Intrinsic instability of premixed flames in cryogenic environment (Effects of unburned-gas temperature and heat loss)

Abstract

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.

The effects of unburned-gas temperature on intrinsic instability of premixed flames at high Lewis numbers under the adiabatic and non-adiabatic conditions

“…The superimposed disturbance evolves, maintaining the sinusoidal shape, where the flame front is defined as the site of maximum reaction rate. The amplitude of a disturbance grows exponentially with time as, a = ai (exp (t)) (4) This behavior is consistent with previous works (Thwe Thwe Aung and Kadowaki, 2015aKadowaki, , 2015b. When the disturbance grows to some degree, the growth rate becomes lower gradually and eventually drops to zero, which is due to the non-linearity brought about by finite amplitude.…”

“…This shows that heat loss inhibited the intrinsic instability which is mainly induced by thermal-expansion effect at Le > 1.0 flames. In our previous work at low Lewis numbers (Thwe Thwe Aung and Kadowaki, 2015b), Scf /Su increases as radiative heat-loss parameter becomes larger, indicating that the heat loss promotes the instability of premixed flames at low Lewis numbers. The present results at high Lewis numbers are perfectly different from the previous results at low Lewis numbers.…”

“…When the heat loss is taken into account, normalized burning velocities become lower. At Le < 1.0, normalized burning velocities of non-adiabatic cellular flames are larger than that of adiabatic ones (Thwe Thwe Aung Kadowaki, 2015b). That is because the difference between the burning velocities of cellular flame and planar flame (Scf -Su) increases when heat loss becomes larger even though the burning velocity of a planar flame decreases owing to heat loss.…”

“…Under the non-adiabatic conditions, the growth rate becomes lower and the unstable range becomes narrower as the radiative heat loss parameter increases. Kadowaki, 2015b), the growth rate is small and unstable range is narrow. This is because the weakness of instability due to diffusive-thermal effect at high Lewis numbers.…”

“…Thermal-expansion effect is the main force to cause the cellular flame at Le = 2.0. At Le < 1 flames, the overshoot of temperature appears at the flame front convex towards the unburned gas, and then lateral movement of the Thwe Thwe Aung, Katusmi and Kadowaki, Mechanical Engineering Journal, Vol.4, No.2 (2017) [DOI: 10.1299/mej.16-00477] cellular flame occurs owing to the strong diffusive-thermal effect (Thwe Thwe Aung and Kadowaki, 2015a;2015b).…”