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Show recycling and the lean combustion. Fujimori et al. [6] had found in their experiments that the N O x emission in the turbulent jet flame with highly preheated air drastically reduced when the flame lifted as high as the flame length, which can be achieved by high speed jets. The numerical simulations carried out by G u o et al. [7] had also shown that the combination of the air preheating and the flue gas recirculation not only improved the combustion efficiency but also decreased the N O x emission in the furnace. In their simulation, however, the improvement on the temperature uniformity had not been evaluated, and the contributions of thermal-NOx and prompt-NOx to total N O x emission had not been predicted respectively. 2. NUMERICAL MODELS AND SIMULATION CONDITIONS Figure 1 shows a schematic diagram of the regenerative burner system which has been widely used for the investigation on the highly preheated air combustion. The system consists of two sets of a ^ main burner, a combustion chamber and a ceramic heat regenerator. The two sets can be operated alternatively by switching a four-way valve for the combustion air and exhaust. The Figure 1 Regenerative furnace system regenerator in this system can preheat the air to the temperatures greater than 1300 °C. In this study, the steady combustion process in one of the two sets was simulated. The simulated combustion chamber and its main sizes are shown in Fig 2. The preheated air from the bottom of the chamber and the fuel from a burner located at a vertical wall form a cross-flow turbulent diffusion flame inside the chamber. The Liquefied Petroleum Gas (LPG) was used as the fuel, and its composition was set as 97 vol% of C3H8 and 3 vol% of N 2 according to the analysis. The flow rate of the fuel was kept as a constant of 0.053 NrnVh at the temperature of 323 K in all simulations. The oxygen concentration in the preheated air was changed from 21 vol% to 4 vol% by air dilution with N2, CO2, H e and flue gas respectively. The air flow rate was kept as a |
