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Show INTRODUCTION In recent years, there has been an increased interest in applying the direct flame impingement (DFI) concept in industrial furnaces to enhance the convective heat transfer rates. In some applications, DFI is emerging as an attractive and cost effective alternative to conventional radiant heating. Use of D FI offers several potential advantages over radiant heating such as increased heat fluxes, which reduces processing time, fuel consumption and undesirable scale fonnation (oxidation), and improves product quality. In addition, with multi-flame DFI, the heating can be locally targeted by adjusting the firing rate of the individual flames. Applications for DFI include continuous heating of tubes and strips, and reheating of billets and slabs. In conventional gas fired furnaces used for these processes, the convective heat transfer coefficient is usually less than 50 W/m2 K (9.0 Btulft2h-F). With DFI, however, it can be increased to several times this value [1]. During the past 10 years, a number of rapid heating technologies, using different jet impingement approaches, have been investigated. The primary differences between these approaches are in the type of jets used and their makeup: • high velocity combustion products generated by special tunnel burners, and · containing little or no combustibles in the impingement zone, or • high velocity flames with intense fuel/oxidant mixing and burning ill the impingement zone. The use of rapid heating technologies based on impinging jets of hot combustion products have grown steadily over the past decade in the metallurgical industry. This is primarily due to the development of burners with high exit velocities [2-5]. Data on flow and heat transfer characteristics of such jets can be found in reference [6]. For many applications, a more effective alternative for rapid heating, however, might be to apply direct impingement of mUltiple flames, or multi-flame DFI, which is the focus of this paper. The approach developed by USTU to accomplish this is relatively simple. It involves installation of several steel nozzles, almost flush with the internal surfaces of the furnace walls, to direct highvelocity premixed gas-air mixture flames at the load surface. No separate combustion chambers or tunnels or flame holders are used. In contrast to most of the investigations by others, which utilize relatively low velocity premixed flames, the USTU approach employs velocities up to Mach = 1 at the nozzles exit to ensure very high heat transfer rates. Although the approach is simple, unlike conventional furnaces equipped with tunnel burners, the high velocity multi-flame DFI system requires more critical furnace design as well as accurate definition of key parameters, including nozzle diameters and their spacing, distance between the nozzles and the metal load, and the firing rate per unit area of the metal surface. Improper specification of these parameters could result in unstable and incomplete combustion and reduced thermal efficiency. A comprehensive review of experimental efforts devoted to DFI is given in three recent papers [7, 8, and 9]. According to these reviews, most of the previous experimental efforts have been concerned with a single jet burning in an ambient environment. Only a few references are made to experiments with impinging jets developing 2 |
