Investigation On Flow Dynamics Of Film Flow Over Topology

In the process of industry, liquid film flow is often applied in distillation tower, absorption tower, heat exchanger and other equipments to get high efficient heat/mass transfer. The topology of the substrate has a very important influence on the flow dynamics of the liquid film, thus changing its efficiency of heat/mass transfer. Using the CFD (Computational Fluid Dynamics) simulation and flow visualization experiment, this thesis investigated the effect of topology structure on film flow.Film flow over topology structures is simulated by open source software OpenFOAM in this thesis. In order to verify the accuracy of the multiphase flow model in OpenFOAM, the results are compared with Nusselt solution, experiments based on PIV (Particle Image Velocimetry) or LIF (Light Induced Fluorescence), and research findings in literatures. With the validated numerical model, a series of investigations of film flow over triangular corrugations are carried out. Following is the main contents of this research:Firstly, in the research of thick film flow over triangular corrugations, it is found that the vortex in the flow field is formulated in two different ways:one kind of vortex is caused by structure, generally appear in the steep valleys, and is not affected by the flow rate of film flow; The other kind of vortex is caused by the fluid inertia, and is directly influenced by flow rate. This phenomenon is also found in the investigation on gravity driven thin film flow.Secondly, the resonance between liquid film and the substrate plate with triangular corrugations is found in this study, which is also discovered on sinusoidal corrugations in literatures. A critical point exists as the flow-rate of the film flow increasing on certain corrugation, where the vortex size, amplitude of the wave on free surface and the phase of the waves change abruptly.Thirdly, it is found that steepness of the corrugation has no effect on the timing of the resonance phenomenon happens, but steeper corrugation can cause larger wave on the free surface and stronger velocity intensity normal to the substrate plate. Increment of these two parameters is beneficial to the enhancement of heat/mass transfer.Fourthly, the investigation shows that the surface tension has a direct relation to the resonance phenomena. Surface tension has the blocking effects on wave formation on the free surface of film flow. The resonance phenomenon happens earlier when the surface tension is smaller. Increase the surface tension can delay film flow resonance point to higher Reynolds number. Surface tension also has strong influence on normal velocity intensity. For film flow with high surface tension, the increase of normal velocity intensity is not linear. A local maximum of normal velocity intensity is encountered at the resonance point. The phase shift of the waves on the free surface of film flow is also investigated in this study.Finally, film flow over the three dimensional corrugations is studied with numerical method. It is found that the fluid basically flows around the three dimensional corrugations. The velocity of the film flow is large in the space between the substrate plates, but it is small in the field close to the trough or crest of the corrugations. The three dimensional corrugations can cause disturbance on film flow in three directions, so the heat/mass transfer of film flow can be greatly improved compare to film flow over two dimensional corrugations.The investigation accomplished in this work can be regarded as groundwork for the study of the mass transfer enhancement. It provides basis knowledge for the optimization design of substrate topology in application with film flow.