Experimental And Numerical Studies On Flow Field And Temperature Field Of Slited Swirl Low NO_x Burner

Natural gas is a kind of clean energy.In recent years,its exploitation and utilization have been rapidly developed.Gas burners which are sufficient,make gas mixed uniformly and discharge low NO_x can meet natural gas utilization.Gas burners can roughly be classified into direct current and swirl burners.The flame of the direct current burner has strong rigidity,but the mixture of gas and the combustion stability are poor.The current with strong entrainment effect in the swirl burner can enhance mixture of gas and air.However,a large amount of kinetic energy is lost,when the airflow passes through the guide vanes.Although a large amount of literature has proved that swirl combustion is one of the means to reduce NO_x emissions,the exact burners are difficult to be produced and costly,and volute-type swirl burners are bulky.Therefore,a new type of combustor should be designed to generate swirling combustion to promote mixture of air and gas,to enhance combustion efficiency and stability,to eliminate localized hot spots and to reduce NO_x emissions.After repeated tests,a slited swirl low NO_x burner has been designed and developed.With structure changed,the flow field in the combustion chamber is affected,which in turn affects the temperature field and NO_x emissions.The main structure of the burner is a conical steel plate.The wall of the burner is spirally slited.The air enters the burner through the slits in the state of swirling flow and is fully mixed with the gas.The experimental results of the burner show that swirl flame can be formed.The flame rigidity is strong and stable.With fuel fully burned,the temperature range in the combustion chamber is small.Without any localized hot spots,the maximum temperature is 1398K measured by a thermocouple.The NO_x concentration of exhaust gas is 58ppm measured by flue gas analyzer at the outlet of furnace,which is lower than the national standard.The experimental results can be served as the reference standard for numerical simulation results.It is studied by numerical simulation that influence of burner structure(number of slits,cone angle and secondary air volume)and operating parameters(thermal power,air-fuel ratio and preheated air temperature)on flow field,temperature field and smoke compositions,etc.The numerical simulation of this paper employs tetrahedral mesh,Realizable k-ε turbulence model,non-premixed combustion model,thermal and prompt NO_x model and DO radiation model.Numerical simulation and experimental results of temperature and flue gas composition are in good agreement and the errors are within a reasonable range.It makes some conclusions,which are:(1)Effects of structure:As the number of slits increases,the mixture area between air and gas increases,the position of the highest temperature moves toward the outlet of the furnace,and NO_x concentration decreases accordingly.As the burner angle increases,the velocity in the direction of the central axis decreases and the area of high temperature zone increases.The maximum temperature moves toward the exhaust port and NO_x concentration increases at the same time.The secondary air volume increases and the velocity inside the firebox decreases.The insufficient amount of oxygen in the initial combustion stage causes the combustion zone and the high temperature zone to move toward the exhaust port,and NO_x concentration decreases.(2)The impact of operating parameters:When the combustion thermal power is small,the flame is deflected upward and NO_x concentration is low.When the power increases,more than half of the rated power,the central temperature of the combustion chamber basically unchanged,but the temperature in combustion chamber corner rises.NO_x concentration decreased slightly.The air excess coefficient range is 1.05～1.20.When the excess coefficient increases,the influence on the flow field is insignificant,the temperature turns slightly higher,and the NO_x concentration is higher.With the energy saving and emission reduction taken into consideration,a smaller air excess coefficient should be selected.As preheated air temperature increases,the overall temperature in furnace increases,which achieves the role of the utilization of flue gas waste heat,but the NO_x concentration doubled.