AN EXPERIMENTAL-STUDY OF A JET WITH LOCAL BUOYANCY ENHANCEMENT

We report here an experimental study of the behaviour of a fully devel oped axisymmetric turbulent jet whose buoyancy is enhanced by volumetr ic heating over the region between two streamwise stations. The buoyan cy enhancement is achieved by ohmic heating of an electrically conduct ing liquid jet, and the measurements are made using a laser Doppler ve locimeter, It is found that, with heating, the axial component of mean velocity can increase appreciably relative to the unheated jet; howev er the turbulent intensity (normalized by the jet centreline velocity) decreases. The shape of the normalized mean velocity distribution acr oss the jet is not significantly affected by the heating, but that of the fluctuating velocity is. The decay of the centreline velocity is c onsiderably slowed down, or even reversed, due to the heating; similar ly the spread rate is arrested at larger values of the Richardson numb er. As a result of the enhanced buoyancy the mass flux in the jet at f irst increases more rapidly than in the unheated jet but further downs tream remains nearly constant over a distance of the order of the leng th of the heat injection region.