The effect of an inline blockage on the formation of a turbulent free jet

In many equilibrium-based models for droplet size prediction, the fundamental parameters of turbulence are removed in favour of bulk quantities like velocity, and diameter. In this work we show that the upstream conditions of the jet are of critical importance and cannot be neglected, as they influence the turbulence field in jets in ways that are not accounted for in models based entirely on bulk parameters. This work presents the results of a single-phase investigation into the effect of upstream disturbances on a turbulent free jet. Reynolds Averaged Navier-Stokes Simulations of turbulent free jets with disturbances (in the form of orifice plates with a β ratio of 0.5) located at varying locations upstream of the pipe exit have been undertaken. Validation against experimental data for an orifice plate in a pipe found the Standard k-ε model to be the most accurate of the RANS models tested for this geometry. The additional turbulence generated from the upstream disturbances was found to be advected downstream and did not dissipate prior to the jet exit. The increased turbulence levels and altered form of the turbulence profile at exit was found to affect the evolution and observed turbulence levels of the free jet. Two distinct ‘modes’ were observed for the influence of the disturbances on the free jet. When the orifice plate is near the exit the free jet is dominated by the behaviour of semi-confined jet forming from the blockage. For the deeper set disturbances, the predicted form of the free jet was similar to that of the free jet from a straight pipe, though the turbulence levels are still elevated at the exit, affecting spreading rates and turbulence levels in the jet. Measures based on averages of TKE and TDR over hemispheres of varying radii are proposed for the development of models that do take into account the turbulent state of the exiting jet.