Horizontal tube falling film evaporator is widely used in refrigeration, ocean thermal energy conversion, food processing, chemical processing, petrochemical and multi-effect desalination (MED) system. In the MED system, the seawater film absorbs sensible and latent heat as it flows over the external tube surface from the steam condensing inside the tube. The latent heat of vaporization is obtained from the internal condensation of steam generated in the previous evaporator and thus has multiple effects. To improve the performance of the heat transfer tube, researchers have focused on enhancement technique like usage of fins, and grooves, roll working of surface, making the surface hydrophilic, and thermal spray coating.
In the present study, open-cell metal foam, a class of metal foams characterized by high porosity (ε>0.85), very high specific surface area and a tortuous flow path, is used to enhance the performance of the falling film evaporator. Horizontal copper tube is wrapped and brazed with thin copper metal foam. A non-intrusive technique based on air-coupled ultrasonic transducer was introduced to study the falling film characteristics over the foam wrapped tube surface. The technique was validated by comparing the measurement of falling film thickness on an inclined plate using conventional contact type ultrasonic transducer. Horizontal tube falling film thickness was measured at different circumferential angles, flow rates, and tube spacing using the ultrasonic technique. Measurements were made for both bare and metal foam wrapped tube.
Measurements obtained were compared with the predictions made by a computational model. Computational study of falling film over metal foam wrapped tube surface require the determination of the metal foam flow properties, permeability and form drag coefficient. To obtain the properties, micro-tomography based CFD study was used. The replica of the metal foam was generated from the tomography images, and placed in a uniform flow arrangement. The variation of pressure drop across the foam was observed for different flow velocities. Based on Hazen-Dupuit-Darcy model, flow properties are obtained at different directions of the metal foam. The properties are incorporated into the CFD model using UDF. A good agreement was observed between the predictions of CFD study and measurement.