Experimental investigation of turbulence modification in bubbly axisymmetric jets

The paper deals with an experimental study of bubbly free and impinging axisymmetric turbulent jets (Re = 12,000) by means of a combination of optical planar methods: a fluorescence-based technique for bubbles imaging (PFBI, its description is given in [1]) and PIV and PTV approaches for measurement of velocity distributions of both continuous and dispersed phases, respectively. The bubbly jets are investigated at different volume gas fractions: 0, 1.2, 2.4, and 4.2%. Basing on 10,000 simultaneously measured instantaneous fields of the local gas fraction and velocities of the both phases, statistics on the mean and fluctuating characteristics for the both phases is estimated up to the third-order moments, including mixed one-point correlations. The paper reports on opposite effects of the volume gas fraction on turbulence in the free and confined jet flows. At a nozzle edge, the bubbles increase the growth rate of turbulence fluctuations, whereas downstream (z/d > 0.3) they suppress the turbulence fluctuations compared to the single-phase flow. Close to the impingement surface, the bubbles, however, significantly intensify turbulence fluctuations due to an increase of the slip velocity between the phases.