Water hydraulics
Reynolds number
- where
- Reynolds number
- fluid density (kg/m³)
- mean velocity (m/s)
- internal pipe diameter (m; any unit you pick is converted)
- dynamic viscosity (Pa·s; any unit you pick is converted)
Explain this equation
Works out the Reynolds number, which tells you whether flow in a pipe is smooth and layered (laminar) or mixed and turbulent. It is needed before choosing a friction factor.
ρ is fluid density and v the mean velocity, together representing the momentum of the flow. d is the internal bore. μ is dynamic viscosity, the fluid's resistance to shearing, which damps out disturbances.
Re is the ratio of momentum to viscous forces. When momentum wins, eddies grow and flow becomes turbulent. It has no units because they all cancel.
Sense check: water at 1 m/s in a 20 mm bore with μ 1 mPa·s gives Re 20,000, well turbulent. Below about 2300 in a round pipe flow is laminar. Warm water has lower viscosity, so Re rises.
Watch out: viscosity changes greatly with temperature and glycol content. Use the right value.
Enter your values to see the result.
Good to know
Fluid properties change with temperature and glycol concentration. Circular-pipe laminar/transition/turbulent behaviour requires appropriate interpretation.
Sources
Results are arithmetic only. Worked examples are not recommended design settings. This library does not provide automatic gas pipe selection, live gas tightness tests, purging procedures, combustion acceptance limits, installation approval or commissioning certification. Manufacturer data and the applicable current method remain essential.