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What are the effects of density, size, geometry and surface area on settling rates of small particles in a gaseous medium?
Why does dust settle yet smoke rises?
Smoke and dust, smoke always seem's to rise, a denser substance than air that should be less buoyant than air ...
Gravity causes hot air to rise because it's less dense than cold air, and gravity causes smoke to fall because it's more dense than air. If the hot air is rising faster than smoke particles fall, then the net direction of travel of the smoke is upwards.The same applies to clouds, water droplets are heavier than air, but they're carried up by rising currents of warm air. The ability of the air to carry particles depends on wind resistance, which depends on cross sectional area, but as the mass goes up with the cube of the size and area by the square, larger particles are less able to be carried by the air than small ones. As the water droplets in a cloud coalesce and grow larger, the air becomes unable to support them and they fall as rain. Similarly, large dust particles will settle out of suspension quicker than small smoke particles.
QuoteWhy does dust settle yet smoke rises?Brownian motion causes small particles to be suspended in the air longer than large particles, all other things being the same.- Combustion is a continual new source of very fine particles- Very fine particles of dust blew away long ago, leaving only the coarser grains behind. One thing that is different between smoke and dust is Density:- Dust is picked up from the ground by moving air that is at air temperature. - Air+dust is denser than air, and will start to settle out immediately- Smoke is picked up from a fire by hot gases of combustion, at well above air temperature- Smoke+combustion gases is lighter than air, and will not start to settle out until the temperature drops to around air temperature.- If the air is not moving fast enough, it doesn't pick up any dust.- If the air is not moving, smoke still rises.