
| Creators | Navneet Kumar, * |
| Contributors | Arakeri, Jaywant H |
| Source Sets | India Institute of Science |
| Language | en_US |
| Detected Language | English |
| Type | Thesis |
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