Adiabatic Laminar Flame Speed Research Articles

Liftoff height of turbulent diffusion flame in water-mist-laden coflowing air stream

In the present study, the enhancement of the liftoff and blowout of a propane/air coflowing diffusion flame by water mist was experimentally investigated in the subsonic range. When the fuel injection velocity Uf is low, an attached laminar diffusion flame is stabilized at the burner rim with a premixed flame base. An increase of Uf leads to the liftoff of the attached flame, and eventually to the blowout of the lifted flame. The upstream laminar flame disappears at the liftoff and a turbulent lifted flame is stabilized in the downstream region. The stability of the lifted flame deteriorates with the increase of the coflowing air velocity Ua and the water mist flow rate Qm. The stable lifted flame can be achieved by the dynamic balance of the local mean gas velocity Ug¯ and the mean turbulent flame propagation speed Up¯ of the premixed flame formed at base of the lifted flame, and Up¯ was estimated by the measurements of Ug¯ by a laser Doppler velocimeter. Up¯ decreases with Qm and is of the order of 1 m/s at the blowout independently of Ua and the mass fraction of water mist Y0. This value is nearly equal to 2.5SL0, where SL0 is the adiabatic laminar flame speed of a stoichiometric propane-air mixture. The liftoff height L increases both with Uf and Qm. For the case without water mist, a unique correlation has been proposed in the previous studies between L and Uf in a non-dimensional form using SL0. By extending this correlation to the diffusion flame in the water-mist-laden air stream, the laminar flame speed influenced by water mist was appraised and compared with the simulation previously reported. Resulting flame speed reduction is 65% of that predicted by numerical simulation, presumably due to the partial evaporation of water mist.

Measurement of the adiabatic flame speed and overall activation energy of a methane enriched H2/CO/CO2/N2 low heating value mixture

The objective of this work is to investigate the effect of the methane blending in the adiabatic laminar flame speed and overall activation energy of a CO/CO2 rich, low calorific value, syngas fuel premixed combustion with air. The measurements were performed in a McKenna flat flame burner at constant equivalence ratio, unburned temperature and pressure. A flame asymptotic model curve-fitted to the measurements provided the overall activation energy and adiabatic laminar flame speed. The mixtures tested presented lower heating values from 4.5 MJ/kg to 50 MJ/kg as the CH4 fraction was changed from 0% to 100%. While the measured laminar flame speed remained approximately constant for all fuels tested, the overall activation energy evidenced the higher reactivity of the mixture blended with 20% CH4. Predictions using GRI-Mech 3.0 and San Diego mechanisms closely followed the measurements of the laminar flame speed for higher methane content, but deviations were found for the fuels with higher hydrogen content. Conversely, the mechanisms underpredicted the values of overall activation energy for all mixtures tested. A sensitivity analysis revealed the competition between the CO wet oxidation and the chain branching formation of OH and the increase in reactivity as the fraction of CH4 increased from 0 was related to a switch from a higher sensitivity of the former by the later.

SUBADIABATIC AND SUPERADIABATIC PERFORMANCE OF A TWO-SECTION POROUS BURNER

ABSTRACT Flames may be stabilized in porous media at velocities either above or below the laminar flame speed. These two regimes are often called superadiabatic and subadiabatic, respectively. In this paper, several burners are investigated experimentally at both superadiabatic and subadiabatic conditions. The upper and lower velocity limits of stable combustion are reported. For equivalence ratios of 0.70 and below, both sub- and superadiabatic performance was seen. For both sub- and superadiabatic performance, the flame was stabilized at or near the interface between the upstream and downstream sections of porous media. The lower velocity limit for superadiabatic performance was extinction of the flame. The upper limit for superadiabatic performance was blowoff, which occurred for all burners at a flow velocity several times the adiabatic laminar flame speed, a phenomenon which is attributed to heat recirculation within the porous media. For equivalence ratios above 0.70, only subadiabatic performance was observed. The lower velocity limit occurred when the flame reached extinction. The upper velocity limit occurred when heat recirculation enhanced the laminar flame speed to a point at which the flame front propagated through the upstream section of porous media and flashed back upstream of the burner. Both the upper and lower velocity limits minimized at or near an equivalence ratio of 1.1. For equivalence ratios between 1.3 and 1.7 and flow velocities between 4 and 9 cm/s, oscillations of the flame front within the downstream section of porous media were observed.

Flame propagation in a nonuniform mixture: Analysis of a slowly varying Triple Flame

For flames propagating through a nonuniform medium a three-flame structure is described. This consists of a fuel-rich premixed flame, a fuel-lean premixed flame, and, starting where these flames meet, a diffusion flame. Such formations have been observed experimentally and probably occur as laminar flamelets in turbulent diffusion flames. A low-heat-release model for such flame structures is developed and solutions are obtained in the limit of slowly varying premixed flames. Under these conditions, it is shown that the Triple-Flame propagation speed depends on the transverse mixture fraction gradient and is bounded above by the maximum adiabatic laminar flame speed of the system.

Entropy production in flames

Thermodynamic foundations of the thermal entropy production are rested on the concept of lost heat , ( Q T ) δT. The thermomechanical entropy production is shown to be in terms of the lost heat and the lost work as δS 8= 1 T Q T δT+δW l where the second term in brackets denotes the lost (dissipated) work into heat. The dimensionless number Π s describing the local entropy production s‴ in a quenched flame is found to be Π s ∼(Pe d 0) −2, where Π s = s‴l 2 k , l = α S u 0 (a characteristic length), k thermal conductivity, α thermal diffusivity, S u 0 the adiabatic laminar flame speed at the unburned gas temperature, Pe d 0 = S u 0D α the flame Peclet number, and D the quench distance. The tangency condition ∂ Pe d 0 ∂θ p = 0 , where θ b = T b T b 0 , T b and T b 0 denoting, respectively, the burned gas (nonadiabatic) and adiabatic flame temperatures, is related to an extremum in entropy production . The distribution of entropy production between the flame and burner is shown in terms of the burned gas temperature and the distance from the burner.