The head change coefficient for branched flows: Why "losses" due to junctions can be negative

Highlights: • The reason for negative "loss" coefficients is revealed on a sound physical background. • The exchange of energy due to diffusion between adjacent branches of a junction is computed. • A method for the computation and visualization of dissipative and diffusive energy change is shown. • Examples in terms of detailed CFD-solutions of the flow through a junction are presented. - Abstract: In this study, the phenomenon of negative "loss"-coefficients reported in various studies about turbulent branched flows through combining junctions is investigated systematically. It turns out, that the "loss" for one branch of a junction and its adjacent ducts only in parts is due to devaluation of mechanical energy, which would be a real loss, so that the term energy change is more appropriate. The other part of the energy change which is not due to a loss is due to a mutual energy transfer between the two partial flows. As a consequence, non-dimensional coefficients should be called energy change coefficients rather than head loss coefficients whenever branching of flows occurs. Furthermore, a method is introduced by which losses and the energy transfer are determined directly in the flow field. The field values allow a quantification and visualization of the local loss and work transfer, while their integral values can be used to quantify the individual contributions of losses and energy transfer within the energy change of a flow through a junction