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Show 6.0 Summary High velocity burners are well suited for firing temperature sensitive products such as ceramics and structural clay products. The mixing and entrainment of such burners improve the heat penetration and temperature uniformity. Manufacturers that are faced with increasing production from their existing kilns must do this with minimal detriment to product quality. Increasing the firing power of their existing burners to achieve higher production will likely result in more scrap. Simply adding excess air to the burners to reduce localized overheating adds to fuel, power and equipment cost. Practical solutions to improving heating uniformity involve modification of the burners by 1) recessing the burners into the wall, 2) reducing the burner nozzle outlet to attain greater jet velocity, and (or) 3) changing the nozzle shape to increase the burner mixing intensity and entrainment. Jets have been studied extensively. A means to predict the jet core length and entrainment for reacting flow is possible using a simple modification to account for both density and mixture fraction effects. Burner performance characteristics, such as an asymmetric velocity profile issuing from the nozzle, or cross flow, can significantly effect experimental results. Additional experimental work is needed to better predict the mixing and entrainment of commercial burner flames and their applications. Several investigators have shown that changing the shape of the jet nozzle can significantly reduce the mixing length and increase the amount of entrainment. Retrofitting a slotted nozzle shape to an existing high velocity burner is a simple solution to improving heat uniformity. Theory, experimental and field results support the application of slotted burners to reduce overheating and improve recirculation of kiln gases. Applications of this technology in the field have been encouraging to date as evidenced by improved product quality and increased production. The application of high velocity burners reaches far beyond the firing of ceramics and structural clay products. There are a growing number of heat treating, forging, and specialty metal processes that specify temperature uniformity to ensure product mechanical properties. Industry standards such as ISO 9000 continue to drive manufacturers toward improved quality. Practical suggestions detailed in this report can benefit a much broader manufacturing base, where a high uniformity heating is necessary. References 1. Gollahalli, S. R., "Combustion Processes Relevant to Burners in H V A C Systems," GRI-92/0404, October 1992. 2. Ho, C. and Gutmark, E., "Vortex Induction and Mass Entrainment in a Small-Aspect-Ratio Elliptical Jet," J. Fluid Mech., 179, pp. 383 - 405, 1987. 3. Reddy, D. R., Steffen, C. J. and Zaman, K. B. M. Q., "Computation of 3-D Compressible Flow from a Rectangular Nozzle with Delta Tabs," ASME 97-GT- 257, 1997. 4. Thring, M. W . and Newby, M. P., "Combustion Length of Enclosed Turbulent Jet Flames," 4rth Symposium on Combustion, pp. 789 - 796, 1952. 5. Ricou, F. P. and Spalding, D. B., "Measurements of Entrainment by Axisymmetrical Turbulent Jets," J. Fluid Mech, 11, pp.21 -32, 1961. 6. Dahm, W . J. A., University of Michigan, private communication, 1997. |