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Abstract
A preliminary study of the convective transfer rates of scalar quantities in a strongly oscillating free jet (+70 m/s to -50 m/s at 100 Hz generated by a pulse combustor) has been performed. This work is described in two parts: This paper discusses the free jet character, whereas the companion paper discusses the impinging jet characteristics associated with the convective heat transfer rate to a plate normal to the jet axis.
In this paper the essential differences between the steady and oscillating jets are described. Rayleigh temperature measurements and flow visualizations (schlieren photography and laser Mie scattering) were used to quantify the jet characteristics. The method of vortex dynamics was used to model the vorticity and velocity fields of the pulsating jet. The modeling results support the conclusions drawn from experimental data.
The steady jet exhibited typical axial and radial temperature profiles for an axisymmetric jet and, hence, it had typical decay rates. The character of the oscillating jet was dominated by the formation of toroidal vortex elements during the positive velocity portion of the cycle. The entrainment of ambient fluid and, hence, the mixing of this fluid with that of the jet was an order of magnitude greater for the oscillating jet than for the steady jet. Thus, the temperature decay rate was also an order of magnitude higher than that of the steady jet. The high peak velocities, finer scale of turbulence, and the coherent toroidal vortex of the oscillating jet are believed to be responsible for the improved scalar transport.
