Flow Measurement MCQ with answer and solution

Concept

In the divergent part pressure rises along the flow (adverse pressure gradient). A small divergence angle stops flow separation and energy loss.

Step-by-step solution

1. In the convergent part, velocity increases and pressure falls. This is a favourable pressure gradient, so there is no separation. 2. In the divergent part, velocity decreases and pressure increases. This is an adverse pressure gradient. 3. If the divergence angle is large, the boundary layer separates from the wall and eddies form. 4. Eddies cause a large head loss. So the divergent angle is kept small (about 5° to 7°) and the convergent angle larger (about 20° to 22°). 5. Answer is (A).

Explanation

Flow in the divergent cone slows down and pressure rises, so the flow can separate from the wall. Separation makes eddies and wastes energy. A small, gradual divergence angle prevents separation and keeps head loss low.

Why the other options are wrong

(B) Venturi meters carry liquids, which are incompressible; the angle has nothing to do with compressibility. (C) In the divergent part velocity decreases, it does not increase. (D) In the divergent part pressure increases (pressure recovery), it does not decrease.

Shortcut

Divergent part = adverse pressure gradient = risk of separation, so keep the angle small.

Common mistake

Thinking velocity increases in the divergent part. Actually it decreases and pressure is recovered.