In most practical situations, turbulent premixed flames are
ducted and, accordingly,
subjected to externally imposed pressure gradients. These pressure gradients
may
induce strong modifications of the turbulent flame structure because of
buoyancy
effects between heavy cold fresh and light hot burnt gases. In the present
work, the
influence of a constant acceleration, inducing large
pressure gradients, on a premixed
turbulent flame is studied using direct numerical simulations.A favourable pressure gradient, i.e. a pressure decrease from unburnt
to burnt
gases, is found to decrease the flame wrinkling, the flame brush thickness,
and the
turbulent flame speed. It also promotes counter-gradient turbulent transport.
On the
other hand, adverse pressure gradients tend to increase the flame brush
thickness
and turbulent flame speed, and promote classical gradient turbulent transport.
As
proposed by Libby (1989), the turbulent flame speed is modified by a buoyancy
term
linearly dependent on both the imposed pressure gradient and the integral
length
scale lt.A simple model for the turbulent flux
u″c″ is also proposed, validated from
simulation data and compared to existing models. It is shown that turbulent
premixed flames
can exhibit both gradient and counter-gradient transport and a criterion
integrating
the effects of pressure gradients is derived to differentiate
between these regimes. In
fact, counter-gradient diffusion may occur in most practical ducted flames.
Effects of pressure gradients on turbulent premixed flames
