Numerical Simulation of the Effect of the Straddle Angle of Preheated Air on the High Temperature Air Combustion

A numerical simulation was carried out on the flow and combustion in an industrial furnace with a multi-jet burner for the application of High Temperature Air Combustion technology. The effect of the straddle angle of the preheated air on the combustion characteristics in the furnace was discussed. The standard -model was used to calculate the flow field and PDF (Probability Density Function) combustion model based on a function was selected to simulate the gas combustion. The radiation was simulated by a Discrete Ordinates method. The NOX was simulated by thermal NOX model. The furnace was a rectangular chamber of 800 mm × 800 mm × 1 400 mm. A circular fuel jet of a diameter of 10mm is at the center of the wall. 5 circular air jets equably distributed around the fuel jet with different straddle angles. The results showed that there was the recirculation zone in the furnace due to the interaction of the jets. The recirculation of the flue-gas strengthened the mixing of the air and fuel, leading to a more even temperature distribution. At the mean while, it changed the local distribution of the fuel and oxygen in the furnace and then influenced the local NOX generation and the final emission. The smaller the straddle angle of the preheated air was, the better the mixing of the fuel, oxygen and the flue gas became, which resulted in a stable flame, an enlarged low-oxygen zone, an equable temperature distribution and a suppressed local high temperature. When the straddle angle of the preheated air increased, the burner disposal is more like the jets with the same axis. The recirculation of the high temperature flue gas in the furnace only played the role to stabilize the combustion and the local low oxygen region will disappear. At the 15% O2 condition, when the straddle angle of the preheated air is 120°, the final NOX emission can be reduced about 65% compared with that when the straddle angle of the preheated air is 360°.