Evolution and Dynamics of the Coherent Vortices in Contraction and Premix Coaxial Jets

Contraction and premix coaxial jets are commonly employed in gasification and combustion systems. Understanding the spatial development of coherent structures is a prerequisite to improve the mixing and stability characteristics. The evolution and dynamics of coherent vortices under the contraction and premix coaxial jets are visualized and discussed using flow visualization technology. The contraction configuration increases the instability of the outer shear layer corresponding to an organized rolling up of the outer vortex rings. In the premixed configuration, the earlier interaction between the inner and outer shear layers induces the precocious development of the inner Kelvin-Helmholtz (K-H) vortices. Two empirical equations are proposed to predict the transition length towards a fully turbulent flow.