Effect of air flow, water spray and nozzle injection characteristic on direct evaporative cooling

Abstract

Evaporative cooling occurs through evaporation of water droplets in air. It works on the principle of injecting water spray directly into the air flow to reduce its temperature by a conversion of sensible heat of the air flow to latent heat of water. Incomplete evaporation and non-uniform temperature distribution are the main concerns of water spray cooling systems. Incomplete evaporation of water droplets may cause corrosion in the heat exchanger and increases operational cost due to water con sumption. Moreover, non-uniform temperature distribution due to gravitational effect can decrease the efficiency of the cooling system. In this study the effect of different parameters (air velocity, relative humidity, droplet velocity, nozzle cone angle, droplet size distribution, turbulence intensity, and nozzle injection angle) on dry bulb tem perature, the evaporated water fraction, and spray cooling efficiency are investigated. Contact surface area between water droplets and air and the residence time are two main characteristics which affect spray cooling efficiency. Increasing spray dispersion and residence time are main focus in our study. The results show that as D, the mean of the Rosin-Rammler distribution, is reduced from 60 to 30 µm, the cooling perfor mance of the system is improved by more than 40%. Also, for a given values of the inlet air velocity, as the inlet air velocity decrease from 3.5 m/s to 1.5 m/s, the spray cooling efficiency of the system improves by more than 29.4%. Moreover, it can be concluded that moving toward the end of duct length where the coolant flow become more tur bulent and wide enough we could reach a better water evaporation, more temperature drop for hot air flow, and consequently, higher spray cooling efficiency.