Investigation of Thermal-Flow Behavior and Droplet Dynamics of Mist Sweeping Impinging Jet Cooling

Abstract

This paper presents a 2-D numerical study to investigate the fluid flow behavior and cooling characteristics caused by injecting tiny water droplets into the sweeping air jet through a fluidic oscillator. An unsteady Reynolds-averaged Navier–Stokes (URANS) simulation accompanied with the k–ω SST turbulence model is used in this study. The movement and evaporation of the mist are simulated by using the discrete phase model (DPM). The study has been conducted for a target wall with a constant heat flux of 3,000 W/m2, jet-to-wall distance of 4D, ReD = 2,500, and a mist/air mass ratio of 5% with a droplet size of 5 microns. A comparison between the cooling performance of steady and sweeping jets is presented for two impingement schemes (i.e., confined and unconfined impingement). The approach of using a slip upper wall boundary condition as an alternative to the unconfined impingement scheme is investigated as well. The results show that adding mist provided a temperature reduction of 5–10% on the target wall in all cases when compared to the air cases. Mist mostly follows the air jet behavior in both steady and sweeping jets in both impingement schemes. The liquid droplet coalescence phenomenon prevails in the sweeping jet case, while it is not as significant in the steady jet case. For the confined impingement, both mist jets provided the similar average temperature reduction. However, the steady mist jet introduced a 58% more target wall shear compared to the sweeping mist jet. For the unconfined impingement, the steady mist jet achieved a better average cooling performance compared to that of the sweeping mist jet. However, the steady mist jet introduced a 72% more target wall shear compared to the sweeping mist jet. Using a slip upper wall boundary condition to reduce the computational time resulted in similar average heat transfer distribution on the target wall to the unconfined case. However, the flow pattern, vortical structures, and droplet dynamics were very different.