Seminar: Doctoral Defenses

Scenarios of drop deformation and breakup in sprays

Speaker: Tímea Kékesi
Organization: KTH, Mekanik
Time: 2017-09-15 10:15
Place: D3


Sprays are used in a wide range of engineering applications, in the food and pharmaceutical industry in order to produce certain materials in the desired powder-form, or in internal combustion engines where liquid fuel is injected and atomized in order to obtain the required air/fuel mixture for ideal combustion. The optimization of such processes requires the detailed understanding of the breakup of liquid structures. In this work, we focus on the secondary breakup of medium size liquid drops that are the result of primary breakup at earlier stages of the breakup process, and that are subject to further breakup. The fragmentation of such drops is determined by the competing disruptive (pressure and viscous) and cohesive (surface tension) forces. In order to gain a deeper understanding on the dynamics of the deformation and breakup of such drops, numerical simulations on single drops in uniform and shear flows, and on dual drops in uniform flows have been performed employing a Volume of Fluid (VOF) method. The studied parameter range corresponds to an intermediate Weber number of We = 20, sufficiently high so that breakup occurs, but still much lower than the limit for abrupt catastrophic breakup, and a range of Reynolds numbers covering the steady wake regime for liquid drops, Re = 20-200. In order to account for varying materials of the liquid in different applications, a set of different density and viscosity ratios are considered, rho* = 20-80, and mu* = 0.5-50 respectively. Single drop simulations show that depending on the Reynolds number, as well as on the density and viscosity ratios, various breakup modes besides the classical bag and shear breakup may be observed at a constant Weber number. The characteristics of the deformation process and the time required for breakup are significantly different for these breakup modes. A criterion on the expected breakup mode in the form of a regime map, and a formula estimating the breakup time is suggested, based on the Reynolds number and material properties of the flow. This breakup time is significantly decreased by gradients in the flow surrounding the drop, for which another empirical model has been suggested. Dual drop simulations show that the interaction scenario between two drops is determined by the above parameters (Reynolds number, density and viscosity ratios), and additionally, the initial relative position of the two drops. It is found that the interaction behaviour of drops in tandem arrangement may be predicted based on data obtained for single drops, such as breakup time and the development of the wake region behind the drop. Furthermore, the region where drops behind another drop are likely to collide with this aforementioned drop is identified as a streak two diameters wide and eight diameters long behind the drop, however, weaker forms of interaction may occur up to distances of twenty diameters between the drops. Results presented in this thesis may be applied to formulate enhanced breakup models regarding the deformation, breakup, and interaction of liquid drops employed in spray simulations.