INTENSIFICATION OF MIXING DUE TO ENHANCED INSTABILITY MODES IN VARIABLE DENSITY AND COUNTERCURRENT JETS

摘要

The paper presents a discussion of the mixing intensification caused by unstable modes in variable density and countercurrent jets and its application to the optimization of gas burners. Very high mixing intensity can be attained in flows under so called absolute instability. The idea of absolute instability proposed originally by Landau relies on the perturbation development both in space and time. If the small perturbation develops in time in a given place it finally reaches the highest possible oscillation amplitude limited only by non-linear flow interactions. Moreover because the perturbation is not convected from its onset it affects its source leading to self-exciting oscillations amplifying the effect of mixing enhancement. According to recent theoretical and experimental studies the absolute instability appears in low-density and countercurrent jets provided that certain critical density or velocity ratios are attained. The countercurrent technology seems to be very promising in application to gas burners optimization since this technology can be combined effectively with the reburning idea leading to very efficient NO_x emission reduction.