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

Numerical investigation on unstable behaviors of cellular premixed flames at low Lewis numbers based on the diffusivethermal model and compressible Navier-Stokes equations

The effects of unburned-gas temperature on the characteristics of cellular premixed flames generated by hydrodynamic and diffusive-thermal instabilities were numerically investigated. Two-dimensional reactive flow was calculated in large space, based on the compressible Navier-Stokes equations including a one-step irreversible chemical reaction. The dynamic behavior of cellular premixed flames, i.e. the coalescence and division of cells, appeared in large space owing to intrinsic instability. The behavior of flame fronts became more unstable with a decrease in unburned-gas temperature, even though the burning velocity of a planar flame reduced. This was due to the strength of thermal-expansion effects and to the enlargement of Zeldovich numbers. We found that the average burning velocity of a cellular flame normalized by that of a planar flame increased as the unburned-gas temperature became lower and the space size became larger. To elucidate the increase of burning velocity, we proposed the new model and showed that the normalized increment factor of burning velocity became larger under low unburned-gas temperature. In addition, we performed fractal analysis to consider the fractal dimension for three-dimensional flow. The obtained fractal dimension corresponding to laminar flames was nearly identical to the experimental and numerical results of turbulent flames.

The effects of addition of carbon dioxide and water vapor on the dynamic behavior of spherically expanding hydrogen/air premixed flames Abstract experimental data under a certain condition but also to create the mathematical model for the prediction of flame propagation velocity under various conditions. Thus, it is significant to understand the characteristics of dynamic behavior of hydrogen/air premixed flames and to elucidate the effects of addition of inert gas, i.e. carbon dioxide CO 2 and water vapor H 2 O. We performed the experiments of hydrogen explosion in two types of closed chambers to observe spherically expanding flames using Schlieren photography. Wrinkles on the flame surface were clearly observed in low equivalence ratios. Analyzing the Schlieren images, the flame propagation velocity depending on the flame radius was obtained. Increasing the addition of inert gas, the propagation velocity decreased, especially in the case of CO 2 addition. The propagation velocity increased monotonically as the flame radius became larger. The appearance of flame acceleration was found, which was caused by the evolution of wrinkles on the flame surface. Moreover, the Markstein length decreased as the concentration of inert gas became higher, indicating that the addition of inert gas promoted the instability of hydrogen flames. Furthermore, the wrinkling factor, closely related with the increment in propagation velocity, decreased as the inert-gas concentration became higher. The wrinkling factor normalized by the propagation velocity of flat flame increased, on the other hand, under the conditions of high inert-gas concentration, except for near the quenching conditions. This indicated that the addition of CO 2 or H 2 O promoted the unstable motion of hydrogen flames, which could be due to the enhancement of the diffusive-thermal effect. Based on the characteristics of dynamic behavior of hydrogen flames, the parameters used in the mathematical model on propagation velocity including flame acceleration was obtained, and then the flame propagation velocity under various conditions was predicted.

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.

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