Physical Mechanisms of Droplet/Turbulence Interaction
Antonino Ferrante - Associate Professor
William E. Boeing Department of Aeronautics and Astronautics
University of Washington, Seattle
The interaction of liquid droplets with turbulence are relevant to both environmental flows and engineering applications, e.g., rain formation and spray combustion. The physical mechanisms of droplet-turbulence interaction are largely unknown. The main goal of this research has been to investigate the physical mechanisms of droplet/turbulence interaction for non-evaporating and evaporating droplets.
Droplets in turbulent flows behave differently from solid particles, e.g., droplets deform, break up, coalesce and have internal fluid circulation. In order to simulate such behavior, we have developed a new pressure-correction method, FastP*, for simulating incompressible two-fluid flows with large density and viscosity ratios between the two phases. The method’s main advantage is that, for example, on a 10243 mesh, FastP*, using the FFT-based parallel Poisson solver, is forty times faster than the method using multigrid. In general, FastP* could be coupled with other interface advection methods such as level-set, phase-field, or front-tracking. We have coupled the pressure-correction method with a volume-of-fluid (VoF) method for its properties of being mass conserving and sharp-capturing of the interface.
We performed direct numerical simulation (DNS) of finite-size, non-evaporating droplets of diameter approximately equal to the Taylor lengthscale in decaying isotropic turbulence. We studied the effects of Weber number, viscosity ratio and density ratio. We derived the turbulence kinetic energy (TKE) equations for the two-fluid, carrier-fluid and droplet-fluid flow. This allows us to explain the pathways for TKE exchange between the carrier turbulent flow and the flow inside the droplet. The role of the interfacial surface energy is explained through the power of surface tension term of the two-fluid TKE equation. Also, we derive the relationship between the power of surface tension and the rate of change of total droplet surface area. This allows us to explain how droplet deformation, breakup and coalescence plays a role on the temporal evolution of TKE. Our DNS results show that increasing Weber number, the droplet to fluid density or viscosity ratios increases the decay rate of the two-fluid TKE relative to that of single-phase flow. Furthermore, we have developed a new methodology for the spectral analysis of multiphase flows. Via analysis of the DNS results both in physical and spectral space, the revealed physical mechanisms and opportunities for modeling will be presented. Recently, we have developed a new VoF-based method to simulate evaporating droplets. The verification and validation of the method, and the DNS results of droplet vaporization in isotropic turbulence will be presented in comparison to theory and experiments.
Driving Innovation in Fire Sprinkler Sprays
Andre W. Marshall - Associate Professor
Director, Fire Testing and Evaluation Center
University of Maryland, MD
Industry University Cooperative Research Centers (IUCRC) | Innovation Corps (I-Corps™)
Division of Industrial Innovation and Partnerships (IIP)
Directorate for Engineering (ENG)
National Science Foundation
2415 Eisenhower Avenue
Alexandria, VA 22314, USA
While advancements abound in many spray technology verticals (e.g. propulsion, coatings, and medical applications), the fire sprinkler spray has received relatively little fundamental research attention. Yet fire sprinkler sprays present interesting engineering challenges. There is no doubt that meeting these challenges would have compelling impact on life safety. Further, the fire suppression system can drive building design and utility in unexpected ways potentially resulting in outsized impact on building economies. From a device design perspective, the essential fire sprinkler is certainly robust, and in some ways elegant. These devices create large-scale, three-dimensional, high stokes number sprays through mechanical impingement. Upon activation, fire sprinkler systems offer protection by wetting fuel not yet involved in the fire limiting its spread. These sprays are designed for penetration, coverage, and uniformity even while the detailed spray features are poorly understood. This persistent knowledge gap has resulted in a highly empirical design process that remains largely a matter of cut and try. The basic research investments that provide foundational insights and enable disruptive technologies lag behind other spray technology verticals. Nevertheless, a body of basic research is forming that honors this quintessential high-stakes engineering problem pitting water against fire. Highlights of recent measurement and modeling advancements will be presented along with their novel tightly integrated analytical frameworks. These advancements have created new opportunities for the research, development, and design of fire sprinkler systems.