Local Structure of Two-Phase Flows in Horizontal Ducts: Effects of Uniform and Suddenly Varying Duct Sections

When considering the flow of a two-phase mixture, the complete characterization of its behavior requires not only the evaluation of its temperature and pressure evolutions, but also the description of the distribution of the phases (the so-called "flow structure"). This is of great importance in problems concerning the analysis of the solicitations undergone by the containing ducts, both from a mechanical point of view and - even more important - when heat transfer at fixed power is involved. In the present paper the description and quantitative visualization of the structure of two-phase flows in horizontal ducts with constant diameter and with section modifications is proposed. The evolution of the flow along the duct is described in local terms by means of section fields of void fraction, flow complexity and local mean temporal length of the gaseous phase. These last are three local, time-averaged quantities which can be easily calculated starting from relatively simple experimental acquisitions of the phase density function.