Investigation into Highly Turbulent Flames with Compositionally Inhomogeneous Inlets

Abstract

A piloted jet burner capable of varying the degree of premixing before the jet exit, from fully non-premixed to near homogeneous, has been the target of a second diagnostic campaign. Multi-scalar measurements were performed at Sandia National Laboratories using Raman-Rayleigh-LIF diagnostics combined with data acquisition and processing techniques that enhance spatial resolution and resolve flame orientation. Spatial oversampling through reduced on-chip binning and wavelet denoising with an adaptive thresholding and reconstruction method has facilitated the resolution of thin scalar gradients. Hydroxyl (OH) iso-surfaces from cross planar laser induced fluorescence have been used as surrogates for mixture fraction surfaces and to recover three dimensional data. Radial measurements were performed at a greater number of axial locations with an increased number of images at regions of interest which were informed by earlier measurements of these flames. The new measurements arising from this research complement existing data, enable more in-depth analysis of flame behaviour and provide further validation for numerical models. The Sydney inhomogeneous piloted jet burner comprises two concentric tubes within a pilot annulus. Variability in the mixture fraction profile at the jet exit is controlled by translating the inner tube upstream of the burner exit. When fuel and air are issued separately from the two tubes, the degree of mixing can be controlled from fully non-premixed at no recession to nearly fully premixed when the inner tube is 300mm from the jet exit. Previous research by S. Meares at the University of Sydney found that, when fuel and air are issued from the inner and outer jets respectively, maximum flame stability occurs at an intermediate recession distance. In this case, mixtures are partially premixed at the jet exit and not unlike the conditions often found in practical combustion systems. The enhanced stability of flames with compositionally inhomogeneity at the jet exit is attributed to premixed-stratified combustion enhancing heat release in the upstream region of those flames. Increased number of data points at the primary heat release zone has enabled multi-level conditioning to compare the effects of stratification on the mass fractions of species, carbon monoxide and hydrogen. This has shown how the direction of stratification has an effect on species’ mass fractions for a limited range of axial locations, namely x/D ≈ 5. Back-supported combustion was associated with greater mass fractions of CO and H2 than front-supported samples with equivalence ratio gradients of similar magnitude. Statistics of a recently developed reaction progress variable defined using oxygen mass fractions have been analysed in this thesis. Joint probability distributions of mixture fraction and reaction progress variable are used to examine trends in local extinction. Additionally, spatially conditioned joint PDFs challenge the commonly used assumption of statistical independence of reaction progress variable and mixture fraction and confirm increased upstream heat release in inhomogeneous flames over homogeneous flames. Lastly, improved diagnostic techniques have enabled reporting of quantitative measurements of scalar dissipation. Trends in detailed measurements of well-resolved, three dimensional scalar dissipation rates are discussed. The effects of spatial resolution are shown to become more significant as compositional inhomogeneity increases. It is found that the difference in one dimensional, χr, and three dimensional, χ, scalar dissipation increases with axial distance and degree of local extinction.