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 unrelenting 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 considerations, 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 relative 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.