The fraction of the gas in a multiphase flow system. Knowledge of the gas void fraction is needed in identifying the type of flow regime and flow behaviour of multiphase flow mixture. It is also needed in the calculation of pressure drop and prediction of fluid residence time for pumping requirements and understand the changes in thermal properties along long pipes such as sub-sea pipelines.

Experimentally, the gas void fraction can be measured by quick-closing valves, capacitance, and optical probes, devices based on X-ray and gamma-ray attenuation, and local electrical conductivity. However, different techniques tend to give different values of the (p. 167) gas void fraction for the same flow. There is, therefore, an inherent uncertainty in the published data on gas void fraction.