Effect of inner/outer secondary air mass flow ratio on the characteristics of the reverse swirl burner

The reverse swirl (RS) burner is a kind of low-nitrogen burner which is suitable for bituminous coal.It couples reverse jet and swirl flow for the stabilization of flame and has the advantages of energy saving and emission reduction.However,the further application of RS burner is limited by the lack of in depth research on its combustion stabilization mechanism.The basic performance of RS burner is influenced by the key factors,i.e.,inner/outer secondary air mass flow ratio,reverse primary air ratio and swirling intensity,etc.In order to explore the effect of different inner/outer secondary air mass flow ratios on the flow and combustion characteristics of RS burner,a burner model with a ratio of 1∶2 to the original 14 MW RS burner was established using the isothermal modeling theory.The boundary of coupled recirculation zone was measured using the coordinate frame which a short yarn was tied to the each grid of the frame.The distributions of mean velocity and RMS velocity were measured by Dantec Multichannel hot wire anemometer.The relative reverse flow rate was calculated based on the boundary and mean velocity of the coupled recirculation zone.The NOx and O2 concentration in the pre-combustion cone was measured by Testo 340 flue gas analyzer on a 14 MW RS burner burning Shenfu Dongsheng bituminous coal.The results showed that with increasing of the inner/outer secondary air mass flow ratio,the area of the annular coupled recirculation zone increased,the length remained unchanged,and the maximum diameter increased from 0.67D to 0.87D.The peak value of reverse flow rate increased from 0.83 to 1.23.In addition,the O2 concentration of the measurement section b increased,and by contrast the concentration of NOx decreased near the precombustion cone.In the coupled recirculation zone of 0.3≤X/D≤0.8,the mean velocity was low but the turbulence was strong.For case2∶5,the peak of RMS velocity reached the maximum near the section X/D=0.8.For case 10∶23 and case 10∶28,the flame was stable and the intensity was high near the section X/D=0.75.Therefore,from the viewpoint of aerodynamic field characteristics combined with combustion characteristics,the coupled recirculation zone in the range of 0.3≤X/D≤0.8 would take action in terms of flame stabilization.Furthermore,a highspeed,low-turbulence air protection layer was formed near the pre-combustion cone,and the O2 concentration in this area was more than 7%,which was expected to inhibit high temperature corrosion on the wall.