Experimental Investigation On Heat Transfer Of Horizontal-tube Falling Film Evaporation

Falling film evaporation outside horizontal tubes is advantageous as an effective heat transfer mechanism, because it has high heat transfer rate, small temperature difference, and high transfer performance at low temperature etc. So it now has been widely used in desalination devices, refrigeration systems, and petrochemical plants etc.This experiment is about falling film evaporation outside horizontal tubes, which is based on the falling film evaporator used in the large-scale desalination. The falling film experimental system is established, which contains the intertube flow and falling film evaporation process for attaining experimental data of intertube falling film modes and factors of affecting falling film evaporation by accurately measurement. Heat transfer coefficient correlations are proposed from the experimental data. Both of the experimental data and heat transfer coefficient correlations are useful for the design of the falling film evaporator used in the large-scale desalination.The evaporation tube is made of plain HAL77-2A aluminium brass with an outer diameter of 25 mm in the experiment. To avoid other influence the tube is heated by internal electrical heaters so that a uniform heat flux is generated and heating power and surface temperature can be easily controlled. Results are attained about local and average heat transfer coefficients changing with the effect of spray density, evaporation temperature and tube pitch etc. The effects of those factors on the heat transfer coefficients are analysed 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. Experimental results indicate that the local heat transfer coefficients and average heat transfer coefficients increase with the increase of spray density, evaporation temperature and tube pitch. Average heat transfer coefficient correlations of different tube pitches are proposed from the experimental data. The flow patterns: the droplet mode-liquid, the droplet and jet mode-liquid, the jet mode-liquid, the jet and sheet mode-liquid are attained after analysing the pictures of the intertube flow, which can help analysing the influence of spray density and tube pitch on the heat transfer coefficient.