Kathryn N. Gabet

Towards High-Repetition Rate Rayleigh and Raman Scattering Imaging in Turbulent Jets and Flames

In this paper we will describe recent advances made in our laboratory in the development of high-repetition-rate Rayleigh and Raman scattering imaging capabilities. High-repetition-rate 1D Raman and 2D Rayleigh scattering imaging capabilities are being developed to image the time-varying mixture fraction and temperature fields in turbulent non-reacting and reacting flows. Initial results using a custom pulse-burst laser system at Ohio State University have demonstrated the ability to capture ten sequential 2D Rayleigh scattering images at a repetition rate of 10 kHz in both turbulent non-reacting jets and non-premixed jet-flames with pulse energies approaching 200 mJ at 532 nm. This paper will also describe the development and pending application of a new higher-energy, long-duration, next-generation burst-mode laser system and the use of higher resolution cameras for high-speed Raman/Rayleigh imaging.

Towards the Development of High-Speed 1D Raman Scattering in Turbulent Non-premixed Flames

In this paper we will describe recent advances made in our laboratory in the development of high-speed (10-kHz acquisition rate) 1D Raman scattering imaging. The ultimate goal is a capability of quantitatively measuring all major combustion species and deducing time-varying mixture fraction profiles in turbulent combustion environments. In this paper we will review our initial results depicting high-speed Raman scattering imaging of O2, N2, CH4, and H2 in a turbulent non-reacting CH4/H2 jet issuing into air (Appl Phys B, 101(1), 2010, p. 1-5). This work represented the first temporally-sequential 1D image sequences of major species measuring via high-speed Raman scattering. This paper will also describe our most recent work towards the application in combustion environments by examining system performance through single-shot measurements (at 10 Hz) in wellcharacterized laminar flames.

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

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.

Ultra High Framing-Rate Laser Diagnostics for High-Speed Reacting and Non-Reacting Flows

The critical features of a burst mode diagnostic imaging system is described, along with representative NO PLIF measurements at 1MHz, in a Mach 10 hypersonic flow, and Rayleigh imaging at 10kHz in a turbulent flame.