On Complex Liquid-Gas Interface Morphology Beyond Spherical Drop and Bubble Through Surface Curvature Distribution (SCD)

Abstract

Abstract Liquid-gas flow are very common in many applications where interactions of interest between both phases are controlled by the state of the liquid-gas interface. This interface can have a very complex morphology but only few morphologic states have been studied intensively: when the interface has small deformation from an initial known state linear instability formalism can be used; when one phase is dispersed in the other one (bubbly cloud or spray), the dispersed phase can be considered as a discrete phase in a carrier phase and the so-called multiphase flow formalism holds. For this latter case droplet and bubble are characterized by their diameter and determining the diameter distribution is generally one of the principals aims of the description. However, there are many other regimes that does not correspond to these three situations. For instance, when liquid and gas volume fraction are between 10% and 90% in a turbulent flow, the so-called dense regime occurs. It may contain bubbles or droplets, but they interact strongly leading to interface deformation that are far from being spherical. Thus, it is not possible to characterize droplets and bubble by their diameter distribution since diameter is not defined. A representative application of this problem concerns atomization process: typically, a liquid flow is forced to interact with an external gas leading to an evolution of the liquid volume fraction from 100% to less than 1%, thus main mechanism of atomization occurs within the dense regime with the aim to produce a spray with a controlled diameter distribution. Accordingly, we have researched a way to describe the interface complex morphology from the early stage of liquid jet destabilization up to the formation of the final spray. Initially, thanks to direct numerical simulation (DNS) we have studied the interface morphology with the surface curvature distribution (SCD) [1] among other possibility [2]. The SCD allows to describe continuously the destabilization of the initial liquid structure, through complex interface such a as ligament, blobs, liquid sheet until the apparition of first spherical structures which ultimately become droplets. Beyond the description of the interface it has been possible to show that a careful analysis of the liquid-gas surface through the SCD allows for determining at early stage of the atomization process the final characteristics of the spray, even its diameter distribution [3]. Present works are developing on two directions: the way to measure directly the surface curvature distribution experimentally thanks to 2PLIF and contour analysis, careful DNS simulation of different break up process to elucidate more generally the evolution and the dynamic of the surface curvature distribution.