OCR Text |
Show 5 The test was carried out as follows 1) The steel sheet was set up on the cooling box. 2) A thermocouple was welded to the steel sheet at the opposite side of blowing, and was inserted from the top of the furnace. 3) W h e n the temperature of the steel sheet rose to between 800 and 1200°C, the steel sheet was dropped into the non-oxidation cooling compartment. 4) After cooling to room temperature, the surface of the steel sheet was observed by visual evaluation, and the surface section was observed with S E M and EPMA, and the amount oxidation/reduction was assessed. As a result, the reducing flame with oxygen enrichment was able to reduce at about a steel sheet temperature of 1200°C or less. Figure 9 shows the range of the combustion conditions in which steel sheet at 1200°C could be reduced. The reducing combustion conditions at a steel sheet temperature of 1200°C is the region enclosed by triangle in Figure 9, at about 0.4-0.6 air ratio, and about 50-80% oxygen concentration range in the combustion air. Flashback, which occurs at an oxygen concentration range of more than 80%, could be prevented by remodeling the design of the burner, and it is expected that the iron oxide can be reduced more under these conditions. The soot, generated at an air ratio of lower than 0.4, could be prevented by raising the flame temperature by other methods. Figure 10 shows the steel sheet surface section heated in the reducing and direct fired flame at an air ratio of 0.5, and a oxygen concentration in the combustion air of 60%. E P M A analysis showed that the oxidation layer on the surface had been reduced by about three microns. It is thought that the reactivity and reducing ability of the flame at low air ratio increased with increasing flame temperature which was as a result of the oxygen enrichment. 4. Numerical Simulation of Reducing Flame 4.1. Simulation Models of the laminar flame It was ascertained from these experiments that the reducing ability was improved by air preheating and oxygen enrichment. To clarify what the factors of this reducing ability were, numerical simulation was applied to the combustion. The numerical simulation is applied using the Finite Volume Method and 1st order Up-Wind Method. S N M burner tested in this report forms turbulent diffusion flame, but numerical analysis was applied to laminar pre-mixed flame for simplification of the calculation. This model is the axis symmetry gas jet flame, and is approximated to the two- |