A venturi tube is a tube or pipe which employs a temporary restriction or narrowing in its length to reduce the pressure and increase the velocity of a fluid or gas passing through it. This phenomenon of simultaneous pressure reduction and velocity increase is known as the venturi effect and has a number of uses such the measurement of airflow and pumping or atomizing of a secondary fluid. There are several types of venturi tube 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 tube will accelerate if that flow is constricted. When this occurs, the pressure of the fluid in the constricted area must decrease to conserve energy. The constriction in a tube is known as a 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 the measurement of air and fuel flow in aircraft systems and the calculation of differential pressure in meteorology.
In addition, the drop in pressure in the venturi tube constriction may 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 flow where the two become mixed. This reaction is used in paint spray guns and airbrushes to draw paint into the compressed air flow where it is atomized. The carburetor on an internal combustion engine makes use of the same reaction to suck fuel into the air being drawn into the engine. A perfume atomizer works in the same way by drawing perfume out of a bottle and into the flow of air from the atomizer bulb.
Venturi tubes make use of several types of constriction profiles ranging from gentle, hourglass shaped restrictions to simple baffles with a small hole in their center. As the efficacy of the venturi effect relies on the maintenance of as much of the initial airflow energy as possible through the entire constriction, drag in this area should be kept at a minimum. For this reason, venturi tubes with gradual changes in profile are the most effective. A venturi tube will typically have an initial entry cone angle of approximately 30 degrees and an exit cone angle of around five degrees. These gradual changes in profile ensure minimum resistance to the fluid flow and the best venturi performance.