Heat transfer from flames in a rotary kiln
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I.
INTRODUCTION
IN a direct-fired rotary kiln, two distinct regions of heat transfer may be identified as shown in Figure l ( a ) - - t h e flame zone (I) and nonflame zone (II). In both zones heat is transferred to the upper surface of the solids bed by radiation and convcction and to the lower (covered) surface by the regenerative action of the rotating kiln wall. However, in the flame zone (Figure l(c)), the radiating gases in the freeboard are found largely within the confines of the visible flame in contrast to the nonflame region (Figure l(b)), where the radiating gases occupy the entire freeboard volume. Thus, in the flame zone the solids and exposed wall receive heat primarily by radiation from a well-defined flame; and owing to the high flame temperatures both convection and regenerative heat flow play only a minor role in the overall heat transfer process. Previous papers in this series ~'2 have addressed the problem of mathematically modeling heat transfer in the nonflame region of a rotary kiln. Therefore, the primary objective of the present study is to describe mathematically the overall heat-transfer process in the flame zone of a kiln. The work may be divided into two major sections: (1) The development of a model to predict temperatures and heat flows in the presence of a freeboard flame; (2) Application of the model to examine the flame characteristics and heat flows as a function of kiln variables. In completing this work, a framework has been developed for the prediction of heat flows within a rotary kiln at any position along its axis. J.P. GOROG, formerly Graduate Student, University of British Columbia, is now with Bacon, Donaldson & Associates Ltd., 2036 Columbia Street, Vancouver, British Columbia V5Y 3El. T.N. ADAMS is with Weyerhaeuser Technology Center, Tacoma, WA 98422. J.K. BRIMACOMBE is Stelco Professor of Process Metallurgy, Department of Metallurgical Engineering, The University of British Columbia, Vancouver, British Columbia V6T IW5, Canada. Manuscript submitted January 18, 1983.
METALLURG[CALTRANSACTIONS B
II.
PREVIOUS WORK
A large number of flame-related studies concerned with fluid flow, mixing, and heat transfer in furnaces has been reported previously, but most are beyond the scope of the present investigation. Much of this work has been reviewed in several publications over the last two decades. 3-8 In comparison, few studies have been undertaken recently on flame characteristics in rotary kilns. Rhuland 9 has studied flame length in a full-size cement kiln and has used a small cold-flow model made of plexiglas to investigate mixing and combustion processes in the kiln. In the model experiments the gas streams were simulated by dilute acid and alkali solutions with thymolphthalein used as an indicator. In the zone of mixing, a blue color, which had the essential appearance and characteristics of a flame, was produced. From the laboratory and plant measurements Rhuland was able to deduce a general equation for flame length in a rotary kiln as a function of the dimension
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