What is laminar flow?
In fluid dynamics, laminar flow is flow regime characterized by high momentum diffusion, low momentum convection pressure and velocity independent from time. A flow can be said to be in laminar when the flow exhibits no turbulence.
This type of flow occurs at very low velocities or high viscosities. In this, the liquid flows in smooth layers with the highest velocity at the centre of the pipe and low velocities at the boundary (wall) of the pipe where the viscous forces hold it back.Laminar flow is qualitatively predicted by low values of Reynolds number.
Reynolds number (RE): it is a dimensionless unit. It is defined as the ratio of the liquid’s inertial force and the viscous force. Thus if RE number is high then the inertial force is greater compared to the viscous force.
##Laminar flowmeter:
Laminar flowmeters are differential pressure type flowmeter used to measure low flow rates where no other flowmeters could measure. The theory of Laminar flowmeter based on the Reynolds number and Hagen–Poiseuille equation to measure the pressure difference. The Hagen–Poiseuille is:
Q = Flow rate
ΔP = Pressure dropped across a length of pipe
D = Pipe diameter
μ = Fluid viscosity
L = Pipe length
k = Coefficient accounting for units of measurement
Construction:
The diameter of the flow pipe is expanded and the laminar flow element is installed in the pipeline. The laminar flow element is one or more numbers of tube with small inner diameter, arranged in such a way as to produce a slowmoving flow velocity.
The expanded diameter of the flow element ensures a lower fluid velocity than in the pipes entering and exiting the element. This decreases the Reynolds number to the point where the flow regime exhibits laminar behaviour.
Due to the small inner diameter of the tubes, the fluid pass through the tube experience higher viscosity. The higher viscosity results in pressure drop across the tube and the difference in pressure drop is measured using differential pressure transmitter.
The tubes are available in different shapes.
Advantages:

Can be used to measure low flow rates.

Ability to measure the flow of high viscous liquid.

Linear relationship between flow rate and pressure drop

No need to use square root extractor as it exhibits linear relationship

Low noise
Disadvantages:

Dependent on fluid velocity

Affected by temperature, so needed temperature compensation