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Show AMERICAN FLAME RESEARCH COMMITTEE 1997 INTERNATIONAL SYMPOSIUM S E P T E M B E R 21 -24,1997 Chicago, Illinois High Uniformity Heating -Theory and Practice Ken Gretsinger North American Manufacturing Co. 4455 E. 71st Street Cleveland, Ohio Abstract Uniform heating is key to high quality and increased production of directly fired ceramics and other temperature sensitive materials. Product temperature uniformity in a combustion process is typically gained by use of wasteful excess air. Increased fuel, power and equipment costs are the penalties for high uniformity. Techniques will be discussed that significantly improve temperature uniformity while minimizing energy waste. One such improvement utilizes a non circular (or slot) exit in conjunction with a high velocity burner to improve both the mixing and entrainment between the flame jet and the surrounding furnace gases. Lab and field testing of the slotted high velocity burner will be presented. 1.0 Introduction Manufacturers of ceramic and structural clay products are experiencing a period of economic growth. Many plants that fire with jet flame burners are operating at maximum production. One of the limitations to further increase production in a kiln is the product quality, which is effected by heat uniformity. Heating uniformity (i.e. product temperature uniformity) is typically accomplished using high velocity burners located in the side walls and (or) the crown (roof) of a kiln. High velocity burners represent an improvement over conventional jet flames in that the mixing, stirring, and entraining action of the jet improves heat penetration and temperature uniformity. One problem in using high velocity, nozzle mix burners is the extension of the flame core, which can result in overheating, or scorching, the product. Reducing the jet velocity to relieve this problem can result in under heating the product located at the center, or bottom, of the kiln. A common approach to solving this problem is to reduce the flame temperature of the burner; however, this results in an overall increase in fuel usage that is associated with heating excess combustion air. Another approach is to construct firing lanes, or passageways within the product itself, so that the flame does not impinge. Within a given kiln structure this represents an under utilization of heating space. There are practical ways to improve uniformity which will be described later in this paper. Of particular interest is modification of the jet shape. Several investigators have demonstrated a means of improving the mixing and entrainment of a turbulent jet by simply altering the shape of the nozzle. In one instance Gollahalli1 (1992) showed that the amount of primary air entrained into a Bunsen-type venturi (inspirator) was greater for non-circular fuel injectors. Comparisons were made between elliptical, rectangular, square and circular shapes. Ho and Gutmark2 (1987) showed that the mass entrainment of an elliptical nozzle (aspect ratio, A R = 2:1) was several times greater than circular or rectangular l |