What is Stokes' law?
Stokes' law describes the drag force acting on spherical particles moving through a viscous fluid at low Reynolds numbers. It's fundamental for calculating terminal settling velocity of particles in air or liquid. See also our Vapor Fraction — Flash Calculation.
How do I calculate particle settling velocity?
Use the Stokes' law formula: v = gd²(ρp - ρm)/(18μ), where g is gravity, d is particle diameter, ρp is particle density, ρm is medium density, and μ is dynamic viscosity.
What is terminal velocity?
Terminal velocity is the maximum constant speed a particle reaches when falling through a fluid. At this point, the gravitational force equals the drag force, resulting in zero acceleration.
When is Stokes' law valid?
Stokes' law is valid for spherical particles at low Reynolds numbers (Re < 1), in the laminar flow regime. For higher Reynolds numbers, other drag equations should be used. You might also find our Required Time — Batch Reactor useful.
What factors affect particle settling velocity?
Settling velocity depends on particle size, particle density, fluid density, fluid viscosity, and gravitational acceleration. Larger, denser particles settle faster in less viscous fluids.
How does temperature affect settling velocity?
Temperature affects fluid density and viscosity. Higher temperatures typically decrease viscosity and density, which can increase settling velocity for particles in gases and most liquids.
What are typical values for air properties?
At 25°C, air density is approximately 1.184 kg/m³ and dynamic viscosity is 1.849×10⁻⁵ Pa·s. At standard temperature and pressure (STP), air density is 1.292 kg/m³.