Flameless combustion in the organised turbulent shear layer

Attention is drawn to the unique pattern of mixing that occurs within the self-similar turbulent shear layer in its ‘organised’ state and the potential it provides for the attainment of high-intensity flameless combustion in situations in which the use of a self-propagating turbulent flame is undesirable or impractical. Heat release of high intensity is possible in such shear layers because the natural organisation of the largest eddy scale can be exploited to both maximise the recirculation of hot burned gas within a combustion chamber and mix the recirculated fluid with the incoming reactants at the molecular scale in a two-stage manner which creates particularly favourable conditions for rapid auto-ignition. An integral Lagrangian modelling approach is presented which captures the essential self-similarity of the two-stage mixing and allows the interaction between the mixing and the reaction kinetics to be analysed predictively in some detail. This involves the use of a network of well-stirred reactors in each of which unsteady intermixing of different fluids and reaction occur simultaneously, the time-varying flows into the individual reactors emulating the growth of the coherent parcels of mixed fluid formed within the real flow. Specimen calculations are presented for the case of hydrogen burning at the high-altitude cruise condition in the core of a modern high-bypass-ratio aircraft engine. The results of these calculations indicate that, in a suitably designed combustor, an adequately high combustion intensity should be readily obtainable with combustion temperature rises as small as 700–1000 K. The approach to the attainment of high-intensity flameless combustion proposed in this paper is inherently versatile and a wide range of other potential applications can be envisaged.