This paper examines the fundamental characteristics of a 2 MW swirl burner/furnace system and in particular the excitation of the Precessing Vortex Core (PVC) in diffusion controlled combustion with natural gas being introduced axially on the axis of symmetry. Previous studies have indicated that under such conditions the PVC is virtually undetectable. The phenomena was investigated via phase averaged LDA and fine wire thermocouple measurements. The PVC influence is shown to extend over virtually all the furnace volume. A processing jet like structure of high velocity is created in which combustion is limited and temperatures are low. This region is responsible for the very high rates of mixing found in such systems. Clear relevance to practical industrial system is indicated.