Thermal Radiometric Forces Reading Chap 8 Thermophoresis Particle

Thermal & Radiometric Forces Reading: Chap. 8 • Thermophoresis: Particle motion in a temperature gradient, from a hotter to a colder region Hot T 1, c 1 dp Cold T 2, c 2 Molecular impacts on a particle (dp < ) in a temperature gradient Q: What if the cylinder is colder than the environment? 2020/12/1 Aerosol & Particulate Research Lab 1

Free Molecular Regime • Thermal force on a particle Continuum Regime · Thermal force on a particle • Velocity · Velocity Q: What if Kn << 1? Q: What if kp>>ka? ka ~ kp? 2020/12/1 Aerosol & Particulate Research Lab 2

Terminal Settling & Thermophoretic Velocity in a Unit Temperature Gradient @ 20 o. C Cumulative Deposition During 100 s by Diffusion & Thermophoresis from Unit Aerosol Concentration in a Unit Temperature Gradient @ 20 o. C (kp/ka=10) 2020/12/1 Aerosol & Particulate Research Lab 3

Applications • Thermal Precipitator Air flow Glass cover slip Heated Wire Brass heat sink @ room temperature Deposition zone Q: Advantages? Can diffusion achieve the same? • Problem: Particle deposition in a heat exchanger reduces the heat transfer. 2020/12/1 Aerosol & Particulate Research Lab 4

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 Dry air Moist air ? 2020/12/1 Aerosol & Particulate Research Lab 5

Reflection 2020/12/1 Aerosol & Particulate Research Lab 6