Observations on the effect of centrifugal fields and the structure of turbulent flames

Abstract

Following a demonstration that hot gas pockets coalesce in plasma jet vortex cores, various burner systems are designed to induce solid body rotation such as either to promote or to impede the transport into reactants of any islands of hot gases. Promotion results in large increases in the burning velocity and in the stability of premixed turbulent hydrocarbon-air flames, and vice versa. Planar imaging by laser-induced fluorescence of OH at high magnifications reveals numerous small islands of hydroxyl in small turbulent flames, especially near the tips and close to blow-out. Comparison with schlieren photographs and a review of other work suggests that these are sectioned inner cores of vortex filaments or of cusps on the flame front. In rotating conical flames these tend to drift towards the axis. OH concentrations within islands suggest that only a few – generally of the larger ones – are expanding centres of reaction; many of the small ones appear to be diffusing remnants of flame. A rough estimate of the centrifugally induced increase in diffusivity is deduced from the shortening, with rate of rotation, of turbulent diffusion flames. Comparison with the changes in burning velocity of premixed flames of similar geometry and rotation rate suggests that promoting the drift of hot gas and radicals into the reactants, in addition to increasing diffusivity, may also produce a slight augmentation of the reaction rate.