Numerical investigations on two-phase flow modes in evaporative condensers

Falling film evaporation over horizontal tubes consists of simultaneous heat and mass transfer processes: in an evaporative condenser it improves the heat rejection from the condensing refrigerant to the air. The liquid flow is generally influenced by viscous, gravity, tension effects, liquid mass flow rate, tube diameter and spacing and distance from the feeding system. In this work, a two-dimensional numerical model of the falling film evaporation on horizontal tubes is presented. The temporal change characteristics of the film flow process were studied and different types of flow (stable film and drops mode) were investigated, by varying the ratio between the water-to-air mass flow ratio. The effect of the tubes arrangement on the flow mode was analyzed too: an increase of 73% of the longitudinal pitch corresponds to an increase of 66.7% of the minimum water mass flow rate that prevents the film break-up. The trade-off curve for a given geometry was obtained: at a specific air mass flow rate, a transition zone between the stable film to the drops mode conditions was individuated, with an uncertainty of 10% referring to a water mass flow rate variation of 10%.