Yiheng Tong

INFLUENCE OF COMBUSTOR GEOMETRY ON SWIRL STABILIZED PREMIXED METHANE-AIR FLAME

Flame structures, blowout limits and emissions of swirlstabilized premixed methane-air flames were studied experimentally in a small atmospheric combustor rig. Combustion sections with rectangular cross section (30mm by 40mm) and circular cross section (inner diameter = 39mm) were used to investigate effects of combustor geometry on the flames performance. Flame structures and instabilities were obtained from CH∗ chemiluminescence captured by a high speed intensified CMOS camera. Maps of flame blowout limits (ΦBO) versus total mass flow rates (m = 70∼130 standard liter per minute, SLPM) were obtained with the combustor inlet flow temperature (Tin) kept at Tin = 397 ± 5K and a flow swirl number of S = 0.6. Emission data of mole fraction of CO in the exhaust gas versus equivalence ratio was obtained under the conditions of Tin= 293 ± 5K and S = 0.66. It is found that the flame became longer and more unstable with decreasing equivalence ratio or increasing total mass flow rates. A strong high-amplitude and low-frequency oscillation was found to be the reason for the flame blowout. A possible reason for flame instability and blowout is presented in the paper. Within the parameters investigated in this study, the equivalence ratio had the strongest impact on flame stabilities and CO emission. Both in the rectangular and circular combustors, when the flame length increased to a critical value (LIBO, which was approximately the same for these two combustors), flame could not be stabilized anymore and blowout occurred. Compared with the rectangular combustor, the circular one had lower blowout limits and was better in stabilizing the flame. Combustor geometry did not significantly affect CO emission in the current study. (Less)

An experimental study of effects of confinement ratio on swirl stabilized flame macrostructures

Swirl stabilized premixed flames are common in industrial gas turbines. The flame shape in the combustor is highly related to the combustion stability and the performance of the gas turbine. In the current paper, the effects of confinement on the time averaged flame structures or flame macrostructures are studied experimentally. Experiments are carried out with swirl number S = 0.66 in two cylindrical confinements with diameters of d1 = 39 mm and d2 = 64 mm and confinement ratio c1 = 0.148 and c2 = 0.0567. All the experiments were carried out in atmospheric. CH∗ chemiluminescence from the flame was recorded to visualize the flame behavior. An inverse Abel image reconstruction method was employed to better distinguish the flame macrostructures. Different mechanisms forming the time averaged M shape flames are proposed and analyzed. It is found that the confinement wall plays an important role in determining the flame macrostructures. The flow structures including the inner and outer recirculation zones formed in the confinement are revealed to be the main reasons that affects different flame macrostructures. Meanwhile, the alternation of flame shapes determines the flame stability characteristics. A smaller confinement diameter forced the flame front to bend upstream into the outer recirculation zone hence forming a M shape flame. A strong noise caused by the interaction of the flame front in the outer recirculation zone with the combustor wall was observed. Another unsteady behavior of the flame in the bigger combustor, which was caused by the alternation of the flame root position inside and outside the premixing tube, is also presented. The V shape flame in the two combustors radiated weaker chemiluminescence but the main heat release zone was elongated than the M shape flame. Other operating conditions, i.e. total mass flow rate of the air flow and the equivalence ratio also affect the flame macrostructures. The flame blowout limits were also altered under different test conditions. The bigger confinement has better performance in stabilizing the flame by having lower lean blowout limits. (Less)

Experimental investigation on the influences of bluff-body's position on diffusion flame structures

Effects of the bluff-bodys position on diffusion flame structures and flame instability characteristics were investigated experimentally. A flame regime diagram together with the corresponding flow fields were proposed to evaluate the influences caused by the alternation of bluff-bodys position. The disk shape bluff-body was placed 10 mm downstream or at the same height with the annular channel exit. The bulk velocity of the annular air flow varied from 0 to 8.6m/s while the central jet fuel velocity ranged from 0 to 30m/s. Various flame patterns including the recirculation zone flame, the stable diffusion jet flame, split-flashing flame and lifted flame were observed and recorded with a high speed camera. It is found that the flame has approximately the same patterns with different bluff-bodys positions, except for cases with high air flow rate (Ua > 6.8m/s) and low fuel flow rate (Uj < 5m/s). Under that operating conditions, placing the disk bluff-body 10 mm above the annular channel could better stabilize the flame. High speed Particle Image Velocimetry (PIV) was also used to get deeper insight into the characteristics of the flow fields and flame stabilization. The size and strength of the recirculation zone downstream of the bluff-body altered with the changing of bluff-bodys position and other operating conditions. The recirculation zone, in the burner with the bluff-body placed 10 mm above the air channel exit, was found larger and stronger than that in the other burner geometry. In the reacting case, a recirculation bubble was formed besides the bluff-bodys outer wall which enhanced the flame stabilization. It is also found that the combustion changed the flow fields by enlarging the recirculation bubbles downstream of the bluff-body. (Less)