Observation of the flame structures emerging at low Damköhler number fields

Abstract

We discussed the flame structure categorized in distributed reaction zone and well-stirred reactor on Borghi’s phase diagram. Recently, it is thought that the flame structure formed in high intensity and small scale turbulence is of thin reaction zone structure. The theory, however, is according to the hypothesis that a laminar flamelet propagates into a high intensity turbulent flow, whose Damköhler number is low. We supposed that the distributed reaction zone is formed when reaction initiates in a low Damköhler number field. In the present study, low Damköhler number conditions were successfully formed in relatively weak turbulence fields, even around a burner rim, by using very lean premixture, which gave a sufficiently long chemical characteristic time. Laser tomography, chemiluminescence detectors, LDV and thermocouple were used simultaneously for observing lean turbulent premixed flames with silica powder seeded. As a result, the transition of flame structure from a distributed reaction zone to a wrinkled laminar flame was observed in detail by traversing the observation point from near-nozzle region to downstream region. Further, the flame structures, in which a distributed reaction structure coexists with a propagating flame structure, were observed in the near-nozzle region. It was concluded that distributed reaction zone structure is formed when reaction start at a low Damköhler number field, while thin reaction zone is formed when a laminar flame propagates into a low Damköhler number field.