Near-Wall Unsteady Premixed Flame Propagation in S.I. Engines 951001
A computational study of the near-wall premixed flame propagation in homogeneous charge spark ignited engines is presented on the basis of a spectral concept accounting for flow-chemistry interaction in the flamelet regime. Flame surface enhancement due to wrinkling and modification of the local laminar flame speed due to flame stretch are the main phenomena described by the model. A high pass filter in the turbulent kinetic energy spectrum associated with the distance between the ensemble-averaged flame front location and the solid surface has been also introduced. In addition a probability density function of instantaneous flamelet positions around the above mean flame front location allows to consider statistical effects in a simplified way. Issues of temperature distribution within the boundary layer and associated heat losses, except for the concept of a thermal quenching distance, are thereby not explicitly taken into account. The findings of this study are interpreted using the arrival time difference at one quenching distance away from the wall between a freely propagating turbulent flame and the near wall flame propagation as described by the present model. Results obtained so far for propane-air mixtures indicate that the extent of flame deceleration in the vicinity of the wall, expressed in crank angle units, increases with increasing engine rotational frequency, leaner fuel-air mixture, increasing exhaust gas recirculation rate, decreasing pressure and decreasing engine size; the overall flame arrival time delay thereby is of the order of 2 to 7 degrees crank angle.