Quantitative Measurements of Temperature and Species in Counterflow Flames Near Extinction

Purdue University, West Lafayette IN

Investigators

Abstract

Accurate measurements of temperature are critical for improving our understanding and modeling of flame structure and pollutant formation in flames. Modern computational models of both laminar and turbulent flames incorporate detailed models of chemical reaction kinetics and transport processes. The rates of chemical reactions in particular can exhibit strong temperature dependence. Consequently, accurate and validated methods for measuring flame temperatures are critical for rigorous comparison of numerical calculations of flame structure and pollutant formation with experiment. Coherent anti-Stokes Raman scattering (CARS) spectroscopy is widely considered to be the most accurate temperature measurement technique for flames but it is difficult to assess accurately the absolute temperature accuracy of the method, mostly because of the lack of other validated methods for measuring temperatures in excess of 2000 K. In this work a rigorous theoretical analysis of the accuracy of temperature measurements using the laser diagnostic method CARS spectroscopy will be performed. In addition, a new type of CARS experimental system will be developed to eliminate some factors which cause increased uncertainty in CARS temperature measurements. The goal of the project is to demonstrate temperature measurement accuracies of better than one per cent, which will enable rigorous evaluation of important aspects of computational flame models. Temperatures will be measured in both near-adiabatic Hencken burner flames and in non-premixed counterflow flames using an advanced high-spectral-resolution scanning CARS system. These measurements are enabled by the recent acquisition by our group of a high-power, single-frequency-mode, tunable titanium:sapphire continuous-wave laser system. The spectral width of the CARS laser beams will be approximately an order of magnitude less than the spectral width of the Raman lines for species such as N2 and H2. Consequently, we will be able to resolve the CARS spectral lines, minimizing the effects of the uncertainties their linewidths. The impact of the vibration-rotation interactions (the Herman-Wallis effect) and the effect of vibrational anharmonicity on the accuracy of temperatures extracted from N2 and H2 CARS spectra will be theoretically investigated. The accuracy of the high-spectral-resolution CARS temperature measurements will be validated in near-adiabatic Hencken burner flames. The validated CARS temperature measurements will then be performed in laminar, counterflow, non-premixed hydrocarbon/air flames, emphasizing measurements of flame structure near extinction. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.