2. Non-dimensional numbers used in condensationSeveral non-dimensional numbers are used in the heat transfer and frictional pressure drop correlations. For the convenience of discussion, this section lists these numbers and distinguishes between similar definitions.2.1. Fluid properties of two-phase flowThe Lockhart-Martinelli parameter, X, describes the separated two-phase flow of four combinations: turbulent liquid and turbu-lent vapor, laminar liquid and turbulent vapor, turbulent liquid and laminar vapor, as well as laminar liquid and laminar vapor [12]. In the case of turbulent liquid and turbulent vapor, Xtt is cal-culated with Eq. (1) [13].