NOTE: It is true that
flow is typically considered "laminar" when the Reynolds
Number is less than 2,300.
But.....
for the proposes of the
design of separation equipment that utilize inclined
plates, a series of baffles or separation media of any
kind, a Reynolds Number of less than 500 is the industry
norm (less than 300 preferred).
The Reynolds Number is a non dimensional parameter
defined by the ratio of
- dynamic pressure (ρ u2) and
- shearing stress (μ u / L)
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
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 (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)
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)
Online Reynolds Calculator
Below are two links to online Reynolds Number
Calculators.
Reynolds Number Calculator
Reynolds Number Calculator
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