The role of flow instability in combustion resonance

• Main Author: Nwagwe, Ikechuku Kwezi
• Published: University of Leicester 1998
• Subjects: 620.1
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.696470

The main objective of this work was to investigate a possible two step hydrodynamic mechanism for the production of combustion resonance in fully premixed gas burners. This two step mechanism was modelled numerically using a two stage procedure. In the first stage, the stability of a low Reynolds number, impinging shear layer to acoustic forcing, similar to that in a burner at resonance, was investigated. This was carried out using an unsteady, higher order, incompressible, finite volume Navier-Stokes solver specifically developed for this purpose. Simulation of the isothermal entry flow for a range of parametric forcing conditions was carried out. Results indicate quite strongly that at Reynolds numbers where the unforced flow is steady, acoustic forcing may result in the roll up of the shear layer into coherent vortices by a convective instability mechanism. Flow visualisation was carried out, primarily using vorticity contours and quantitative measurements were made of the vortex characteristics. In the second stage of the work, the interaction of vortices (representative of those formed at the entry of the burner and simulated in the first stage of this work) with a laminar flame front were calculated using a simple, inviscid, Lagrangian flame model. The flame was represented as a discontinuity across which cold unburnt reactants were transformed into hot combustion products. Preliminary results from this study indicate the usefulness of this method for investigating flame-vortex interaction of the type thought to occur in these burners, though further refinement of the model is required. The results of this study would therefore indicate that flow instability may be the primary mechanism by which resonance occurs in fully premixed gas burners of a design investigated in this work, though at very low Reynolds numbers this would become increasingly difficult.