The effects of unburned-gas temperature and radiative heat loss on the intrinsic instability of premixed flames in cryogenic environment were studied. Unsteady reactive flow was calculated numerically, based on the compressible Navier-Stokes equation including chemical reaction and radiative heat loss. As the unburned-gas temperature became lower, the growth rate decreased and the unstable range narrowed, which was due to the reduction of the burning velocity of a planar flame. Considering radiative heat loss, the growth rate was smaller and the unstable range was narrower. On the other hand, the normalized growth rate increased as the unburned-gas temperature became lower. This was due to the strength of thermal-expansion effects and to the enlargement of Zeldovich numbers. Furthermore, cellular flames appeared owing to intrinsic instability. As the unburned-gas temperature became lower, the normalized burning velocity of a cellular flame increased. When radiative heat loss was considered, the normalized burning velocity of a cellular flame increased at small Lewis numbers and remained at Lewis number of unity.

We elucidated the diffusive-thermal instability of premixed flames with low unburned-gas temperature under the adiabatic and non-adiabatic conditions. Numerical calculations of two-dimensional unsteady reactive flows were performed, based on the diffusive-thermal model equation. Lewis numbers smaller than unity were adopted, and radiative heat loss was treated. As the unburned-gas temperature became lower, the growth rate decreased and the unstable range narrowed, which was due to the decrease 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 increased and the latter widened. This was due to the enlargement of Zeldovich numbers. Taking account of radiative heat loss, the normalized growth rate was large and the normalized unstable range was wide. This indicated that the heat loss had a pronounced influence on the diffusive-thermal instability of premixed flames with low unburned-gas temperature. Moreover, the cellular-shape flame fronts formed owing to diffusive-thermal instability. The burning velocity of a cellular flame normalized by that of a planar flame increased as the unburned-gas temperature became lower and the heat loss became greater. This was because of the enlargement of Zeldovich numbers and the pronounced influence of heat loss.

Effects of pressure and heat loss on the unstable motion of cellular-flame fronts caused by intrinsic instability in hydrogen-air lean premixed flames

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