Burst-mode 100 kHz N2 ps-CARS flame thermometry with concurrent nonresonant background referencing.

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 to produce 607 nm broadband Stokes pulses with 120cm-1 bandwidth, along with a narrowband 532 nm pump/probe beam. A 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 benchmarked 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 high-speed thermometry for a wide range of fuels and combustion applications.

10 kHz two-color OH PLIF thermometry using a single burst-mode OPO.

10-kHz hydroxyl radical (OH) two-color planar laser-induced fluorescence (TC-PLIF) thermometry was demonstrated with a single burst-mode optical parametric oscillator (OPO) and a single camera. A fast, dual-wavelength switched seed laser enabled a high-energy, high-repetition-rate burst-mode laser to generate two 10-kHz pulse trains at wavelengths of ${\sim}{354.8}\;{\rm nm}$. The two pulse trains are colinear with 3 µs time interval between the pulse pairs. The injection-seeded OPO efficiently converts the burst-mode laser output to 285.62 and 285.67 nm to excite the ${Q}_2({12})$ and ${P}_1({8})$ OH transitions. PLIF images were collected from each of the two excitation transitions, and intensity ratios from the images were used to determine local temperatures. The development of fast, dual-wavelength switching, burst-mode OPO technology significantly reduces the experimental complexity of the high-speed TC-PLIF thermometry and simplifies its implementation in harsh combustion and flow test facilities.

Compact fiber-coupled UV-NIR hyperspectral imaging sensor for characterizing ultra-high temperature ceramic materials.

A compact fiber-coupled hyperspectral imaging sensor (HSIS) operating within the range of ultraviolet to near-infrared (UV-NIR) wavelengths is designed and developed for the remote recording of two-dimensional (2D) spectrally resolved thermal radiation and chemiluminescent emission from ultra-high-temperature ceramics (UHTCs). Using simulations, the entire system is optimized to improve the collection efficiency and minimize aberrations. The design, construction, and characterization of the HSIS sensor are discussed in detail. We present the 2D spectrally resolved measurements of the simultaneous thermal radiation and BO2∗ chemiluminescent emission from a commonly used UHTC (HfB2-SiC) material under high-heat-flux conditions. Our results show that BO2∗ chemiluminescence corresponds directly to material ablation and can be used to track the formation of the protective heat-resistant glass/oxide layer. Furthermore, the temperature measurements demonstrate the heat distribution properties of the sample and indicate the locations at which BO2∗ chemiluminescence is possible. These results highlight the application prospects of the compact fiber-coupled HSIS for high-temperature material characterization in practical arc-jet facilities with limited optical access.

10 kHz 2D thermometry in turbulent reacting flows using two-color OH planar laser-induced fluorescence.

10 kHz two-color OH planar laser-induced fluorescence (PLIF) thermometry was demonstrated in both laminar Hencken flames and turbulent premixed jet flames using two injection-seeded optical parametric oscillators (OPOs) pumped by a high-speed three-legged burst-mode laser. The two burst-mode OPOs generate ∼5mJ/pulse at 282 nm and 286 nm to excite the Q1(5) and Q1(14) transitions of the A2Σ+←X2Π (1,0) system of OH, respectively. PLIF images were collected simultaneously from each of the two transitions and ratios of intensities from the two images were used to determine local temperatures. Analyses of flame curvature, temperature, and the correlation in time of these two quantities are also discussed. The results from this work are promising for the use of this technique in more complex flow environments and at, potentially, even higher repetition rate.

Concentration and pressure scaling of CH2O electronic-resonance-enhanced coherent anti-Stokes Raman scattering signals.

Nanosecond electronic-resonance-enhanced coherent anti-Stokes Raman scattering (ERE-CARS) is evaluated for the measurement of formaldehyde (CH2O) concentrations in reacting and nonreacting conditions. The three-color scheme utilizes a 532 nm pump beam and a scanned Stokes beam near 624 nm for Raman excitation of the C-H symmetric stretch (ν1) vibrational mode; further, a 342 nm resonant probe is tuned to produce the outgoing CARS signal via the 101403 vibronic transition between the ground (X~1A1) and first excited (A~1A2) electronic states. This allows detection of CH2O at concentrations as low as 9×1014molecules/cm3 (55 parts per million) in a calibration cell with CH2O and N2 at 1 bar and 450 K with 3% uncertainty. The measurements show a quadratic dependence of the signal with CH2O number density. Pressure scaling experiments up to 11 bar in the calibration cell show an increase in signal up to 8 bar. We study pressure dependence up to 11 bar and further apply the technique to characterize the CH2O concentration in an atmospheric premixed dimethyl ether/air McKenna burner flame, with a maximum concentration uncertainty of 11%. This approach demonstrates the feasibility for spatially resolved measurements of minor species such as CH2O in reactive environments and shows promise for application in high-pressure combustors.

Femtosecond laser activation and sensing of hydroxyl for velocimetry in reacting flows.

A molecular tagging method for velocity measurements in reacting environments such as propulsion devices and high-temperature combustion-assisted wind tunnels is described. The method employs a femtosecond (write) laser to photodissociate H2O, a common combustion product, into a locally high concentration of OH radicals. These radicals are tracked by planar laser-induced fluorescence (PLIF) from the A2Σ-X2Π (1-0) vibrational band excited by a time-delayed 284 nm (read) laser sheet. As a variant of hydroxyl tagging velocimetry, the source laser can also be used to dissociate nitrogen for femtosecond laser electronic excitation tagging velocimetry to mark the time-zero location of the write laser for velocimetry in non-reacting regions using the same imaging system without OH PLIF. The OH tracer lifetime is studied in a hydrogen-air Hencken burner operating at Φ=0.5-1.8 to evaluate the tracking capability for velocimetry over a range of conditions. Effects of changing read laser wavelength, excitation energy, and influence of background flame emission are also studied. The data processing methodology and results are described for tracking displacements with 9-25 µm uncertainty in a hydrogen diffusion flame. This method presents several advantages in operational convenience and availability of laser sources, and it provides an avenue for improvements in the repetition rate, precision, and applicability over previously demonstrated hydroxyl tagging schemes.

WIDECARS multi-parameter measurements in premixed ethylene-air flames using a wavelength stable ultrabroadband dye laser.

Width-increased dual-pump enhanced coherent anti-Stokes Raman spectroscopy (WIDECARS) measurements were used to determine the temperature and major species mole fractions in laminar, premixed, ethylene-air flames operating at atmospheric pressure. Conventional ultrabroadband dye lasers for WIDECARS, which use Pyrromethene dyes, have historically suffered from day-to-day wavelength shifting. To overcome this problem, a new ultrabroadband dye laser was developed in this study to provide a stable wavelength and power generation. A new dye laser pumping scheme and a mixture of Sulforhodamine 640, Kiton Red 620, and Rhodamine 640, was used to generate the desired FWHM ${\sim}{15}\;{\rm nm}$∼15nm (${410}\;{{\rm cm}^{ - 1}}$410cm-1) bandwidth. The WIDECARS measured mole fraction ratios of ${{\rm CO}_2}$CO2, CO, and ${{\rm H}_2}$H2 with ${{\rm N}_2}$N2 agreed well with chemical equilibrium calculations.

Fiber-coupled, UV-SWIR hyperspectral imaging sensor for combustion diagnostics.

A fiber-coupled, hyperspectral imaging sensor (HSIS) ranging from ultraviolet (UV) to short-wavelength-infrared (SWIR) wavelengths is developed for remote detection of planar [two-dimensional (2D)], spectrally resolved flame emission. The key component of the sensor is a dimension-reduction 2D-to-1D (one-dimensional) fiber-optic array that contains 1024 fibers and features high-UV optical transmission (>30% transmission at 310-340 nm, >90% at 340-2000 nm), wide operational wavelengths (300-2400 nm), and a compact and robust design (full length <5 cm). The flame-emission signals are transmitted to the remote HSIS through a 3-m-long, UV-grade, imaging fiber bundle that consists of 30,000 single-mode fibers. The design of the 2D-to-1D fiber array, the fiber-characterization process, and the sensor development are discussed in detail. 2D spectrally resolved measurements of CH*, OH*, and C2* distribution are made in premixed laminar flames. Improved chemiluminescence-based fuel/air ratio measurements using spectrally resolved detection are demonstrated. The results of the current study indicate that implementation of fiber-coupled HSIS is feasible in practical gas-turbine-engine test facilities with limited optical access.