Visualization of water droplet deformation and breakup in a continuously accelerating flow field

Abstract

A new specific direct illumination technique is proposed for studying the deformation and breakup process of droplets that are exposed to a continuously increasing flow field. This type of flow field is in contrast to the traditional shock-tube experiments where droplets are suddenly exposed to a constant high airstream. In order to generate a continuously accelerated flow field the rotating arm facility at INTA is used. Droplets are allowed to fall in the path of an incoming airfoil mounted at the end of a rotating arm. Under certain conditions, these droplet deform and breakup before impinging on the airfoil. The incoming airfoil is a key element in the illumination technique. Droplets of 0.8 mm of diameter immersed in the flow field generated by an airfoil model of chord 0.690m approaching at 80 m/s were chosen for this study and were found to undergo a breakup process similar to the bag and stamen breakup found in the literature. Each of the stages at this type of breakup process was investigated by means of this new direct illumination technique and a high resolution camera. These images were compared to those images obtained using shadowgraph illumination technique and a high speed camera under the same conditions. Five different stages were studied and characterized. The final objective in this study was to visualize and identify fluid structures that occur during the deformation and breakup process in a continuously accelerated flow field in order to develop and fine tune physical-mathematical models. KeywordsDroplet Breakup, Visualization, Accelerating Flow. IntroductionDroplet aerobreakup has been studied mainly in shock tube [1,2] or wind tunnels facilities [3,4,5], where the droplet suddenly experiences a high constant air speed. Two comprehensive reviews on this field are those of Guildenbecher et al. [6] and Theofanous [7]. In the first one, attention was paid to the breakup morphology and a similar classification of breakup mode as the one found in Pilch and Erdman [8] was established: Vibrational breakup, Bag Breakup, Multimode (Bag and Stamen), Sheet Stripping (Sheet Thinning), and Catastrophic Breakup. The second review made a good discussion on the two main mechanisms leading to breakup: RayleighTaylor piercing and shear-induced entrainment. However, these two reviews [6,7] cover data obtained in facilities where droplets are suddenly exposed to a high constant velocity air stream. This is in contrast with the problem studied in this paper, where droplet are initially in a quiescent flow and then the air velocity starts to increase gradually with an acceleration that also increases, until the acceleration and the velocity reach values that allow for droplet breakup. The problem is a non-stationary problem and transient effects need to be considered. Accelerating and decelerated non-uniform flow fields have only been studied for non-deformable spheres [9,10], or droplets that are small enough to neglect deformation [11] and it has proved to be a very complex problem since there still ex...

Droplet deformation and breakup in shear flow of air

“…Moreover, in continuously accelerated flow, the overall morphology of the droplet changes continuously during the breakup process. Garcia et al 30 experimentally found that the deformation of the droplet went through the oblate spheroid shape, hat-like shape, mushroom-like shape, parachute-like shape, and fountain-like shape when the droplet was exposed to a continuously increasing flow.…”