Opposed jets with small separations and their implications for the extinction of opposed flames

Abstract

 Profiles of static pressure and the mean and rms of velocity fluctuations in opposed isothermal jets were measured with a combination of static-pressure probes, hot-wire and laser-Doppler velocimetry. The static pressures close to the axis at the outlet planes of the two jets increased as the separation was reduced below three diameters, with measurable and increasing radial flow below 0.8 diameters. This, in turn, reduced the mean axial velocity on the axis of symmetry. The mean strain rate along the stagnation plane was near constant over much of the impingement region for separations greater than 1.0 diameter and a maximum occurred at radial distances which increased as the jet separation was reduced below this value. The maximum values stemmed from the exit-plane profiles and were larger than the bulk strain rate by a factor which increased with radial distance from 2 to 5 for separations of 0.8 and 0.2 diameters.

The non-uniform radial profile of mean strain rate caused local extinction of premixed flames to occur initially in the region of the maximum value and, with further increase in the bulk, and therefore in mean strain rate, it moved radially inwards to produce complete extinction. The bulk strain rate for complete extinction increased non-linearly with separation so that it was almost three times larger at 0.2 than at 0.8 diameters.