When is a convergent-divergent nozzle used in relation to steam turbine nozzles?

A convergent-divergent nozzle, often referred to as a de Laval nozzle, is specifically designed to allow the flow of gas, including steam, to accelerate to supersonic speeds. This type of nozzle is particularly used when the flow needs to transition from subsonic to supersonic conditions, which is influenced by the relationship between exit and inlet pressure.

The correct choice highlights that a convergent-divergent nozzle is utilized when the exit pressure is less than approximately 0.577 times the inlet pressure. This ratio is crucial because it indicates the critical pressure drop necessary for achieving supersonic flow. When the exit pressure falls below this threshold relative to the inlet pressure, the nozzle can effectively expand the steam and facilitate the acceleration required to maximize efficiency in steam turbines.

In scenarios where the exit pressure is considerably lower than this critical ratio, utilizing a convergent-divergent nozzle allows for optimized expansion and energy extraction from the steam as it passes through the turbine. This results in improved performance and efficiency in the turbine system.

Other options mention varying pressure ratios or conditions that don't align with the requirements for achieving optimal flow characteristics in a convergent-divergent nozzle. Understanding these principles is essential for engineers to design effective turbine systems and ensure high