Heat transfer characteristics in horizontal tube bundles for falling film evaporation in multi-effect desalination system

• 2 dimensional modelling of falling film evaporation
• Prediction of film coefficient and evaporation for different tube sizes
• Experimental studies on falling film evaporation
• Comparison of in-line staggered tube layouts in tube bundle
• Comparison of CFD, experimental data and published literature

Horizontal tube falling film evaporation finds wider applications in multi-effect distillation system in recent years. The latent heat released inside the tube due to condensation of vapor is transferred to the falling film on the outer surface of the tube resulting in convective evaporation of water film. The evaporator consists of multiple rows and columns of horizontal tubes. Heat transfer from condensing film inside the tube bundle is more or less uniform while there is a large variation in heat transfer outside the tube bundle. This paper focuses on Computational Fluid Dynamics (CFD) analysis of falling film evaporation of seawater on a single tube and bundle of tubes using ANSYS Fluent 13.0®. CFD results are validated with published data available in the literature and also with experimental studies carried out. The effect of feed rate, tube diameter, wall temperature, etc. on the heat transfer is studied. Local film coefficient around the tubes of 19.05, 25.4 and 50.8 mm ∅ for different film Reynolds numbers is discussed. It is observed that convective evaporation heat transfer performance increases with feed rate, but decreases with tube diameter. In-line tube configuration is found to be better compared to staggered tube configuration.