ADVANCEMENT OF COHERENT ANTI-STOKES RAMAN SCATTERING SPECTROSCOPY FOR APPLIED MEASUREMENTS IN COMBUSTION AND HIGH SPEED FLOW ENVIRONMENTS

Laser diagnostic techniques are a powerful tool for the understanding of multiphase dynamic environments related to the development of energetic materials [1 ]–[3 ], aeronautical and space propulsion systems, and novel high energy shock physics [4 ], [5 ]. In particular, Coherent Anti-Stokes Raman Scattering (CARS), first applied in 1973 [6 ] has been employed for high fidelity measurements of temperature[7 ], concentration[8 ]–[10 ] and pressure [11 ] for novel combustion environments [12 ] such as high pressure burners [13 ], pool fires[14 ], and rotating detonation engines (RDE)[15].

The current work focuses on (i) advancing the sensitivity of CARS for minor species concentration measurements, and (ii) advancing the repetition rate of CARS temperature measurements from the 100s Hz to 100s kHz for flame conditions and real-world applications such as high enthalpy shock tubes and blasts.

In part (i) quantitative nanosecond electronic resonance enhanced Coherent Anti-Stokes Raman Scattering spectroscopy (ERE- ARS) is presented for for the measurement of formaldehyde (CH2O) concentrations in reacting and non-reacting conditions. The three-color scheme utilizes an electronically resonant probe which allows detection of CH2O at concentrations as low as 9×1014 molecules/cm3 (55 parts per million) in a calibration cell with CH2O and N2 with a maximum of 3% uncertainty.The pressure dependence is studied up to 11 bar, and the technique further applied to characterize the CH2O concentration in an atmospheric premixed dimethyl ether/air McKenna Burner flame, with a maximum concentration uncertainty of 11%.

(ii)A burst-mode nitrogen (N2) picosecond vibrational coherent anti-Stokes Raman scattering (ps-VCARS) system is presented for accurate flame thermometry at 100 kHz repetition rate. A frequency-tripled ps burst-mode laser is used to pump a custom optical parametric generator/amplifier (OPG/OPA) to produce 607 nm broadband Stokes pulses with 120 cm−1 bandwidth, along with a narrow-band 532 nm pump/probe beam. A novel simultaneous shot-to-shot nonresonant background (NRB) measurement is implemented to account for Stokes spectral profile and beam overlap fluctuations. The 100-kHz ps-VCARS data are bench-marked in a near-adiabatic CH4/air Hencken calibration flame with an accuracy of 1.5% and precision of 4.7% up to peak flame temperatures. The use of N2 VCARS and simultaneous NRB measurements enables accurate high-speed thermometry for a wide range of fuels and combustion applications.

The developed burst-mode ps-VCARS system described above was applied first to a turbulent jet flame, and then to a fireball environment in parallel with pyrometry temperature measurements to simultaneously determine the gas and the particle phase. Results from nitrocellulose and from black powder fireballs are shown. The system was also employed to further the understanding of the physics occurring in a high enthalpy shock tube at Sandia National Laboratories. Here, the 100 kHz measurement allowed to measure the pre-shock, incident and reflected shock conditions at temperatures between 300K and ?5,000 K.

Furthermore, it was found that development of an accurate model for picosecond CARS was needed to accurately analyze the data. For this, a novel approach is presented to model the pump and stokes pulses as a summation of time delayed impulse response functions, which are probed by an appropriately placed probe pulse, and then incoherently summed over the time interval of interest. This model enables to improve the accuracy of the spectral fitting, and hence the temperature accuracy of the measurement technique. Extensive validation and comparison to other CARS models is shown to explain the space of validity of the model. The combination of a developed burst-mode system, a single shot nonresonant bandwidth referencing technique, and a physically accurate fitting model have made it possible to put

together an instrument to reliably perform measurements in various field environments attemperatures from 700 to >4,000 K with an uncertainty of 4.8%.

Finally, improvements to the ps-CARS model, future work and other possible laser architectures are discussed.