Flow Meters: Laminar Flow vs Turbulent Flow

what is the difference between laminar flow and turbulence flow?

Laminar flow and turbulent flow are two important aspects to take into account when measuring flow. When does a turbulence effect occur? What is the effect on your flow meter? And very important: What can you do to minimise the disadvantageous effects of turbulent flow?

All kinds of questions which come to mind when working with flow meters.
In this blog I’ll share my top 3 tips with you to minimise the disadvantageous effects of turbulent flow.

Laminar flow in flow meter with 'bypass' sensor

Working principle of 'bypass' sensor, with Laminar Flow Element (LFE)

Why is laminar flow important for flow meters?

It depends on the type of flow meter how turbulent flow can affect your measurement. If you have a thermal flow meter based on the ‘bypass’ working principle, then part of the main flow goes through a restriction and a small part of the flow goes through the actual sensor.

The ratio between these two flows is determined by the pressure drop over the sensor and the restriction in laminar flow. The turbulence effect will disturb this ratio. As the instruments with bypass sensor are often used for very precise measurements, the turbulence effect can have a huge effect on the measurement results.

In conclusion, thermal flow meters – based on the ‘bypass’ principle - behave best using a laminar flow.

What is turbulence effect?

''Turbulence is a dangerous topic which is often at the origin of serious fights in the scientific meetings devoted to it since it represents extremely different points of view, all of which have in common their complexity, as well as an inability to solve the problem”. Marcel Lesieur, 1987.

In practice you will encounter a turbulent flow quite often. A turbulent flow can be caused by (too many) restrictions in an installation, such as valves or adapters, in combination with a high velocity of the used fluid. This effect is known as ‘turbulence effect’. 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 well. As it takes some time for a turbulent flow to get laminar again, it is important to use the right pipe length.

Turbulent flow

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.

Laminar flow profile vs Turbulent flow profile

Turbulent flow versus Laminar flow

How do you know if it is laminar flow or turbulent flow?

In general it can be said that there are two types of flows: a laminar flow and a turbulent flow. You can see in the picture that laminar flow has been visualised by an experiment using ink in a cylindrical tube. The ink has been injected into the middle of a glass tube through which water flows. When the speed of the water is still low, the ink does not appear to mix with water, the stream lines are parallel; this is called laminar flow.

If the speed of the water increases, a sudden change will occur at a certain speed. The flow completely disrupts and the water turns homogeneous through the ink. The stream lines are chaotic, not linear anymore, which is called turbulent flow.

The importance of the Reynolds number (Re)

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

These variables 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).

3 Tips to minimise the disadvantageous effects of turbulent flow?

If you use thermal mass flow meters based on the ‘bypass’ sensor principle, I advise you to do the following:

1) Try to prevent restrictions in your process, such as valves, adapters and elbow couplings:

Do not mount the flow meter directly behind a restriction, such as a valve. If you cannot arrange this, than use a turbulence filter between the valve and flow meter or use a flow meter with integrated turbulence filter.
Limit the number of elbow couplings close to a flow meter as much as possible.

2) Limit the speed of your flow by using the right pipe length. In general, I advise to use a minimal pipe length of:

10x the pipe diameter, at the inlet of the instrument
4x the pipe diameter, at the outlet of the instrument (flow meters only)
For gas flow rates > 100 l/min it is common to use as a minimum a 12mm or ½” pipe.

3) Use a ‘turbulence’ filter in your flow process.

The turbulence filter will filter the flow before it reaches the sensor and makes it laminar again. Nowadays, flow meters often have such a filter integrated in the flow meter (for example Bronkhorst EL-FLOW series) or have an extended flow path inside the flow meter (for example the Bronkhorst Low delta P flow meters).

It all depends on the process and application

It depends very much on the application what the consequences are of turbulent flow. In semicon processes for example, particularly in coating processes such as layer deposition, turbulent flow is a no-go! A stable process is essential here. However, in other coating processes, like flame spray techniques, the impact of turbulences will be less due to the high pressure in the flow. If you need advise on choosing the best flow meter for your application, please let us know.

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Extended flow path inside a flow meter

What is the difference between laminar flow and turbulent flow?