The lower temperatures associated with lean premixed combustion generally lead to lower NOx emissions; however, the benefit of lean premixed combustion may be lost if the fuel and air are poorly mixed. In this paper, we describe the development of an inexpensive fiber optic probe capable of measuring the extent of mixing. The fuel concentration is determined by laser light absorption at 3.39 μm over a short path length created by using infrared transmitting fiber optics. A hydrogen-piloted, CH4-in-air turbulent flame with a variable fuel injection location is used to vary the degree of mixedness at the burner exit. We use the optical probe to measure the level of mixedness (nonreacting) at the burner exit. The level of mixing and the mean concentration profiles are also measured by using planar laser-initiated Rayleigh scattering. NOx measurements are reported for several mixing distances. We show that at lean conditions (=0.6), incomplete mixing causes a dramatic increase in NOx production because of the exponential temperature dependence of NOx formation about =0.6. We also numerically investigate how the extent of mixing affects NOx production at various equivalence ratios and pressures. Modeling the effect of incomplete mixing on NOx formation is done with a distribution ofconvolved with numerical results from a perfectly stirred reactor in series with a plug flow reactor. The model does an excellent job of predicting the NOx increase caused by incomplete mixing at lean conditions. Model predictions at higher pressures that are typical of gas turbine conditions show good agreement with available data. In particular, for lean premixed combustion, NOx is not a function of pressure if the air and fuel are well mixed.
ASJC Scopus subject areas
- Chemical Engineering(all)
- Fluid Flow and Transfer Processes
- Physical and Theoretical Chemistry
- Energy Engineering and Power Technology
- Fuel Technology
- Mechanical Engineering