This article presents an application of a large-scale structural mixing model(Broadwell et al., 1984) to the blowout of turbulent reacting jets discharging perpendicularly into a unconfined cross flow. Experimental observations, therefore, aim to identify the existence of a large-scale vortical structure exerting an important effect upon flame stabilization. The flame/flow visualization using direct photography, a Reactive-Mie-Scattering (RMS) technique and high speed motion analyser, and the concentration measurement using gas chromatography are employed to confirm the role and existence of large structural vortices.
In an analysis of a common stability curve, a plausible explanation can be made that the phenomenon of blowout is related only to the mixing time scale of the two flows. The propane concentration decaying along the trajectory of the maximum concentration in X-Z plane shows that the flame stabilization fails on the contrary if the mixing becomes exceedingly fast. For the lower stability limits, the distinctive features of the flame shape are found that the flame formed due to the reaction keeps up the same shape (counter-rotating vortices pair) as the non-reacting jet by the strong effect of these vortical motions. Also, the fluctuation in the flame base of lift-off flame near blowout is on the order of local jet diameter at the blowout distance, which is consistent with the mixing process distance of organized large-scale vortical motion. In particular, it is noted that the blowout distances, projected to the direction of the cross stream, are always fixed at about $12d_o$ (X_b.o=12d_o)$, even though the blowout distances decrease slightly with the curvilinear coordinate. This is probably the most remarkable observation in the present work. The constancy of blowout distance makes it possible to formulate the blowout parameter epsilon proposed by Broadwell et al.
The blowout stability limits are formulated in terms of flame stabilization mechanism based on the large-scale organization of entrainment and mixing observed in turbulent shear flows. Namely, the blowout parameter ε have been formulated from the well-known experimental equations on the self-similarity of non-reacting jet in a cross flow. Measurements of the lower blowout limits in the liftable flame agree qualitatively with the blowout parameter ε but show quantitatively a difference. Good agreement between the results calculated by modified blowout parameter ε' and experimental results confirms the important effect of a large-scale structure in specifying the stabilization feature of blowouts.
Consequently, it is ascertained that the large-scale structure vortical motion play an important role in flame stability in turbulent nonpremixed jet flames with cross flows, especially the blowout.
본연구는 주유동에 수직으로 분사한 반응성 난류 분류 화염에서 화염 날림에 대한 미케니즘을 파악하고자 Broadwell 등이 자유 분류 화염에서 제안한 거대와구조 혼합 모델을 적용한 연구이다. 이를위해 화염 안정화에 중요한 영향을 미치는 거대 구조의 와운동 존재를 실험적으로 관찰하였다. 화염 안정화 선도 분석에서 복잡한 3 차원 난류 횡분류 화염의 안정화 특성이 주유동과 분사 분류의 두유체 간의 시간차원, 즉 혼합시간과 관련 있음이 예측되었다. 실험적 관찰에서 가장 놀라운 관측은 화염 날림이 모든 경우의 속도비에서 항상 일정한 거리에서 발생한다는 사실이다. 이러한 화염 날림 위치에 대한 정보로 거대와구조 혼합 모델의 화염 날림 변수를 수식화 할 수 있었다. 화염 날림 모델을 적용한 결과 실험값과 이론값이 정성적인 일치을 보였으나 정량적인 차이를 보여 새로운 변수 도입이 요구됨을 알았다. 새로 제안한 수정된 화염 날림 변수를 수식화하여 비교한 결과 실험값과 모사된 이론값이 잘 일치함을 알았고 따라서 난류 횡분류 화염에서의 화염 날림으로 대표되는 화염 안정화 기구가 거대 구조의 와운동에 의해 이루어 지고 있음을 알 수 있었다.
