Flame propagation in
narrow channels:
What Darrieus and
Landau didn't tell you
Abstract
The
flame speeds and wrinkling spectra of premixed flames propagating in quasi-2D
channels (Hele-Shaw cells) were studied using CH4 and C3H8
fuels with N2 and CO2 diluents. Upward, downward and horizontal propagation configurations
were tested for varying mixture strength and thus laminar burning velocity (SL). In this way the effects of buoyancy,
thermal expansion, heat loss and Lewis number were studied. Wrinkling and
thus flame speed enhancement was observed even for downward propagating
(buoyantly stable) flames have high Le (diffusive-thermally stable) due to the
effects of thermal expansion (Darrieus-Landau, DL) and viscosity
increase (Saffman-Taylor, ST) across the front. The quasi-steady flame speed (UT) was always
higher than (SL), typically by a factor of 3. Values of UT/SL
correlated well with a scaled growth rate parameter (K) based on the
Joulin-Sivashinsky model of flame instabilities in narrow channels due to DL,
ST and buoyancy effects. The observed correlation was UT/SL
= 1 + K, thus K serves a role similar to u' in turbulent combustion in the
laminar flamelet regime. Wrinkling spectra exhibited a marked change as
the cell thickness decreased due to a change in the dominant instability
mechanism from DL to ST. Flame
wrinkling in the plane of the cell and front curvature in the transverse
dimension are found to be of similar importance in affecting UT. These results indicate that the
behavior of practical flames in confined geometries such as internal combustion
engines or gas turbines is quite different from that inferred from laboratory
experiments conducted in open geometries such as Bunsen, counterflow or
V-flames, where thermal expansion is relaxed in the transverse directions.