The Effects of Unburned-Gas Temperature and Heat Loss on the Diffusive-Thermal Instability of Premixed Flames

The effects of unburned-gas temperature and heat loss on the diffusive-thermal instability of premixed flames were studied by two-dimensional unsteady calculations of reactive flows, based on the diffusive-thermal model equation, under the conditions of constant temperature jump through flame fronts. As the unburned-gas temperature became higher, the growth rate increased and the unstable range widened at Lewis numbers smaller than unity, which was due mainly to the increase of the burning velocity of a planar flame. As for the growth rate and unstable range normalized by the burning velocity of a planar flame, the former decreased and the latter narrowed. This was due to the reduction of Zeldovich numbers. In addition, the normalized growth rate increased and the normalized unstable range widened when the heat loss was taken into account. This indicated that the heat loss had a pronounced influence on the diffusive-thermal instability of premixed flames with high unburned-gas temperature. Furthermore, the cellular shape of flame fronts formed owing to diffusive-thermal instability. The normalized burning velocity of a cellular flame decreased as the unburned-gas temperature became higher, and increased when the heat loss was taken into account. Compared with high-temperature premixed flames where the adiabatic flame temperature was constant, the normalized level of instability intensity was low. This was because of small Zeldovich numbers.