When the exit pressure of a steam turbine nozzle is less than 0.577 times the inlet pressure, which type of nozzle is utilized?

When the exit pressure of a steam turbine nozzle is less than 0.577 times the inlet pressure, a convergent-divergent nozzle is employed. This type of nozzle is designed to achieve supersonic flow, which is essential in scenarios where the pressure drop across the nozzle is significant.

The convergent section of the nozzle accelerates the steam to sonic speed as it approaches the throat. Once the steam passes through the throat, the divergent section allows the steam to expand further, reducing its pressure and increasing its velocity even more. This expansion and acceleration are crucial for efficient turbine operation in low exit pressure conditions.

Overall, the convergent-divergent nozzle is optimally suited to operating efficiently when there is a significant drop in pressure, enabling the steam to perform more work as it enters the turbine stages. Other nozzle types, like the convergent or rotary types, wouldn't facilitate this necessary acceleration and expansion under the given conditions.