Computational Fluid Dynamics (CFD) Modeling of Falling Film Heat Transfer Over Horizontal Tube for Multi-effect Desalination (MED) Evaporator

Horizontal-type falling film evaporators are commonly used in multi-effect desalination (MED) plants, refrigeration, air conditioning and waste heat recovery application. In falling film evaporators, a thin liquid film covers the tube, and heat is transferred from the heating medium inside the tube to the thin film. The heat transfer in falling films is governed by the film hydrodynamics, temperature gradient and tube material. In order to analyze film heat transfer at microscopic level, computational fluid dynamics (CFD) tool is implemented in this work. A 25.4 mm horizontal tube with an impingement height of 5 mm is selected as the computational domain. Water enters at 65 °C that is the top brine temperature of MED plant. The liquid load and temperature difference varied from 0.01 kg/(m s) to 0.09 kg/(m s) and from 2 °C to 10 °C, respectively. It is found that the heat transfer coefficient is very high at the stagnation point and then decreases, as the flow moves along the tube surface. The recirculation in the film enhances the heat transfer. Furthermore, increasing liquid load from 0.01 to 0.09 kg/(m s) with ΔT = 5 °C, increase the heat transfer coefficient by 231%.