Quantification and Accuracy of a CMOS-Based Raman Scattering Imaging System for High-Speed Measurements in Flames

Abstract

In this paper we will describe recent work in our laboratory towards the quantification of a high-speed (> 10 kHz) combined 1D Raman-Rayleigh scattering imaging system utilizing CMOS-based cameras. While our previous work has demonstrated the ability to acquire high-speed Raman/Rayleigh scattering images using a pulse burst laser system (Gabet et al., 2010), further study of the acquisition system is necessary for quantitative results. For the majority of high-speed imaging experiments, CMOS cameras are used because conventional CCD cameras cannot operate at sufficiently high acquisition rates to capture the full range of temporal scales and fluctuations in turbulent flows. Unlike CCD cameras, which typically have uniform and linear pixel response, each pixel on CMOS cameras has a unique response which needs to be characterized individually (Patton et al., 2011; Weber et al., 2011). In addition, CMOS cameras are known to exhibit increased levels of noise, particularly when coupled with an image intensifier. Careful examination and calibration of CMOS-based acquisition systems is of particular importance to understand their limitations and accuracy for low-signal applications such as Raman scattering. This paper will focus on quantifying the precision and accuracy of Raman/Rayleigh scattering measurements of major species, temperature, and mixture fraction using our CMOS-based 1D Raman/Rayleigh system in a series of near-adiabatic H2/air flames and turbulent H2/N2 jet flames. A detailed analysis of the spectral response and signal-to-noise ratio (SNR) of major species (H2, N2, H2O, and O2) and temperature is presented. The ability to measure “single-shot” scalar values accurately in turbulent flames is assessed by comparing scalar results in the DLR H3 (50% N2/50% H2 Re=10,000) turbulent jet flame to previous work (Meier et al., 1996). The ultimate goal of our research is to measure the time-varying profiles of all major combustion species and deduce temporally resolved mixture fraction profiles in turbulent combustion environments.