Characterization of Flow-Focusing and Flow-Blurring Modes of Atomization

Abstract

This paper presents a characterization of flow-blurring (FB) and flow-focusing (FF) modes of atomization using a new atomizer, which enables continuous transition from FB to FF regimes by controlling the recess of a liquid tube. The atomizer has optical access and features flexibility in controlling the Weber number (We) and the air/liquid mass flow ratio (ALR). The Phase Doppler Particle Analyzer (PDPA) technique is employed here along with long-distance microscopic and direct imaging to observe and characterize the atomization regime. Experiments were carried out with water and air as the working fluids at room temperature and in an atmospheric pressure environment. The results show that operation in the FB regime generates smaller droplet sizes in the spray core compared to the FF regime. However, the benefit of FB decays with increasing We, and eventually the difference between both regimes decreases. Among the distinctive features of FB sprays found only in the near-field region are: a more noticeable change in the mean droplet size with the radial direction, more pronounced polydispersity, larger centerline axial velocity and larger axial and radial velocity fluctuations. Liquid tube recess was also found to be an important parameter in controlling the spray behavior in the FB regime. It was found to impact the droplet size, the values and shape of the axial velocity variation curves, as well as the range of radial velocity values. It is also shown that, at a high enough We, effective differentiation between FB and FF atomization regimes became harder and rather limited to the visual confirmation of the presence of a bubble in the liquid tube.

Review of Flow Blurring Atomization: Advances and Perspectives

“…They concluded that the finer sprays resulting from FB are due to the generation of these microjets which result in small droplets and a uniform droplet size distribution . Such jets have been consistently identified in earlier and later work , and have some consistency with the “tree-root” description of higher ALR effervescent atomization . In the “tree-root” model, analytical predictions of the droplet size are possible from the thickness of the liquid sheet that forms at the exit, as demonstrated in refs and , although such an analysis has not yet been conducted in flow blurring atomizers.…”

“…This decreasing benefit with ALR shown in ref and Figure was later confirmed by de Azevedo et al using ethanol, which showed that both the Sauter mean diameter and mass median diameter were inversely proportional to ALR. Jaber et al further confirmed the dependence of ALR while also opting to present the reduction of droplet size as a function of the Weber number. Among all of the studies in flow blurring atomization, a common conclusion is the central role of ALR in defining the effectiveness of the atomization process, and it is worth noting that in effervescent atomization, it is also the ALR (and not the Weber number, as common in other air-assisted atomization methods), which is used to delineate between atomization regimes. , …”

“…Since the early work of Gañán-Calvo in 2005, flow blurring atomization has been used for spraying very viscous or viscoelastic liquids for manufacturing applications, , and it has also been used in combustion . A key advantage it has over effervescent atomization is its simpler design and also stable mode of operation. ,, The bifurcation of the air into the liquid nozzle also induces substantial local turbulent mixing prior to ejection from the orifice, and this has a number advantages for combustion applications.…”

Special Issue in Memory of Professor Mário Costa

“…The flamelet model was tested for a coal flame LES as one of the first simulations conducted on the new configuration of the investigated flame and showed good performance. Jaber et al 12 introduce a flow-blurring (FB) atomizer design with optical accessibility to investigate characteristics and differences between FB and flow-focusing (FF) modes over a range of the Weber number (We) using water and air as the working fluids. It was found to impact the droplet size, the values and shape of the axial velocity variation curves, and the range of radial velocity values.…”

A laser-driven optical atomizer: photothermal generation and transport of zeptoliter-droplets along a carbon nanotube deposited hollow optical fiber

“…[14][15][16][17][18] In response to these demands, various liquid jetting systems have been developed, where liquid droplets have been ejected and delivered through a nozzle to a target area with droplet diameter/volume distributions artificially controlled by physical impulses. [19][20][21] These previous approaches commonly require three driving mechanisms: (1) ejection of the liquid, (2) formation of liquid droplets, and (3) delivery of the droplets to the outer environment. Meanwhile, several driving mechanisms, such as ultrasound, 22,23 flow focusing, 24,25 and electrohydrodynamic jetting, 26,27 have been investigated.…”