On dissipation intermittency and its modelling in terms of turbulence-chemistry interactions

Abstract

A key challenge in combustion research is to model complex turbulence-chemistry interactions during extinction and re-ignition of non-premixed ames. These are caused when the scalar dissipation, , is above (or below) certain limits such that the diffusive temperature ux from the ame is much larger (or smaller) than chemical heat-release. is characterised by its small-scale intermittency, i.e. large uctuations in are frequent and localised in space. This behaviour severely constrains its resolvability and has implications for modelling extinction/re-ignition processes. Scalar eld statistics from the direct numerical simulation (DNS) database of a spatially-evolving, turbulent jet ame with multi-step chemistry by Pantano (2004) are studied using three different approaches, namely analysis of dissipation-spectra, direct investigation of dissipation contours and spatial filtering of the instantaneous dissipation signals. Out of these the spatial filtering method is found to be most suited for capturing the intermittent dissipation length scales and an Reδ-1 scaling is proposed for 'adequate' -resolution in turbulent jet ame experiments/simulations. Furthermore, a Multiple Mapping Conditioning (MMC) approach with two reference variables is used to model extinction/re-ignition in inhomogeneous turbulent jet ames. A new sub-model for the convective velocity term is employed that does not need to presume Gaussian statistics and consistent closures for MMC drift and diffusion coefficients are derived. Effect of temperature uctuations on scalar diffusivity is also accounted for. Joint scalar PDFs, conditional dissipation and conditional species predictions from MMC, and also conditional species concentrations from conventional singly- and doubly-conditioned moment closure (CMC1 and CMC2), are assessed against the Pantano DNS database. CMC1 expectedly over-predicts extinction and does not capture re-ignition, whereas extinction and re-ignition effects on species concentrations (including atomic H-radical) are captured satisfactorily using CMC2 and MMC. However, MMC scores over CMC2 because models for the joint scalar PDF evolution and conditional dissipation are self-contained in the former.