Experimental investigations on the comparison of developments between transient jets and evolving jet diffusion flames, and corresponding jet flames with stagnant point diffusion flames have been described. A high speed Schlieren visualization has been used to examine the developing processes for several flow conditions. Measurement programs on the developing process have included the jet tip penetration velocity and the jet width of the primary vortex. A qualitatively compensated measurement of maximum flame temperature, which is based on the ion signal, has been employed to inspect flame responses to characterized time-varying strain rate. The flame responses are obtained at two conditions for the slowly time-varying strain rate and the case of flame extinction, and analyzed to depict similarity between a stagnant point diffusion flame and an evolving jet diffusion flame.
From the visualization and the measurements on the developing process, the developing behaviors of the evolving jet diffusion flame are greatly different from that of the transient jet and thus the jet tip penetration velocities are also modified. In the initial stage, the fuel jet motion is primarily along the nozzle centerline in the evolving jet diffusion flame while a radial motion is rigorous in the transient jet. The growth of primary vortex in the transient jet is properly explained through a impulsively started laminar vortex described by Pullin. The existence of flame moves the roll up of the primary vortex downstream. However, once the rollup is started, the jet of the evolving jet diffusion flame spreads faster than that of the transient jet as a consequence of expansion due to the exothermicity of the combustion process. Finally, the primary fuel jet in the transient jet enlarged and highly deformed through interaction with continuous vortices. However, in the presence of flame, only a primary vortex structure survives and the jet width remains nearly constant in the last stage even though the jet flame is still being developed. It is also found that the jet tip penetration velocity varies with downstream distance from the nozzle exit and an increase in Res gives rise to a higher tip penetration velocity.
From the investigation on flame behaviors near the jet tip, the time varying-strain rate can be properly characterized with the jet tip penetration velocity and the jet width of the primary vortex. The time variation with low strain rates, in which illustrates the flame behavior of the upper branch far from extinction in the well- known S-curve, is experimentally confirmed to produce a quasi-steady flame response. The time variation with strain rates in the case of flame extinction, in which may experience the middle branch of the S-curve in the transition to flame extinction, indicates an unsteady effect of flame response. Consequently, the experimental flame responses in the evolving jet diffusion flame correspond to those numerically depicted by previous researchers in a stagnant point diffusion flame. It is therefore found that the flame responses near jet tip depend on time histories of characterized strain rates in the developing process and is expected that one can use this combustion field as a test field for a nonsteady flame behavior.
