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A new similarity analysis method is proposed in this book through a complete theoretical derivation. It leads to establishing systems of complete similarity governing mathematical models for deep investigations of laminar forced convection and its film flows for resolving the challenges in the research. The proposed novel similarity variables, the dimensionless velocity components, directly describe momentum field, which shows that the new similarity analysis method is different from the traditional Falkner–Skan transformation with the dimensionless function variable f(η) for indirect description of the momentum field.With the new similarity analysis method, it is convenient to consider variable physical properties, especially to conveniently treat the interfacial physical matching conditions of two-phase film condensation and even to conveniently investigate the effect of noncondensable gas on the film condensation, compared to those with the traditional Falkner–Skan-type transformation. A series of results on rigorous analysis and calculation are reported for effects of the viscous thermal dissipation and variable physical properties on heat transfer of laminar forced convection, and a series of related prediction equations are provided. Furthermore, the complete similarity mathematical models for forced film condensation of pure vapour and vapour–gas mixture are developed, respectively, based on the new similarity analysis method, in which the coupled effect of the condensate liquid film flow and the induced vapour film flow is considered. The vapour film flow for the general forced film condensation involves vapour momentum and temperature boundary layers, while the laminar forced film condensation of vapour–gas mixture flow involves the additional concentration boundary layer. An even big challenge is resolved for rigorous calculation of the interfacial vapour saturation temperature, a decisive issue of heat and mass transfer for laminar film condensation of vapour–gas mixture. Then, it is realized to rigorously evaluate heat and mass transfer of the forced film condensation of vapour–gas mixture. Furthermore, a condensate mass–energy transformation equation is created under the new similarity analysis system, for a better clarification of the internal relations between heat and mass transfer of the forced film condensation.