Comparative evaluation of flame observables for simplified scalar dissipation rate measurements

The scalar dissipation rate at the stoichiometric surface (χ _stoich) is the quantity controlling the structure of strained laminar diffusion flamelets. However, its measurement necessitates the simultaneous acquisition of multi-scalar data and requires a significant amount of experimental effort and resources. In this paper, we provide a comparative study to evaluate the potential of several flame observables to yield a simplified measurement of χ _stoich. Simplified experimental techniques stem from the realization that χ _stoich scales as the inverse square of an appropriately defined thickness of the mixing layer. We examine several flame observables and characterize the accuracy with which the thickness of their zones follows an inverse square root scaling with the imposed strain rate in a steady counterflow diffusion flame (where strain is proportional to χ _stoich). In a flat, nitrogen diluted, counterflow, methane/oxygen diffusion flame, the scalar dissipation rate was first measured directly using line Raman imaging of major species and a N ₂-molecule based definition of the mixture fraction. Additionally, LIF measurements of the hydroxyl radical (OH) and formaldehyde (HCHO) as well as Raman measurements of carbon monoxide (CO) were performed. This provided an array of combustion products, combustion intermediates and their combinations (e.g. [HCHO]x[OH], [CO]x[OH]) which were used as observables to define a thickness δ. For each of the flame observables, the scaling of δ with strain rate (and therefore χ _stoich) was measured.