Flow structure in near-nozzle region of gas jet flames

Abstract

A flow visualization study of the near-nozzle regions of a cold jet, an attached flame, and a lifted flame of propane jet exiting a contoured nozzle with uniform velocity profile in quiescent air is presented. The observed flowfields and celerities of the coherent structures in the three cases are compared. The results show that com- bustion reactions occur away from both jet boundary and cores of coherent structures. The presence of flame retards the growth of the coherent structures and increases their celerity substantially. HE interaction of flames and flow structure in combus- tion systems has been the subject of several investiagations in the past. Many of these studies were concerned with pre- mixed flames. In recent years, however, considerable effort has been devoted to understand the flame/flow interaction in reacting plane shear layers13 and axisymmetric gas jets.47 Such studies are motivated by the necessity to understand the roles of coherent structures in the mixing rates.89 This infor- mation is needed to improve the burning characteristics of dif- fusion flames and to develop theoretical models for their prediction. A review of the uses of coherent structures was presented recently by Coles.10 An understanding of the detailed flow structure and its interaction with reaction zones is also crucial to unfold the stability mechanism of diffusion flames, including liftoff and blowout phenomena, which has been a topic of controversy in recent years.11'12 Here, we present some results from a flow visualization study in the near-nozzle region of propane gas jet flames where the flowfield is essen- tially an axisymmetric shear layer. The results not only con- firm the presence of large-scale structures in the flames but also reveal some characteristic features of their behavior in the presence of combustion. We focused our attention on the near-nozzle region in order to understand 1) the changes in flow structure of cold jets caused by the presence of a surrounding burner-attached flame, and 2) the liftoff and reattachment processes of flames.