Self-excited jet-precession Strouhal number and its influence on downstream mixing field

Abstract

Luxton et al. (1987) discovered naturally occurring precession of a nonswirling, axisymmetric jet discharging into a large cylindrical chamber. The fluid-mechanical (fluidic) nozzle which generates this precessing jet (PJ) flow has found applications in industrial burners, providing stable flames, reduced pollutant emissions, and fuel savings. The low-frequency precession phenomenon is the key to achieve those benefits. However, its complexity is such that a detailed understanding of it is still in its infancy. The present study investigates the jet precession from a fluidic nozzle with a configuration similar to that of Luxton et al. It firstly assesses the variation in the precession frequency with the chamber length and jet velocity and then seeks to extend previous definitions of the precession Strouhal number for the flow emerging from the chamber outlet. Previous definitions are based on the characteristic velocity and length scales of the flow at the chamber inlet. The effects of both Strouhal and Reynolds numbers on the downstream mixing field are examined by measuring the total pressure variation along the nozzle axis at different values of these numbers. The influence of the Strouhal number is also explored by visualizing the PJ flames. It is demonstrated that the jet precession frequency increases approximately linearly as either the nozzle chamber length or the jet velocity increases, for the present range of conditions in which the precession occurs continuously. Also, the precession Strouhal number is found to have far stronger influence on the downstream mixing field than does the Reynolds number.