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Show 2.2 Experimental arrangement of indirectly gas-fired furnace The experimental arrangement of the indirectly gas-fired furnace tested in this study is shown in figure 5. Figure 6 illustrates a cross-section of the thermal insulating wall of this furnace. The wall is constructed from two layers of a water jacket (75mm) and ceramic bord (200mm) outside to inside of the furnace. O n the ceiling wall of this furnace, a pair of U-type gas-fired radiant tubes are mounted The total input of these burners is 26kW. Figure 7 shows a cross-section of the U-type radiant tube used in this investigation. Both end of the radiant tube are set through the ceiling wall, and hot air generated by the burner at one side of the tube flows through the tube and is exhausted at the other side of the tube. During this process, flue gas does not exchange heat directly with the atmosphere in the furnace. Alternatively, the heat energy is conducted indirectly by radiative heat transfer between the high temperature tube surface and furnace elements in the furnace. Mass of the indirectly gas-fired furnace elements are tabulated in table 2. 2.3 Experimental conditions For the directly gas-fired furnace, we examined the heat response of the furnace system during a transient heating process from room temperature to a target temperature (77SK). At the beginning of the experiment, we set the furnace temperature at room temperature (288K). Uniformity of furnace temperature was confirmed before each trial. Two burners were controlled to drive at maximum input until the furnace temperature attained to the target temperature (773K). In this condition, the fuel gas flow rate and the supplying pressure are 2.12 m3/h and 1961 Pa (Higher calorific value is 27.14kW , lower calorific value is 24.55/ciy), and the air ratio is set at 2.43. These values were fixed through the experiment. The flow rate and the supplying pressure of the fuel gas were monitored using a flow meter and a pressure gauge during each trial. In this study, we focused on the effect of the rate of circulation of hot air on the thermal response of the furnace. W e tested four rotating speeds for the circulating fan, 906, 1086, 1260 and 1480 r.p.m.. However, in these experiments, since circulating rates may change with atmospheric temperature, we identified the fan rotating speed as a parameter of the representative circulating rate. These rotating speeds were within the fan capacity. Here work Reynolds number was calculated from a work length scale (50mm), the mean velocity value at the furnace center, and kinematic viscosity. The work Reynolds numbers corresponding to each fan rotating speed are tabulated in table 3. In order to measure the transient temperature change of the work, a carbon steel brick was set above the loading roller. Its dimension were bOW x 50D x 50// mm. Three sheathed K type thermocouples (outer diameter 1.6mm) were used to measure the brick temperature. T w o of the three were set near the brick surface, and one was set at the center of the brick. At the exit of the exhaust pipe, the flue gas flow rate and temperature were monitored using a sheathed K type thermocouple (O.D. 1.6mm) and a Pitot tube |