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Show INTRODUCTION Turns and Myhr (1991) have examined the relationship between global NOx emissions and flame luminosity in free, turbulent, jet diffusion flames by varying fuel type. For hydrocarbon fuels, they observed that an increase in global residence time correlated with an increase in radiant fraction, reduction in flame temperature, and a reduction in normalized NOx emissions. Their dimensionless description of jet flames provides a good collapse of data for a wide range of fuel types and nozzle diameters. An alternative description of the NOx emission characteristics of jet flames has been presented by R0kke et al. (1992, 1994). Their description is based on the stretched laminar flamelet theory applied to jet flames and ignores the effects of radiation. A different set of dimensionless parameters is derived which also provides good collapse of the data. Both of the above descriptions build on the growing body of research which recognizes the importance of the mixing characteristics of jet flames. For example, Chen and Driscoll (1990) show how the mixing characteristics of a turbulent jet diffusion flame influence NOx emissions via characteristic mixing time scales. They found that for hydrogen flames, NOx emission indices (EINOx ) are well correlated with the length of the flame, so that a decrease in global residence time is associated with a reduction in EINOx' By contras4 for methane flames, the correlation is very weak. They attributed the difference to prompt NO, which is absent in hydrogen flames. No measurements of the radiant characteristics of the flames were performed. More recently, Turns and Brooks (1994) found that altering the mixing of fuel and air by injecting air jets radially into gaseous jet flames tends to increase NOx emissions slightly. However, they were unable to obtain reliable information regarding the radiation characteristics of the flames due to the lack of similarity between the various flames. While the previously cited studies recognize the importance of the mixing characteristics of the jet flow, the use of self-similar flow types, which are an inherent feature of free, turbulent, jet diffusion flames, does not enable an independent investigation of the influence of the scale and intensity of turbulence on flame stretch, flame radiation, and flame emissions. In fact, they assume similarity to simplify their analyses (Chen & Driscoll, 1990; Turns & Myhr, 1991). The distinguishing feature of the present investigation is that it makes use of a precessing jet flame, described below, to provide a wide range of dominant mixing scales, while maintaining the same jet diameter, exit velocity, and fuel type. This enables mixing parameters to be altered, or controlled, in contrast to simple jet flames, to provide further insight into the mechanisms of NOx generation in jet flames and the means whereby NOx emissions can be reduced. 2 |