Flame temperatures along a laminar premixed flame with a non-uniform stretch rate

We investigated experimentally the effects of a spatially non-uniform stretch rate on the flame temperature. A flame surface with a non-uniform stretch rate was formed by creating a wrinkled laminar premixed flame in a spatially periodic flow field of a lean propane/air mixture. The measured flame temperature was lower/higher than the adiabatic flame temperature at flame segments with positive/negative stretch rates. This was a result of the effects of flame stretch and preferential diffusion for Lewis number greater than unity. The flame temperature estimated using the conventional flame stretch theory, which is based on a uniform stretch rate along the flame surface, did not agree quantitatively with the measured temperature. Therefore, we revised the theory, taking into account heat transfer along the flame surface, and then produced estimates that agreed with the measured temperature. We found that the effect of flame stretch and preferential diffusion is changed along the flame surface which has spatially non-uniform stretch rate, causing a temperature gradient along the surface, which in turn transfers heat and changes the flame temperature. Thus, heat transfer along the flame surface is an important factor in estimating flame temperature. In addition, a second temperature gradient appears downstream just behind the flame, because the temperature of the burned gas is also non-uniform. Therefore, conductive heat transfer is believed to occur between the flame and the burned gas. The effect of the downstream heat transfer is not as large as that of the heat transfer along the flame surface.