Investigation On Heat Transfer And Flow Of Falling Film Evaporation Outside Horizontal-tube

Horizontal-tube falling film evaporation has long been used for its advantage of high heat transfer coefficient at low temperature and small temperature difference. It has been wide application in desalination, food processing, refrigeration engineering, oil refining, chemical engineering, and many other fields. Incorporated with falling film evaporation as the main technology, low temperature evaporation multi-effect desalination not only owns such advantages above, but also could produce high quality fresh water, have high resistant to fouling, and keep high corrosion-resistant and is becoming one of the mainstream technologies in desalination power plant. Carrying out studies on horizontal tube falling film evaporation technology provides important technical and theoretical basis for designing more effective water desalination equipments or falling film evaporators for other applications.For obtaining the accurate data and mechanism, experiments of heat transfer study, flow pattern study, and numerical simulation have been carried out for falling film evaporation. The main contents of this paper are as follows:(1). Experiments are carried out to explore influencing factors for falling film evaporation. The influence of spray density and tube layout on heat transfer coefficient are discussed through flow pattern and water film velocity. Influencing area at different spray density and with different tube layouts are studied by comparisons between local heat transfer coefficient and average heat transfer coefficient, meanwhile, an optimal spray Reynolds number Recr is defined to thoroughly analyze heat transfer process within the falling film and the influences of each factor on heat transfer performance. The impact of heat flux on heat transfer coefficient is discussed based on the modes of heat exchange. Variations of local heat transfer coefficient and average heat transfer coefficient with evaporation temperatures and working fluids are discussed by means of the change of properties of working fluids. A special law of heat transfer coefficient with the evaporation temperature is first discovered and explained with seawater as working fluid in this paper. The result exhibits that heat transfer laws of falling film evaporation with other working fluids might not be applicable for sea water desalination.(2). For different tube layouts, the falling film are observed, photographed and comparative discussed. Through the observation experiments, the physical process of working fluid falling downwards and film spreading are discussed in detail. The Reynolds number needed and influencing factors for the transition of flow patterns are deeply studied and used to further confirm the influence of flow pattern on heat transfer performance within experiment results.(3). The Navier-Stokes equation chosen as governing equation, VOF (Volume of Fluid) method selected to trace the change of film on free surface, the three-dimensional numerical simulation was conducted on falling film flow. Through the comparison between the simulation results and that of the experiments, it was found that the results of three dimensional simulations fitted well with that of the experiments which showed the feasibility of and reliability of such numerical simulation. Moreover, through the analysis of flow field diagram numerical simulation and film distribution map, the flowing process and film spreading law are further discussed and better explain the physical law exhibited in experiments. That make the simulation results play an significant role in microscopic analysis of experimental results.