Characteristics of sprays produced by coaxial non-swirling and swirling air–water jets with high aerodynamic Weber numbers

Abstract

This paper describes part of an experimental study on shear-based spray formation. A laminar liquid jet was ejected inside co-annular non-swirling and swirling air streams. The aerodynamic Weber numbers (WeA) and swirl numbers (S) of the flow cases ranged from 256 to 1426 and from 0 to 3.9, respectively. The aim of this paper is to investigate how S and WeA influence the breakup of a central laminar liquid jet within the fiber-type atomization regime. High-speed shadowgraphy was utilized to visualize the spray behavior, while phase Doppler interferometry was utilized to measure the droplet size and velocity distributions. It was found that as S increases, atomization is improved in a way that the droplets are blown outward from the central axis of the nozzle. However, for some specific flow cases, the median droplet diameter (D) does not appear to be related to S. These specific flow cases are discussed in this work. For S≥0.3, the upward motion of droplets located at the central axis of the nozzle was observed, which was caused by recirculating air flows. In addition, it was found that when S increases to 2.5, recirculating air flows start to penetrate to the water exit, which momentarily stops portions of the central laminar water jets from exiting. This pattern will be called turn-off behavior in this report. In order to study the underlying mechanisms behind the turn-off behavior, proper orthogonal decomposition was performed on the shadowgrams. It was found that the timing of turn-off initiation is random.