Optical probe for in-situ measurements of air-to-fuel ratio in low emission engines

The lower temperatures associated with lean premixed combustion generally lead to lower NOX emissions; however, the NO_x emissions also depend on how well the air and fuel are mixed. In this paper, we describe the development of an inexpensive fiber optic probe capable of measuring the extent of turbulent mixing. The fuel concentration is determined by laser absorption at 3.39 urn over a short path length created by infrared transmitting fiber optics. We use this probe to show that NO_x formation depends significantly on the extent of fuel-air mixing as well as the overall stoichiometry. A hydrogen piloted, QHU in air turbulent flame with a variable fuel injection location is used to vary the degree of mixedness at the burner exit. The level of mixing and the mean concentration profiles are also measured by using Planar Laser Initiated Rayleigh Scattering. Nitrogen oxides measurements are reported for several mixing lengths. We show that at lean conditions, incomplete mixing causes an increase in NO_x production because of the unfavorable temperature dependence of NO_x formation at Φ = 0.6. We also show that the optical probe is capable of measuring the extent of mixing of the fuel-air mixture.