Metal Foam, Falling film evaporator, Heat and mass transfer, heat transfer enhancement
There is an acute scarcity in availability of water across the world. As per United Nations Development Programme (UNDP) report on Human Development almost one-fifth of the world’s population, live in areas of physical water scarcity. To combat this situation establishment of desalination plants is considered as one of the most feasible option. Multi-Effect Desalination systems are thermal based system with horizontal falling film evaporators (HTFFE). HTFFE are featured by evaporation from the liquid film over the surface of the tube and simultaneous condensation inside the tube. Condensation heat transfer coefficient is higher than evaporative heat transfer and can be improved by using heat transfer enhancement technique . Metal foams which are characterized by high porosity and large volumetric heat transfer coefficient is a promising material for thermal application . Numerical analysis was carried out with Aluminium foam wrapped evaporator tubes and enhancement in heat transfer was found out. Metal foam characteristics, foam thickness, porosity and permeability was optimized to give maximum heat transfer rate. Computational analysis was carried out using ANSYS Fluent 16.1. Sources terms are added using User Defined Functions (UDF) to continuity and energy equation to capture the evaporation phenomena happening from the liquid-vapor interface. Computations are carried out at saturation condition of 64.6 0C for falling film Reynolds number of range 500 to 1000. It is observed from the computational study that the heat transfer coefficient increases with decrease in the porosity and increases with the flow rate. Effect of foam thickness is not linear on the heat transfer rate, there are two competing effects. Increase in the foam thickness allow more fluid to be in contact with the metal part which tends to increase heat transfer and simultaneously increase in foam thickness will also lead to increase in the conductive resistance due to increase in falling film thickness. References: Abraham, R. and Mani, A., 2015. Heat transfer characteristics in horizontal tube bundles for falling film evaporation in multi-effect desalination system. Desalination, 375, pp.129-137. Arjun, Jayakumar and Mani, A., 2017. Computational analysis of high porosity aluminium foam for heat and mass transfer enhancement in falling film evaporator. 6th Asian Symposium on Computational Heat Transfer and Fluid Flow, Paper No. 225. .