Know your Reynolds numbers !

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So you have 30 diameters of straight pipe, job done yes ? Well, not always !
If a flow meter needs to measure flow across the “transition” zone between Reynolds numbers 2,000 to 4,000, it will inevitably suffer a significant loss in accuracy at one end of the flow range. This is because the “flow profile” will change from a parabolic curve associated with Laminar flow (≤ 2,000 Re) to a “flatter” curve at Turbulent flow conditions (≥ 4,000 Re). The difference in centre line velocity can be as much as 30%.

So what are Reynolds Numbers ? In fact, they don’t have a dimension, they are a ratio of a fluid’s momentum to the frictional “drag” force of the pipe wall.

Basically, at very low (Laminar) flows, friction affects are higher and the velocity at the pipe wall is lower. This produces the characteristic peak flow on the centre line. As the velocity increases the “drag” of the pipe wall reduces & the gas becomes what is known a “Turbulent” meaning there is greater degree of mixing in the gas.

In both cases the flow profile is consistent & steady, it’s just the nature of the flow profile that changes.

As an example, if we were measuring air flow in a 2” pipe, (20°C / 1 BarA), 1 – 10 Nm3/hr, the transition zone would be between 2.08 to 8.3 Nm3/hr. This corresponds to a Reynolds numbers between 1,000 & 4,000. Below 2.08 Nm3/hr the flow would be Laminar & above 8.3 Nm3/hr it would be Turbulent.

The Vortab – VIP flow conditioner will produce it’s own flow profile irrespective of the Reynolds number and eliminate the errors associated with flow measurement across the transition zone.