AN EXPERIMENTAL-STUDY OF A JET WITH LOCAL BUOYANCY ENHANCEMENT

We report here an experimental study of the behaviour of a fully developed axisymmetric turbulent jet whose buoyancy is enhanced by volumetric heating over the region between two streamwise stations. The buoyancy enhancement is achieved by ohmic heating of an electrically conducting liquid jet, and the measurements are made using a laser Doppler velocimeter, It is found that, with heating, the axial component of mean velocity can increase appreciably relative to the unheated jet; however the turbulent intensity (normalized by the jet centreline velocity) decreases. The shape of the normalized mean velocity distribution across the jet is not significantly affected by the heating, but that of the fluctuating velocity is. The decay of the centreline velocity is considerably slowed down, or even reversed, due to the heating; similarly the spread rate is arrested at larger values of the Richardson number. As a result of the enhanced buoyancy the mass flux in the jet at first increases more rapidly than in the unheated jet but further downstream remains nearly constant over a distance of the order of the length of the heat injection region.