Session Q35: Atomization and Sprays II

Subtle interplay of rim and bag instabilities in drop atomization

Research on liquid drop atomization until now has assumed different deforming segments of a fragmenting fluid drop to be influenced by instabilities which either act independent of each other or occur one after the other. The inherent coupling between the instabilities though evident, has been largely overlooked. In this work we investigate the breakup of a single drop as it deforms into a bag bounded by a rim upon being subjected to aerodynamic forces emanating from an air nozzle. Our analysis reveals the hidden interplay of instabilities acting along the rim and the bag as we observe a close relationship between the thickness of the rim, bag and the radial extent of the deformed drop based on fundamental laws of conservation. This interdependence helps us determine the exact relation between the wavelength of instabilities acting on the bag and the rim which we extend to predict the resulting drop sizes and establish their dependence on the aerodynamic disruptive forces. The scaling laws derived from these calculations and verified by our experimental observations show that the rim drop sizes weakly depend on the oncoming air flow while the bag sizes are dominantly affected by it.   

A D2 law for supercritical droplet vaporization

To achieve higher efficiencies in rocket, jet, and diesel engines, the combustion pressures are increasing, meaning the fuels are often injected under supercritical conditions. Supercritical fluids are unique as they no longer have a liquid-gas phase equilibrium or surface tension. However, supercritical droplets have been observed in injection experiments. The common explanation is the droplets form due to a local subcritical pressure region caused by the fluid mixture, but the exact mechanism is still unknown. We provide an alternative physical mechanism for the formation of supercritical droplets. We demonstrate the formation of droplets in a pure supercritical fluid without surface tension, and it is driven by heat transfer and unique fluid properties. Also, like subcritical droplets, the supercritical droplets follow the $D^2$ law for vaporization. The interface temperature is at the pseudo boiling temperature, similar to subcritical droplets, as the interface temperature is at the saturation temperature. At supercritical conditions, pseudo boiling supersedes boiling, so the pseudo boiling temperature supersedes the saturation temperature. With these results, we provide a new physical mechanism that causes the formation of supercritical droplets without surface forces.   

Direct numerical simulations of turbulent jets: vortex-interface-surfactant interactions

Many industrial applications involve the breakup of a jet into emulsions. A fundamental understanding of the physical mechanisms that control the droplet size distribution can aid the improvement of mixing/shearing devices. Additionally, those streams are usually contaminated with surface active agents triggering the formation of surfactant-induced Marangoni stresses. Here, we study the effect of surfactants on the spatio-temporal evolution of turbulent jets using three-dimensional numerical simulations that employ an interface-tracking/level-set method that accounts for surfactant-induced Marangoni stresses. The present study builds on our previous work (Constante et al., 2021, J. Fluid Mech., 922, A6) in which we examined in detail the vortex-surface interaction in the absence of surfactants. Numerical solutions are obtained for a wide range of Weber and elasticity numbers in which vorticity production is generated by surface deformation and surfactant-induced Marangoni stresses. The present work demonstrates, for the first time, the crucial role of Marangoni stresses, brought about by surfactant concentration gradients, in the formation of coherent, hairpin-like vortex structures. These structures have a profound influence on the development of the three-dimensional interfacial dynamics. We also present theoretical expressions for the mechanisms that influence the rate of production of circulation in the presence of surfactants for a general, three-dimensional, two-phase flow and highlight the dominant contribution surfactant-induced Marangoni stresses.   

A study of the role of droplet fluid properties on airborne pathogen transmission

This research aims to study the physics of airborne transmission of pathogens such as SARS-CoV-2 concerning the underlying character of droplets. These droplets encapsulate the virus and are formed and emitted through speaking, sneezing, and coughing processes. The research utilizes Computational Fluid Dynamics(CFD) with Detached Eddy Simulation (DES) to study how the droplet character relates to dispersion. The model of the droplet (Lagrangian) includes size, breakup, forces (drag, pressure, gravity), and evaporation, inherited within the droplet flow. The CFD model studies these cases by approximating speech (5 min) and coughs (2min) and releases a range of sizes within the buccal cavity of a model human mouth within a ventilated room. Our previous research found out that the flow of these droplets and aerosols released during the respiratory phenomena can be modified by using foods and it is believed that these foods alter the formed droplet viscosity and salivation rate (hence film thickness) therefore modulating the resulting droplet distribution. This research will further study the behavior of each of the droplet’s fluid properties: size, viscosity, and flow rate, concerning the transmission of droplet flow. The aim is to observe and analyze if these fluid properties play a role in limiting the transmission of droplets. Distance traveled by the droplets can be measured by using the vertical probes placed at certain distances away from the human body. These results can be presented as the concentration of droplet particles for the different cases. Comparative plots can give us insight into the effect of droplet fluid properties on limiting droplet transmission. The conclusion will be drawn based on the results observed and can suggest to us the important fluid dynamics properties that have to be modified, helping prevent airborne transmission associated with COVID-19, influenza, and others.

    

Characterizing Interface Topology in Multiphase Flows using Skeletons

The unsteady motion of a gas-liquid interface, such as during splashing or atomization, often results in complex liquid structures embedded in the ambient fluid. Here we explore the use of skeletonization to identify the minimum amount of information needed to describe their geometry. We skeletonize a periodic liquid jet by a modification of a recently introduced approach to coarsen multiphase flows while retaining a sharp interface. The process consists of diffusing an index function and at the same time moving the interfaces with it, until they "collapse" into each other and form skeletons. The skeleton represents the basic topology of the jet and we also keep track of how much the interface is moved (or how much volume is "accumulated") during the process, which can be used to approximately reconstruct the jet. We explore various quantitative measures to characterize and distinguish the skeletons. Those include standard morphometrics such as branch length distribution, after segmenting the skeletons into branches, and a more sophisticated representation of the skeleton structures called Topology Morphology Descriptor (TMD), to obtain an "equivalent" description of the skeletons by retaining information about the topology in a compact way.   

Author not Attending

Present work studies the effect of surrounding gas medium on the stability of a radially expanding liquid sheet by taking into account the formation of gas boundary layer whilst neglecting liquid viscosity. The stability of the liquid sheet is analysed using perturbation theory. Small perturbations are applied to a Blasius-type base state system resulting in a linear model. Sinuous modes are significantly more unstable than the varicose modes therefore, spatial instability in sinuous modes is investigated. The results are compared with those obtained by using Kelvin Helmholtz instability, and give us a better understanding of the stability of a thin liquid sheet.   

Dynamic wetting in the curtain coating by the Volume-of-Fluid method and Navier-slip model Part 2: the slip length and the inertial effects

The dynamic wetting failure in the curtain coating setup is studied numerically by solving the 2D two-phase Navier-Stokes equation subject to the Navier-Slip boundary condition and a constant contact angle at the grid scale using the Volume of Fluid method. Previously, the results were shown to recover the logarithmic curvature singularity and the hydrodynamic assist property for the reduced model with the slip-length of 10 microns [Kulkarni, Y., Fullana, T., Zaleski, S. (2021). APS Division of Fluid Dynamics Meeting Abstracts (pp. P29-008)]. We now show that the problem involves highly coupled length scales. The inertial effects can not be neglected up to a scale of 10 microns making the apparent contact angle depend on the Reynolds number as well and proving the reduced model slip length to be unphysical large. By decreasing the slip lengths to a few hundreds of nanometers we observe a shift in the stability window. Finally, we compare against the experiments of Blake, Bracke and Shikhmurzaev (1999) and recover the experimentally observed nonlocal hydrodynamic effects on the scale of 20 microns qualitatively. The effect of slip length reduction is seen and the threshold between stable and unstable solution is displaced towards the experimental one.   

Phase Field Modeling and Numerical Algorithm for Dielectrowetting

We present a phase field-based method for numerical simulations of two-phase dielectric fluids and their interactions with solid wall surfaces and external electric fields. A physically consistent phase field model attained based on the conservation laws and thermodynamic principles is discussed first. Then we present an efficient numerical algorithm for solving the coupled system of governing equations. In particular, our algorithm requires only the solution of linear algebraic systems with constant and time-independent coefficient matrices upon discretization, even though the physical model involves two fluids with different dielectric constants. As a result, the presented algorithm is computationally highly efficient. Several examples of dielectrowetting simulations will be presented to demonstrate the performance of the presented method, with comparisons between the simulation and the experimental data or theoretical predictions.   

On Hydrodynamics of Dry Slag Granulation of LD/BOF Slag: Analytical Modeling

Experimental trial of dry granulation process has been widely performed to granulate blast furnace (BF) slag, in addition to, heat recovery from molten material. In this study, analytical formulation has been obtained for Dry Slag Granulation (DSG) process of Spinning Disc Atomizer (SDA) using literature survey and fundamental principles. An expression has been derived to obtain diameter of droplet formed, after granulation, by rotating disc method. In accordance with the three phases of the problem, i.e. thin viscous film formation, ligament formation, and droplet formation, mathematical models have been developed, separately, for each phase. Further, the obtained expression has been validated with established benchmarks. Further study has been also done on effects of rotating speed of disc and liquid flow-rate on diameter of droplets formed.