Filtered Rayleigh scattering thermometry in premixed flames—Part I: Laminar flames and the effects of mixture composition approximation

Abstract

Filtered Rayleigh scattering (FRS) is a proven laser-based measurement technique that can be used to determine gas-phase properties in the presence of interference from unwanted surface or particulate scattering. In combustion environments, an important issue for conversion of measured FRS signals into temperature is the strong dependence on the local gas mixture composition. Previously, the importance of accurately characterizing the local chemical state has not been systematically evaluated in terms of FRS-based thermometry. Simultaneous quantitative measurements of all pertinent species are quite challenging and involve complex and expensive experimental setups. Thus, there have been previous FRS-specific approaches developed to approximate species composition that circumvent additional multi-scalar measurements. This paper is one of a two-part series that first seeks to evaluate the sensitivity of derived temperature estimates to the local composition and assess existing simplifying assumptions for approximating the unknown composition in laminar premixed flames. This paper uses a combination of simulations and experiments in laminar premixed flames of various fuels and equivalence ratios to understand the importance of composition knowledge/approximation for accurate FRS thermometry results. In general, results show that there is a need to reliably represent the most abundant species and their associated Rayleigh–Brillouin scattering spectral contributions. Approximations of the local composition by a single species lead to significant error (up to 50%) and non-physical results over a broad range of flame conditions. Finally, a simple but robust framework based on state relations from laminar flame calculations is recommended for accounting for composition effects. This approach leads to accurate ($<$2% error) and reliable FRS-based temperature results in laminar premixed flames without the need for simultaneous species concentration measurements.