Steam nozzle back pressure vs design exit pressure: identify the nozzle condition for expansion. Given: Back pressure (ambient) is lower than the designed exit pressure of the nozzle. Choose the correct technical term that describes the nozzle condition.

Concept/ApproachFor a fixed-geometry convergent–divergent nozzle, the designed exit pressure is the ambient pressure at which the nozzle ideally expands the fluid to match the surroundings.If the actual back (ambient) pressure is lower than this designed value, the nozzle has not expanded enough — the exit pressure remains higher than ambient — and therefore the jet needs further expansion outside the nozzle.

DefinitionUnderexpanded nozzle: exit static pressure > ambient (back) pressure; needs additional expansion externally (expansion fans form at the lip).Overexpanded nozzle: exit static pressure < ambient pressure; external compression shocks form.

Final AnswerUnderexpanded nozzle.