Coaxial plumes: Theory and experiment

Abstract

Boussinesq, turbulent plume flows have been successfully described by the well-established model of Morton et al (1956), however, the more complicated case of a coaxial plume consisting of an inner circular plume and an outer annular plume is more challenging to describe theoretically. The difficulties in question arise because of the turbulent exchange of mass, momentum and buoyancy between the inner plume, the outer plume and the ambient. The present study explores the possibility of using a double plume model for coaxial plumes. Model predictions are compared against analogue experimental measurements where, in the latter case, we employ a planar laser-induced fluorescence technique for purposes of visualizing the flow and for measuring concentrations of coaxial plumes. The plumes in question issued from a specially-designed coaxial nozzle, are of moderate Reynolds number (approximately 500) and are of intermediate flux-balance parameter indicating a plume that is either slightly lazy or slightly forced. A whole-field comparison of the scalar concentration between theory and experiment is conducted to obtain the optimal entrainment coefficients. The advantage and limitation of using coaxial plumes to abate the visible plume discharged by cooling towers is discussed.