Influence of strain and heat loss on flame stabilization in a non-adiabatic combustor

Abstract

Experiments on swirl stabilized burners in a non-adiabatic combustor show flame stabilization only at the inner shear layer, whereas turbulent combustion models typically predict stabilization at both layers. In the present work, it is argued that these deviations are due to the presence of strain and heat loss in the outer shear layer. The combined influence of these effects on a laminar flame was analyzed using an asymmetric counterflow configuration with detailed chemistry. An empirical relation for the laminar flame speed including these two effects was proposed and implemented into a turbulent combustion model. With this extension, it was possible to get a flame that exhibits the correct stabilization behavior only at the inner shear layer with a flame length similar to the experiments. ∗ Corresponding author: tay@td.mw.tum.de Proceedings of the European Combustion Meeting 2009 Introduction Flame stabilization by swirl is commonly used in technical applications like gas turbine combustors. The swirling flow creates a vortex breakdown leading to an inner recirculation zone (IRZ) at the burner exit. The inner recirculation zone transports hot products and radicals from the flame back to the unburnt mixture, supporting the continuous ignition of the flame and the stabilization at the inner shear layer (see Fig. 1). Additionally, an outer recirculation zone (ORZ) develops in the combustor, presenting a similar flow pattern. Quite interesting, many swirl-stabilized burners do not exhibit significant heat release in the outer shear layer. This behaviour was observed for the burner investigated in this paper and in other burners like the one presented in Fig. 1 [1]. Fig. 1: Top: OH chemiluminescence measurement using a swirl burner in a square section non-adiabatic combustor [1]. Bottom: Scheme for the shear layers and recirculation zones. In Fig. 1, experimental results of OH* chemiluminescence emissions are shown with contours as measure of the heat release, and the velocity obtained by particle image velocimetry is represented by vectors. The distribution of the OH* emissions corresponds to a line of sight integration over the depth of the combustion chamber. It can be seen that the flame stabilizes only at the inner shear layer. The stabilization behaviour of the flame at the outer shear layer is strongly affected by the presence of both flame stretch (defined as the fractional rate of change of the flame surface area [2]) and heat losses. The latter appear since the products transported in the ORZ are cooled by the combustor wall cooling. Flame stretch is created by the combined effects of strain and curvature. Nevertheless, in highly turbulent flows with a Karlovitz number higher than 0.1, the influence of curvature is very small [3]. Thus, the curvature is neglected for this study and only effects of strain are taken into account. The combined occurrence of strain and heat losses is especially relevant, since flames are more sensitive to strain effects under non-adiabatic conditions, what could eventually lead to a total extinction of the flame in zones affected by heat losses [4-9]. Some turbulent combustion models take into account effects of strain to promote extinction considering an extinction stretch rate [10,11] but do not consider this enhanced sensitivity to stretch under nonadiabatic conditions, leading to a flame stabilized at the inner and outer shear layers as seen in Fig. 2 in a cross plane of a square section combustor. In order to obtain the correct flame stabilization, the combined influence of strain and heat losses has to be taken into account by the combustion model. Fig. 2: Reaction source term at cross plane from CFD simulations using a swirl burner in a square section non-adiabatic combustor [1]