It is well known that steadily propagating planar premixed-gas flame fronts are unstable to a number of disturbances, notably that due to thermal expansion as first analyzed by Darrieus (1938) and Landau (1944). However, in most laboratory experiments the effects of the Darrieus-Landau (DL) instability are masked by the use of open geometries such as Bunsen, counterflow or V-flames, where thermal expansion is relaxed in the transverse directions. With this motivation the front speeds and wrinkling spectra of premixed flames propagating in quasi-2D channels (Hele-Shaw cells) were studied in order to avoid suppression of DL instabilities. CH4 and C3H8 fuels with N2 and CO2 diluents were used to assess the effects of Lewis number (Le) and thus diffusive-thermal (DT) instabilities. Upward, downward and horizontal propagation configurations were tested to assess the effects of buoyancy and thus Rayleigh-Taylor (RT) instabilities. Varying mixture strengths and thus laminar burning velocities (SL) were employed to assess heat loss effects.

Wrinkling and thus flame speed enhancement was observed even for downward propagating (RT stable) flames have high Le (DT stable) due to the effects of DL and the viscosity increase (Saffman-Taylor, ST) instability across the front. The quasi-steady wrinkled 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. The viability of modeling this type of front propagation using a modified level-set (G equation) approach is discussed.