PhD ThesisThis thesis focuses on fundamental physical understanding and modelling of turbulent
premixed flame-wall interaction by using Direct Numerical Simulation (DNS)
data. Three-dimensional compressible simulations of turbulent premixed flame-wall
interaction have been carried out for head-on quenching (HOQ) of statistically planar
flames by an isothermal inert wall and also for oblique quenching of a V-flame by two
isothermal inert sidewalls (top and bottom walls). Simulations have been conducted
for different values of Damköhler, Karlovitz and global Lewis numbers (i.e. Da, Ka
and Le), and the chemical mechanism is simplified by a single-step Arrhenius type
irreversible chemistry for the sake of computational economy in the interest of a detailed
parametric analysis. The flame-wall interaction has been characterised in terms of wall
heat flux magnitude and wall Peclet number (i.e. normalised wall normal distance).
It has been found that the maximum wall heat flux magnitude decreases, whereas
the minimum wall Peclet number (which quantifies the flame quenching distance)
increases with increasing Lewis number in the case of laminar head-on quenching of
planar flames. However, the minimum wall Peclet number for Le < 1.0 turbulent
premixed flames has been to be smaller than the corresponding laminar value, whereas
the minimum Peclet number in the case of turbulent flames with Le ≥ 1.0 remains
comparable to the corresponding laminar values. It has been found that heat loss
through the wall and flame quenching in the vicinity of the wall significantly affect
dilatation rate distribution in the near-wall region, and has influences on the behaviours
of the invariants of the velocity gradient tensor, which in turn influences statistical
behaviours of flow topology and enstrophy distribution in the near-wall region. The
statistical behaviours of vorticity and enstrophy transports in the near-wall region and
the distribution of flow topologies within the flame, and their evolution with flame
quenching have been analysed in detail using DNS data, and important fundamental
physical insights have been gained regarding the flame-quenching processes associated
with the flame-wall interaction.
The DNS data has been explicitly Reynolds averaged to analyse the statistical
behaviours of turbulent kinetic energy, scalar variance, turbulent scalar flux, FlameSurface Density (FSD) and scalar dissipation rate (SDR) and their transport in the
near-wall regions. It has been found that existing closures of these quantities do not
adequately capture their near-wall behaviours and in this thesis modifications to the
existing closures have been proposed based on a-priori DNS analysis to account for
the wall effects in such a manner that the modified closures perform well both near
to and away from the wall. Furthermore, it has been found both FSD and SDR
based conventional reaction rate closures do not adequately capture the mean reaction
rate close to the wall, and the current analysis offers alternative reaction rate closure
expressions both in the contexts of FSD and SDR based modelling approaches. Thus,
the current thesis offers a unified modelling strategy for premixed flame-wall interaction
in the context of Reynolds Averaged Navier-Stokes (RANS) simulations for the very
first time.
Finally, in order to validate the findings based on simple chemistry DNS, a limited
number of DNS calculations of head-on quenching has been conducted using a multistep
chemical mechanism for methane-air combustion. It has been found that the
statistics of wall heat flux magnitude and wall Peclet number obtained from detailed
chemistry simulations are in good qualitative and quantitative agreements with the
corresponding results from simple chemistry DNS. However, detailed chemistry DNS
reveals the presence of heat release at the wall during early stages of flame quenching,
whereas heat release remains identically zero at the wall for simple chemistry DNS. In
spite of this difference, an FSD based reaction rate closure which was proposed based
on a-priori analysis of simple chemistry DNS has been found to work also for detailed
chemistry DNS data without any modification. This provides the confidence in the
models which have been proposed based on the analysis of simple chemistry DNS data