Contents lists available at ScienceDirect Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng Research Paper Numerical study of the drift and evaporation of water droplets cooled down by a forced stream of air Oskar Javier González Pedraza, J. Jesús Pacheco Ibarra, Carlos Rubio-Maya ⁎ , Sergio Ricardo Galván González, Jorge Alberto Rangel Arista Faculty of Mechanical Engineering, Edif. W, Central Campus, Universidad Michoacana de San Nicolás de Hidalgo, Morelia, Michoacán C.P. 58030, Mexico HIGHLIGHTS • A numerical simulation of water droplets falling in a forced air stream was performed. • Suitable size of water droplets for reducing drift and evaporation was estimated. • Mass evaporated was between 0.2 and 1.2% of the total droplet mass. • Droplet diameters between 4 and 10 mm are suitable for reducing water losses. • Diameter higher than 3 mm and air velocities lower than 5 m/s avoid drifting. ARTICLE INFO Keywords: Numerical simulation Water losses Water droplets Evaporation Drift ABSTRACT Evaporation is the basic heat transfer mechanism to reduce temperature of water in a cooling tower. Drift is a phenomenon in which water particles are carried by the leaving air stream causing water losses. In both pro- cesses the droplet size plays an important role for an effective cooling and minimum losses. A numerical si- mulation of water droplets falling in a forced air stream was performed by means of an Eulerian-Lagrangian reference framework. The aim of this work is to investigate water droplet size, inlet air temperature and inlet air velocities that reduce water losses. Particularly, the study is focused on the assessment of water losses caused by evaporation, as well as to determine the suitable size of water droplets for reducing water losses caused by drift. The mathematical model includes improvements to represent in a more realistic manner the heat and mass transfer mechanisms. One of these improvements is related to the convective heat transfer coefficient that for this study varies according to the temperature as well as to the instantaneous velocities of the continuous and dispersed phases. The results shows that the amount of mass evaporated for particles of 1 mm in diameter was around 1.2% of the total droplet’s mass. On the contrary, for particles of 8 mm that percentage was around 1% for the same residence time. Results also indicate that the minimum diameter of water droplets should be higher than 3 mm and air velocities lower than 5 m/s, in order to avoid drifting. 1. Introduction Cooling towers are devices widely utilized in industry to dissipate heat from different heat rejection components and processes to the ambient air. Since the basic heat transfer mechanism to reduce water’s temperature is the process of evaporation, significant amounts of water are demanded. Losses of water are essentially found in three ways: evaporation, drift and blowdown, being evaporation and drift the most significant. In a cooling tower the energy performance and amount of water losses depend on the correct design and proper management of recirculated water. Therefore, both aspects require a comprehensive understanding of the heat and mass transfer mechanisms occurring between air and water, which in turns allow improvements for reduc- tion of water consumption and for achieving better thermal perfor- mance. Merkel [1] introduced the first mathematical theory about cooling towers that describes the heat and mass transfer phenomena between water droplets and air flowing inside the cooling tower. Although this theory is the most employed for sizing and performance estimation of cooling towers, water losses due to evaporation are not considered. Such a process is relevant because it causes an increase of temperature and moisture inside the cooling tower. Since Merkeĺs model does not https://doi.org/10.1016/j.applthermaleng.2018.07.011 Received 24 January 2018; Received in revised form 14 June 2018; Accepted 3 July 2018 ⁎ Corresponding author at: Group of Energy Efficiency and Renewable Energy (GREEN-ER), Mexico. E-mail address: rmaya@umich.mx (C. Rubio-Maya). Applied Thermal Engineering 142 (2018) 292–302 Available online 04 July 2018 1359-4311/ © 2018 Elsevier Ltd. All rights reserved. T