The Effect of Nozzle Operating Conditions on the Droplet Flow and Heat Extraction Characteristics of Air-Mists

摘要

Although, it is accepted that droplet size, d_d, droplet velocity, u, and droplet number flux, N, are the independent flow characteristics that affect the boiling heat extracted by air-mists, very little experimental information has been reported regarding their variation with the nozzle operating conditions. This work describes the results of a systematic study - at room temperature - undertaken to examine such variation under the high droplet density conditions occurring in continuous casting secondary cooling. A state of the art visualization system was used to measure simultaneously drop size and velocity distributions through a technique called Particle/Droplet Image Analysis (PDIA). Averages of these quantities were used in conjunction with local water impact flux, w, measurements to estimate the droplet number flux at different positions, along the virtual impact plane. The measurements indicate that at constant w the increase in air-nozzle pressure, p_a, leads to a decrease in droplet volume mean diameter, d_(30), and to increases in volume weighted mean velocity, u_(z,v), and droplet number flux. The net effect of these changes is an increase in droplet impact pressure with p_a in a manner consistent with the increase exhibited also by the boiling heat flux. Furthermore, the high sensibility of the PDIA system to determine d_d and u allowed a deeper validation of a computational fluid dynamic (CFD) model developed for simulating air-mist jets, a very good agreement was obtained.