Venturimeter diffuser behavior: How does the flow velocity change in the divergent (diffuser) portion of a venturimeter under normal operation?

Introduction / Context:
A venturimeter accelerates fluid in the convergent section to a high velocity at the throat, then decelerates it in the diffuser (divergent section) to recover static pressure. Understanding how velocity changes in each segment is essential for correct use and interpretation of differential-pressure readings.

Given Data / Assumptions:

• Incompressible, steady flow.
• Well-designed diffuser with modest divergence angle to limit separation.
• Negligible elevation changes over the instrument length.

Concept / Approach:
Continuity requires v * A ≈ constant for incompressible flow. The diffuser has increasing cross-sectional area; therefore, velocity decreases. Bernoulli with losses shows the accompanying rise in static pressure (pressure recovery).

Step-by-Step Solution:

Verification / Alternative check:
Manometer connections at the throat and downstream of the diffuser typically show lower head at the throat and higher head downstream, consistent with decreased velocity in the diffuser.

Why Other Options Are Wrong:

• Remains constant: Would require constant area; not the case.
• Increases: Opposite of diffuser action.
• Depends upon mass of liquid: Kinematics here depend on geometry and continuity, not system mass.

Common Pitfalls:
Thinking velocity keeps increasing through the meter; confusing diffuser with convergent section.

Final Answer:
decreases