Two-Phase Flow Calculator
Two-phase pressure drop by the Lockhart–Martinelli separated-flow model with the Chisholm C parameter, from the single-phase gradients of each phase.
When to use this calculator
Use where gas and liquid share the line — a reboiler return, a flashing condensate line, a wellhead flowline, a relief header carrying carry-over. Two-phase pressure drop is not the sum of the phases and is not close to either one alone: the gas accelerates the liquid and the interface adds drag, so the real gradient can be tens of times the liquid-only value. The model computes each phase as if it flowed alone, forms the Martinelli parameter, and applies the two-phase multiplier.
Required inputs
- Liquid and gas mass flow rates
- Density and viscosity of each phase
- Pipe inside diameter, length, and roughness
- Flow orientation and the Chisholm C parameter for the regime pair
Expected outputs
- Single-phase pressure gradient for each phase flowing alone
- Martinelli parameter X
- Two-phase multiplier φ_L²
- Two-phase pressure gradient and total drop
Formula overview
Gradients in kPa/m or Pa/m consistently for both phases — X is a ratio, so the unit cancels as long as both are computed on the same basis.
Lockhart–Martinelli separated flow with the Chisholm correlation:
X² = (dP/dx)_L / (dP/dx)_G Martinelli parameter
φ_L² = 1 + C/X + 1/X² two-phase multiplier
(dP/dx)_TP = φ_L² · (dP/dx)_L
C depends on whether each phase is laminar or turbulent flowing alone:
20 turbulent–turbulent, 12 laminar–turbulent,
10 turbulent–laminar, 5 laminar–laminar.Worked example
A line where each phase alone would give:
liquid only (dP/dx)_L = 0.5 kPa/m
gas only (dP/dx)_G = 2.0 kPa/m
both turbulent, so C = 20
X² = 0.5 / 2.0 = 0.25 → X = 0.5
φ_L² = 1 + 20/0.5 + 1/0.25 = 1 + 40 + 4 = 45
(dP/dx)_TP = 45 × 0.5 = 22.5 kPa/m
Forty-five times the liquid-only gradient, and more than ten times the
gas-only value — neither single phase gives any useful indication.Common mistakes
- Estimating two-phase drop from the dominant phase. The multiplier is an order of magnitude in ordinary conditions, so no single-phase calculation is a usable approximation.
- Ignoring the flow regime. Lockhart–Martinelli is a pressure-drop correlation, not a regime map. Slug flow imposes cyclic forces on bends and supports that a pressure gradient never reveals, and the mechanical consequences can matter more than the drop.
- Applying it to a vertical or inclined line without the elevation term. The correlation as used here addresses frictional loss; in a riser the static head of the mixture depends on liquid holdup, which is a separate calculation.
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