Investigation of conditional statistics in premixed combustion and the transition to flameless oxidation in turbulent opposed jets

Abstract

The current work focuses on extracting useful statistical information from Particle Image Velocimetry (PIV) measurements, complemented by Hot Wire Anemometry (HWA) measurements. Experiments were conducted in the turbulent opposed jet geometry, which is a canonical configuration for understanding the fundamentals of flow and combustion. Novel fractal grids were used to generate turbulence in the flow, and turbulent characteristics of the flow field were extracted to ascertain the relative effects of flow configuration on the flow characteristics, including lengthscale and energy information, using energy spectra and Proper Orthogonal Decomposition (POD). Lean premixed flames were measured using PIV, and novel techniques used to extract unconditional velocity statistics, bulk motion and rotation effects, conditional statistics, turbulent burning velocities, flame surface density (FSD), thickness of turbulent flame brush and instantaneous reaction zone, flame surface area, conditional dissipation, as well as conditional POD (CPOD). These were made accessible via density segregation, a multi-step image processing algorithm, used to detect flame isocontours directly from PIV images. Relative effects of flow and chemistry were investigated for lean premixed twin opposed jet flames, using methane, propane, ethylene, cyclopentane and JP-10 as fuels. In addition, the transition to flameless oxidation from conventional premixed flames was also characterised using methane, cyclopentane, cyclopentene and JP-10, via a Damkohler number analysis.