Modeling and parametric studies of heat transfer in a direct-fired continuous reheating furnace
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INTRODUCTION
THIS paper presents a mathematical system model of a direct-fired continuous reheating furnace. The model was developed to identify the design and operating parameters that significantly affect furnace performance. The details of the model are provided, followed by the results from a parametric investigation of some of the more important furnace parameters. The paper culminates with a summary of the major conclusions found during the study. Since the load moves steadily and continuously through the furnace, calculations are performed on a steady-state basis, but many details of the procedure are similar to those of a previously developed transient model for a batch reheating furnace. ~l~ Understanding of combustion kinetics, chemically reacting flow, turbulent transport processes, and heat transfer in industrial and commercial furnaces and equipment is incomplete, even though natural gas has been used as a fuel for many decades. In the past, design of direct-fired natural gas fumaces and combustion systems has been mainly empirical. The complex physical and chemical processes involved in flames have, until more recently, eluded detailed analytical description. During the last two decades, the challenge of increasing productivity, improving thermal performance, and reducing combustiongenerated pollution have attracted considerable attention from researchers and designers, leading to significant progress in science-based engineering design, t2,3,41 Phenomenological mathematical models, based on fundamental principles of fluid mechanics, thermodynamics, and heat transfer, have proven useful as tools in developing a basic understanding of heat transfer in furnaces. However, the computer resources required by K.S. CHAPMAN, Assistant Professor, is with the Department of Mechanical Engineering, Kansas State University, Manhattan, KS 66506. S. RAMADHYANI, Associate Professor, and R. VISKANTA, Professor, are with the School of Mechanical Engineering, Purdue University, West Lafayette, IN 47907. Manuscript submitted June 7, 1990. METALLURGICAL TRANSACTIONS B
these fundamental models make them prohibitively expensive for the furnace designer. System models, which are based on the phenomenological models but include substantial simplifying assumptions, provide design and operating information relatively quickly and inexpensively, enabling the designer to explore a wide range of options for improving system performance. Heating processes, including product reheat and heat treatment, in furnaces such as that shown in Figure 1 can benefit from the application of system modeling techniques. System models can be used to predict the thermal efficiency and fuel consumption or the design of equipment and construction materials required to achieve an optimum thermal efficiency and heating rate. This information is valuable in estimating the operating and capital costs of a new heating system. Additionally, system models enable the quality of the heated product to be assessed by predicting both its temperature uniform
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