Nozzle classification by area change:\nA steam nozzle is termed a convergent nozzle when its cross-sectional area from inlet to outlet ________ continuously.

Introduction / Context:
Nozzles are classified by how their flow area changes along the axis. This classification dictates acceleration or deceleration for subsonic and supersonic regimes and is foundational for turbine and rocket nozzle design.

Given Data / Assumptions:

• Steam/gas as a compressible working fluid.
• Geometry alone defines the term (independent of actual Mach number attained).
• Convergent means monotonically decreasing area.

Concept / Approach:
A convergent nozzle has a decreasing cross-sectional area from entrance to exit. In subsonic flow, such a duct accelerates the fluid; the maximum Mach number attainable with a purely convergent nozzle is M = 1 at the exit if the flow is choked. To reach supersonic speeds, a divergent section must follow a throat (convergent–divergent, or De Laval nozzle).

Step-by-Step Solution:

Verification / Alternative check:
Classic compressible-flow equations show that for subsonic flow, dV > 0 when dA < 0, consistent with the definition.

Why Other Options Are Wrong:

• Increases: That is a divergent nozzle.
• First increases then decreases: Reverse of a C–D nozzle.
• Remains constant: That is a constant-area duct, not a nozzle.

Common Pitfalls:
Assuming convergence alone guarantees M > 1; a divergent section after a throat is required for supersonic acceleration.

Final Answer:
decreases