The primary air of a low-NOx coaxial swirling burner is visualized by using glycol as smog tracer. The information of the visual flow field is input into a computer through image-capturing card with CCD camera as the image-capturing element. The boundary of the visual zone, i.e., the interface of the primary and secondary airs, is obtained by image processing. Fractal dimension (FD) of the boundary is examined and found to change from 1.10 to 1.40 with S-1, S-2 and zeta(1). When FD is small, the complex level of the interface is low, and mixture between the primary and secondary airs is weak near exit of the burner at the initial phase of combustion. This is stratified flow. When FD is big, mixture becomes strong near exit of the burner. It has been proposed that the flow with FD ranging from 1.10 to 1.20 is stratified flow favoring the reduction of NOx yield and the flow with FD from 1.25 to 1.40 is mixed flow producing significant amount of NOx. The mechanisms of the formation of stratified flow and mixed flow are analyzed. The corresponding S-1, S-2 and zeta(1) of these flows are given. (C) 2004 Elsevier Ltd. All rights reserved.