What does steam quality x represent in a two-phase mixture?

• A. Temperature of steam
• B. Mass fraction of vapor in a two-phase mixture; x = m_vapor /(m_vapor + m_liquid)
• C. Pressure of saturated steam
• D. Mass of liquid in mixture

Answer: (B)

Steam quality represents the mass fraction of vapor in a saturated two-phase mixture. It is defined as x = m_v / (m_v + m_l), where m_v is the mass of vapor and m_l is the mass of liquid. This value ranges from 0 to 1: x = 0 means all liquid, x = 1 means all vapor. In a saturated mixture, properties can be found by mixing rules that interpolate between the saturated liquid and saturated vapor values using x, for example v = x v_g + (1 − x) v_f and h = x h_g + (1 − x) h_f. Note that temperature and pressure describe the state, while quality indicates how much of the mass is in the vapor phase.