Effects of soret diffusion on the laminar flame speed of n-butane-air mixtures

The effect of Soret diffusion on flame speed of the freely-propagating planar premixed flames was computationally studied for n-butane/air mixtures using a detailed reaction mechanism and transport. The results show that Soret effect almost has no influence on the n-butane/air flame speed, and the sensitivity analysis of flame speed shows that only very light species H radical is involved in the ranked key reactions. That is to say, the Soret diffusion of H radical has no influence on the flame speed. However, the Soret effect of H plays an important role on hydrogen/air flame speed. We found that the Soret diffusion flux of the kth species just exists in the overlap region which consists of the rear part of “driving force” gradT/T profile and the front part of mole fraction profile of the kth species in the freely-propagating planar premixed flame. Compared with hydrogen/air flame, the investigation shows that for the n-butane/air flame, due to the inherent feature of total heat release rate, the high temperature largely reduces the “driving force” gradT/T especially on the rear part of its profile and compared with the peak of temperature gradient the location of “driving force” peak is closer to the unburned side, on the other hand the broader consumption layer of H weakens the front part of mole fraction of H and makes the peak of H mole fraction far away from the unburned side. Thus, the overlap region is narrow and low values of “driving force” and mole fraction of H embed in it.