Applying Spectral Proper Orthogonal Decomposition (SPOD) to concurrent multi-spectral infrared and retroreflective shadowgraph imagery of open turbulent flames.

Previous literature cites direct noise sources, stemming from unsteady heat release, and indirect noise sources, stemming from vorticity fluctuations in accelerating flow, as the major contributing sources of acoustic noise in combusting flows. The current study seeks to employ concurrent multi-spectral infrared imaging with retroreflective shadowgraph imagery on open turbulent flames to further characterize the relationship between the combusting region of the flame with the thermally induced buoyant plume. A Spectral Proper Orthogonal Decomposition (SPOD) technique is applied to both datasets to extract coherent spatial structures that are oscillating at specific frequencies. In this fashion, the effect dominant frequencies in the combustion region (such as flame intermittency) have on the plume flow-field in its entirety can be viewed directly. Pre-mixed and diffusion laboratory-scale flames are presented from various fuel types, including propane, natural gas, and aspen wood. Geometry and fuel dependent variations are discussed.