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Venturi Tube
Brief description
A Venturi tube is a fluid dynamics device consisting of a constricted throat section positioned between two conical sections. By forcing fluid to flow through a narrowed throat, the fluid velocity increases while its static pressure drops according to Bernoulli’s principle. This localized low-pressure zone can be used to measure fluid flow velocity, draw in and mix secondary liquids or gases (aspiration), or atomize liquids.
Use / Function
- Primary use: Measuring flow rate in closed piping systems and inducing fluid suction or mixing without moving mechanical parts.
- Secondary uses: Fuel-air mixing in carburetors, suction creation in water aspirators, chemical dosing, and draft induction in industrial chimneys.
- Scale: Laboratory glassware to municipal water pipelines and heavy chemical processing facilities.
Operating principle
Bernoulli’s Principle and Continuity Equation:
- Inlet Flow: Fluid enters the wider upstream cylindrical section of the tube at a baseline static pressure ($P_1$) and velocity ($V_1$).
- Throat Constriction: As fluid enters the smoothly narrowing convergent cone, its cross-sectional area decreases. To maintain mass conservation (Continuity Equation), fluid velocity accelerates to $V_2$ at the throat.
- Pressure Drop: The increase in kinetic energy causes a proportional reduction in static pressure ($P_2$) at the throat ($P_2 < P_1$).
- Fluid Suction / Differential Measurement: Connecting a side tap or port at the narrow throat enables measuring the differential pressure ($\Delta P = P_1 - P_2$) or drawing secondary fluids into the high-velocity stream.
- Pressure Recovery: Fluid then enters a gradually expanding divergent cone, slowing down and recovering most of its original static pressure with minimal frictional turbulence losses.
How to create it
1. Constructing the Convergent Cone
- Take a rigid pipe of Copper, Brass, or Glass.
- Form or machine a convergent cone section with a smooth reduction angle (typically 20° to 22°).
2. Machining the Throat Section
- Fabricate a short, uniform cylindrical neck at the narrow end of the convergent cone, with a cross-sectional area roughly one-fourth to one-half of the main pipe.
- Drill a clean, burr-free pressure tap or aspirator port perpendicular to the inner wall of the throat.
3. Constructing the Divergent Cone
- Join a longer, gradually expanding cone (typically 5° to 7° expansion angle) to the exit of the throat to ensure smooth pressure recovery without flow separation.
- Mount an upstream pressure tap in the full-diameter inlet section before the convergent cone.
4. Calibration and Connection
- Connect the inlet tap and throat tap to a differential pressure gauge or U-tube manometer.
- Calibrate flow rate formulas based on tube geometry and fluid density.
Materials needed
- Body Pipe: Glass, Copper, Brass, or carved hard Wood sealed with resin.
- Taps & Tubing: Thin copper or glass tubes for differential pressure ports.
- Working Fluid: Water, air, or industrial gases.
Variants and improvements
- Venturi Meter: Equipped with pressure taps and a calibrated manometer for precise fluid flow rate measurement in water supply networks.
- Venturi Injector / Ejector: Uses the low throat pressure to draw chemicals, fertilizers, or steam into a main flow stream.
- Carburetor Venturi: Incorporates a fuel jet inside the throat to atomize liquid fuel into incoming air streams for internal combustion engines.
Limits and risks
- Throat Erosion & Cavitation: High fluid velocities and steep pressure drops can cause cavitation in liquid systems, eroding the throat lining.
- Frictional Losses: If the divergent expansion angle is too steep (greater than 15°), flow separation occurs, leading to energy loss and pressure turbulence.
- Clogging: Particulate matter or sediment in the fluid stream can block small pressure taps or restrict the narrow throat.