Chemiluminescence of Burner-Stabilized Premixed Laminar Flames

ABSTRACT

The OH*, CH* and CO${ }_2^{\ast }$ chemiluminescence signals of methane/air premixed laminar flames stabilized over a nonadiabatic porous plug burner are compared to the signals measured from a nearly adiabatic conical flame in a series of experiments. The impact of reactant stream temperature is also characterized. A numerical study based on 1-D flame models then follows to support the experimental results. It is found both in experiments and in simulations that the linear relationship between the mixture flowrate and the chemiluminescence intensities is no longer valid when flames are closely attached to the burner surface due to the heat transfer between the flame and the burner. The transition between the linear and the nonlinear regimes is identified as the gas flow velocity drops below the adiabatic laminar burning velocity calculated at the bulk temperature of the flow leaving the burner. When the mass flowrate is kept constant, preheating of the reactant stream increases the chemiluminescence intensity for a freely propagating flame, but has almost no impact for a burner-stabilized flame. It is finally found that the OH* and CH* chemiluminescence intensities correlate with the burnt gas temperature for the adiabatic but also the nonadiabatic flames. The underlying physical mechanisms are discussed. Finally, the evolution of the CH*/OH* ratio with the inlet gas velocity is discussed.

KEYWORDS:

• Flame chemiluminescence
• non-adiabatic combustion
• preheating
• heat losses
• operating point control

Nomenclature

$\lambda$	=	Wavelength
$\varphi$	=	Equivalence ratio
$\rho$	=	Density
${\xi }_{\mathrm{l}oss}$	=	Heat loss fraction
$A_{21}$	=	Rate of radiative decay
$c$	=	Concentration
$f_0$	=	Mass burning flux
$h_0$	=	Inlet gas enthalpy
$I$	=	Chemiluminescence emission intensity
$I_s$	=	Specific chemiluminescence emission intensity
$k$	=	Thermal conductivity
$P$	=	Flame power
$q$	=	Heat flux
$Q_{21}$	=	Global rate of quenching
$S_L$	=	Adiabatic laminar burning velocity
$T_0$	=	Reactant stream temperature at burner outlet
$T_b$	=	Burnt gas temperature
$T_u$	=	Reactant stream temperature before entering the burner
$u_0$	=	Mean bulk velocity of reactant stream at burner outlet
$V$	=	Diffusion velocity
$Y$	=	Mass fraction
$y$	=	Fluorescence yield of excited species
exp	=	Superscript for experimental results
sim	=	Superscript for simulation results

Acknowledgments

The authors gratefully acknowledge the support of Bosch Thermotechnologie. They also wish to thank David Charalampous for his help with spectrometry and Yannick Le Teno for his help on the design of the experimental setup.

Additional information

Funding

This work was supported by FUI (Fonds Unique Interministériel).