Dense spray evaporation as a mixing process

Abstract

We explore the processes by which a dense set of small liquid droplets (a spray) evaporates in a dry, stirred gas phase. A dense spray of micron-sized liquid (water or ethanol) droplets is formed in air by a pneumatic atomizer in a closed chamber. The spray is conveyed in ambient air as a plume whose extension depends on the relative humidity of the diluting medium. Standard shear instabilities develop at the plume edge, forming the stretched lamellar structures familiar with passive scalars. Unlike passive scalars however, these lamellae vanish in a finite time, because individual droplets evaporate at their border in contact with the dry environment. Experiments demonstrate that the lifetime of an individual droplet embedded in a lamellae is much larger than expected from the usual d 2 law describing the fate of a single drop evaporating in a quiescent environment. By analogy with the way mixing times are understood from the convection-diffusion equation for passive scalars, we show that the lifetime of a spray lamellae stretched at a constant rate γ is t v = 1 γ ln(1+φ φ), where φ is a parameter that incorporates the thermodynamic and diffusional properties of the vapor in the diluting phase. The case of time-dependent stretching rates is examined too. A dense spray behaves almost as a (nonconserved) passive scalar.

Host-to-Host Airborne Transmission As a Multiphase Flow Problem For Science-Based Social Distance Guidelines

“…is the Spalding mass number, where Y d is the mass fraction of water vapor at the droplet surface and Y p is the mass fraction of water vapor in the surrounding puff. Under the assumption that N u and B m are nearly constant for small droplets, the above equation can be integrated [24] to obtain the following law (mapping) for the evolution of the droplet:…”

Interaction of droplet dispersion and evaporation in a polydispersed spray

“…Therefore, given our past experience with various types of experiments with mono-dispersed droplet generators and after careful consideration, we decided to use a commercial atomizer which produces a polydispersed droplet size distribution that is typical to injectors used in several combustion applications, and can be quantified without evaporation, and we use this as a reference to evaluate the evolution of the evaporation rate in the spray. However, it is also noted that, earlier, droplet evaporation experiments in sprays with nearly mono-sized droplets (around 10 µm) have been reported (for instance, see Rivas & Villermaux 2016). Use of such sprays has some advantages, since it allows for well-defined boundary conditions for droplet sizes to assist comparison with spray simulations.…”

“…the regression rate of the droplet surface may vary nonlinearly with time. For instance, in their experiments studying the evaporation and mixing processes in a dense spray plume, Rivas & Villermaux (2016) and Villermaux et al (2017) demonstrated that the lifetime of an individual droplet embedded in a cloud of droplets is much larger than predicted by the conventional d 2 -law for a single drop evaporating in a quiescent environment. In a turbulent spray, due to the wide range of the droplet size distribution in sprays, different dynamic behaviour of droplet dispersion and interaction with the surrounding gas leads to formation of instantaneous clusters of droplets in sprays (Zimmer et al 2003;Lian, Charalampous & Hardalupas 2013).…”

Особенности Испарения Капли При Лучистом И Конвективном Нагреве

“…Закономерности испарения капель играют важную роль при решении ряда прикладных задач для теплотехнических систем и технологических процессов [1,2]. Различным аспектам нагрева и испарения отдельных капель и капельных систем посвящены многочисленные публикации, связанные с численным моделированием [3][4][5][6][7] и экспериментальными исследованиями [8][9][10][11][12][13][14] процесса испарения. Большинство экспериментальных результатов исследования скорости испарения капель жидкости получено при их нагреве в условиях кондуктивного, конвективного или комбинированного радиационноконвективного теплообмена [2,8,14].…”

“…Большинство экспериментальных результатов исследования скорости испарения капель жидкости получено при их нагреве в условиях кондуктивного, конвективного или комбинированного радиационноконвективного теплообмена [2,8,14]. Скорость испарения капель определялась при их нагреве на твердой подложке [11], в условиях обдува неподвижной капли высокотемпературным потоком воздуха [2,[8][9][10], при движении капель через газообразные продукты сгорания [2,[12][13][14] или через полый цилиндрический нагреватель [2,12].…”