1. Experiments on Strained Premixed Flames in the Distributed Reaction Regime
Alessandro Gomez, Department of Mechanical Engineering, Yale University, USA
In turbulent premixed combustion, experimental investigation of the non-flamelet regime, often called broken reaction zone regime or distributed reaction zone regime, still remains an open challenge. The non-flamelet regime corresponds to a situation of discontinuous flame sheets locally extinguished by turbulent eddies and is expected in very intense turbulence, at turbulent Karlovitz number greater than 100. Conditions of strong turbulence and chemistry interaction are particularly hard to reproduce in a laboratory-scale burner and rare evidence of the non-flamelet regime under such conditions has been reported to date. In this project we demonstrated how highly turbulent flames with real flame effects, such as heat losses and high strain rates, can be established in a compact, bench-top experiment, that is amenable to computational modeling and even direct numerical simulation. The configuration shows a rich phenomenology, including local extinction and reignition, as evidenced by OH fluorescence. The so-called extended Borghi diagram is used to identify various turbulent combustion regimes, with turbulent velocity and integral length scales normalized with respect to the laminar flame speed and the flame thickness. The extended Borghi diagram is an idealization. The present burner allows for a systematic examination of the effects of strain and heat losses. It should enable us to redraw a modified Borghi diagram accounting for these realities and redefine the boundaries of the various regimes. Also noteworthy is the fact that the achievable experimental conditions for these premixed flames overlap part of the domain of IC engines.
OH planar laser induced fluorescence showing a wrinkled, singly connected flame on the left, and a broken one on the right
Extended Borghi digram with the burner operational domain in the dashed trapeze in the figure