Fluid which may develop more than one phases at certain pressure and temperature conditions.
The most popular multi-phase fluid are:
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It is important to remember that multi-phase fluid may collapse to a single-phase state at certain pressure and temperature conditions.
For example, a 2-phase oil + gas fluid may degrade to single-phase state when pressure goes up above bubble point.
See also
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Natural Science / Physics / Thermodynamics / Thermodynamic system / Phase