The effect of fractal grid generated turbulence on the structure of premixed flames

Abstract

In this thesis fractal grids as a new type of turbulence generators for premixed combustion applications are investigated. Fractal grids produce turbulence fields which differ from those formed by regular turbulence grids such as perforated plates or meshes. Fractal grids generate high turbulence intensities over an extended region downstream of the grid at relatively low blockage ratios (sigma ≈ 35%). Additionally, the integral scale of the flow remains almost constant downstream of the grid. This thesis examines the effect of fractal grid generated turbulence on the structure of premixed flames using a set of four different fractal square grids and one regular square grid which acts as a reference case. It is found that for flames stabilised at the same downstream position, flames in the turbulence field of fractal grids show more intense corrugation, a higher flame surface density and a larger turbulent burning velocity compared to flames in regular grid generated turbulence. This is the direct result of the increased turbulence level produced by the fractal grids and demonstrates the potential benefit of using fractal grids for applications in premixed combustion. As an example for possible applications, fractal grids are used as turbulence generators to investigate the geometric alignment between flames and the principal strain-rate axes of a turbulent flow. The statistical analysis reveals that turbulence-flame alignment strongly depends on the distance between the flame surface and the location where the strain-rate field is evaluated. This dependency also helps to interpret findings of previous alignment studies.