Theoretische Untersuchung des Wärme- und Stoffübergangs am laminaren Fallfilm

In this thesis a model is presented describing a laminar film flow over a vertical isothermal plate while heat and mass transfer is occurring (see section 3). Most common assumptions, such as constant property data and constant film thickness, have been cancelled. The hydrodynamic model results in longitudinal and transversal velocity components and their evolution in the entrance region. Heat and mass transfer occurs simultaneously and is modelled with respect to uni-directional diffusion, release of differential heat of solution as well as heat flow due to interdiffusion. A simplified model (equimolar diffusion, constant film thickness and streamwise velocity) is described in section 3.6. For validation, the results of this is compared to an established analytical model with comparable assumptions in section 4.1. For a sub-cooled inlet temperature equal to the wall temperature a very good agreement is observed. However, due to instabilities in the analytical model a deviation occurs when the inlet temperature is different to the wall temperature. In section 4.2 the influence of different hydrodynamic conditions at the inlet of the flow is illustrated and discussed using a fully developed velocity profile (Nußelt flow) and a plug flow at the inlet for comparison. Even if a Nußelt profile is assumed, it develops further since mass is absorbed or desorbed. It is found that the onset of absorption occurs at shorter flow length when applying a plug flow at the inlet. If the film is initially in equilibrium, this results in a 9.3% increase in absorbed mass over a length of 0.03m as compared with the Nußelt flow. A fluid with a viscosity five times the one of lithium bromide solution but sharing comparable properties apart from that, leads to lower overall heat and mass transfer rates. If the respective fluids are saturated at the inlet, the accumulated mass flux absorbed by lithium bromide solution is 2.2 times higher than the one absorbed by a high viscous fluid. However, when a plug flow is applied and the fluid is sub-cooled, ab initio the absorbed mass flux is slightly higher for a high viscous fluid. Assuming a sub-cooling of 3K at the inlet, lithium bromide solution now only performs around 11% better as compared with a high viscous fluid over the considered length of 0.03m. This finding suggests that in applications with high viscous working fluid horizontal tubes with a small diameter may increase the performance of the absorber and desorber respectively. In section 4.4 the influence of uni-directional diffusion is shown by a comparison between the results of two models. The detailed one accounts for uni-directional diffusion along with a transversal velocity component and a variable film thickness. In the simplified model, equimolar is assumed with no transversal velocity component and consequently a constant film thickness. It is found that the disregard of uni-directional diffusion leads to an underestimation between 24% and 28% regarding mass flux absorbed. The link for downloading the source code may be found in the appendix section A.3. The publications associated with this thesis can be found in the appendix section B. Absorption, Desorption, Heat transfer, Mass transfer, Film flow, Laminar film, Flat plate, Isothermal plate, Modelling, Hydrodynamics.