In two phase flow what is hold up?

• A. The fraction of pipe volume occupied by the lighter phase
• B. The ratio of velocities
• C. The pressure drop due to density difference
• D. When the lighter phase flows faster than the denser phase

Answer: (D)

Hold-up in two-phase flow is the fraction of the pipe volume that a particular phase occupies. In gas–liquid flow, this is often described as the portion of the pipe volume filled by the lighter phase (gas) compared to total volume. It is a true volume fraction and not a statement about how fast the phases move, nor about the pressure drop caused by density differences. Therefore, describing hold-up as the fraction of pipe volume occupied by the lighter phase best captures the concept. The other options refer to speeds or pressure effects, which are not hold-up.