Nozzle with Friction – Primary Effects on Steam Flow and Quality What is the principal combined effect of internal friction on the flow of wet steam through a nozzle, considering mass flow and moisture content at the exit?

Introduction / Context:
Real steam nozzles are not frictionless. Wall friction and boundary-layer growth introduce irreversibilities that alter discharge, velocity, pressure distribution, and condensation behavior. Recognizing the signs of these changes is important for diagnosing performance shortfalls on test stands and in service.

Given Data / Assumptions:

• Flow of wet or potentially condensing steam through a converging or converging–diverging nozzle.
• One-dimensional, steady modeling with friction and associated entropy rise.
• Back pressure low enough to make compressibility effects relevant.

Concept / Approach:

Friction converts some of the available isentropic enthalpy drop into internal energy (heating) rather than kinetic energy. This reduces the achievable velocity at a given pressure ratio and effectively lowers the discharge for the same upstream stagnation conditions and back pressure. Moreover, in wet-steam conditions, irreversible mixing and wall heat transfer promote additional condensation, reducing dryness fraction and thus increasing wetness (mass fraction of liquid). The result is a lower mass flow than ideal and a wetter exhaust.

Step-by-Step Solution:

Verification / Alternative check:

Empirical discharge coefficients for steam nozzles (Cd < 1) capture reduced mass flow relative to isentropic predictions. Moisture measurements downstream of high-load nozzles commonly show greater wetness than ideal models predict.

Why Other Options Are Wrong:

Options claiming increased mass flow contradict the universal trend of Cd < 1 with friction. Decreased wetness is unlikely in condensing flows with friction. A claim of ‘‘no change’’ ignores real, measurable losses.

Common Pitfalls:

Confusing static temperature trends (which can locally rise due to frictional dissipation) with overall discharge behavior; the key combined effect remains reduced mass flow rate and increased moisture.

Final Answer:

decrease the mass flow rate and to increase the wetness of steam