Breakup of a laminar liquid jet by coaxial non-swirling and swirling air streams

Abstract

This thesis describes 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 4 to 1426 and from 0 to 3.9, respectively. High-speed shadowgraphy was utilised to obtain data on the first droplet locations, breakup lengths of the liquid jets, and two-dimensional wave spatiotemporal spectra for the jets. In order to detect the large-scale instabilities of the central liquid jet, proper orthogonal decomposition (POD) was performed on the high-speed shadowgraphic images. Stereo particle image velocimetry (SPIV) was utilised to investigate the annular air flow fields with S in the range of 0−2.5. Phase Doppler interferometry (PDI) was utilised to measure the droplet size and velocity distributions. It was found that air swirl promotes the morphological development of the jets with S in the range of 1.2−2.5. Both the breakup length and axial distance between the first droplet separation and the nozzle exit reduce as Weₐ and S increase. In terms of the air flow fields, radial expansion of the annular swirling air jets was observed, and the annular swirling jets expand radially further as S goes up. Central reversal air flows appear near the nozzle exit when S ≥ 1.2, and some small droplets are blown upwards to the nozzle exit by these central reversal air flows. In terms of large-scale instabilities, flapping is the dominant instability across most of the flow cases (as revealed by the first POD mode). Wavy and explosive breakup appear as the secondary breakup modes when Weₐ is low (≤ 110). In the absence of the central reversal air flows, the temporal frequencies of the instabilities of the air-water interfaces increase as S goes up. It was found that the central reversal air flows tend to stabilize the air-water interfaces. The spatial frequencies of the instabilities of the airwater interfaces remain low (≤ 0.06 mm−1) across all the flow cases which produce long wave structures.

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. Those specific flow cases are discussed in this work. For S ≥ 0.3, 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 behaviour in this thesis. In order to study the underlying mechanisms behind the turn-off behaviour, proper orthogonal decomposition (POD) was performed on the shadowgrams. It was found that the timing of turn-off initiation is random.

In general, there are three breakup mechanisms including turbulence in the liquid, aerodynamic forces acting on the gas-liquid interface, and cavitation inside the nozzle. In this project, shear forces are isolated from the other breakup mechanisms. How shear forces and air swirl influence the breakup of a co-annular air-water jet was investigated.