Effect of Mach number on droplet aerobreakup in shear stripping regime
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RESEARCH ARTICLE
Effect of Mach number on droplet aerobreakup in shear stripping regime Zhaoguang Wang1 · Thomas Hopfes1 · Marcus Giglmaier1 · Nikolaus A. Adams1 Received: 20 April 2020 / Revised: 27 July 2020 / Accepted: 28 July 2020 © The Author(s) 2020
Abstract The present experimental study investigates the shear stripping breakup of single droplets in subsonic and supersonic gaseous flows. In contrast to most research that places emphasis on the Weber number (We), we focus on the individual effects exerted by flow Mach (M∞) and Reynolds numbers (Re). Millimeter-sized droplets made of either ethylene glycol or water are exposed to shock-induced flows. Shadowgraph and schlieren images of the breakup process are recorded by an ultrahigh-speed camera. The experimental We is constrained at 1100, while M∞ is varied from 0.3 to 1.19 and Re from 2600 to 24,000. A systematic analysis of the experiment series reveals that the breakup pattern alters with M∞ although a constant We is maintained. The classical stripping behavior with fine mist shed from the peripheral sheet changes to rupture of multiple bags along the periphery at M∞ = 0.63, and further to stretching of ligament structures from the leeward surface at M∞ = 1.19. The corresponding breakup initiation is delayed and the resultant fragments are sized less uniformly and distributed over a narrower spread. In terms of the early-stage deformation, droplets experience less intense flattening and slower sheet growth at higher M∞. The change of Re introduces additional variations, but only to a minor extent.
* Zhaoguang Wang [email protected] 1
Chair of Aerodynamics and Fluid Mechanics, Technical University of Munich, 85748 Garching, Germany
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193
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Experiments in Fluids
(2020) 61:193
Graphical abstract
Subsonic vs Supersonic Droplet Breakup at Constant Weber Number 1100 Mach Number
separation zone
oblique shock
less flattening kinks shifted downstream liquid sheet
multiple bags
Fragment Distribution
Breakup Pattern
1.2
normal shock
Early-Stage Deformation
Wave Dynamics
0.3
ligaments
larger fragments over narrower spread (The length scale of images in the last row is half of that in the first three rows.)
1 Introduction Droplet breakup, also termed secondary atomization, refers to the fragmentation of a droplet subjected to aerodynamic forces. This phenomenon is relevant in diverse applications, such as fuel injection (Reitz and Diwakar 1986), spray coatings (Mostaghimi et al. 2002) and metal powder production (Lagutkin et al. 2004). It has been widely recognized that the breakup morphology is primarily determined by the Weber number (We) and the Ohnesorge number (Oh) (Lane 1951; Hinze 1955):
We = 𝜌g u2g d0 ∕𝜎
(1)
√ Oh = 𝜇d ∕ 𝜌d d0 𝜎,
(2)
where ρg and ug are the density and the velocity of the gas flow, and d0, σ, μd and ρd are the initial diameter, the surface tension, the dynamic viscosity and the density of the liquid droplet, respectively. The Weber number represents the ratio between the disrupt
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