Instability of premixed lean hydrogen laminar tubular flames

Abstract

Combustion at the conditions found in practical applications is extremely complex and must be well characterized to support the next generation of combustors. This task is extremely difficult both experimentally and numerically, where the fast timescales and small spatial scales restrict complete characterization. This complexity is mitigated for numerical simulations through the use of computationally efficient models that approximate problem physics. Limitations arise in validating these approximations, due to the inability to experimentally measure the required quantities combined with the prohibitive numerical cost of performing detailed simulations. Fundamental investigations address this difficulty—simplified flame geometries can be examined in more complete detail and can be used to support the validation of modeling approximations.

Presented here is the experimental and numerical characterization of premixed laminar tubular flames fueled with dilute lean hydrogen mixtures. This fundamental flame geometry exhibits structure similar to practical turbulent flames (locally varying curvature with extinction zones), yet retains a time independent 2D planar structure. This aspect greatly reduces the experimental and computational requirements, permitting: (1) a more complete experimental and numerical characterization and (2) detailed numerical experiments to directly validate modeling approximations.

Experimental measurements using non-intrusive laser diagnostic techniques are presented that provide 2D spatially resolved and quantitative measurements of temperature, major species, and two minor species (H and OH) through flame cross-sections. The detailed flame structure is then simulated using a 2D fully-implicit primitive variable finite difference formulation that includes multicomponent transport and detailed chemical kinetics. Comparisons between the experimental and numerical data sets are presented and show overall good agreement, though with significant quantitative discrepancies. These discrepancies are examined, and are further investigated with numerical experiments to better elucidate the dependence of cellular flame appearance on experimentally controlled variables. The cellular tubular flame is found to be highly sensitive geometry that may be used for validating diffusive transport modeling approximations. This capability is exemplified through the development of a simple and accurate approximation for thermal diffusion (i.e. Soret effect) that is suitable for practical combustion codes.