Effect of Mach number on droplet aerobreakup in shear stripping regime

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 ultra-high-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. Graphical abstract

Droplet aerobreakup under the shear-induced entrainment regime using a multiscale two-fluid approach

“…Overall, the predominant pattern is the further decrease of the secondary droplets' sizes inside the dispersed mist, which is also reflected in the droplet population in Figure 8(i), highlighting an increase and dominance of the smallest scales over time. A distribution of uniformly small-scaled fragments is also demonstrated in the experiments of Wang et al [25] at subsonic postshock flows. In case 2, shown in Figure 9(ii), the creation of smaller-scaled droplets, driven by the local turbulence and collision, remains dominant for the mist evolution with a minor decrease compared to case 1.…”

“…During the evolution of aerobreakup, the coalescence region expands, driven by the increasing local flow vorticity. Similarly, in the study of Wang et al [25], the presence of larger fragments among the dominant microdroplets is observed at the advanced stages of aerobreakup under supersonic postshock conditions. As discussed in [25] and in agreement with the present subgrid scale analysis, these nonuniform fragments coalesce into larger secondary droplets, as imposed by the local flow conditions and the limited spanwise spread on the produced dense mist.…”

“…Similarly, in the study of Wang et al [25], the presence of larger fragments among the dominant microdroplets is observed at the advanced stages of aerobreakup under supersonic postshock conditions. As discussed in [25] and in agreement with the present subgrid scale analysis, these nonuniform fragments coalesce into larger secondary droplets, as imposed by the local flow conditions and the limited spanwise spread on the produced dense mist. At the same time, the secondary breakup shows a considerable and gradually increasing contribution to the mist evolution over time, as depicted in Figure 9(iii).…”

“…Furthermore, recent studies in the literature investigate the effect of the postshock flow on the initiation and evolution of the aerobreakup. Wang et al [25] examined the effect of the gas stream conditions on the macroscopic breakup pattern and the final dispersion of the produced secondary structures for a constant Weber number at 1100 and varied postshock flow Mach numbers in the range of 0.3-1.19. Specifically, the mist penetration and the fragment sizes show a dependency on the gas stream conditions and, thus, a narrower mist of less uniform fragments is observed at the advanced stages of aerobreakup under supersonic postshock conditions.…”

“…The attempts to obtain droplet size distributions from high-quality experimental data visualizations in the up-to-date literature, employed by Hsiang and Faeth [27], [28], [29], Villermaux [30], and Xu et al [31], are restricted to cases with moderate breakup, falling in the transition zone between the RTP and SIE regimes. Recent experimental studies of the SIE regime, such as the works of Theofanous [16], Theofanous et al [22], and Wang et al [25], provide a thorough investigation of the dominant physical mechanisms that influence the development of the dispersed mist. However, a quantification of the obtained fragment sizes inside the mist is not available.…”