|
|
Reynolds Number
An introduction and definition of the
dimensionless Reynolds Number - with online calculators
Sponsored Links
The Reynolds Number, the non-dimensional velocity, is defined by the ratio of
·
dynamic pressure (ρ u2)
and
·
shearing stress (μ u / L)
and
can be expressed as
Re = (ρ
u2) / (μ u / L)
=
ρ u L / μ
= u L / ν (1)
where
Re =
Reynolds Number (non-dimensional)
ρ = density (kg/m3, lbm/ft3 )
u = velocity based on the actual cross section area of
the duct or pipe (m/s, ft/s)
μ = dynamic viscosity (Ns/m2, lbm/s ft)
L = characteristic length (m, ft)
ν = kinematic viscosity (m2/s, ft2/s)
Reynolds Number for a Pipe or Duct
For a pipe or duct the characteristic length is the hydraulic diameter. The Reynolds Number for a duct or pipe can be expressed as
Re = ρ
u dh /
μ
= u dh / ν (2)
where
dh = hydraulic diameter (m, ft)
Reynolds Number for a Pipe or Duct in common Imperial
Units
The Reynolds number for a pipe or duct can also be expressed
in common Imperial units like
Re = 7745.8 u dh / ν (2a)
where
Re =
Reynolds Number (non dimensional)
u = velocity (ft/s)
dh = hydraulic diameter (in)
ν = kinematic viscosity (cSt) (1 cSt = 10-6 m2/s
)
The Reynolds Number can be used to determine if flow
is laminar, transient or turbulent. The flow is
·
laminar when Re
< 2300
·
transient when 2300
< Re < 4000
·
turbulent when Re
> 4000
Example -
Calculating Reynolds Number
A Newtonian fluid with a dynamic or absolute
viscosity of 0.38 Ns/m2 and a specific gravity of 0.91 flows through a 25 mm diameter pipe with a velocity of 2.6 m/s.
The density can be calculated using the specific gravity like
ρ = 0.91 1000 (kg/m3)
= 910 kg/m3
The Reynolds Number can then be calculated using
equation (1) like
Re = 910 (kg/m3) 2.6 (m/s) 25 (mm) 10-3 (m/mm)
/ 0.38 (Ns/m2)
=
156 ((kg m / s2)/N)
= 156 ~ Laminar flow
1 (N) = 1 (kg m / s2)
Related Mobile Apps from The Engineering ToolBox 
·
Reynolds Number - Calculator App
- free apps for offline use on mobile devices.
Online Reynolds Calculator
The calculator below can be used if the density and the absolute (dynamic) viscosity of a fluid is known. The calculator is valid
for incompressible flow - flow with fluids or gases without compression - as
typical for air flow in HVAC systems or similar.
The default values are based on an air at 60 oF, 2 atm and density 0.146 lbm/ft3,
flowing 20 ft/s between two metal sheets with
characteristic length 0.5 ft.
Dynamic
(absolute) viscosity is 1.22
10-5 lbm/s
ft.
The calculator below can be used when the kinematic viscosity of a fluid is known. The default values are for water at60oC with a kinematic viscosity of 1.13 10-6 m2/s in a schedule 40 steel pipe. The characteristic length (hydraulic diameter) of the pipe is 0.102 m.
Sponsored Links
Search the Engineering ToolBox
Custom Search
Related
Topics
·
Fluid Flow and Pressure Drop - Pipe lines - fluid flow and pressure loss - water, sewer, steel pipes,
pvc pipes, copper tubes and more
·
Fluid Mechanics - The study of fluids - liquids and gases. Involves various properties
of the fluid, such as velocity, pressure, density and temperature, as functions
of space and time.
·
Piping Systems - Dimensions of pipes and tubes - dimensions, materials, capacities -
pressure drop calculations and charts - insulation and heat loss diagrams
·
Water Systems - Hot and cold water systems -
design properties, capacities, sizing and more
Related
Documents
·
Colebrook Equation - Calculate friction loss coefficients in pipes, tubes and ducts
·
Darcy-Weisbach Equation for Pressure and Head Loss - The Darcy-Weisbach formula can be used to calculate pressure or head
loss due to friction in ducts, pipes and tubes
·
Dynamic, Absolute and Kinematic Viscosity - An introduction to dynamic, absolute and kinematic viscosity and how
to convert between CentiStokes
(cSt), CentiPoises (cP), Saybolt Universal Seconds (SSU), degree Engler and
more
·
Entrance Length and Developed Flow - The entrance length is the length in a tube or duct until the flow velocity
profile is fully developed after an obstruction
·
Fluids - Kinematic Viscosities - Kinematic viscosities of some common fluids - motor oil, diesel fuel,
peanut oil and many more
·
Friction Coefficient at Laminar Flow - Laminar flow and friction coefficients
·
Hydraulic Diameter - Hydraulic diameter of ducts and tubes
·
Laminar, Transitional or Turbulent Flow - When calculating heat transfer or pressure and head loss it is
important to know if the fluid flow is laminar, transitional or turbulent
·
Major loss in Ducts, Tubes and Pipes - Major loss - head loss or pressure loss - due to friction in ducts,
pipes and tubes
·
Moody Diagram - Moody Diagram to estimate friction
coefficients
·
Water - Absolute or Dynamic Viscosity - Absolute, or dynamic viscosity of water in centipoises for
temperatures between 32 - 200oF
·
Water Flow in Tubes - Reynolds Number - Reynolds Number in pipes transporting clean cold water
Engineering ToolBox - SketchUp Edition -
Online 3D modeling!
Engineering ToolBox -
SketchUp Edition - enabled for use with the amazing,
fun and free Google SketchUp
Translate the ToolBox
Arabic - Chinese (Simplified) - Chinese (Traditional) - Dutch - French - German - Italian - Japanese - Korean -Portuguese - Russian - Spanish - - Select Your own language . .