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. The seawater film absorbs sensible and latent heat in the MED system 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 has multiple effects. To improve the heat transfer tube's performance, 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, very high specific surface area, and a tortuous flow path, is used to enhance the falling film evaporator's performance. Horizontal copper tube is wrapped and brazed with thin copper metal foam. Experimental and numerical study of heat transfer characteristics of falling film over metal foam layered horizontal tube was carried out. Numerical study of falling film over metal foam wrapped horizontal tube requires the determination of flow and thermal properties of the metal foam. Micro-tomography (μ-CT ) based CFD study was used to obtain the metal foam properties. A new image-based technique was used to estimate the effective thermal conductivity (ETC) of metal foam. The method can determine the ETC of metal foam composing of different materials in the solid, pore, and hollow parts of the metal foam. The technique is highly feature preserving and eliminates the computationally expensive surface generation and volume mesh generation processes required in the μ-CT based CFD study. ETC obtained from the present study was also compared with the values obtained from the ETC models available in the literature.
Heat transfer studies were carried out in an evaporator of a multi-effect desalination experiment system which can accommodate tube bundles. It was observed that the falling film heat transfer coefficient was enhanced 2.7 times by the application of metal foam over the plain horizontal tube. The measurements obtained from hydrodynamic and heat transfer studies were compared with the predictions made by a computational model and are in good agreement. Effect of foam thickness, foam permeability, foam porosity, feed rate, and degree of subcooling on heat transfer rate was also studied.
Scale formation due to dissolved salts and suspended particles is unfavourable for the desalination plant and necessitates the use of antiscalants for effective control of scale formation. In the present study, plain aluminium and copper tube, thermal spray coated alumina over an aluminium tube, and copper metal foam wrapped over copper tube were tested. The last two tubes were developed in-house to enhance the heat and mass transfer rate in multi-effect desalination system. Effect of temperature, flow rate, and seawater turbidity on scale formation was studied. Total scale deposit weight was observed after every 16 hrs of operation for a total run time of 128 hrs. Energy-dispersive X-ray spectroscopy and scanning electron microscopy were applied to investigate the mineralogy and microstructure of the deposits.