Prof. Jiakuan Yang Huazhong University of Science and Technology Air Pollution Control Engineering

Lecture 8 Lecture 8 The nature of Particulate Pollutants Particulate pollutants (the most common air pollutants) Volatile Organic Compounds (VOCs) Sulfur Oxides Nitrogen Oxides The four major air pollutants

Lecture 8 Lecture 8 The nature of Particulate Pollutants Nonhomogeneous pollutants Particulate pollutants are not chemically uniform, have different size, shape, and chemical compositions.

Lecture 8 Lecture 8 The nature of Particulate Pollutants Ⅰ Diameter of particles Ⅱ Primary and secondary particles Ⅲ Forces acting on particles Ⅳ Terminal gravitational settling velocity Ⅴ Behavior of Particles in the Atmosphere Ⅵ Summary

Ⅳ Terminal gravitational settling velocity (1) Forces acting on particles in a fluid Assuming a single particle, no the electrostatic and Van der Waals. Forces between the particle and other particles can be ignored. buoyancy gravity

Ⅳ Terminal gravitational settling velocity buoyancy weight Drag force a (1) Forces acting on particles in a fluid Particle falls down, getting an acceleration v : v 0 Drag force takes place. Initially, v = 0

Ⅳ Terminal gravitational settling velocity (1) Forces acting on particles in a fluid buoyancy gravity Drag force a μ ---- the viscosity of the fluid According to Newton's law for viscosity

Ⅳ Terminal gravitational settling velocity (1) Forces acting on particles in a fluid The drag force increases as the velocity increases until it equals the gravity minus the buoyancy. At this time, the velocity is terminal setting velocity, the sum of the forces acting is zero, so the particle continues to move at a constant velocity.

Ⅳ Terminal gravitational settling velocity (1) Forces acting on particles in a fluid Why drag force is not considered in the air parcel model?

Ⅳ Terminal gravitational settling velocity (1) Forces acting on particles in a fluid Stokes’ law So ρ part can be ignored, buoyant 0

(2)the assumptions of stokes law ① The fluid is continuous. ④ the terms involving velocities squared are negligible. ③ Newton’s law of viscosity holds ② The flow is laminar. Ⅳ Terminal gravitational settling velocity

(3) Why are we interested in settling velocity? Ⅳ Terminal gravitational settling velocity What is the relationship between v t and diameter of particles? What is the relationship between v t and air pollution?

(3) Why are we interested in settling velocity? 1000 μ V t = 600 cm/s 1 μ V t = cm/s 10 μ V t = 0.6 cm/s P210, Figure 8.1

(3) Why are we interested in settling velocity? VtVt u = Vertical wind velocity For smaller particles, u > V t Suspending in the atmosphere for a long time. For larger particles, u < V t Particles will settle to the ground, not causing air pollution.

(4) Particles too large for Stokes’ Law Ⅳ Terminal gravitational settling velocity ① the flow of fluid around the sphere becomes turbulent. ② velocities squared ≠0 Then the principal assumptions of stokes’ law become inapplicable.

(4) Particles too large for Stokes’ Law Ⅳ Terminal gravitational settling velocity 雷诺数 Reynolds number Reynolds number for a particle is a dimensionless ratio of the inertial force acting on a mass of fluid to the viscous forces acting on the same mass of fluid in the same flow.

(4) Particles too large for Stokes’ Law Ⅳ Terminal gravitational settling velocity Reynolds number stokes’ law works satisfactorily. When, When, flow becomes turbulent. Laminar flow

(4) Particles too large for Stokes’ Law Ⅳ Terminal gravitational settling velocity Drag coefficient 阻力系数 A p = surface area of particle.

(4) Particles too large for Stokes’ Law Ⅳ Terminal gravitational settling velocity Drag coefficient

(4) Particles too large for Stokes’ Law At the terminal settling velocity, Ⅳ Terminal gravitational settling velocity Settling velocity

(4) Particles too large for Stokes’ Law Ⅳ Terminal gravitational settling velocity Settling velocity when R p <0.3, Stokes’ law

(4) Particles too large for Stokes’ Law Ⅳ Terminal gravitational settling velocity Settling velocity When 0.3≤R p ≤1000

(5) Particles too Small for Stokes’ Law Ⅳ Terminal gravitational settling velocity Continuous medium assumption becomes inaccurate.  Real gases, liquids, and solids are not truly continuous but are made up of atoms and molecules.  When a particle becomes as small as or smaller than the average distance between molecules, then its interaction with molecules changes.

(5) Particles too Small for Stokes’ Law Continuous medium assumption becomes inaccurate. λ = mean free path of gas molecule, ≈0.07μm A = an experimentally determined constant, Aλ/D Cunningham correction factor

(5) Particles too Small for Stokes’ Law Ⅳ Terminal gravitational settling velocity

(5) Particles too Small for Stokes’ Law Ⅳ Terminal gravitational settling velocity Page 227,Table 8.1 Calculated applicability range of Stokes’law, alone, and with simple form of the Cunningham correction factor. Formula Permission error 10%1% Stokes’ law Stokes’ law with C 16 < d < 30 μ 1.6 < D < 70 μ 0.36 < d <30 μ 0.1 < D < 70 μ

(6) Stokes’ Stopping Distance Ⅳ Terminal gravitational settling velocity How far do the particles travel before it is stopped by viscous friction. V0V0 FdFd Horizontal motion

(6) Stokes’ Stopping Distance Ⅳ Terminal gravitational settling velocity inertial force viscous forces

(7) Aerodynamic Particle Diameter Stokes’ law Stokes’ law with Cunningham correction factor Stokes’ Stopping Distance

(7) Aerodynamic Particle Diameter Microns, aerodynamic For example, Page226

(8) Diffusion of Particles Ⅳ Terminal gravitational settling velocity =Diffusivity, m 2 /s k= Boltzmann constant C= Cunningham correction factor

Ⅴ Behavior of Particles in the Atmosphere Particle diameter, μ D(surface)/d(diameter) Hot vapor condensation agglomeration Chemical conversion Rainout and washout Mechanical generation Gravity settling

Ⅴ Behavior of Particles in the Atmosphere Production process Size range 0.05~0.1 μ 0.1~1 μ 2~100 μ condensation Chemical conversion Mechanical generation types secondary primary state Liquid or tar secondary Liquid or tar solid Removal mechanism agglomeration Rainout and washout Gravity settling example Tobacco smoke Exhaust gases Coal dust

Ⅵ Summary Page 242

This Lecture and the Next Lecture  This Lecture:  Chapter 8 Page 209~227 Page 239~243  The Next Lecture:  Chapter 9 page 249~328

Exerciser Page Page

DISCUSSION Topics about Air pollution:  Introducing yourself  Interesting news or information about air pollution your having read or heard  Your opinions on this Air Pollution course  Your suggestions for Chinese Air Pollution  Other familiar issues about Air Pollution