How is dryness fraction (steam quality) defined?

• A. Mass fraction of liquid in a steam-water mixture
• B. Mass fraction of vapor in a steam-water mixture
• C. Ratio of vapor pressure to total pressure
• D. Total mass of steam per unit volume

Answer: (B)

Dryness fraction, or steam quality, is the portion of a saturated steam–water mixture by mass that is in the vapor phase. It is defined as x = m_v / (m_v + m_l), meaning the mass of vapor divided by the total mass. This is why the correct choice is the mass fraction of vapor in the mixture. The concept links directly to mixture properties: for a saturated mixture, the specific enthalpy is h = x h_g + (1 − x) h_f, and x ranges from 0 (all liquid) to 1 (all vapor). It is not a pressure ratio or a density, which would refer to vapor pressure relative to total pressure or the mass per unit volume, respectively.