Thermal fluid dynamic modelling of a water droplet evaporating in air

Water saving is one of the biggest issues that the world will soon have to deal with, considering the unre- lenting population growth and the not uniform global distribution of fresh water sources. Agriculture alone is responsible, in many countries, for some 70% of its usage and consequently finding ways to save water in agriculture would produce a particularly significant result in such struggle. Sprinkler irrigation is one of the most diffused irrigation techniques employed in agriculture. Based on these initial consider- ations, the present paper is aimed at describing and understanding the dynamic and thermal– fluiddynamic behaviour of a water droplet travelling from the nozzle outlet to the ground through dry and moist air, in function of all the variables involved in the process, namely: droplet initial diameter, droplet initial velocity, water temperature, air temperature, diffusion coefficient of water in air, air rela- tive humidity, droplet inlet inclination, solar and environmental radiation, wind vectorial velocity. After a full analytical modelling of the phenomenon, a numerical implementation based on Runge–Kutta fourth order method was made. The effect of the above reported parameters is broadly discussed also in relation to their in-flight evolution; as foreseen in previous papers, also air friction proves not to be negligible when assessing in-flight droplet evaporation. Some comparisons with well established literature data contribute to the model validation.