Curtis (velocity-compounded) impulse stage behavior: In a velocity-compounded impulse turbine, what happens to the steam velocity as it passes through the second row of moving blades?

Introduction / Context:Velocity compounding (Curtis staging) splits the large nozzle jet velocity across multiple moving rows with an intervening set of guide (fixed) blades. Understanding how velocity and pressure evolve through each row is key to analyzing impulse stages and estimating diagram efficiency.

Given Data / Assumptions:

• Impulse principle: pressure drop occurs mainly in the nozzle and fixed guides; moving blades ideally see constant pressure.
• Two (or more) moving rows with a fixed guide (re-directing) row between them.
• Energy is extracted from the jet by turning it in the moving blades.

Concept / Approach:In an impulse stage, moving blades convert part of the incoming kinetic energy into work. Thus, across each moving row, the absolute velocity magnitude decreases as energy is extracted, while the static pressure ideally remains approximately constant. The fixed guide between moving rows re-directs the flow (changing direction and sometimes redistributing velocity components) without adding net energy.

Step-by-Step Solution:

Verification / Alternative check:Velocity triangles drawn for a Curtis stage show diminishing absolute outlet velocities after each moving row, while static pressure across moving rows is nearly constant, confirming the trend.

Why Other Options Are Wrong:

• Velocity increases/constant: Opposite to energy extraction behavior.
• Pressure remains constant: True for moving rows, but the question asks about velocity change specifically.

Common Pitfalls:Mixing the behavior of fixed and moving rows; assuming pressure drops in moving blades (that is characteristic of reaction stages, not pure impulse).

Final Answer:velocity decreases