Influence of thermal wall boundary condition on scalar statistics during flame-wall interaction of premixed combustion in turbulent boundary layers

Abstract

The influence of thermal wall boundary conditions on the behaviour of scalar statistics during flame-wall interaction (FWI) of premixed flames within turbulent boundary layers has been investigated. Three-dimensional direct numerical simulations (DNS) of two different flow configurations for flames interacting with chemically inert isothermal as well as adiabatic walls in fully developed turbulent boundary layers have been performed. The first configuration is an oblique wall interaction (OWI) of a V-flame in a turbulent channel flow and the second configuration is representative of head-on interaction (HOI) of a planar flame in a turbulent boundary layer. The turbulence in the non-reacting conditions for these simulations is representative of a friction velocity based Reynolds number of Reτ=110 while the combustion characteristics are representative of stoichiometric methane–air mixture under atmospheric conditions. Differences in the mean behaviours of the progress variable and non-dimensional temperature have been observed for the two configurations in response to different wall boundary conditions. Mean scalar variance of the progress variable and temperature and their respective scalar dissipation rates have been investigated and it is found that the variances of the progress variable and temperature remain coupled in the case of adiabatic walls, while significant differences exist between these quantities in the vicinity of the wall under isothermal wall conditions for both flow configurations. The mean scalar dissipation rates of progress variable and temperature during FWI decrease towards the wall in both flow configurations under different thermal wall boundary conditions, but in the case of isothermal wall conditions a higher scalar dissipation rate for the non-dimensional temperature is observed when compared with the scalar dissipation rate for the progress variable. The behaviours of the mean turbulent scalar fluxes of progress variable and the non-dimensional temperature suggest that the scalar fluxes can show both gradient and countergradient type behaviour depending on the flow configuration and also on the proximity of the flame to the wall. In the case of V-flame OWI, the choice of the wall boundary condition has a significant influence on the behaviour of the scalar fluxes, whereas in the case of turbulent boundary layer HOI, no significant differences in the scalar fluxes are observed for different wall boundary conditions.