https://www.jpcomplex.ir/Content/media/image/2013/08/772_orig.pdf

Electrochemical technologies in wastewater treatment

Discrete particle simulation of particulate systems: Theoretical developments

The paper provides an overview of the challenges and progress associated with the task of numerically predicting particle-laden turbulent flows The review covers the mathematical methods based on turbulence closure models as well as direct numerical simulation (DNS) In addition, the statistical (pdf) approach in deriving the dispersed-phase transport equations is discussed The review is restricted to incompressible, isothermal flows without phase change or particle-particle collision Suggestions are made for improving closure modelling of some important correlations

On predicting particle-laden turbulent flows

Discrete particle simulation of particulate systems: A review of major applications and findings

UNLABELLED A large number of infectious diseases are believed to be transmitted between people via large droplets and by airborne routes. An understanding of evaporation and dispersion of droplets and droplet nuclei is not only significant for developing effective engineering control methods for infectious diseases but also for exploring the basic transmission mechanisms of the infectious diseases. How far droplets can move is related to how far droplet-borne diseases can transmit. A simple physical model is developed and used here to investigate the evaporation and movement of droplets expelled during respiratory activities; in particular, the well-known Wells evaporation-falling curve of droplets is revisited considering the effect of relative humidity, air speed, and respiratory jets. Our simple model considers the movement of exhaled air, as well as the evaporation and movement of a single droplet. Exhaled air is treated as a steady-state non-isothermal (warm) jet horizontally issuing into stagnant surrounding air. A droplet is assumed to evaporate and move in this non-isothermal jet. Calculations are performed for both pure water droplets and droplets of sodium chloride (physiological saline) solution (0.9% w/v). We calculate the droplet lifetimes and how droplet size changes, as well as how far the droplets travel in different relative humidities. Our results indicate that a droplet's size predominately dictates its evaporation and movement after being expelled. The sizes of the largest droplets that would totally evaporate before falling 2 m away are determined under different conditions. The maximum horizontal distances that droplets can reach during different respiratory activities are also obtained. Our study is useful for developing effective prevention measures for controlling infectious diseases in hospitals and in the community at large. PRACTICAL IMPLICATIONS Our study reveals that for respiratory exhalation flows, the sizes of the largest droplets that would totally evaporate before falling 2 m away are between 60 and 100 microm, and these expelled large droplets are carried more than 6 m away by exhaled air at a velocity of 50 m/s (sneezing), more than 2 m away at a velocity of 10 m/s (coughing) and less than 1 m away at a velocity of 1 m/s (breathing). These findings are useful for developing effective engineering control methods for infectious diseases, and also for exploring the basic transmission mechanisms of the infectious diseases. There is a need to examine the air distribution systems in hospital wards for controlling both airborne and droplet-borne transmitted diseases.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1600-0668.2007.00469.x

How far droplets can move in indoor environments--revisiting the Wells evaporation-falling curve.

The turbulent diffusion mechanism of particles in a round air jet are studied both theoretically and experimentally, with particular attention to the relative velocity between particles and fluid, overshooting effect of particles, and the distributions of fluid properties in space. The results indicate that the particle diffusivity decreases with the increase of the particle inertia. In general, the turbulent diffusivity of particles in an air jet is smaller than that of fluid scalar quantities. The particle inertia and the fluid large eddies, which are expressed by the Stokes number and the integral scale, respectively, play an important role in the transport mechanism of particles.

Particle turbulent diffusion in a dust laden round jet

Numerical simulation of air and particle motions in bubbling fluidized bed of small particles

The reduction of pressure drop due to dust loading in a conventional cyclone

A theoretical equation is derived for the collision rate of aerosol particles in a homogeneous and isotropic turbulent system. This equation takes into account the relative velocity between fluid and particles. The calculated results indicate that the relative velocity between fluid and particles is the main factor in the turbulent coagulation (agglomeration, coalescence) of unequally sized particles in an air flow. This hold true, even when the particle sizes are less than 1 micron. For particles of equal radii the coagulation coefficient reaches its minimum value, because the effect of motion relative to the fluid now becomes zero and only the spatial variation of turbulent motion remains to cause collisions between the particles. For particles following a fluid motion completely, as in a water stream, the equation for the collision rate reduces to the Saffman and Turner equation.

Shinichi Yuu

Papers

Particle turbulent diffusion in a dust laden round jet

Numerical simulation of air and particle motions in bubbling fluidized bed of small particles

The reduction of pressure drop due to dust loading in a conventional cyclone

Collision rate of small particles in a homogeneous and isotropic turbulence

Numerical simulation of the high Reynolds number slit nozzle gas–particle jet using subgrid-scale coupling large eddy simulation