Effect of supersaturation inside a steam nozzle:\nDuring metastable expansion (supersaturation), which of the following qualitative effects is observed within the nozzle?

Introduction / Context:
Supersaturation occurs in fast nozzle expansions when condensation lags behind the thermodynamic equilibrium. Designers care about how this affects available energy conversion, velocities, and blade erosion downstream. One key qualitative outcome is the change in effective heat drop between inlet and exit conditions in the flow passage.

Given Data / Assumptions:

• Rapid expansion of steam through a nozzle.
• Condensation kinetics delayed (metastable vapour persists).
• Comparison at nominally similar inlet and outlet pressures to equilibrium cases.

Concept / Approach:
If condensation is delayed, less latent heat is released inside the nozzle during the same pressure decrease. The flow retains higher vapour content (higher dryness fraction). This effectively increases the portion of enthalpy drop available for conversion into kinetic energy across the nozzle, often described as an increase in the heat drop and hence a higher ideal exit velocity compared with the fully equilibrated wet expansion at the same pressure ratio.

Step-by-Step Solution:

Verification / Alternative check:
Wilson line observations and measured pressure-velocity profiles in steam nozzles show increased velocity (and thus greater effective heat drop) before spontaneous condensation sets in and forms a visible fog region downstream.

Why Other Options Are Wrong:

• Decrease dryness fraction: Supersaturation raises, not lowers, dryness fraction prior to nucleation.
• Decrease specific volume: Drier mixture has, if anything, higher specific volume at the same pressure.
• Increase entropy: The metastable path is not an isentropic improvement; entropy generation is primarily associated with non-equilibrium and shock/viscous effects, but the simple qualitative hallmark is increased effective heat drop, not a guaranteed entropy rise along the nozzle passage considered ideally.

Common Pitfalls:
Assuming “more heat drop” always means more efficiency; once nucleation occurs, droplet drag and shocks may introduce losses.

Final Answer:
increase the heat drop