Daniel R. Richardson

Evaluation of Mixing Processes in a Non-Premixed Rotating Detonation Engine Using Acetone PLIF

The fuel and air mixing processes in an optically accessible non-premixed rotating detonation engine (RDE) are visualized using acetone planar laser induced fluorescence (PLIF) imaging. The acetone PLIF images are used to observe the transient fuel injection processes and evaluate the extent of partial premixing upstream of the detonation wave. The acetone PLIF images complement past OH* chemiluminescence images which showed the instantaneous size and shape of the detonation structure, oblique shock wave, and possible presence of deflagration between the fuel-fill zone and expansion region containing detonation products. The acetone PLIF data presented in this work represents a recent and ongoing experimental investigation that provides insightful information on the transient processes in the RDE. The acetone PLIF images of the non-reacting flow show an impinging jet in crossflow consistent with the fuel injection scheme of the current RDE design. A recirculation zone with minimal fuel concentration is observed in the outer corner near the fuel injection surface of the annular detonation channel. The acetone PLIF images of the reacting flow indicate that there is a purging period (60 – 75 μs corresponding to 18 – 22 % of the cycle) in which fuel is not being injected into the channel after the detonation wave travels past a particular fuel jet. This observation suggests that the high-pressure detonation wave inhibits the inflow of fuel during the purging period. The application of established experimental techniques such as acetone PLIF and OH* chemiluminescence imaging is providing new insights into RDEs. The results provide benchmark measurements that are useful for evaluating RDE models and simulations, improving fundamental understanding of the detonation structure in RDEs, and identifying critical design parameters that influence RDE operation and performance.

Single-Pulse Femtosecond Coherent Anti-Stokes Raman Scattering (CARS) Spectroscopy for Temperature Measurements at Data Rates of 1 kHz

[Abstract] Single-laser-shot temperature measurements at a data rate of 1 kHz are demonstrated using femtosecond coherent anti-Stokes Raman scattering (CARS). The single-laser-shot measurements were performed by using a chirped probe pulse to map the time-dependent frequency-spread decay of the Raman coherence into the spectrum of the CARS signal pulse. Temperature is determined from the spectral shape of the chirped-probe femtosecond CARS signal for probe delays of approximately 2 picoseconds with respect to the impulsive pumpStokes excitation of the Raman coherence.

Phase conjugate digital inline holography (PCDIH)

Digital inline holography (DIH) provides instantaneous three-dimensional (3D) measurements of diffracting objects; however, phase disturbances in the beam path can distort the imaging. In this Letter, a phase conjugate digital inline holography (PCDIH) configuration is proposed for removal of phase disturbances. Brillouin-enhanced four-wave mixing produces a phase conjugate signal that back propagates along the DIH beam path. The results demonstrate the removal of distortions caused by gas-phase shocks to recover 3D images of diffracting objects.

Simultaneous Single-Shot Thermometry and Minor-Species Detection via Femtosecond Fully Resonant Electronically Enhanced CARS (FREE-CARS)

Femtosecond time-resolved, fully resonant electronically enhanced coherent anti-Stokes Raman scattering (FREE-CARS) spectroscopy, incorporating a two-color excitation scheme, is used to demonstrate selective and sensitive gas-phase detection of minor species in mixtures and reacting flows.

Comparison of Chirped-Probe Femtosecond and Hybrid Femtosecond/Picosecond CARS for Gas-Phase Thermometry

A comparison of two femtosecond coherent anti-Stokes Raman scattering (CARS) thermometry techniques is presented. While similar accuracies can be obtained with each technique, the hybrid femtosecond/picosecond technique was found to be less complex.

Femtosecond Fully Resonant Electronically Enhanced CARS (FREE-CARS) for Simultaneous Minor-Species Detection and Single-Shot Thermometry

Femtosecond time-resolved, fully resonant electronically enhanced coherent anti-Stokes Raman scattering (FREE-CARS) spectroscopy, incorporating a two-color excitation scheme, is used to demonstrate selective and sensitive gas-phase detection of minor species in mixtures and reacting flows.

Characterization of Inverse Diffusion Flames by Planar Laser Induced Fluorescence of CO and OH

Two-color OH planar laser induced fluorescence (PLIF) thermometry and two-photon CO PLIF are used to characterize inverse diffusion flames. These flames are important tools to aid in understanding the secondary reaction zones in gas turbine engines that result from film-cooling air reacting with fuel-rich packets exiting from the combustor. For the experiments performed here, the exhaust from a well-stirred reactor is channeled to a test section where three different film-cooling geometries are used to create inverse diffusion flames. The two-color OH PLIF data shows the area with significant reactions as well as a two dimensional temperature field. The two-photon CO PLIF shows the consumption of the CO as a primary fuel in the secondary reaction zone.

Concentration Measurements in CO/N2 and Ar/N2 Gas Mixtures using Femtosecond Coherent Anti-Stokes Raman Scattering

Single-laser-shot concentration measurements in CO/N2 and Ar/N2 gaseous mixtures are performed using femtosecond (fs) coherent anti-Stokes Raman scattering (CARS) with a chirped probe pulse. Measurements are performed in a heated gas cell at temperatures of 300, 600, and 900 K for the complete range of binary gas mixtures. Due to the broadband nature of the ultrafast pulses, Raman transitions of CO (2145 cm -1 ) and N2 (2330 cm -1 ) are probed simultaneously. A theoretical model is developed to extract temperature and concentration measurements from the experimental data. The best agreement between theory and experiment was found for probe-pulse time delays from 0 to 2 picoseconds (ps). Preliminary concentration measurement results are promising when the lasers are tuned to the minor species Raman transition frequency. Measurements performed in Ar/N2 mixtures are used to explore detection limits in environments with strong nonresonant backgrounds. Experimental data is also reported utilizing polarization suppression of the nonresonant background and further analysis will be performed in the future. These measurements will help to determine the effect of resonant and nonresonant gases on temperature measurements based on the N2 spectral shape.

Chirped-Probe-Pulse Femtosecond Coherent Anti-Stokes Raman Scattering for Single-Laser-Pulse Flame Temperature Measurements

Single-laser-pulse temperature measurements are made at 1000 Hz by femtosecond coherent anti-Stokes Raman scattering (CARS) with a chirped-probe-pulse. The temporal decay of the Raman coherence is mapped onto the frequency of the CARS signal.

Temperature Measurements in Flames at 1000 Hz Using Femtosecond Coherent Anti-Stokes Raman Spectroscopy

[Abstract] Single-laser-shot temperature measurements at a data rate of 1 kHz are demonstrated using femtosecond coherent anti-Stokes Raman scattering (CARS) spectroscopy. The excitation of gas-phase Raman lines with spectral widths of 3 GHz by pump and Stokes beams with spectral widths of 3000 GHz is very efficient provided that the pump and Stokes beams are Fouriertransform-limited. The single-laser-shot measurements were performed by using a chirped probe pulse to map the time-dependent frequency-spread decay of the Raman coherence into the spectrum of the CARS signal pulse. Temperature is determined from the spectral shape of the chirped-probe femtosecond CARS signal for probe delays of approximately 2 picoseconds with respect to the impulsive pump-Stokes excitation of the Raman coherence. Fs CARS spectra with very high signal-to-noise ratios are acquired from laminar flames, forced unsteady flames, and turbulent flames. The fs CARS spectrum is not affected by collisional line shapes in contrast to ns CARS spectroscopy. However, the fs CARS spectrum is affected by the spectrum and phase of the pump, Stokes, and probe beams, and the effect of departures from the assumptions of Fourier-transform-limited pump and Stokes beam and a linearly chirped probe beam are discussed. I. Introduction Single-pulse coherent anti-Stokes Raman scattering (CARS) spectroscopy of gas-phase resonances using femtosecond (fs) lasers is discussed. Femtosecond CARS offers two potential major advantages compared with nanosecond CARS; i.e., CARS as usually performed with nanosecond pump and Stokes lasers. These potential advantages are (1) a significant increase in the signal-to-noise ratio of the CARS signal and (2) the capability of performing real-time temperature and species measurements at data rates of 1 kHz or greater. The potential for real-time measurements at frequencies of interest in turbulent flames is the result of the commercial availability of femtosecond laser systems with pulse energies of a few uf06dJ to a few mJ and with repetition rates of 1 kHz up to 250 kHz. If techniques for determining single-pulse temperatures and/or concentrations can be developed, time series measurements can be performed in turbulent flames and flows at data rates that are faster than turbulent fluctuation frequencies. The potential for significant noise reduction is a result of the