OBSERVATION OF DROPLET GROUP COMBUSTION IN TERMS OF SIMULTANEOUS MEASUREMENT OF MIE SCATTERING AND SPECTRAL LUMINOSITY FROM SPRAY FLAMES

Detailed structure of spray flames has significant influence on burnin g rates, pollutant emissions, and flame stability. The purpose of the present study is to elucidate the ''group combustion'' structure of sp ray flames with or without gaseous fuel in terms of simultaneous monit oring and visualizing OH-radical chemiluminescence, CH- and/or C-2-ban d flame luminosity, and Mie scattering of droplet clusters and subclus ters in the vicinity of an apparent flame front. It was found that, in the case of a small liquid fraction in the fuel and a small average d iameter of droplets; gas-phase reaction occurred in premixed combustio n mode inside the periphery of each droplet cluster, whereas in the ca se of a large liquid fraction in the fuel and a large average diameter of droplets, gas-phase reaction accompanied by OH-radical chemilumine scence occurred outside the periphery. In the latter case, the intense pulsating emission signals in the CH and/or C-2 bands that did not sy nchronize with the emission signal in the OH band were detected in the downstream region. This implies that small dense subclusters survived downstream of the flame front, burning with an intense solid-body lig ht emission. These results suggest that the spray flame has a group st ructure-not a simple one, but a complicated one determined by the beha vior of droplet clusters and subclusters.