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Show The principle of flame stabilization by low-swirl is counter to the conventional high-swirl methods emphasizing the formation of a T R Z . In fact, formation of flow recirculation and T R Z s are deliberately avoided in low swirl burners. Originally intended as a laboratory research tool to investigate fluid mechanical and combustion processes of ultra lean premixed flames, the L S B design is deceptively simple. Its operating principle exploits the propagating nature of premixed turbulent flames instead of the traditional approach to "anchor" the flame. The version with an air-jet swirler is essentially an open tube with the swirler fitted to its mid section (Figure la). The role of the swirler is to generate swirling motion in an annular region at the periphery of the burner's exit tube. This allows the central core of the flow to remain undisturbed (i.e. no rotational velocity within the core). W h e n this flow exits the burner tube, the angular momentum in the flow periphery establishes a radial mean pressure gradient that uniformly diverges the non-swirling reactant core. Consequently, the mean flow velocity in the core region decreases linearly without forming a T R Z . This velocity "down-ramp" enables the flame to propagate upstream against the decelerating flow, self-sustaining itself at the position where the local flow velocity equals the flame speed. The L S B operates with a swirl number, Sg, between 0.02 to 0.1. This is m u c h lower than the minimum Sg of 0.6 required for the high-swirl burners. ATTRIBUTES OF THE LOW-SWIRL BURNER As low swirl generates a divergent and non-recirculating flow, it provides an ideal flowfield to support stable propagation of lean premixed flames. W e found that the non-dimensional swirl number varies only slightly with input power, fuel type, flow velocity, turbulent conditions and burner configuration (i.e. throat diameter and swirl injection angle). Defining and predicting the swirl rates as functions of these parameters form the basis of scaling laws for the LSB. Knowledge of lean premixed flame processes, in particular the turbulent flame speeds for different gaseous fuels, provides the scientific foundation for further development of the LSB. There are currently two versions of LSB. The original version uses an air-jet swirler. M u c h of the research activities on fundamental combustion processes and technology transfer have been performed using an air-jet L S B [8, 9]. The second version of the L S B uses a guide-vane swirler [10]. The guide-vane is preferred for commercial applications because it does not need a secondary blower or control unit for the swirl flow. Due to its simple design, a commercial L S B should have significantly lower capital operating and maintenance costs than current ultra-low N O x burners. It can be made of c o m m o n materials. In the laboratory, w e have demonstrated a L S B made with P V C pipe sections to show that the burner does not receive or retain a significant amount of heat from the flame. This feature, in the absence of F G R , means that the burner has a very short response time to accommodate any load change. This is ideal for load following and turndown and thus increasing system efficiency and operational flexibility. As none of the L S B components are expected to degrade over the life of the burner, the cost to maintain and operate L S B should also be comparatively low. 4 |
