Quantification of combustion regime transitions

• Main Author: Hampp, Fabian
• Other Authors: Lindstedt, R. Peter ; Beyrau, Frank
• Published: Imperial College London 2015
• Subjects: 621.402
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.686331

The current work provides fundamental understanding of combustion regime transitions from distributed reactions towards the corrugated flamelet regime through a novel application of the multi-fluid approach of Spalding. Aerodynamically stabilised premixed flames were studied in a back-to-burnt opposed jet configuration featuring fractal grid generated multi-scale turbulence (Re ≃ 18,400 and Ret > 350). The chemical timescale was varied via the mixture stoichiometry, fuel reactivity and excess enthalpy with rates of strain exceeding the laminar flame extinction point. Rayleigh thermometry was performed to quantify the reaction zone broadening with large low temperature regions observed. Simultaneous Mie scattering, OH-PLIF and PIV were used to quantify the encounter of intermediate fluid states (i.e. mixing, mildly and strongly reacting) in addition to reactants and combustion products. A physical interpretation was provided for the individual fluid states. The analysis showed self-sustained flames in low strain regions with a collocated and pronounced dilatation for higher Damköhler numbers. By contrast, highly strained regions resulted in an auto-ignition related burning with attenuated dilatation and increased vorticity levels. The variation of the excess enthalpy - in particular for low Damköhler number combustion - illustrates the dominant influence of the burnt gas state on the dilatation and burning mode, with a distinct impact on the scalar flux also evident. The fuel reactivity showed a clear effect on the burning mode transitions, with explicit differences in the resulting flow field. The flow conditions were analysed in terms of Damköhler and Karlovitz numbers based on chemical timescales corresponding to laminar flames and auto-ignition events. The thesis provides novel insights into the underlying conditions leading to combustion regime transitions by means of (i) the evolution of multi-fluid probability, (ii) interface, (iii) mean flow field, (iv) conditional velocity and (v) conditional strain statistics evaluated as a function of the Damköhler number. (vi) The combustion mode influence on the scalar transport is discussed and (iv) a tentative 3D regime diagram is provided. The data illustrate the potential of a multi-fluid delineation to quantify a wide range of burning modes of relevance to low polluting combustion technologies.