Critical Pressure Ratio for Steam – Identify the Initial State from the Value 0.546 If the measured critical pressure ratio (static back pressure to upstream total/static as defined for choking) for steam is approximately 0.546, the steam at inlet is generally considered to be in which initial condition?

Introduction / Context:
The critical pressure ratio p*/p0 (or p2/p1 depending on convention) marks the onset of choking in nozzle flow. For steam, its value depends slightly on the thermodynamic state (wet, saturated, or superheated) because the effective specific heat ratio and real-gas effects vary with quality and temperature. Recognizing typical benchmark values helps infer inlet condition when only the ratio is known.

Given Data / Assumptions:

• Nozzle or orifice with steady flow of steam to a lower-pressure region.
• Critical pressure ratio reported as about 0.546.
• Classical steam-nozzle practice where saturated vs. superheated states have slightly different characteristic ratios.

Concept / Approach:

Empirical and semi-theoretical treatments show that for steam initially dry saturated at the nozzle inlet, the critical pressure ratio is close to 0.546. For superheated steam, an often-quoted value is around 0.577 (reflecting an effective specific heat ratio nearer 1.3–1.33 rather than that typical at saturation). Wet steam values can differ further due to mixture properties. Therefore, observing 0.546 suggests the inlet is dry saturated steam.

Step-by-Step Solution:

Verification / Alternative check:

Design handbooks and nozzle test data reproduce these ratios for standard conditions; deviations arise with significant moisture or high superheat, but 0.546 is widely associated with saturated entry.

Why Other Options Are Wrong:

‘‘Superheated’’ corresponds to a higher critical ratio around 0.577. ‘‘Wet’’ is not tied to 0.546 as a typical value and, depending on quality, can show different behavior. ‘‘None of these’’ and ‘‘compressed liquid’’ do not match compressible vapor flow at choking.

Common Pitfalls:

Mixing up the definition (p*/p0 versus its inverse) and confusing air’s value (~0.528) with steam’s saturated value (~0.546). Always be clear about state and ratio orientation.

Final Answer:

dry saturated steam