Fractal turbulence enhancing low-swirl combustion

The use of fractal grids in a low-swirl burner can significantly increase the turbulent combustion rate, realizing a higher power density in these flames. The standard turbulence generating blocking grid has been replaced by one consisting of a pattern of cruciform structures of different sizes, forming a multi-scale grid derived by truncating an underlying fractal structure. It is shown that the turbulence is intensified when comparing the flow behind the multi-scale grid to the reference situation, where a standard single-scale grid is used. This increase is expressed by more than doubling of the r.m.s. of the velocity fluctuations, while only marginal changes in pressure drop are observed. From the energy spectrum of the velocity it becomes clear that not only the largest scales are more energetic; also smaller scales are introduced as the spectrum is further extended into the high-frequency range. By means of planar OH-LIF the flame geometry was assessed, showing an increase in flame surface density and widening of the flame brush as well as much finer wrinkling of the flame front for the cases involving a multi-scale blocking grid. Here the turbulent flame speed (local consumption speed) is doubled. The grid parameters that were varied are the level of 'fractality' and the blockage. For both properties their effect on the flow and combustion are evaluated. The blockage mainly affects the stabilization mechanism, while the level of 'fractality' determines the increase in turbulence and combustion rate. Finally, it is shown that the low NOx emission levels that characterize the low-swirl mode of combustion are not affected. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.