The importance of the Reynolds number (Re)
How do we achieve the goal of increase and decrease? In theory the flow pattern depends on four variables:
- Diameter of the tube
- Speed of the fluid
- Density of the fluid
- Dynamic viscosity of the fluid
The factors combined provide the so called
Reynolds number (Re), an important parameter that describes whether flow conditions lead to
laminar flow or
turbulent flow. In general it can be said that a
laminar flow occurs at a low Reynolds number (≤ ca. 2300) and a
turbulent flow occurs at a high Reynolds number (≥ ca. 3000). In between these two numbers (Re 2300-3000) you have a
‘transitional flow’, meaning the flow can be laminar or turbulent (numbers mentioned are for a cylindrical tube).
When can turbulence effect occur?
As mentioned before, turbulence effect is a common effect which can occur in installations using (too many) restrictions, such as valves or adapters, in combination with a high velocity of the used fluid. In every restriction, the flow has been disrupted and the speed of the gas will change. Besides the usage of restrictions, the pipe length is something to take into account. As it takes some time for a turbulent flow to get laminar again, it is important to use the right pipe length.
A turbulent flow is something you would like to prevent at the inlet of your flow measurement instrument, as it can affect the accuracy of your measurement. It is preferable to have a laminar flow just before your flow instrument. However, the instrument itself used as flow controller, with a valve behind the meter, can cause a turbulent flow again.
Not all kinds of flow meters experience this as disadvantageous. Mainly
thermal flow meters using the bypass principle are sensitive for this effect. Flow meters based on the
Coriolis,
CTA (Constant Temperature Anemometry) or
Ultrasonic principle are independent of turbulence.