A DNS study of the physical mechanisms associated with density ratio influence on turbulent burning velocity in premixed flames

Artikel i vetenskaplig tidskrift, 2018

Data obtained in 3D direct numerical simulations of statistically planar, 1D weakly
turbulent flames characterised by different density ratios σ are analysed to study the
influence of thermal expansion on flame surface area and burning rate. Results show
that, on the one hand, the pressure gradient induced within a flame brush owing to heat
release in flamelets significantly accelerates the unburned gas that deeply intrudes into
the combustion products in the form of an unburned mixture finger, thus causing largescale
oscillations of the burning rate and flame brush thickness. Under the conditions
of the present simulations, the contribution of this mechanism to the creation of the
flame surface area is substantial and is increased by σ, thus implying an increase in the
burning rate by σ. On the other hand, the total flame surface areas simulated at σ =
7.53 and 2.5 are approximately equal. The apparent inconsistency between these results
implies the existence of another thermal expansion effect that reduces the influence of σ
on the flame surface area and burning rate. Investigation of the issue shows that the flow
acceleration by the combustion-induced pressure gradient not only creates the flame
surface area by pushing the finger tip into the products, but also mitigates wrinkling of
the flame surface (the side surface of the finger) by turbulent eddies. The latter effect
is attributed to the high-speed (at σ = 7.53) axial flow of the unburned gas, which is
induced by the axial pressure gradient within the flame brush (and the finger). This
axial flow acceleration reduces the residence time of a turbulent eddy in an unburned
zone of the flame brush (e.g. within the finger). Therefore, the capability of the eddy
for wrinkling the flamelet surface (e.g. the side finger surface) is weakened owing to a
shorter residence time.