Mixing enhancement in axisymmetric turbulent isothermal and buoyant jets

Measurements of the velocity and concentration in axisymmetric, turbulent, isothermal and buoyant jets have been performed with laser-Doppler velocimetry and planar and point laser-induced fluorescence to quantify the mixing enhancement achieved by periodic forcing when the jet exit has a fully-developed turbulent pipe flow, a situation less well-studied than the case of laminar initial conditions. It was found that forcing at Strouhal numbers around 0.6 enhances mixing in the developing region of the jet and this enhancement increased with increasing amplitude of excitation, consistent with results of initially-laminar jets. The initial turbulence intensity did not have any effect, but an increase in the initial lengthscale of the turbulence, controlled by a perforated plate inside the nozzle, caused faster mixing. In agreement with previous experiments, the initial conditions of the jet did not affect the far-field rate of decay, but the jet-fluid concentration there was significantly reduced by forcing due to the increased mixing during the early stages of development, an effect that can be described by a smaller virtual origin in decay laws of jet decay. These results are independent of the Froude number because the initial conditions have an influence only in the early stages where the flow is still momentum dominated.List of Symbols A normalised excitation amplitude, defined by A = u'/U₀ - D nozzle diameter - f jet-fluid concentration - F mean f - f r.m.s. f - Fd Froude number, defined by Fd=U₀²/(gDT₀) - g acceleration of gravity - I fluorescent intensity - I_inc incident light intensity - I_ref light intensity of the reference flow - K decay constant - L_hf concentration halfwidth - M mixing enhancement, defined by U_cl/U_cl,st=0 at x/D=5 - r radial coordinate - Re Reynolds number, defined by Re=U₀D/v - [Rh] concentration of Rhodamine B - St Strouhal number, defined by St=D/U₀ - T₀ temperature of jet fluid - T temperature of outer fluid - T₀ temperature difference (= T₀–, T) - u r.m.s. axial velocity - u r.m.s. of the sinusoidal velocity fluctuation due to forcing - U mean axial velocity - U_cl mean axial centreline velocity - U_cl,st=0 mean axial centreline velocity for an unforced jet - U_max U at the centre of the nozzle exit - U₀ bulk velocity at nozzle exit - x streamwise coordinate - X₀ virtual originGreek coefficient of thermal expansion - kinematic viscosity of the jet fluid - forcing frequencyThe experiments described here have been performed together with Mr. J. Sakakibara. Acknowledgments are also due to Prof. H. Longmire, of the University of Minnesota, for helpful discussions on forcing. This work was done while E.M. visitied Keio University with the financial assistance of TEPCO.