Thermal and emission characteristics of a turbulent swirling inverse diffusion flame

The thermal and emission characteristics of a swirl-stabilized turbulent inverse diffusion flame (IDF) burning liquefied petroleum gas (LPG) were studied experimentally and the results of visible flame lengths, flame temperatures, in-flame gaseous species concentrations and global pollutant emissions were reported. The flame shape and length of the swirling IDF and a non-swirling IDF were compared. The swirling IDF is featured by a large internal recirculation zone (IRZ), which plays an important role in stabilizing and shortening the flame. Compared with the non-swirling IDF, the swirling IDF is shorter, wider and more stable. For the swirling IDF, both temperature and species distributions are uniform in the IRZ. Comparison of the radial NOx/temperature distributions indicates that the thermal NO mechanism plays a leading role in NOx formation, since the high-temperature IRZ favors thermal NO production. The effects of air jet Reynolds number (Re) and overall equivalence ratio (F{cyrillic}) on centerline temperature and emission index were examined. The main finding is that the IRZ which is large in size and high in temperature dominates the thermal and emission characteristics of the swirling flame. Efforts were made to compare the global NOx and CO emissions of the swirling and non-swirling IDFs. It was found that strong swirl and lean combustion are two key factors for reducing NOx emission. However, the decreasing NOx emission is compromised by increasing CO. Under stoichiometric and rich conditions, EINOx of the swirling IDFs is slightly higher, but the EICO is significantly lower. Further comparison of EINOx with other studies indicates that the swirling IDF can achieve low NOx emission.