What is the Venturi effect and how does it work in a nebuliser?

A Venturi or Venturi tube is a system used to speed up the flow of fluid by constricting it in a cone-shaped tube.

As it is restricted, the fluid must increase its velocity, which reduces its pressure and produces a partial vacuum.

As fluid leaves the constriction, its pressure then increases back to pipe level.

Giovanni Battista Venturi

What is the Venturi effect?

The Venturi effect is named after its discoverer, Italian physicist Giovanni Battista Venturi.

The main operating principle of the Venturi effect is that the velocity of an incompressible liquid - in this case either your sample or standard - increases as the diameter in which the fluid passes though also grows proportionately.

In layman's terms, as the diameter of the tube decreases, the same amount of energy and mass is contained within a smaller space and must therefore be expelled with a greater magnitude of velocity out of the system.

This relates back to one of the most basic principles in science of Newton’s third law of motion. That every action must have an equal and opposite reaction.

In terms of fluid dynamics, the continuity equation can also be applied to the Venturi principle. Its official definition is “the continuity equation states that the rate at which mass enters a system is equal to the rate at which mass leaves the system plus the accumulation of mass within the system”.

What are the limiting factors in the Venturi principle?

There is one main reason for the limit of the Venturi principle.

Where fluid velocity approaches the speed of sound, the flowrate through the nebuliser hits a critical point where any further pressure decrease downstream will not result in a net flow rate increase.

This can cause the venturi effect to stutter or even cease to exist entirely.

How does a Venturi work in a nebuliser?

With a nebuliser, an aerosol is generated by passing air flow through a Venturi in the nebuliser body.

This then forms a low-pressure zone which pulls droplets up through a feed tube from a solution or suspension of sample into the nebuliser body.

In turn, this creates a stream of atomised droplets which flow into the mixing chamber. Higher air flows lead to a decrease in particle size and an increase in output.

The nebulisers we manufacture for all our BWB-Tech flame photometers utilise the Venturi effect to form a fine and stable mist that does not change.

To do this, the air compressor must emit a very constant pressure and the needle and orifice must be cut with precision to ensure regularity in their circular shape.

A small deviation from this will cause fluid to impact the irregularity and build up into a small droplet that will then be expelled from the orifice.

This can then get pushed into the burner head and cause small spikes in the light emitted as a larger mass of ions have been excited in the flame.

The continuous development of our nebuliser over several years gives you the most stable readings possible from your instrument.