Steam Nozzles – Definition of a Divergent Nozzle In steam and gas dynamics, classify a nozzle as ‘‘divergent’’ when considering how its flow area varies along the flow direction from entrance, through any throat if present, to the exit. Choose the statement that correctly describes a divergent nozzle.

Introduction / Context:
A clear understanding of nozzle geometry is essential in steam turbines, gas turbines, ejectors, and rocket engines. The words divergent, convergent, and convergent–divergent (C–D) describe how the flow area changes along the nozzle centerline and directly control whether the flow accelerates subsonically, reaches sonic conditions, or expands to supersonic speeds.

Given Data / Assumptions:

• Nozzle refers to a duct guiding flow with varying cross-sectional area.
• Steady, quasi-one-dimensional behavior is assumed for conceptual discussion.
• ‘‘Divergent’’ refers solely to geometry (area change), not to pressure ratio or Mach number outcome by itself.

Concept / Approach:

A divergent nozzle is one whose flow area increases monotonically from inlet to outlet. By contrast, a convergent nozzle has monotonically decreasing area. A convergent–divergent nozzle (de Laval) first converges to a minimum area (the throat) and then diverges. Whether a divergent section accelerates or decelerates the flow depends on the local Mach number: subsonic flow decelerates in a diverging passage, while supersonic flow accelerates in it. The geometric definition, however, remains independent of regime.

Step-by-Step Solution:

Verification / Alternative check:

Standard nozzle sketches show a bell or flared shape for a divergent section. In C–D designs, the downstream portion is the divergent section following the throat; a purely divergent nozzle has no upstream convergent part by definition.

Why Other Options Are Wrong:

‘‘Decreases continuously’’ defines a convergent nozzle. ‘‘First decreases then increases’’ defines a convergent–divergent nozzle. ‘‘Constant area’’ is a duct, not a nozzle, and ‘‘none of the above’’ is unnecessary given a correct option.

Common Pitfalls:

Equating ‘‘divergent’’ with ‘‘supersonic’’ unconditionally; the flow regime depends on the pressure ratio and presence of a throat, not solely on geometry.

Final Answer:

when the cross-section of the nozzle increases continuously from entrance to exit