Combustion-transition interaction in a jet flame

Abstract

The transition between laminar and turbulent flow in a round jet flame is studied experimentally. Comparison is made between transition in nonburning and burning jets and between jet flames with systematic variation in initial Reynolds number and equivalence ratio. Measurements are made using laser anemometry, miniature thermocouples, ionization probes, laser Schlieren, and high-speed cine films. Compared with the cold jet, the jet flame has a longer potential core, undergoes a slower transition to turbulence, has lower values of fluctuating velocity near the burner with higher values further downstream, and contains higher velocity gradients in the mixing layer region although the total jet width does not alter greatly in the first twenty diameters. As in the cold jet, transitional flow in the flame contains waves and vortices and these convolute and stretch the initially laminar interface burning region. Unlike the cold jet, which has Kelvin-Helmholtz instabilities, the jet flame can contain at least two initial instabilities: an inner high-frequency, combustion-driven instability and an outer, low-frequency instability that may be influenced by buoyancy forces.