• Journal of Advanced Marine Engineering and Technology
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• v.33 no.2
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• pp.259-266
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• 2009

An experimental study was conducted to investigate the effects of forcing amplitude on the flow structure near the nozzle exit of forced jet diffusion flames. The jet was excited up to the blowout occurrence by a considerable large amplitude with a periodic velocity fluctuation at the tube resonating frequency. In the attached flame regime, we disclosed the very interesting result newly that adding of a moderate forcing amplitude caused the jet flame to become longer in spite of being forced. Particular attention is focused on the turnabout mechanism of vortex roll-up around the elongated flame, which has not been reported previously, and on the inner coherent structure of the forced jet in the attached flame regime. From the velocity and flow visualization results, it was ascertained that the surrounding air due to the occurrence of negative velocity parts was suck into the fuel nozzle. To aid in understanding the rotating phenomenon of coherent structure, we present a schematic diagram of the turnabout mechanism of vortex roll-up. The mechanism of vortex turnabout phenomenon can be easily understood by considering the positive and negative velocity amplitudes about the instantaneous velocity of the forcing flow, as shown in this diagram.

• Fire Science and Engineering
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• v.23 no.1
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• pp.55-62
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• 2009

A general combustion characteristics of forcing nonpremixed jet in laminar flow rates have been conducted experimentally to investigate the effect of forcing amplitude with the resonant frequency of fuel tube. There are two patterns of the flame lift-off feature according to the velocity increasing; one has the decreasing values of forcing amplitude on the lift-off occurrence when a fuel exit velocity is increasing, while the other has the increasing values. These mean that there are the different mechanisms in the lift-off stability of forced jet diffusion flame. Especially, the characteristics of attached jet flame regime are concentrically observed with flame lengths, shapes, flow response and velocity profiles at the nozzle exit as the central figure. The notable observations are that the flame enlogation, in-homing flame and the occurrence of a vortical motion turnabout have happened according to the increase of forcing amplitude. It is understood by the velocity measurements and visualization methods that these phenomena have been relevance to an entrainment of surrounding oxygen into the fuel nozzle as the negative part of the fluctuating velocity has begun at the inner part of the fuel nozzle.