On the influence of the exit conditions on the entrainment rate in the development region of a free, round, turbulent jet

Abstract

The development region of a free round turbulent jet was experimentally investigated in order to determine the dependence of the jet development on the exit conditions. In particular the influence of the exit conditions on the entrainment rate in both the axisymmetrix mixing layer region and at a distance of 20D from the nozzle was investigated.

The entrainment rate at 20D depends noticeably on the exit boundary layer state, i.e. laminar or turbulent, and on the turbulence intensity of the core. A laminar exit boundary layer showed the highest value of entrainment rate. A turbulent one reduced the entrainment rate at 20D by 15%. A grid placed at the nozzle exit, in order to increase the turbulence intensity of the core, reduced the entrainment rate at 20D by 40%. It must be mentioned that this grid also disturbed the exit boundary layer.

An empirical relation for the entrainment rate at the end of the mixing layer region as a function of the mixing layer parameters, is successful in predicting the entrainment rate at 20D within an accuracy of 5%. This result suggests a strong dependence of the jet development region on mixing layer processes, i.e. the development of organized structures and their coalescence. Possibly there is no asymptotic region which is fully independent of the mixing layer processes, and which is not more or less influenced by the exit conditions via the influence on these processes.