Particle concentration and local mass flux measurements in two-phase flows with PDA. Application to a study on the dispersion of spherical particles in a turbulent air jet

The Phase Doppler Anemometry (PDA) technique is suitable for spherical particle concentration and local flux measurements in dispersed two-phase flows. When particles have three-directional paths, accurate measurements can be achieved if an integral method of calculation over the effective probe volume and an efficient auto-calibration process are employed. An experimental study is carried out on an axisymmetric two-phase jet. A post-processing of the measurements has been developed to obtain a quite complete characterisation of the particle dispersion throughout the flow. The axial local flux distribution is normalised on the transversal sections with the injected mass rate for each particle size class (by means of correction coefficients). Particle concentration and radial fluxes are calculated from the axial flux and velocity results. A balance of particles over a cylindrical volume permits to calculate further estimates of the particle radial flux, used for testing of the results. The detailed study of the spatial distribution of particles has been made up to 10 nozzle diameters (10D) from the jet exit, in five transversal sections. The local axial flux profiles show two development sub-zones: (i) the near to nozzle zone is dominated by the exit conditions. Mainly, a wide distribution of particle path directions produces a strong radial particle transference (directional classification); (ii) the second sub-zone shows a more regular development controlled by the drag of the mean gas velocity field together with radial particle transport by gas turbulence, rebounding crashes of particles and other effects.