Quantifying Fuel/Air Unmixedness in Premixing Nozzles Using an Acetone Fluorescence Technique

The ability of a lean-premixed combustion system to minimize emissions while maintaining combustion stability over the operating curve relies upon how well the fuel nozzle premixes the fuel and air. As the level of premixing increases, NOx emissions at a given flame temperature decrease until a perfectly premixed condition is achieved. The objective of this paper is to quantify the level of premixing achieved by a premixing nozzle using an acetone fluorescence technique and determine its relationship to NOx emissions and combustion stability. The technique of using acetone fluorescence has been used as a fast and quantitative diagnostic to map the fuel-air distribution. This technique has been applied to the development of a lean premixing nozzle to measure the fuel air distribution at the fuel nozzle exit plane. In this study, the fuel air distribution is presented as 2-D images. The average fuel/air ratio and the standard deviation are calculated at various annular regions to determine the distribution as a function of radius. A single unmixedness parameter (σ/μ) over the entire annulus is also calculated to allow relative ranking of the various fuel nozzle configurations. The fluorescence data is acquired for various nozzle hardware configurations in an atmospheric test facility. Fuel and air flow conditions are determined by scaling engine conditions to cold flow conditions and matching the fuel to air momentum ratio at the fuel injection site. Measured fuel/air distributions, 6 mm downstream of the nozzle exit plane, from the acetone fluorescence technique are correlated to emissions and acoustic measurements made at full pressure and temperature conditions in a single-nozzle test rig. The paper includes a description of the acetone fluorescence technique, the method for optimizing the fuel/air distribution through changes to the main gas fuel injection array, and correlations made between the fuel/air distribution, nozzle geometry, power setting, emissions and combustor acoustics.