ν = μ / ρ = kinematic viscosity (m 2 /s, ft 2 /s)
For a pipe or duct the characteristic length is the hydraulic diameter .
L = d h
d h = hydraulic diameter (m, ft)
The Reynolds Number for the flow in a duct or pipe can with the hydraulic diameter be expressed as
Re = ρ u d h / μ
= u d h / ν (2)
d h = hydraulic diameter (m, ft)
The Reynolds number for a pipe or duct expressed in Imperial units
Re = 7745.8 u d h / ν (2a)
Re = Reynolds Number (non dimensional)
u = velocity (ft/s)
d h = hydraulic diameter (in)
ν = kinematic viscosity (cSt) (1 cSt = 10 -6 m 2 /s )
The Reynolds Number can be used to determine if flow is laminar, transient or turbulent. The flow is
In practice laminar flow is only actual for viscous fluids - like crude oil, fuel oil and other oils.
A Newtonian fluid with a dynamic or absolute viscosity of 0.38 Ns/m 2 and a specific gravity of 0.91 flows through a 25 mm diameter pipe with a velocity of 2.6 m/s .
Density can be calculated from the specific gravity of the fluid and the density of the specific gravity reference water 1000 kg/m 3 - as
ρ = 0.91 (1000 kg/m 3 )
= 910 kg/m 3
Reynolds Number can then be calculated using equation (1) like
Re = (910 kg/m 3 ) (2.6 m/s) (25 mm) (10 -3 m/mm) / (0.38 Ns/m 2 )
= 156 ((kg m / s 2 )/N)
= 156 ~ Laminar flow
1 (N) = 1 (kg m / s 2 )
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This calculator can be used if density and absolute (dynamic) viscosity of the fluid is known. The calculator is valid for incompressible flow - flow with fluids or gases without compression - as typical for air flows in HVAC systems or similar. The calculator is generic and can be used for metric and imperial units as long as the use of units are consistent.
Default values are for air at 60 o F , 2 atm pressure and density 0.146 lb m /ft 3 , flowing 20 ft/s between two metal sheets with characteristic length 0.5 ft . Dynamic (absolute) viscosity is 1.22 10 -5 lb m /s ft .
The calculator below can be used when kinematic viscosity of the fluid is known. The calculator is generic and can be used for metric and imperial units as long as the use of units are consistent.
Default values are for water at 20 o C with kinematic viscosity 1.004 10 -6 m 2 /s in a schedule 40 steel pipe . The characteristic length (or hydraulic diameter) of the pipe is 0.102 m .
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The study of fluids - liquids and gases. Involving velocity, pressure, density and temperature as functions of space and time.
Dimensions of pipes and tubes, materials and capacities, pressure drop calculations and charts, insulation and heat loss diagrams.
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Online calculators, figures and tables with dynamic and kinematic viscosity of liquid and gaseous butane, C4H10, at varying temperarure and pressure, SI and Imperial units.
Friction loss coefficients in pipes, tubes and ducts.
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Calculate pressure loss - or head loss - in ducts, pipes or tubes.
Online calculator, figures and tables showing dynamic and kinematic viscosity of ethylene, C2H4, also called ethene or acetene, at varying temperature and pressure - Imperial and SI Units.
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