The influence of low-temperature chemistry on partially-premixed counterflow n-heptane/air flames

The continuous research towards novel combustion technologies, operating at high temperatures and pressures, has recently raised the question of the influence of low temperature chemistry in such conditions. As a first step in identifying the potentially critical conditions where low-temperature oxidation might play a major role, this theoretical study investigates the steady-state flame evolution of 1-dimensional, partially-premixed counterflow diffusion flames. To this purpose, n-heptane is used as reference species, as representative of real fuels. Its kinetic mechanism, including the low- and high-temperature oxidation paths of n-alkanes, is indeed well-validated. Considering fuel-rich and lean conditions, the parametric space of (i) inlet temperature, (ii) pressure and (iii) strain rate is explored. It is found that in a delimited range of operating conditions, a wide, distributed reaction zone is obtained. Different dynamics are observed in rich and lean conditions, exhibiting, respectively, a two-stage oxidation and a premixed-flame behavior: in particular, by increasing the fuel temperature, the diffusion-reaction structure typical of premixed flames is progressively lost, and an autoignition front is established. A two-stage ignition significantly affects the flame width, as well as velocity and composition profiles. Moreover, the competition between low- and high-temperature chemistry results in the presence of multiple solutions and cool flames. Overall, the impact of low temperature chemistry on ignition and flame structure was quantified through regime diagrams, which show that modification on flame regime occur in a vast region. It is also concluded that without considering these reaction paths, the ignition region would be underestimated.