Thermal

1
Thermal Radiometric Forces
Reading Chap. 8

• Thermophoresis Particle motion in a temperature
  gradient, from a hotter to a colder region

Molecular impacts on a particle (dp lt ?) in a
temperature gradient
Q What if the cylinder is colder than the
environment?
2
Free Molecular Regime

• Continuum Regime
• Thermal force on a particle
• Velocity

• Thermal force on a particle
• Velocity

Q What if Kn ltlt 1? Q What if kpgtgtka? ka
kp?
3
Terminal Settling Thermophoretic Velocity in a
Unit Temperature Gradient _at_ 20 oC
Cumulative Deposition During 100 s by Diffusion
Thermophoresis from Unit Aerosol Concentration
in a Unit Temperature Gradient _at_ 20 oC (kp/ka10)
4
Applications

• Thermal Precipitator
• Problem Particle deposition in a heat exchanger
  reduces the heat transfer.

Q Advantages? Can diffusion achieve the same?
5
Other Radiometric Forces

• Photophoresis Particle motion under the
  influence of asymmetric light absorption within a
  particle
• Radiation Pressure Pressure results from a
  direct momentum transfer by the deflection and
  absorption of the light (Example?)
• Diffusiophoresis Particle motion under the
  influence of a concentration gradient of
  surrounding gas molecules

6
Reflection