Reaction zone structure in flameless combustion

We present a study of the combustion limit and reaction zone structure of non-premixed counterflow flames of highly preheated air and methane diluted with nitrogen. First, we measured the flammable region experimentally in the range of air temperatures up to 800 K. In the case of flame stretch rates smaller than 20 s^-1, experiments were conducted under microgravity. All the microgravity tests were conducted at the JAMIC drop tower facility in Hokkaido, Japan. The results show that the extinction limits become broader with increasing air temperature. All the extinction curves are C-shaped and exhibit the radiation extinction branch. That is, the configurations of the extinction curves are the same as those of conventional combustion at air temperatures from 300 K to 800 K. Second, we computed the flammable region and reaction zone structure of these flames by using the conventional one dimensional flame code with detailed chemistry. The flammable regions obtained by experiment agree with the computated ones. This shows that the present computation could also represent the phenomena well for high-temperature air combustion. Based on this, computation was extended to higher air temperatures (up to 1900 K) at which microgravity experimentation is not possible. When the air temperature was higher than 1300 K, extinction limits disappeared. In this temperature range, combustion continues even under extremely fuel-lean conditions such as 1% methane in nitrogen. Reaction zones without any temperature peaks were observed. Methane and oxygen leakage through the flame occurred, and they coexisted there. This is like the Liñán's premixed flame regime, in other words, a reaction-time-dominated reaction zone structure. In these regions, NO_x emission is very low, and this may lead to the low levels of NO_x emission of high-temperature air combustion.