Centrifugal compressor prewhirl — effect on inlet absolute velocity In centrifugal compressor practice, “prewhirl” refers to intentionally imparting swirl to the incoming air before it meets the impeller eye. The process that allows the air to enter the impeller blades at a __________ absolute velocity (without changing the mass flow rate or the meridional velocity of flow) is termed prewhirl.

Introduction / Context:
Prewhirl is a widely used inlet treatment in centrifugal compressors. By giving the incoming air a controlled tangential component (usually in the direction of impeller rotation), the relative incidence at the impeller leading edge can be optimized. This reduces losses and can extend the stable operating range without altering the meridional (through-flow) velocity or the mass flow rate.

Given Data / Assumptions:

• Flow approaching the impeller eye is given a finite tangential velocity by inlet guide vanes.
• Mass flow rate and meridional velocity component are maintained.
• Focus is on how the absolute inlet velocity magnitude seen by the impeller changes.

Concept / Approach:
Velocity triangles govern blade incidence. Adding prewhirl (in the direction of wheel rotation) reduces the relative inlet velocity and the work input required per unit mass. For constant meridional component, a favorable tangential component changes the absolute velocity vector so that its magnitude, relevant to incidence and shock formation at leading edges, is effectively reduced at entry.

Step-by-Step Solution:
Establish inlet velocity triangle with components: V_m (meridional), V_t (tangential due to prewhirl).With prewhirl in the direction of rotation, the blade-relative velocity W = V − U decreases at the leading edge.Maintaining mass flow and V_m while adding favorable V_t leads to a reduced effective absolute entry magnitude for the target incidence.Therefore, the description that fits prewhirl is that the air enters at a reduced absolute velocity.

Verification / Alternative check:
Design texts show prewhirl lowers inlet relative Mach number and mitigates leading-edge shock or separation, consistent with a reduced effective entry velocity for a given blade speed.

Why Other Options Are Wrong:
Increased velocity would worsen incidence. “Unchanged” ignores the vector change. “Reversed” is not used in practical prewhirl and would cause severe incidence.

Common Pitfalls:
Confusing absolute velocity reduction with meridional velocity; prewhirl targets the swirl component while preserving through-flow.

Final Answer:
reduced