ME437/537 G. Ahmadi 1 ME 437/ME 537 PARTICLE TRANSPORT, DEPOSITION AND REMOVAL Goodarz Ahmadi Department of Mechanical and Aeronautical Engineering Clarkson University Potsdam, NY 13699-5725ME437/537 G. Ahmadi 2 Air pollution and smog. Particle trajectories in a hot gas filtration vessel.ME437/537 G. Ahmadi 3 Sample glass fiber particles.ME437/537 G. Ahmadi 4 Dust storm over the red sea.ME437/537 G. Ahmadi 5 Birth place of stars.ME437/537 G. Ahmadi 6INTRODUCTION TO AEROSOLS Definition: Aerosol is a suspension of solid or liquid particles in a gas. Dust, smoke, mists, fog, haze, and smog are various forms of common aerosols. Aerosol particles are found in different shapes (isometrics, platelets, and fibers) and different sizes. For irregular shaped particles, different equivalent diameters are defined. Examples of equivalent diameters are: • Equivalent area diameter, • Feret’s diameter (maximum distance edge to edge); • Stoke’s diameter (diameter of a sphere with the same density and the same velocity as the particle); • Aerodynamic diameter (diameter of a sphere with the density of water and the same velocity as the particle). The range of diameters of common aerosol particles is between 0.01 and 100 µm. The lower limit of 10 nm roughly corresponds to the transition from molecule to particle. Particles larger than 100 µm normally do not remain suspended in air for a sufficient amount of time. Noting that the mean free path for air is about 0.07 µm and visible light has a wavelength band of 0.4 – 0.7 µm, the mechanical and optical behaviors of particles are significantly affected by their size. Particles greater than 5 – 10 µm are usually removed by the upper respiratory system. But particles smaller than 5 µm can penetrate deep into the lung and become a health hazard. Typical ranges of values for aerosol parameters for aerosols are listed in Table 1. The corresponding values for air (N2) are also shown in this table for comparison. Table 1 - Parameters of Aerosol in the Atmosphere Aerosols Air Number Density (Number/cm3) 100-105 1019 Mean Temperature (K) 240 – 310 240 – 310 Mean Free Path Greater than 1m 0.06 µm Particle Radius 0.01 – 10 µm 2 ×104− µm Particle Mass (g) 1018− - 109− 4.6 × 1023− Particle Charge (in Elementary Charge Units) 0 – 100 Weakly Ionized Single Charge The important relevant dimensionless groups relevant the motion of aerosols are listed in Table 2.ME437/537 G. Ahmadi 7 Table 2 – Dimensionless Groups Knudsen Number d2Knλ= Mach Number fc||Mfpvv −= Schmidt Number 4dnDSc2fλ=ν= Brown Number |'v||'v|)vv(Brfp2/12,f2,p== Reynolds Number nKM4d||Re =ν−=fpvv Here the following symbols are defined: λ = Mean Free Path ν = Kinematic Viscosity d = Particle Diameter D = Diffusivity vp = Particle Velocity v’ = Thermal Velocity vf = Fluid (Air) Velocity n = Number Density cf = Speed of Sound Here superscript "f" corresponds to fluid and superscript "p" denotes particle. In these equations the root mean square fluctuation velocity is given by 2/1ff)m/kT8(|'v| π= and λ=νfc5.0 The mean free path of the gas is given as Pd2kTnd212m2mπ=π=λ Here n is the gas number density, mdis the gas molecule (collisional) diameter, -23101.38k ×= J/K is the Boltzmann constant, P is pressure, and T is temperature. For air, nm361.0dm= and PT1.23)m( =µλ , P is in Pa, and T is K.ME437/537 G. Ahmadi 8Table 3. Aerosol Characteristics 410− 310− 210− 110− 010 110 210 310 410 Electromagnetic x-Ray UV Vis Infrared Microwaves Wave Definition Solid Fume Dust Liquid Mist Spray Soil Clay Silt Sand Gravel Atmospheric Smog Cloud/Fog Mist Rain Typical Viruses Bacteria Human Hair Particles Smoke Coal Dust Beach Sand Size Analysis Microscope Method Electron Microscope Sieving x-Ray Diffraction Ultra Centrifuge Sedimentation Gas Cleaning Ultrasonics Settling Chamber Centrifugal Liquid Scrubber Air Filter HE Air Filter Impact Separators Thermal Separators Electrostatic Separators Diffusion Air 2105−× 4105−× 510− 7103−× 9102−× 10102−× 11102−× Coeff. s/cm2 Water 6105−× 7105−× 8105−× 9105−× 10105−× 11105−× 12105−× 13105−× Terminal (S=2) Air 610− 5105.1−× 4102−× 3107−× 6.0 50 600 3105.2 × Velocity s/cm Water 1010− 9106−× 7106−× 5106−× 3106−× 6.0 12 58 410− 310− 210− 110− 010 110 210 310 410 Particle Diameter, µm Particle Diameter,