The structure of turbulent flames was examined in a premixed-charge, spark-ignition ported engine using a three-dimensional visualization technique with 10 ns time resolution and 350 µm best spatial resolution. The engine had a pancake chamber, a compression ratio of 8, a TDC swirl number of 4 and was operated at 300, 1200 and 2400 rpm with stoichiometric and lean propane/air mixtures.
The second and third harmonic beams of an Nd-YAG laser (532 nm and 355 nm), along with the two strongest beams (first Stokes (683 nm) and first anti-Stokes (436 nm)) from a hydrogen Raman shifter pumped by the second harmonic were used to create four parallel laser sheets each of less than 300 microns thickness. The laser sheets were passed through a transparent quartz ring in the cylinder head parallel to the piston top with vertical separations between successive sheets ranging from 1.5 to 0.9 mm. The light from the four sheets was scattered by sub-micron TiO2 seeding particles and collected through a transparent quartz window in the cylinder head by a 100 × 100 intensified diode array camera operated at a luminous gain of 103. The Field of view was 16 × 16 mm at the center of the chamber. The collection optics included a quadrupling multivision prism with four different color filters in front of each of its facets and a video lens which allowed imaging areas from each of the four planes to occupy almost a quarter (44 × 44 pixels) of the 100 × 100 diode array.
The simultaneous images on four different planes provide the first three-dimensional information of the flame structure in an engine. Over 100 flames were analyzed for each of the six cases.
The three stoichiometric flames are in the reaction sheet regime where islands of reactants were expected. The three lean flames are between the reaction sheet and the distributed combustion regimes where islands of products and reactants were expected. No island of products or reactants was observed in any of the flames. In about 10% of the lean cases “fingers” of products were found. Fractal analysis was performed on all the laminar fronts and our earlier finding that their wrinkles exhibit fractal behavior is now confirmed for all conditions. The fractal dimension increases with engine speed and charge leanness but eventually it may level off. The differences among the instantaneous fractal dimension in the four planes were taken as measures of the inhomogeneity in the geometry of the flame wrinkles and were found to be significant. There is a correlation between the fractal dimension in various planes over a distance of the order of the integral length scale.