CFD analysis of the impact of physical parameters on evaporative cooling by a mist spray system

The evaporation of droplets in a turbulent two-phase flow is of importance in many engineering applications. Water droplet evaporation in spray systems, for example, is increasingly used in public spaces and near building surfaces to achieve immediate cooling and enhance the thermal comfort in indoor and outdoor environments. The complex two-phase flow in such a system is influenced by many parameters such as continuous phase velocity, temperature and relative humidity, drop size distribution, velocity and temperature of the droplets and continuous phase-droplet and droplet-droplet interactions. Most of these parameters are not easily varied independently. To gain insight into the performance of the system, however, detailed knowledge of the impact of every parameter is important. Computational Fluid Dynamics (CFD) is a useful tool for performing such parametric analyses. To the best of our knowledge, a detailed analysis of the cooling performance of a water spray system under different physical conditions has not yet been performed. This paper provides a systematic parametric analysis of the evaporative cooling provided by a water spray system with a hollow-cone nozzle configuration. The analysis is based on grid-sensitivity analysis and validation with wind-tunnel measurements. The impact of several physical parameters is investigated: inlet air temperature, inlet air humidity ratio, inlet air velocity, inlet water temperature and inlet droplet size distribution. The results show that for a given value of inlet water temperature (35.2 °C), as the temperature difference between the inlet air and the inlet water droplets increases from 0 °C to 8 °C, the sensible cooling capacity of the system improves by more than 40%. In addition, injecting water droplets with a temperature higher than the dry-bulb temperature of the air can still provide cooling, although the amount of cooling reduces considerably compared to the case with water at lower temperatures. It is also shown that as . D-, the mean of the Rosin-Rammler distribution, is reduced from 430 to 310 μm, the cooling performance of the system is improved by more than 110%. For a given value of . D-, the cooling is enhanced for wider drop-size distributions.