Mixing Of A Starting Turbulent Jet In Homogeneous And Isotropic Turbulence

Abstract

The mixing of a starting turbulent jet, injected with an injection velocity of U = 18.8 m/s from a tube with a diameter of D = 4mm (Reynolds number Re = 4800, characteristic time scale t* = D/U = 0.21 ms at the nozzle exit), is experimentally investigated in the presence of homogenous and isotropic turbulence (HIT) using Planar Laser-induced Fluorescence (PLIF). The root-mean-square (RMS) velocity of the ambient HIT is 0.38 and 0.63 m/s, corresponding to turbulent Reynolds numbers of Reλ = 190 and 250, respectively. Spatial distributions of mixture fraction and scalar dissipate rate (SDR) were measured within an axial range of y = 0 – 10 D and at different times after the start of injection (ASIs) up to 21.4 t*. Ambient HIT did not alter the location or size of the leading vortex ring in the initial formation stage. However, it was able to enhance the entrainment of ambient fluids in this stage and deform/transport both the leading and the trailing vortex rings at later stages, despite that the ASIs examined were only comparable to its Kolmogorov timescale. Ambient HIT also introduced a significant increase to the fluctuations of the jet penetrations length but only a marginal reduction (less than 10%) to the averaged jet penetration speed. PDFs of the ensemble averaged mixture fraction within the whole jet region showed increased probability of low mixture fraction values – and therefore indicating enhanced entrainment and mixing – in the presence of ambient HIT. Ensemble averaged distributions of the SDR showed no significant alterations by the ambient HIT in the initial formation stage of the leading vortex ring. At later stages, while the high SDRs in the leading vortex ring remained concentrated in a layer along the edge of the vortex ring for Reλ = 0, it became increasingly uniformly distributed with a lower maximum value as the ambient HIT increased. Similar, but less significant, impact of the ambient HIT on the SDR fields was observed in the trailing vortex rings, while no noticeable impact was found in the shear layer close to the nozzle exit.