On the transition modes and mechanisms for laminar to turbulent lifted jet diffusion flames at normal- and micro-gravity

Abstract

Experiments of laminar-to-turbulent jet diffusion flames were conducted in microgravity (μg) and normal gravity (ng) to investigate effects of liftoff and buoyancy on the flame transition. Characteristics of flame shape and flame stability during the transitional process as well as the transition mechanism were examined. Theoretical analyses were conducted by considering influences of buoyant flow on flame shape. Results show that for a lifted flame the transition from laminar to turbulent is mainly caused by incipient turbulent flow near the flame base, and the flow turbulence arises from fuel jet breakup developed upstream of the flame. Buoyancy has little effects on the flame transition mechanism. It was found therefore that the ranges of jet Reynolds number corresponding to the transitional stage in ng and μg are nearly the same (1720 ∼ 2650). During the transitional process, the flame heights in μg were approximately doubled comparing to that in ng, and the flame liftoff heights in μg were persistently smaller than that in ng. Besides, the transitional flames in μg appeared to increase soot intensity comparing to those in ng. In the absence of buoyancy intrusion, limits of liftoff and blowout were both extended. In addition, a 2/3 power law correlation between the non-dimensional flame height and the flame Froude number was derived with certain theoretical basis. It was found to agree well with the experimental data for transitional and turbulent lifted flames.