| Falling film evaporation outside horizontal tubes is advantageous as an effective heat transfer mechanism, because it has high heat transfer rate, small temperature difference etc. So falling film evaporation now has been widely used in desalination devices, refrigeration systems, and petrochemical plants etc.Process of horizontal tube falling film evaporation in desalination of sea water is researched in the thesis. Based on the measurement of tube wall temperature at different heat flux and spray density, the variation law of heat transfer coefficient of falling film evaporation is analyzed and discussed. The local heat transfer coefficients around the circumference of the tube show a great discrepancy. The heat transfer coefficient at the top of the tube is apparently larger than that at the bottom of the tube. However in the complete wetting condition, heat transfer performance of porous tube reduces with the increasing of heat flux, heat transfer performance of smooth tube increase with the increasing of heat flux. Increasing the spray height of liquid distributing pipe can significantly improve the heat transfer coefficient of smooth tube and porous tube.For the more thorough research of horizontal tube falling film evaporation heat transfer and mass transfer mechanism, this paper uses FLUENT to simulate liquid falling film distribution and horizontal tube falling film evaporation process. Through simulation study, liquid film of porous tube wall at the lower heat flux is under boiling state, the liquid film of smooth tube wall in the same heat flux density is mainly under free evaporation, this is mainly due to1) the surface porous layer can effectively reduce the velocity and stability of liquid flow, so that liquid can fully contact surface, which strengthening heat transfer, thereby strengthening the mass transfer process;2) the surface porous layer provides additional nucleation sites, provides more favorable conditions for boiling. |
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