PULSE COMBUSTION - TAILPIPE EXIT JET CHARACTERISTICS

A preliminary study of the convective transfer rates of scalar quantit ies in a strongly oscillating free jet (+70 m/s to -50 m/s at 100 Hz g enerated by a pulse combustor) has been performed. This work is descri bed in two parts: This paper discusses the free jet character, whereas the companion paper discusses the impinging jet characteristics assoc iated with the convective heat transfer rate to a plate normal to the jet axis. In this paper the essential differences between the steady a nd oscillating jets are described. Rayleigh temperature measurements a nd 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 pu lsating jet. The modeling results support the conclusions drawn from e xperimental data. The steady jet exhibited typical axial and radial te mperature 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 po rtion of the cycle. The entrainment of ambient fluid and, hence, the m ixing of this fluid with that of the jet was an order of magnitude gre ater for the oscillating jet than for the steady jet. Thus, the temper ature decay rate was also an order of magnitude higher than that of th e steady jet. The high peak velocities, finer scale of turbulence, and the coherent toroidal vortex of the oscillating jet are believed to b e responsible for the improved scalar transport.