A numerical and experimental investigation of “inverse” triple flames
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abstract
Tribrachial or triple flames represent a class of partially premixed flames that generally contain three spatially distinct but synergistically coupled reaction zones, namely a rich premixed, a lean premixed, and a nonpremixed reaction zone. The generally considered flow arrangement for burner-stabilized triple flames involves a rich mixture issuing from a central port and a lean mixture from two outer ports, which we call a reference configuration (RC). Herein, we examine an inverse configuration (IC) in which a fuel-lean stream is flanked by two fuel-rich streams. The reaction zone topology in this configuration is richer and more complex compared to that in a RC flame. A numerical-experimental investigation is conducted to characterize the fundamental differences and similitude between the RC and IC methane–air triple flames in both spatial and mixture fraction based coordinates. The detailed structure of IC triple flames and their response to variations in the rich and lean equivalence ratios are examined. Finally, the transient behavior of the flames in both configurations is described. The IC and RC flames have markedly different spatial structures. In the inverse configuration, the global flame contains five reaction zones. The predicted and measured topologies of the various reaction zones are in excellent agreement. The modified mixture fraction (ξ) is found to be effective in characterizing the structure of both RC and IC flames. The scalar profiles in terms of ξ clearly illustrate the similitude between the two flames. The three reaction zones in the RC flame have a structure that is similar to that of the corresponding five reaction zones of the IC flame. The two nonpremixed (or the two rich premixed reaction zones) in the IC flames are not differentiated in terms of ξ. Both the flames are subjected to a buoyancy-induced instability at normal gravity that generates large vortex structures, which cause the reaction zones to flicker. The flame–vortex interaction is initiated in the nonpremixed reaction zone for the IC flame and in the lean premixed reaction zone for the RC flame. Consequently, the IC flame flickers with higher amplitude but lower frequency as compared to the RC flame. There is good agreement between the measured and predicted flickering frequencies for the two flames. As φrich is increased, the height of the two rich premixed reaction zones in the IC flames increases, their tips open, and the chemical activity in these zones decreases. While the oscillation frequency is essentially constant, the oscillation amplitude increases as φrich is increased. The effect of increasing φlean is to enhance chemical activity in the lean premixed zone. However, the two rich premixed zones and the outer nonpremixed zone are relatively unaffected by variations in φlean. The oscillation amplitude and frequency also remain unaffected by variations in φlean.
