MRI: Development of a High Repetition Rate Raman Scattering Instrument for Combustion Research

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Proposal Number: CTS 9977481Principal Investigator: N. ClemensThis Major Research Instrumentation (MRI) award supports the development of a spontaneous Raman scattering instrument. Spontaneous vibrational Raman scattering has proven to be one of the most important diagnostic techniques in combustion research owing to its ability to provide spatially resolved information on the concentration of several species simultaneously. However, conventional Raman scattering systems employ high-power pulsed lasers where the rate at which they can acquire data is limited by the pulse repetition rate (typically 5 to few hundred Hz). The problem with low sampling rates is that they restrict measurements to essentially random sampling, so that statistical information can be extracted, but important time-correlation information cannot be captured.It is proposed to develop a spontaneous Raman scattering instrument that will enable essentially continuous single-point measurements of all major species (hence mixture fraction) and temperature simultaneously in a turbulent flame, at a rate of 10 kHz or more. A core element of this effort will be the development of a high-repetition rate laser system that operates at visible wavelengths. In order to accomplish this, the proposes will work with a partner in industry to develop a laser that meets the demanding requirements for the proposed instrument. The proposed laser will be a diode-pumped Nd:YAG laser that will be frequency doubled to a visible (green) wavelength. It will operate at an average power of 150 W in the green with a repetition rate of 10 kHz and above; such a laser would be the first of its kind developed commercially. Once the laser is delivered, substantial work will be required to develop an instrument that can be used to make meaningful measurements in actual large-scale laboratory flames. Despite the fact that the average power of the laser is quite high, the energy per pulse is not (15 mJ at 10 kHz), and thus substantial effort will be required to maximize the light collected. This will be accomplished by utilizing a multi-pass cell so that the laser energy in the probe volume is increased by an order-of-magnitude. The system will employ separate custom-designed fast collection optics and high efficiency interference filters. This system will yield excellent signal-to-noise ratios on a single-shot basis, for all major species in an atmospheric pressure turbulent methane flame.