Influence of the Exit Conditions on the Entrainment Rate in the Development Region of a Free, Round, Turbulent Jet

摘要

The influence of the exit conditions on the entrainment rate in the axisymmetric mixing layer region and at a distance of 20D of the nozzle was investigated. The entrainment rate at 20D depends on the exit boundary layer state, and on the turbulence intensity of the core. A laminar exit boundary layer shows the highest entrainment rate. A turbulent one reduces the entrainment rate at 20D by 15%. A grid placed at the nozzle exit, in order to increase the turbulence intensity of the core, reduces the entrainment rate at 20D by 40%. An empirical relation for the entrainment rate at the end of the mixing layer region as a function of the mixing layer parameters predicts the entrainment rate at 20D within an accuracy of 5%. Possibly there is no asymptotic region fully independent of the mixing layer processes, and not influenced by the exit conditions via the influence on these processes.