A venturi tube narrows to increase the velocity and decrease the pressure of fluid or gas passing through it, known as the venturi effect. It has various uses, including measuring airflow and pumping or atomizing a secondary fluid. The most effective venturi profiles have a gradual change in profile to minimize drag.
A venturi tube is a tube or pipe that employs a temporary restriction or narrowing in its length to reduce pressure and increase the velocity of a fluid or gas passing through it. This phenomenon of simultaneous reduction in pressure and increase in velocity is known as the venturi effect and has numerous uses such as measuring airflow and pumping or atomizing a secondary fluid. There are several types of venturi profiles in general use with a gradual change in profile being the most effective.
The laws of physics dictate that a fluid or gas flowing in a pipe will accelerate if that flow is restricted. When this occurs, the fluid pressure in the confined area must decrease to conserve energy. The constriction in a pipe is known as the venturi and the simultaneous increase in flow velocity and decrease in pressure as the venturi effect. The pressure change characteristics of this phenomenon are used to perform tasks such as measuring air and fuel flow in aircraft systems and calculating differential pressure in meteorology.
Additionally, the pressure drop in the constriction of the venturi tube can be used to pump or atomize a secondary fluid. If a second fluid is introduced into the restricted area of the venturi tube, the partial vacuum created by the lower pressure will draw it into the primary fluid stream where the two mix. This reaction is used in paint spray guns and airbrushes to draw paint into the compressed air stream where it is atomized. The carburetor of an internal combustion engine uses the same reaction to draw fuel into the air drawn into the engine. A perfume atomizer works the same way by drawing perfume out of a bottle and into the airflow from the atomizer bulb.
Venturi tubes use different types of constriction profiles ranging from delicate hourglass-shaped restrictions to simple baffles with a small hole in the center. Since the effectiveness of the venturi effect relies on maintaining as much energy as possible of the initial airflow through the entire constriction, drag in this area should be minimized. For this reason, venturi tubes with smooth profile changes are most effective. A venturi will typically have an initial inlet cone angle of about 30 degrees and an outlet cone angle of about five degrees. These gradual changes in profile ensure the least resistance to fluid flow and the best venturi performance.
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