Heat flux and flow topology statistics in oblique and head-on quenching of turbulent premixed flames by isothermal inert walls

Abstract

Direct Numerical Simulations (DNS) of oblique wall quenching of a turbulent V-flame and head-on quenching of a statistically planar flame by isothermal inert walls have been utilised to analyse the statistics of wall heat flux, flame quenching distance in terms of the distributions of flow topologies and their contributions to the wall heat flux. The flow topologies have been categorised into 8 generic flow configurations (i.e. S1-S8) in terms of three invariants of the velocity gradient tensor (i.e. first, second and third , and respectively). It has been found that nodal (i.e. strain rate dominated) flow topologies are major contributors to the wall heat flux when it attains large magnitude in the head-on quenching configuration, whereas focal (i.e. vorticity dominated) topologies contribute significantly to the wall heat flux in the case of oblique flame quenching. These differences in the heat transfer mechanisms contribute to the differences in wall heat flux and flame quenching distance between head-on quenching and oblique quenching configurations. The maximum wall heat flux magnitude in the case of oblique flame quenching has been found to be greater than that in the corresponding turbulent head-on quenching case. By contrast, the minimum wall Peclet number, which quantifies the flame quenching distance, in the case of oblique quenching has been found to be smaller than that in the case of head-on quenching. Lai et al., 2017a-d) in comparison to the vast body of literature on DNS of turbulent reacting flows away from the walls. Poinsot et al. (1993) used two-dimensional DNS to analyse the flame-vortex interaction in head-on quenching (HOQ) of premixed flames where the mean direction of flame propagation is normal to the wall. This two-dimensional analysis also provided the quantification of the maximum wall heat flux and the minimum wall Peclet number (i.e. normalised quenching distance) in the case of HOQ, which has been confirmed recently by a three-dimensional DNS based analysis by Lai et al. (2016a). The analysis of Lai et al. (2016a) further demonstrated the influences of the characteristic Lewis number on the maximum wall heat flux and the flame quenching distance. Lai et al. (2017a,b) also analysed turbulent kinetic energy and enstrophy transports in the near-wall region for HOQ of premixed turbulent flames based on simple chemistry DNS data, and revealed that the presence of wall and wall-induced quenching significantly affect the statistical behaviours and modelling of the unclosed terms of the turbulent kinetic energy equation and baroclinic torque and dilatation contributions to the enstrophy transport. Recently, Sellmann et al. (2017) and Lai et al. (2017c) have addressed the modelling of the FSD and turbulent scalar flux transports using DNS data of HOQ of statistically planar premixed turbulent flames. The algebraic closures of FSD in the context of Large Eddy Simulations (LES) have also recently been analysed by Lai et al. (2017d). Lai and Chakraborty (2016b,c) and Lai and Chakraborty (201...

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