“Paradox” of flow reversal caused by protective wall-jet in a pipe

Some fluids must not come into direct contact with solid walls, from which they have to be separated by a co-flowing wall-jet of protective fluid. In a pipe with the faster wall-jet at the wall, the core flow may be expected to be move faster. After all, the wall friction is eliminated and the radial momentum transport from the wall-jet towards the axis may be expected to increase the central flow velocity. What actually happens is the very opposite: the central flow is slowed down and may even reverse its direction so that it flows upstream, against its original directions as well as the direction of the wall-jet. This unexpected reversal was found [V. Tesař, 1978–1980] in radioactivity detectors. Early simple measurements and contemporary hypothesis about its origin (entrainment into the wall-jet) were recently vindicated by numerical flowfield computations. These were performed in a more general way, for a whole family of related geometries, and provide an input information for an analytical model based on control volume balance. The resultant control volume model is useful for designing devices using wall-jet flows in a confined space.