A VOLUMETRICALLY HEATED JET - LARGE-EDDY STRUCTURE AND ENTRAINMENT CHARACTERISTICS

We report here an experimental study of a round vertical liquid jet th at, after achieving a self-preserving state, is subjected to volumetri c heating between two diametral stations. The heat injection is achiev ed by applying a voltage across the stations, the jet fluid having bee n rendered electrically conducting by the addition of acid. Using lase r-induced fluorescence, digital image processing and laser-Doppler ane mometry, the flow properties of the jet have been studied in detail. I t is found that, with sufficient heating, the jet no longer grows line arly with height, and the decay of both centreline velocity and turbul ence intensity is arrested, and may even be reversed just beyond the z one of heat addition; nevertheless the entrainment decreases, which is at variance with the hypotheses often made for modelling it. This beh aviour is here attributed to the disruptive influence that, as the pre sent experiments show, the volumetric heating has on the large-scale v ortical structures in the jet, which are known to be largely responsib le for the engulfment of ambient fluid that is the first step in the e ntrainment process. It is shown that a new non-dimensional heat releas e number correlates the observed data on changes in jet width. An inte gral model that would describe the effect of local heating is proposed , and implications for cloud development in the atmosphere are discuss ed.