The Venturi Effect

Venturi Effect
Venturi Effect

The venturi effect is a constriction within a pipe (classically an hourglass shape) that varies the flow characteristics of a fluid (either liquid or gas) travelling through the tube.  As the fluid velocity in the throat is increased there is a consequential drop in pressure.  Italian scientist Giovanni B Venturi (1746-1822) was the first to observe this phenomenon.

The fact that a pressure drop accompanies an increased flow velocity is fundamental to the laws of fluid dynamics.  Swiss mathematician, Daniel Bernoulli, derived the interrelationship between pressure, velocity and other physical properties of fluid in 1738. Classically, his theorem is used in the design of aircraft wings to create lift from the flow of air over the wing profile.

Choked Flow

The limiting case of the Venturi effect is when a fluid reaches the state of choked flow, where the fluid velocity approaches the local speed of sound. When a fluid system is in a state of choked flow, a further decrease in the downstream pressure environment will not lead to an increase in the mass flow rate. However, mass flow rate for a compressible fluid will increase with increased upstream pressure, which will increase the density of the fluid through the constriction (though the velocity will remain constant). This is the principle of operation of a de Laval nozzle. Increasing source temperature will also increase the local sonic velocity, thus allowing for increased mass flow rate but only if the nozzle area is also increased to compensate for the resulting decrease in density.

Expansion of the section

The Bernoulli equation is invertible, and pressure should rise when a fluid slows down. Nevertheless, if there is an expansion of the tube section, turbulence will appear and the theorem will not hold. Notice that in all experimental Venturi tubes, the pressure in the entrance is compared to the pressure in the middle section. The output section is never compared with them.

There are basically two applications for a venturi.  By attaching manometers to three sections of the tube, the pressure drop can be measured and the flow rate through the throat calculated. This is termed a Venturi Meter.

More commonly, a venturi can use this negative pressure to draw a second fluid into the primary flow.  An example of this would be a jet pump or an eductor. This effect has found many applications across a range of industries. However, the basic mechanism has remained the same for almost 200 years.

In the mid 1990s David RM Short, Plumber, Former Fire-fighter and Koi Carp Keeper, brought naïve logic and lateral thinking to bear to fundamentally re-invent the venturi . His award winning invention was subsequently evaluated and qualified by the Techische Universitat Hamburg-Harburg fluid dynamics department in Germany (see Performance Data). The patented Airmaster not only delivers improved performance over conventional venturis, it can also extend its utility by enabling precise adjustment of the process.

A conventional venturi carries inherent limitations.  The constriction strangles the primary flow resulting in backpressure that can burden a pump with unnecessary load increasing energy costs and shortening its serviceable life.  In addition, the venturi effect is notorious fickle fluctuating with slight changes in flow rate, temperature, viscosity and other parameters.  The Airmaster leaves behind these penalties offering benefits for new and existing applications.

Fluid dynamics is a challenging topic and the venturi effect is frequently misunderstood or misrepresented.

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