Effect of nozzle friction on jet speed How does internal friction (irreversibility) within a steam nozzle influence the exit velocity of the steam jet for a given inlet and back pressure?

Introduction / Context:Nozzle friction converts part of the available enthalpy drop into entropy rather than directed kinetic energy. Appreciating this effect is crucial for predicting jet speed and turbine work.

Given Data / Assumptions:

• Same inlet state and back pressure for the comparison.
• Real nozzle with friction vs. ideal isentropic nozzle.
• Negligible heat transfer to surroundings.

Concept / Approach:In an ideal nozzle, the isentropic enthalpy drop fully converts to kinetic energy: V^2/2 = Δh_isentropic. With friction, the actual kinetic energy equals η_nozzle * Δh_isentropic, where 0 < η_nozzle < 1. Thus, exit velocity is lower.

Step-by-Step Solution:Write V_actual^2 / 2 = η_nozzle * Δh_isentropic.For fixed inlet and exit pressures, friction raises entropy and reduces effective Δh to kinetic energy.Therefore, V_actual < V_isentropic; exit velocity decreases.

Verification / Alternative check:Measured nozzle coefficients (velocity coefficient < 1) quantify this reduction; test data show lower exit Mach numbers and speeds than ideal predictions.

Why Other Options Are Wrong:

• (b) Choking does not eliminate friction losses; it limits mass flow but velocity still falls from the ideal.
• (c) Internal viscous heating increases entropy, not useful kinetic energy.
• (d) Mass flow changes are secondary and do not increase exit kinetic energy beyond ideal.
• (e) Friction affects magnitude strongly.

Common Pitfalls:Assuming choked flow guarantees ideal velocity; choking fixes Mach at the throat, not exit losses.

Final Answer:It decreases the exit velocity due to kinetic energy loss