Low-pressure multipass Raman spectrometer.

Nonintrusive, quantitative measurements of thermodynamic properties of flows associated with propulsion systems are pivotal to advance their design and optimization. Laser-based diagnostics are ideal to provide quantitative results without influencing the flow; however, the environments in which such flows exist are often not conducive for such techniques. Namely, they often lack the optical accessibility required to facilitate the delivery of incident laser radiation and the subsequent collection of induced signals. A particularly challenging, yet crucial, task is to measure thermodynamic properties of plumes issuing from thrusters operating within a vacuum chamber. Large chambers used to simulate the vacuum of space generally lack optical ports that can facilitate complex laser-based measurements. Additionally, the near-vacuum environments within such chambers coupled with the ability of thrusters to efficiently expand the gas flowing through their nozzles lead to plumes with prohibitively low number densities (pressures below 1 Torr). Thus, there is a need to develop a diagnostic system that can offer high throughput without the use of free-space optical ports. Moreover, facilities where propulsion systems are tested typically lack vibrationally isolated space for diagnostic equipment and accurate climate control. As a result, such a high-throughput system must also be compact, versatile, and robust. To this end, the present work describes a fiber-coupled, multipass cell, spontaneous Raman scattering spectroscopy system. This system is intended to provide accurate temperature measurements within low-pressure environments via H2 rotational Raman thermometry. Proof-of-principle measurements are successfully performed at pressures as low as 67 Pa (500 mTorr). Techniques to maintain the signal-to-noise ratio at lower pressures, and the trade-offs associated with them, are discussed and evaluated. Finally, the ability of this system to facilitate additional quantitative measurements is also discussed.

20 kHz CH2O and OH PLIF with stereo PIV.

Planar laser-induced fluorescence (PLIF) of hydroxyl (OH) and formaldehyde (CH2O) radicals was performed alongside stereo particle image velocimetry (PIV) at a 20 kHz repetition rate in a highly turbulent Bunsen flame. A dual-pulse burst-mode laser generated envelopes of 532 nm pulse pairs for PIV as well as a pair of 355 nm pulses, the first of which was used for CH2O PLIF. A diode-pumped solid-state Nd:YAG/dye laser system produced the excitation beam for the OH PLIF. The combined diagnostics produced simultaneous, temporally resolved two-dimensional fields of OH and CH2O and two-dimensional, three-component velocity fields, facilitating the observation of the interaction of fluid dynamics with flame fronts and preheat layers. The high-fidelity data acquired surpass the previous state of the art and demonstrate dual-pulse burst-mode laser technology with the ability to provide pulse pairs at both 532 and 355 nm with sufficient energy for scattering and fluorescence measurement at 20 kHz.