Venturimeter diffuser (divergent) section: How does the static pressure change in the divergent portion under normal operation?

Introduction / Context:
A venturimeter consists of a convergent section, throat, and a divergent (diffuser) section. The diffuser is designed to recover static pressure by converting part of the kinetic energy gained in the convergent/throat back into pressure energy with controlled deceleration.

Given Data / Assumptions:

• Steady, incompressible flow through a venturimeter.
• Negligible elevation change across short length.
• Well-designed diffuser with modest divergence angle to avoid separation.

Concept / Approach:
From Bernoulli's principle with losses: p/ρg + v^2/(2g) ≈ constant (minus small hf). In the diffuser, area increases, velocity decreases, and thus static pressure increases as kinetic energy is converted back to pressure. The goal is effective pressure recovery with minimal losses.

Step-by-Step Solution:

Verification / Alternative check:
Manometer tappings show lower head at the throat and higher head downstream in the diffuser compared to the throat, confirming pressure recovery for subsonic incompressible flows.

Why Other Options Are Wrong:

• Remains constant: Would require zero deceleration and no area change.
• Decreases: Opposite of diffuser behavior (that is the convergent effect).
• Depends upon mass of liquid: Static pressure change is governed by area/velocity changes and losses, not the total mass present.

Common Pitfalls:
Assuming pressure keeps dropping through the entire meter; ignoring that the diffuser is explicitly for pressure recovery; choosing too large a divergence angle, which can induce separation and reduce recovery.

Final Answer:
increases