Process Analysis of Exergy Loss in Combustion Field.

2010

JTST

Asymptotic analysis on premixed flames with high-temperature combustible mixtures was performed to elucidate the instability of flame fronts. Planar premixed flames with sufficiently low Mach numbers were treated, and a one-step irreversible chemical reaction with sufficiently high activation energy was assumed. Through the asymptotic analysis, we obtained the formula for the burning velocity, depending on the temperature of burned and unburned gases and on the mass fraction of fuel. To investigate the instability of high-temperature premixed flames under the constant-enthalpy conditions, we obtained the instability intensity which depended on the burning velocity and on the temperature ratio of burned and unburned gases, where the burned-gas temperature was constant. As the unburned-gas temperature became higher, the instability intensity became higher, except for premixed flames with low reaction exponents under the conditions of constant-density unburned gases. This was because that the instability intensity was determined by the competition between the increase of the burning velocity and the decrease of the temperature ratio.

“…The matching yields ( ) ( Substituting Eqs. (5), (6), and (11) into Eq. (19), we obtain the formula for the mass flux density as follows: …”

mentioning

confidence: 99%

Asymptotic Analysis on High-temperature Premixed Flames: Instability of Flame Fronts under the Constant-enthalpy Conditions

2010

JTST

“…(3)- (4) In view of the energy saving, we have to investigate the reduction of irreversibility in combustion, i.e. the reduction of exergy loss.…”

Section: Introductionmentioning

confidence: 99%

Effects of the Unburned-Gas Temperature and Lewis Number on the Intrinsic Instability of High-Temperature Premixed Flames

Yahata

Kobayashi

2011

JTST

The effects of the unburned-gas temperature and Lewis number on the intrinsic instability of high-temperature premixed flames under the constant-enthalpy conditions were investigated by two-dimensional unsteady calculations of reactive flows.A sinusoidal disturbance with sufficiently small amplitude was superimposed on a planar flame to obtain the relation between the growth rate and wave number, i.e. the dispersion relation. As the unburned-gas temperature became higher, the growth rate increased and the unstable range widened. This was due to the increase of the burning velocity of a planar flame. In addition, the obtained numerical results were consistent with the theoretical solutions in small wave-number region. As the Lewis number became smaller ( larger ), the growth rate increased ( decreased ) and the unstable range widened ( narrowed ), which was due to diffusive-thermal effects. The dispersion relation yielded the linearly most unstable wave number, i.e. the critical wave number. The critical wave number increased as the unburned-gas temperature became higher. Thus, the critical wavelength shrank, and then the cell size shrank. To clarify the characteristics of cellular flames induced by intrinsic instability, a finite disturbance with the critical wavelength was superimposed. The superimposed disturbance evolved, and a cellular-shaped front formed. In all Lewis numbers, the behavior of cellular flames became milder as the unburned-gas temperature became higher, even though the growth rate increased. The normalized burning velocities of cellular flames decreased monotonously.This was because that the thermal-expansion effects became weaker owing to the decrease of the difference in temperature between the burned and unburned gases, which was generated by the conditions of constant enthalpy, i.e. constant burned-gas temperature.

“…1,2) As for the reduction of exergy loss, the preheating of combustible mixtures due to heat recirculation is often used. 3,4) From the viewpoint of energy savings, reducing the irreversibility in combustion, i.e. decreasing entropy generation, must be studied.…”

Section: Introductionmentioning

confidence: 99%

Intrinsic Instability of Premixed Flames at Sufficiently Low Activation Energy

Kaewpradap

AEROSPACE TECHNOLOGY JAPAN

2010

The intrinsic instability of premixed flames at sufficiently low activation energy was studied by two-dimensional unsteady calculations of reactive flows, based on the compressible Navier-Stokes equation including a one-step irreversible chemical reaction. A sinusoidal disturbance with small amplitude was superimposed on a planar flame to obtain the relation between the growth rate and wave number, i.e. the dispersion relation, and the average local burning velocity of a cellular flame. When the Lewis number was smaller (larger) than unity, the growth rate and the average local burning velocity decreased (increased) as activation energy became lower, because of the decrease of the Zeldovich number. When the Lewis number was unity, the growth rate decreased slightly at sufficiently low activation energy, and the average local burning velocity was almost constant. In addition, the dependence of the average local burning velocity on the Lewis number became weaker at sufficiently low activation energy.