An experimental study on the liftoff of a co-flowing non-premixed turbulent methane flame: effect of the fuel nozzle geometry
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The effect of the fuel nozzle geometry on the liftoff phenomenon of turbulent methane diffusion flame with and without a co-airflow is investigated experimentally. This investigation consists of two parts. In the first part, the effect of the internal geometry of a circular nozzle is examined. This was accomplished via varying the nozzle diameter, orifice length to diameter ratio (L/D), and (3) the contraction angle. These geometrical parameters were aimed to create a wide range of test conditions of the ensuing jet flow. The strength of the co-airflow was also varied to evaluate its impact on the jet flame liftoff parameters. The second part consists of investigating the effect of the fuel nozzle exit orifice geometry on the flame liftoff. This was achieved by employing a rectangular nozzle with an exit aspect ratio of 2 and a circular nozzle. Particle Image Velocimetry (PIV) technique was used to characterize the velocity field of the turbulent jets issuing from these nozzles. Also, a high speed imaging technique was employed to determine the flame liftoff height. The flame results showed that the fuel nozzle having the greater L/D or smooth contraction has higher liftoff velocity. In addition, the results revealed that the rectangular nozzle has a lower liftoff velocity. The effect of the nozzle diameter on the liftoff, however, was found to depend on the co-airflow strength. The corresponding turbulent jet flow characteristics showed that higher levels of jet near-field turbulence results in a lower flame liftoff velocity regardless of the nozzle internal geometry. Moreover, the results showed that a nozzle with the lowest L/D or with smooth contraction has the lowest flame liftoff height. The PIV results revealed that a circular jet, which spreads faster and generates higher near-field turbulence, generates a flame with its base sitting closer to the nozzle. The results revealed also that the rectangular fuel nozzle, which, in general, has lower liftoff height, produces higher turbulence intensity in the jet near-field and faster spread along the minor axis of the nozzle which is an indication of the presence of relatively more turbulent flow structures (which is induced by the nozzle’s exit asymmetry). The results confirmed that higher jet spread rate in the near-field in conjunction with higher turbulence level result in an increased flame propagation speed (in line with Kalghatgi’s lifted diffusion flame stability theory), and hence make it possible for a flame to stabilize at a relatively lower height from the nozzle.