Evolution of reactive surfaces in turbulent flows

Abstract

The growth of reactive surfaces in turbulent flows is of intrinsic interest to turbulent premixed combustion modeling. In the regimes of turbulent combustion relevant to technological flows, the global fuel burning rate is directly proportional to the area of the propagating reactive front. This dissertation aims to investigate the growth rate of the area of reactive surfaces in unsteady flow configurations and identify its functional dependence on various parameters, particularly the Reynolds number.

First, a canonical flow configuration of spherical turbulent premixed flame in decaying isotropic turbulence is considered. A mathematical framework based on the flame surface density function is developed to analyze a database of large-scale direct numerical simulations. The surface area enhancement through turbulent wrinkling is found to be proportional to the product of the thickness of the turbulent flame brush; a region of space where the flame is found over repeated experiments, and the peak flame surface density within the brush. Both these quantities are studied in detail through their evolution equation derived within the proposed framework.

Subsequently, the analysis is extended to the investigation of a more realistic flow configuration of the swirling von-Kármán flow device. The device consists of a set of counter-rotating impellers that generate intense turbulence through mean shear. In particular, the Reynolds number dependence of the evolution of large surfaces in shear-generated turbulence is addressed briefly.