抄録
It is well known that the combustion in SI engines typically takes place in the wrinkled laminar flame region. According to the concept of the conventional models of turbulent burning velocity in this turbulence condition, the ratio of the turbulent burning velocity to the local burning velocity is proportional to the ratio of the turbulent flame surface area to the laminar flame surface area. The area ratio is well approximated to be proportional to the ratio of the turbulence intensity to the local burning velocity, which is usually taken to be equal to the original laminar burning velocity. However, in our previous works, the mean local burning velocity turned out to be changed from the original laminar burning velocity due to the preferential diffusion effect and it was found to be an important factor as dominating the turbulent burning velocity. This fact seems to contradict the concept of the conventional models. In the present study, an attempt is made to investigate directly the local propagation characteristics of methane and propane premixed turbulent flames in the wrinkled laminar flame region. A laser tomography technique is used to obtain the local flame configuration and movement. And quantitative analyses are performed. The local flame front curvature 1/r and the local flame burning velocity S_F are quantitatively obtained as the key parameters of the turbulent combustion. In the first place, the observation of sequential flame tomograms shows that the turbulent flame front can be classified into the active and inactive part depending on its geometric configuration. In the next place, the values of S_F are obtained directly from the sequential flame tomograms as the local burning velocity. Figure shows a typical probability density function (pdf) of the acquired 1/r normalized by the preheat zone thickness η_o, and a distribution of the acquired S_F normalized by S_<LO> for the laminar flame and the turbulent flame, respectively. It is found that the values of S_F of the turbulent flame are not a constant value as S_<LO>, but to be distributed over a wide range depending on the local curvature of the turbulent flame. In addition, the mean value of S_F for the methane mixture has a tendency to become larger as decreasing its equivalence ratio, whereas that of the propane mixture to become smaller.
