Three-dimensional simulation of droplet breakup and evaporation in attemperator

Abstract

An attemperator (also called a desuperheater) sprays water into superheated steam to modulate its temperature to a desired set point. Here, the sprayed water must evaporate completely before it reaches the thermowell installed downstream of the attemperator, as the amount of water sprayed into the flow varies according to the steam temperature measured by the thermowell. Therefore, to develop optimal attemperator designs, it is vital to accurately predict the evaporation distance of the sprayed water and find suitable methods to reduce this distance. In this study, droplet breakup and evaporation in various attemperator design configurations were numerically simulated in the three-dimensional domain, considering both primary and secondary droplet breakup. The simulation model was validated by comparing the simulation results of four commissioning conditions with the corresponding transient thermal analysis results from a previous study. Subsequently, four attemperator designs were analyzed using the validated simulation model to obtain useful design insights. The simulation results demonstrated that all droplets should break up into droplets that are below a certain size to ensure complete evaporation. In addition, a venturi-type thermal liner can enhance droplet breakup by accelerating and decelerating the steam flow, resulting in faster droplet evaporation. Finally, the evaporation distance can be reduced by using multiple smaller-sized nozzles, which produce smaller initial droplets.