Two-Phase Flow Quality
Two-Phase Flow Quality
Whenever a fluid is partially boiling or flashing — inside a boiler tube, a refrigerant evaporator, or a wellhead choke — the mixture flowing through the pipe is part liquid and part vapor, and almost every two-phase pressure-drop and heat-transfer correlation is written in terms of the vapor mass fraction, called quality. Quality is a purely compositional quantity (mass of vapor over total mass), distinct from void fraction (volume of vapor over total volume), and confusing the two is a frequent source of error since vapor occupies a hugely disproportionate volume fraction compared to its mass fraction.
Flow quality is x = mv/(mv + ml), the vapor mass flow rate divided by the total mass flow rate. where m_vap2 is the vapor mass flow rate and m_liq2 is the liquid mass flow rate in the same two-phase stream.
Inputs
m_vap2 := 0.2 kg/s=0.2 kg/s
m_liq2 := 1.8 kg/s=1.8 kg/s
Quality is simply the vapor's share of the total mass flowing, which is why it always falls between 0 (all liquid) and 1 (all vapor).
x_qual := m_vap2/(m_vap2 + m_liq2)=0.1
Results
A quality of 0.1 means the stream is only 10% vapor by mass, but because vapor is far less dense than liquid, that same stream can occupy well over half the pipe cross-section by volume — this mismatch is why two-phase pressure drop correlations need quality as an explicit input rather than assuming it tracks volume fraction. Quality trending toward 1 along a boiler tube signals approach to dry-out, a critical design limit.
Assumptions & limitations
- Thermodynamic equilibrium between phases assumed (no significant slip in composition).
- Homogeneous mixing is not assumed — quality is a mass-based ratio only.
- Steady-state mass flow rates for each phase.
- Does not account for flow regime (bubbly, slug, annular, etc.).