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The structure and dynamics of a hydrogen-air rotating detonation engine (RDE) are described based on 100-kHz laser absorption spectroscopy measurements of water temperature at four simultaneous locations within the detonation channel. The analysis focuses on the evolution of the flowfield over a 200 ms period for three separate air mass flow rate cases. Two-dimensional unwrapped visualizations of the temperatures show a flowfield structure containing regions with the detonation front, combustion products, oblique shock, and refilling reactants, qualitatively agreeing with previous simulations and experiments. A major conclusion is that water from the combustion products is measured throughout all space and time in the RDE, including near the injector, implying the presence of performance loss processes such as burning upstream of the detonation wave or the back recirculation of combustion products with fresh fuel–air. By analyzing the elevated temperatures of the reactants during the refill process, one estimation for the mass fraction of combustion products in the reactants is as high as 20–30% on average. This product mass fraction is found to be inversely proportional to the bulk air mass flow rate and decreases as time progresses. This indicates these non-ideal processes are more significant closer to RDE ignition for poorer performing operating conditions. For the largest air mass flow case, water temperatures near the nominally cold plenum conditions likely corroborate the presence of a recirculation region on the RDE inner body. Analysis of inter- and intra-cycle temperature dynamics further support non-ideal processes occurring behind the detonation wave and during the refill process. As a whole, the data indicates that the RDE performance is better as time progresses away from ignition or for higher air mass flow rates. These data are also important for comparison with numerical models.

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 ∼ 5 m J / p u l s e at 282 nm and 286 nm to excite the Q 1 ( 5 ) and Q 1 ( 14 ) transitions of the A 2 Σ + ← X 2 Π (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.

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 ∼ 354.8 n m . 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.

Quantitative, one-dimensional (1D), single-laser-shot, fuel–air ratio (FAR) measurements in both laminar and turbulent methane–air flames were conducted using time-gated nanosecond-laser-induced breakdown spectroscopy (ns-LIBS) line imaging. In the laminar methane–air flames at a pressure of 1–11 bar, hydrogen (Hα) and nitrogen (NII) atomic emission lines at 568 and 656 nm, respectively, were selected to establish a correlation between the line intensities and the local FAR. The spatial calibration profiles of the N/H ratios in the flames at various pressures were obtained in one dimension. The effects of the laser energy and pressure on the stability and precision of the 1D FAR measurements were investigated. It was observed that the N/H correlation is significantly reduced at ∼11 bar, which sets the limits of the 1D LIBS-based FAR measurements. Single-laser-shot 1D FAR measurements were conducted in a turbulent flame at atmospheric pressure, and multiline LIBS was performed to extend the measurement area of interest. Spatially and spectrally resolved line LIBS can provide the local FAR with a spatial resolution of ∼0.1 mm. These results hold promise for the utilization of ns-LIBS for spatially resolved 1D FAR measurements in turbulent flames at elevated pressures.

Krypton (Kr)-based tagging velocimetry is demonstrated in a K r / N 2 jet at 100 kHz repetition rate using a custom-built burst-mode laser and optical parametric oscillator (OPO) system. At this repetition rate, the wavelength-tunable, narrow linewidth laser platform can generate up to 7 mJ/pulse at resonant Kr two-photon-excitation wavelengths. Following a comprehensive study, we have identified the 212.56 nm two-photon-excitation transition as ideal for efficient Kr-based velocimetry, producing a long-lived ( ∼ 40 µ s ) fluorescence signal from single-laser-pulse tagging that is readily amenable to velocity tracking without the need for a second “read” laser pulse. This long-lived fluorescence signal is found to emanate from N 2 —rather than from Kr—following efficient energy transfer. Successful flow velocity tracking is demonstrated at multiple locations in a high-speed K r / N 2 jet flow. The 100 kHz repetition rate provides the ability to perform time-resolved velocimetry measurements in high-speed and even hypersonic flow environments, where standard velocimetry approaches are insufficient to capture the relevant dynamics.

In this work, a variable-pulse-oscillator is developed and coupled with a burst-mode amplifier for generation of high-energy laser pulses with width of 100 ps to 1 ms and near-Gaussian temporal pulse shape. Pulse energy as high as 600 mJ is demonstrated at 1064 nm, with a super-Gaussian spatial profile and beam quality as good as 1.6 times the diffraction limit. A time-dependent pulse amplification model is developed and is in general agreement with experimentally measured values of output pulse energy and temporal pulse shape of the amplified pulses. Key performance parameters (pulse energy, temporal pulse shape, and spatial beam profile and quality) are analyzed as a function of pulse width across seven orders of magnitude. Additionally, the model is used to elucidate deviations between the simulated and experimental data, showing that the relationship between pulse width and output pulse energy is dominated by the variable-pulse-width oscillator performance, not the burst-mode amplifier.

A burst-mode laser system is developed for hybrid femtosecond/picosecond (fs/ps) rotational coherent anti-Stokes Raman scattering (RCARS) at megahertz rates. Using a common fs oscillator, the system simultaneously generates time synchronized 1061 nm, 274 fs and 1064 nm, 15.5 ps pulses with peak powers of 350 MW and 2.5 MW, respectively. The system is demonstrated for two-beam fs/ps RCARS in N 2 at 1 MHz with a signal-to-noise ratio of 176 at room temperature. This repetition rate is an order of magnitude higher than previous CARS using burst-mode ps laser systems and two to three orders of magnitude faster than previous continuously pulsed fs or fs/ps laser systems.

Simultaneous dual-plane stereoscopic particle-imaging velocimetry (DS-PIV) and hydroxyl planar laser-induced fluorescence (OH-PLIF) was performed at 10 kHz repetition rate in a highly turbulent swirling jet flame. The burst-mode laser system provides sufficient 532-nm dual-pulse energy enabling high-speed large-area dual-plane stereo-PIV measurement. The scattered light from the two planes was separated onto respective cameras using orthogonal polarizations. A diode-pumped solid-state Nd: YAG/dye laser system produced the narrowband excitation 283-nm beam for the 10-kHz OH-PLIF imaging. The combined diagnostics produced simultaneous, temporally resolved two-dimensional fields of OH and two-dimensional, three-component velocity fields, facilitating the observation of the interaction of fluid dynamics with flame fronts.

Conventional residual stress mitigation techniques involve long processing times at high temperatures and/or mechanical loading to build plastic compressive stress below the surface. In this study, we present a new residual stress mitigation methodology at near ambient temperature in less than a minute. This is demonstrated on a welded joint of 316 L stainless steel, where low-frequency DC current pulses are shown to recrystallize the specimen and reduce residual stress. We present experimental evidence of ∼30 % reduction in electrical resistance, which corresponded to ∼40 % decrease in both microhardness and residual stress, measured by the X-ray diffraction tests. Similar improvement was qualitatively observed through significant decrease in the low-angle grain boundary density, which also reflects the decrease of the residual stress. The technique can be applied to relieve residual stress in conditions difficult for the conventional processing, such as locations with extreme space constraints or objects that cannot be heat treated.

Broadband femtosecond (fs) two-photon laser-induced fluorescence (TP-LIF) of the B1Σ+←X1Σ+, Hopfield-Birge system of carbon monoxide (CO) is believed to have two major advantages compared to narrowband nanosecond excitation. It should (i) minimize the effects of pressure-dependent absorption line broadening and shifting, and (ii) produce pressure-independent TP-LIF signals as the effect of increased quenching due to molecular collisions is offset by the increase in number density. However, there is an observed nonlinear drop in the CO TP-LIF signal with increasing pressure. In this work, we systematically investigate the relative impact of potential deexcitation mechanisms, including collisional quenching, forward lasing, attenuation of the source laser by the test cell windows or by the gas media, and a 2+1 photoionization process. As expected, line broadening and collisional quenching play minor roles in the pressure-scaling behavior, but the CO fs TP-LIF signals deviate from theory primarily because of two major reasons. First, attenuation of the excitation laser at high pressures significantly reduces the laser irradiance available at the probe volume. Second, a 2+1 photoionization process becomes significant as the number density increases with pressure and acts as a major deexcitation pathway. This work summarizes the phenomena and strategies that need to be considered for performing CO fs TP-LIF at high pressures.

Two-color, planar laser-induced fluorescence (PLIF)-based two-dimensional (2D) thermometry techniques for reacting flows, which are typically developed in the laboratory conditions, face a stiff challenge in their practical implementation in harsh environments such as combustion rigs. In addition to limited optical access, the critical experimental conditions (i.e., uncontrolled humidity, vibration, and large thermal gradients) often restrict sensitive laser system operation and cause difficulties maintaining beam-overlap. Thus, an all fiber-coupled, two-color OH-PLIF system has been developed, employing two long optical fibers allowing isolation of the laser and signal-collection systems. Two OH-excitation laser beams (∼283 nm and ∼286 nm) are delivered through a common 6 m long, 400 µm core, deep ultraviolet (UV)-enhanced multimode fiber. The fluorescence signal (∼310 nm) is collected by a 3 m long, UV-grade imaging fiber. Proof-of-principle temperature measurements are demonstrated in atmospheric pressure, near adiabatic, CH4/O2/N2 jet flames. The effects of the excitation pulse interval on fiber transmission are investigated. The proof-of-principle measurements show significant promise for thermometry in harsh environments such as gas turbine engine tests.

A burst-mode nitrogen ( N 2 ) 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 120 c m − 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 C H 4 / a i r Hencken calibration flame with an accuracy of 1.5% and precision of 4.7% up to peak flame temperatures. The use of N 2 VCARS and simultaneous NRB measurements enables high-speed thermometry for a wide range of fuels and combustion applications.

Three-dimensional imaging of the tracer concentration field is demonstrated at a rate of 20 kHz in a turbulent gaseous free jet. The acetone tracer species that is seeded into the jet is excited by the fourth-harmonic output of a Nd:YAG burst-mode laser. The laser beam is expanded and re-collimated to a 35-mm-diameter beam and passed through a diffuser to reduce spatial variations within the beam profile. Seven high-speed cameras coupled to high-speed intensifiers with vertical stereoscopes collect fourteen simultaneous views of the fluorescence emission. The images are processed and a linear relationship between signal intensity and acetone concentration was verified. Eight of the fourteen processed views are then reconstructed into volumes using a simultaneous multiplicative algebraic-reconstruction technique, limited by the software. The mixing behavior is characterized and compared with turbulent-jet scaling laws and data from the literature at similar conditions. The accuracy, precision, and limitations of the technique are discussed, and potential strategies for implementation in reacting flows are considered.

Flash x-ray radiography is used to capture quantitative, two-dimensional line-of-sight averaged, single-shot liquid distribution measurements in impinging jet sprays. The accuracy of utilizing broadband x-ray radiation from compact flash tube sources is investigated for a range of conditions by comparing the data with radiographic high-speed measurements from a narrowband, high-intensity synchrotron x-ray facility at the Advanced Photon Source (APS) of Argonne National Laboratory. The path length of the liquid jets is varied to evaluate the effects of energy dependent x-ray attenuation, also known as spectral beam hardening. The spatial liquid distributions from flash x-ray and synchrotron-based radiography are compared, along with spectral characteristics using Taylor's hypothesis. The results indicate that quantitative, single-shot imaging of liquid distributions can be achieved using broadband x-ray sources with nanosecond temporal resolution. Practical considerations for optimizing the imaging system performance are discussed, including the coupled effects of x-ray bandwidth, contrast, sensitivity, spatial resolution, temporal resolution, and spectral beam hardening.

We have considered the Raman scattering in molecular media. Applying two laser fields in a two-photon resonance with vibrational transition, we have studied the role of rotational levels for excitation of vibrational coherence. It is shown that the molecular vibrational coherence strongly depends on the effect of coherent population trapping for rotational levels. The obtained results are important for applications of Raman spectroscopy to molecular detection in engineering, chemical, and biological applications.

We report the high-speed imaging of multi-species and multi-parameter combustion diagnostics for turbulent non-premixed jet flames using a three-legged burst-mode laser system. Simultaneous OH/CH2O planar laser-induced fluorescence and Rayleigh-scattering imaging measurements at a 10-kHz rate are obtained. OH and CH2O concentrations, flame temperatures, and heat-release rates are simultaneously acquired in two-dimensions at 10 kHz.

Two-dimensional (2D) Rayleigh scattering (RS) imaging at an ultrahigh repetition rate of 100 kHz is demonstrated in non-reacting flows employing a high-energy burst-mode laser system. Image sequences of flow mixture fraction were directly derived from high-speed RS images. Additionally, a 2D instantaneous flow velocity field at 100 kHz was obtained through optical-flow-based analysis of the RS images. In further analysis of both the mixture fraction and flow velocity field, the result for the centerline mixture fraction agreed well with the scaling law. The demonstrated high-speed RS technique in conjunction with optical-flow-based analysis provides non-intrusive, simultaneous measurements of the flow mixing and velocity field, extending the measurement capability of the RS technique to high-speed non-reacting and reacting flows.

We report on the development of a robust fiber-coupled long-wavelength infrared (LWIR) hyperspectral sensor suite for accurate and reliable non-contact surface temperature measurements in propulsion systems with limited optical access. We first experimentally investigate various state-of-the-art LWIR optical fibers and identify the ideal fiber for efficient coupling and transmission of LWIR signals. The effects of the fiber material, structure, bending, and thermal heating on LWIR fiber transmission are characterized. Subsequently, we discuss the development of a fiber-coupled LWIR hyperspectral sensor using a multi-mode polycrystalline fiber. The temperature measurement accuracy and precision of the sensor are determined using a well-calibrated blackbody radiation source and heated thermal barrier coating. The sensor is integrated into a homemade water-cooled probe housing and environmental protection box and subsequently used for reliable combustor liner temperature measurements in a high-pressure, liquid-fueled combustor rig with no built-in optical access. We also discuss the measurement challenges associated with flame interference and potential solutions. The LWIR sensor shows significant promise in its application to surface temperature measurements, and our findings can aid propulsion system engineers and researchers in system design and operation optimization.

Optical measurement techniques have become powerful tools for the detailed study of the chemistry and physics of reacting flows, and plasmas. Traditional combustion diagnostics based on continuous-wave and low-repetition-rate ns- pulsed lasers continue to dominate fundamental studies and applications; however, revolutionary advances in the science and engineering of both ultrashort-pulse (femtosecond) lasers and high-repetition-rate (burst-mode) lasers are driving the advancement of existing diagnostic techniques and enabling the development of new measurement approaches. The ultrashort pulses afforded by femtosecond laser systems provide tremendous peak powers—allowing nonlinear signal generation with broad spectral coverage—and unprecedented temporal resolution for studying chemical kinetics and dynamics. The high pulse-repetition rates of ultrashort-pulse amplifiers as well as ns- and ps-pulse burst-mode lasers allow previously unachievable data-acquisition bandwidths for the study of turbulent time series and combustion instabilities. More importantly, the high pulse energies emanating from these advanced laser systems afford the ability to extend measurement capabilities beyond point-wise measurements to multi-dimensional (line [1D], planar [2D], or even volumetric [3D]) imaging. The rapid growth of ultrafast laser-based spectroscopic measurements has been fueled by the need to achieve the following: 1) time-resolved single-shot measurements 2) simultaneous detection of multiple species, 3) spatially resolved measurements, 4) interference-free measurements (collisional broadening, photolytic dissociation, etc.), and 5) higher dimensionality (line, planar, or volumetric). Several state-of-art ultrafast-laser–based spectroscopic techniques and their remarkable developments will be reviewed in meeting one or all of the above five needs for measurements of temperature and key chemical species concentrations in reacting flows and plasmas.Optical measurement techniques have become powerful tools for the detailed study of the chemistry and physics of reacting flows, and plasmas. Traditional combustion diagnostics based on continuous-wave and low-repetition-rate ns- pulsed lasers continue to dominate fundamental studies and applications; however, revolutionary advances in the science and engineering of both ultrashort-pulse (femtosecond) lasers and high-repetition-rate (burst-mode) lasers are driving the advancement of existing diagnostic techniques and enabling the development of new measurement approaches. The ultrashort pulses afforded by femtosecond laser systems provide tremendous peak powers—allowing nonlinear signal generation with broad spectral coverage—and unprecedented temporal resolution for studying chemical kinetics and dynamics. The high pulse-repetition rates of ultrashort-pulse amplifiers as well as ns- and ps-pulse burst-mode lasers allow previously unachievable data-acquisition bandwidths for the study of turbulent time se...

Detailed characterizations of picosecond laser electronic excitation tagging (PLEET) in pure nitrogen ( N 2 ) and air with a 24 ps burst-mode laser system have been conducted. The burst-mode laser system is seeded with a 200 fs broadband seeding laser to achieve short pulse duration. As a non-intrusive molecular tagging velocimetry (MTV) technique, PLEET achieves “writing” via photo-dissociating nitrogen molecules and “tracking” by imaging the molecular nitrogen emissions. Key characteristics and performance of utilization of a 24 ps pulse-burst laser for MTV were obtained, including lifetime of the nitrogen emissions, power dependence, pressure dependence, and local flow heating by the laser pulses. Based on the experimental results and physical mechanisms of PLEET, 24 ps PLEET can produce similar 100 kHz molecular nitrogen emissions by photodissociation, while generating less flow disturbance by reducing laser joule heating than 100 ps PLEET.

Multiphoton-resonance enhancement of a rare-gas-assisted nitrogen femtosecond-laser electronic-excitation-tagging (FLEET) signal is demonstrated. The FLEET signal is ideal for velocimetric tracking of nitrogen gas in flow environments by virtue of its long-lived nature. By tuning to three-photon-resonant transitions of argon, energy can be more efficiently deposited into the mixture, thereby producing a stronger and longer-lived FLEET signal following subsequent efficient energy transfer from excited-state argon to the C ( 3 Π u ) excited state of nitrogen. Such resonant excitation exhibits as much as an order of magnitude increase in this rare-gas-assisted FLEET signal, compared to near-resonance excitation of seeded argon demonstrated in previous work, while reducing the required input excitation-pulse energies by two orders of magnitude compared to traditional FLEET.

Modal decompositions of unperturbed and acoustically driven injector flows from shear coaxial jets are implemented using dynamic-mode decomposition, which is a natural approach in the search for collective oscillatory behaviour in nonlinear systems. Previous studies using proper orthogonal decomposition had revealed the most energetic pairs of coherent structures in injector flows. One of the difficulties in extracting lower-energy coherent structures follows from the need to differentiate robust flow constituents from noise and other irregular facets of a flow. The identification of robust features is critical for applications such as flow control as well, since only they can be used for the tasks. A dynamic-mode decomposition based algorithm for this differentiation is introduced and used to identify different classes of robust dynamic modes. They include (1) background modes located outside the injector flow that decay rapidly, (2) injector modes – including those presented in earlier studies – located in the vicinity of the flow, (3) modes that persist under acoustic driving, (4) modes responding linearly to the driving and, most interestingly, (5) a mode whose density exhibits antiphase oscillatory behaviour in the observation plane and that appears only when $J$, the outer-to-inner-jet momentum flux ratio, is sufficiently large; we infer that this is a projection of a mode rotating about the symmetry axis and born via a spontaneous symmetry breaking. Each of these classes of modes is analysed as $J$ is increased, and their consequences for the flow patterns are discussed.

Megahertz-rate hydroxyl radical planar laser-induced fluorescence (OH-PLIF) was demonstrated in a hydrogen/air rotating detonation combustor for the first time, to the best of our knowledge. A custom injection-seeded optical parametric oscillator (OPO) pumped by the 355 nm output of a high-energy burst-mode laser produced narrowband pulses near 284 nm for OH excitation. The system generated sequences of more than 150 ultraviolet pulses with 400 µJ/pulse at 1 MHz and 150 µJ/pulse at 2 MHz. The order of magnitude improvement in the repetition rate over prior OH-PLIF measurements and in the number of pulses over previous megahertz burst-mode OPOs enables spatiotemporal analysis of complex detonation combustion dynamics.

Optical diagnostics of highly dynamic supersonic and hypersonic flows requires laser sources with a combination of high pulse intensities and fast repetition rates. A burst-mode Nd:YAG laser system is presented for increasing the overall energy of 532 nm pulse trains by ∼ 100 × and the number of high-energy pulses by 30 × for extended duration megahertz-rate flow diagnostics. At a lower repetition rate of 100 kHz, unprecedented energies near 1 J/pulse are achieved at 532 nm over a 1.1 ms burst. The laser performance is characterized and demonstrated for megahertz-rate laser-induced breakdown spectroscopy in a Mach 2 turbulent jet.

A robust approach for acquiring background-free, multitransition absorption spectra under single-laser-shot conditions is demonstrated using broadband, ultrashort laser pulses. This technique—referred to as time-resolved optically gated absorption (TOGA)—exploits the inherent differences in timescales between broadband, femtosecond-duration light sources and the longer-duration responses of narrowband atomic or molecular absorption features. An optical temporal gate, based on frequency mixing via sum-frequency generation or difference-frequency generation, is used to isolate these long-lived time-domain absorption features from the ultrashort component associated with the broadband absorption light source. A proof-of-principle demonstration of TOGA is provided using atomic Rb as an absorbing medium. Application of this technique toward single-laser-shot simultaneous detection of hydroxyl radical concentration and the corresponding local temperature is also demonstrated in a reacting flow. These results indicate that TOGA can provide spectrally resolved, broadband, background-free absorption measurements at laser-source repetition rates.

Burst-mode femtosecond laser electronic excitation tagging (FLEET) of nitrogen is introduced for tracking the velocity field in high-speed flows at kilohertz–megahertz (kHz–MHz) repetition rates without the use of added tracers. A custom-built Nd:glass femtosecond laser is used to produce 500 pulses per burst with pulses having a temporal separation as short as 1 µs, an energy of 120 µJ, and a duration of 274 fs. This enables 2 orders of magnitude higher measurement bandwidth over conventional kHz-rate FLEET velocimetry. Characteristics of the optical system are described, along with a demonstration of time-resolved velocity measurements with ∼ 0.5 % precision in a supersonic slot jet.

An imaging system for analysis of particle fields and product gases in a multiphase blast is proposed. The approach combines a kHz-MHz-rate burst-mode laser with particle image velocimetry (PIV) or tomographic feature tracking velocimetry (FTV) to resolve flowfields over a wide range of time scales.

A compact-footprint (0.18 m2) flash-lamp-pumped, burst-mode Nd:YAG-based master-oscillator pulsed-amplifier laser is reported with a fundamental 1064 nm output of over 14 J per burst. A directly modulated diode laser seed source is used to generate 10 ms duration arbitrary sequences of 500 kHz doublet or MHz singlet pulses for flow-field velocity or species measurements, respectively. Flexible pulse widths are used to balance the energy distribution of pulse doublets and achieve second-harmonic conversion efficiencies up to 42%. Burst-mode laser performance characteristics, measurement accuracies in turbulent flows, and prospects for kHz–MHz flow-field diagnostics are discussed.

High-repetition-rate interferometric-Rayleigh-scattering (IRS) velocimetry is implemented and demonstrated for non-intrusive, high-speed flow-velocity measurements. High temporal resolution is obtained with a quasi-continuous burst-mode laser that is capable of providing bursts of 10-msec duration with pulse widths of 10–100 nsec, pulse energy > 100 mJ at 532 nm, and repetition rates of 10–100 kHz. Coupled with a high-speed camera system, the IRS method is based on imaging the flow field though an etalon with 8-GHz free spectral range and capturing the Doppler shift of the Rayleigh-scattered light from the flow at multiple points having constructive interference. The seed-laser linewidth permits delivery of a laser linewidth of < 150 MHz at 532 nm The technique is demonstrated in a high-speed jet, and high-repetition-rate image sequences are shown.

Transmission of intense ultrashort laser pulses through hollow-core fibers (HCFs) is investigated for molecular-tagging velocimetry. A low-vacuumed HCF beam-delivery system is developed to transmit high-peak-power pulses. Vacuum pressure effects on transmission efficiency and nonlinear effects at the fiber output are studied for 100 ps and 100 fs laser beams. With a 0.1 bar vacuum in the fiber, transmission efficiency increases by ∼30%, while spectral broadening is reduced. A 1 m long, 1 mm core metal-dielectric-coated HCF can transmit ∼45 mJ/pulse and ∼2.9 mJ/pulse for 100 ps laser pulses (at 532 nm) and 100 fs laser pulses (at 810 nm), respectively. Proof-of-principle, single-laser-shot, fiber-coupled, ps and fs laser-based, nitrogen electronic-excitation tagging velocimetry is demonstrated in a free jet. Flow velocities are measured at 200 kHz to capture high-frequency flow events.

Ultrafast laser generates controlled plasma even at high gas pressures (40 bars), which helps in increasing the measurement stability of the laser-induced breakdown (LIBS) based fuel-air ratio measurement.

We demonstrate a robust ultrafast-laser technique, referred to as time-resolved optically gated absorption (TOGA) spectroscopy, for acquiring broadband, background-free, single-laser-shot absorption spectra for use in combustion diagnostics.

Simultaneous 10-kHz OH-PLIF/CH2O-PLIF/PIV (Rayleigh scattering) measurements in DRL-A non-premixed flames were demonstrated using a three-leg burst-mode laser system. High-speed multi-species concentrations, heat-release rate, and flow velocity field (temperature) were measured.

We report on modifications to the widely used astrophysical code, FLASH (Fryxell, Olson et al., 2000) that enable accurate simulations of chemically reacting flows with heat addition. The enhancements to FLASH include the implementation of extensive hydrogen–air and methane–air chemistry through multiple, detailed mechanisms (Smooke, 1991; Katta and Roquemore, 1995; Mueller, Kim et al., 1999; Billet, 2005), accomplished by building on the existing infrastructure of nuclear reaction network solvers. The chemical reaction network is represented as a system of coupled ODEs, that are then solved either through the Kaps–Rentrop (Rosenbrok) method (Kaps and Rentrop, 1979) or the Bader–Deuflhard method (Bader and Deuflhard, 1983), supplemented by a sparse matrix package for solving linear systems of equations. Furthermore, an existing gamma-law equation of state solver was modified to describe multiple species, each with temperature-dependent properties necessary for realistic simulations of combustion applications. The calculation of temperature-dependent transport properties of constituent species is accomplished through a comprehensive expansion of the materials database. We take advantage of the capability in FLASH to handle the diffusion of heat, mass, and momentum either through an update of the fluxes of each quantity across cell faces, or by directly solving a diffusion equation for each, and the relative merits of each approach for reacting flows are discussed. The capabilities of the modified tool are extensive and in some instances unique, and are documented in detail, along with numerical properties. We also present results from validation of the above capabilities through comparison with analytical solutions, and published numerical and experimental data of chemically reacting flows. Our validation cases include comparison of temporal evolution of species and temperature in a well stirred reactor, comparison of adiabatic flame temperature data, advection of reacting and non-reacting 1D fronts, 2D laminar premixed methane–air flame in a Bunsen burner configuration, shock-driven combustion of an initially circular hydrogen bubble, and a reacting Richtmyer–Meshkov instability not studied previously.

Tracer-free mixture-fraction measurements were demonstrated in a jet using femtosecond-laser electronic-excitation tagging. Measurements were conducted across a turbulent jet at several downstream locations both in a pure-nitrogen jet exiting into an air-nitrogen mixture and in a jet containing an air-nitrogen mixture exiting into pure nitrogen. The signal was calibrated with known concentrations of oxygen in nitrogen. The spatial resolution of the measurement was ∼180 μm. The measurement uncertainty ranged from 5% to 15%, depending on the mixture fraction and location within the beam, under constant temperature and pressure conditions. The measurements agree with a mixture fraction of unity within the potential core of the jet and transition to the self-similar region.

Cycle-resolved measurements of H2O temperatures and number densities taken within the detonation channel of a hydrogen—air rotating detonation engine (RDE) at a 100 kHz repetition rate using laser absorption spectroscopy are presented. The laser source used is an MEMS-tunable Vertical-Cavity Surface Emitting laser which scans from 1330 to 1360 nm. Optical access into and out of the RDE is achieved using a dual-core fiber optic. Light is pitched into the RDE through a sapphire window via a single-mode core, retroreflected off the mirror-polished inner radius of the RDE annulus, and collected with the multi-mode fiber core. The resulting absorption spectra are used to determine gas temperatures as a function of time. These measurements allow characterization of the transient-temperature response of the RDE.

Single-laser-shot femtosecond rotational coherent anti-Stokes Raman scattering (fs-RCARS) temperature measurements are performed across a 6-mm line in a turbulent, sooting ethylene jet flame to characterize temperature gradients. A 60- fs pulse is used to excite many rotational Raman transitions in N 2 , and a 160-ps pulse is used to probe the Raman coherence. The spatial resolution of the measurements is 500 μm in the direction of beam propagation and 50 μm in the transverse directions. Measurements have been performed at multiple locations in the jet flame, and the measured temperature are similar to previously recorded point measurements. Future work will include performing simultaneous laser-induced incandescence (LII) measurements to measure soot volume fraction to perform joint statistical analysis of the sooting turbulent flame.

Application of time-resolved ultrashort-pulse-laser–based laser-induced breakdown spectroscopy (LIBS) is proposed for diagnostics in hydrocarbon flames at elevated pressures. Earlier study in nanosecond-laser–based LIBS for measuring fuel-to-air (F/A) ratios in stable hydrocarbon flame at high pressures is reported to have high measurement instabilities. A correlated measurement of time-resolved LIBS and electron number density measurement is carried out to develop an understanding of the source of the aforesaid measurement instability. The planned correlated measurements of LIBS and electron number density with short picosecond to femtosecond excitations have potentials to significantly reduce the instability in F/A ratio measurements at high pressure.

Femtosecond (fs)-duration laser pulses are well suited for two-photon laser-induced-fluorescence (TPLIF) imaging of key atomic species such as H, N, and O in gas-phase reacting flows. Ultrashort pulses enable efficient nonlinear excitation, while reducing interfering photochemical processes. Furthermore, amplified fs lasers enable high-repetition-rate imaging (typically 1–10 kHz) for capturing the dynamics of turbulent flow fields. However, two-dimensional (2D), single-laser-shot fs-TPLIF imaging of the above species is challenging in most practical flow fields because of the limited ultraviolet pulse energy available in commercial optical parametric amplifier (OPA)-based tunable laser sources. In this work, we report the development of an efficient, fs frequency-quadrupling unit [i.e., fourth-harmonic generator (FHG)] with overall conversion efficiency more than six times greater than that of commercial OPA-based systems. The development, characterization, and application of the fs-FHG system for 2D imaging of H atoms in flames are described in detail. The potential application of the same laser system for 2D imaging of N and O atoms is also discussed.

100-kHz particle image velocimetry (PIV) is demonstrated using a double-pulsed, burst-mode laser with a burst duration up to 100 ms. This enables up to 10,000 time-sequential vector fields for capturing a temporal dynamic range spanning over three orders of magnitude in high-speed turbulent flows. Pulse doublets with inter-pulse spacing of 2 µs and repetition rate of 100 kHz are generated using a fiber-based oscillator and amplified through an all-diode-pumped, burst-mode amplifier. A physics-based model of pulse doublet amplification in the burst-mode amplifier is developed and used to accurately predict oscillator pulse width and pulse intensity inputs required to generate equal-energy pulse doublets at 532 nm for velocity measurements. The effect of PIV particle response and high-speed-detector limitations on the spatial and temporal resolution are estimated in subsonic turbulent jets. An effective spatial resolution of 266–275 µm and temporal resolution of 10 µs are estimated from the 8 × 8 pixel correlation window and inter-doublet time spacing, respectively. This spatiotemporal resolution is sufficient for quantitative assessment of integral time and length scales in highly turbulent jets with Reynolds numbers in the range 15,000–50,000. The temporal dynamic range of the burst-mode PIV measurement is 1200, limited by the 85-ms high-energy portion of the burst and 30-kHz high-frequency noise limit.

Two-photon-absorption laser-induced fluorescence of Kr was explored using both nanosecond- and femtosecond-duration laser excitation sources. Fluorescence signals following two-photon excitation at two wavelengths (212.56 nm and 214.77 nm) were compared while varying laser pulse duration, energy, and excitation wavelength as well as pressure and Kr mole fraction in mixtures with nitrogen. Our findings show that stronger fluorescence was observed when the excitation wavelength was tuned to 212.56 nm, regardless of the excitation-pulse duration. Moreover, an approximate 100-fold signal enhancement from nanosecond excitation (∼3 mJ/pulse, 10 ns duration) was observed as compared to femtosecond excitation (∼6 μJ/pulse, 90 fs duration).

Thomson scattering (TS), as a popular and reliable diagnostic technique, has successfully measured electron temperatures and electron number densities of plasmas for many years. However, conventional TS techniques using Nd:YAG lasers operate only at tens of hertz. Here, we present the development of a high-repetition-rate TS instrument based on a high-speed, pulse-burst laser system to greatly increase the temporal resolution of measurements. Successful instrument prototype testing was carried out by collecting TS light from laboratory helium and argon plasmas at 10 kHz. Calibration of the instrument detection sensitivity using nitrogen/oxygen rotational Raman scattering signal is also presented. Quantitative electron number densities and electron temperatures of the plasma were acquired at 10 kHz, for stable plasma discharges as, respectively, ~0.9 eV and ~5.37 × 1021 m−3 for the argon plasma, and ~1 eV and ~6.5 × 1021 m−3 for the helium plasma.

The fuel/air ratio (FAR) in a methane-air Hencken flame at pressures of 1–5 bar is measured using the chemiluminescence-based method. Emission spectra are used to investigate the effects of pressure on the OH* (308 nm), CH* (430 nm), and C2* (500 nm) emissions and the effect on equivalence ratio determination from the ratios of these emission peaks. Both OH*/CH* and C2*/CH* ratios are linear to FAR at atmospheric pressure. At elevated pressures, C2*/CH* remains roughly linear to FAR, while OH*/CH* becomes highly nonlinear. There are significant spectral contributions from continuum radiation at higher pressures, likely due to increasing soot production. Therefore, while it is a truly passive and nonintrusive diagnostic method, the use of chemiluminescence for FAR determination at high pressures could be limited. Possible improvements to the measurement setup and future studies are discussed.

Gas composition detection and analysis in high-pressure environments, such as those encountered in combustors, power plants, and planetary bodies, are important for scientific and engineering research. Conventional laser-based sensing using nanosecond (ns)-laser-induced breakdown spectroscopy (ns-LIBS) remains unreliable at elevated pressures because of the relatively high measurement uncertainty. To circumvent this problem, we propose to employ femtosecond-laser-induced plasma spectroscopy (FLIPS) to achieve a stable and reliable gas sensing at elevated pressures. A comprehensive study on the differences in the measurement quality of FLIPS and ns-LIBS for high-pressure nitrogen gas is conducted. The results show that the signal level and stability of ns-LIBS significantly decrease with the increase in pressure. In contrast, the signal level of FLIPS increases with the increase in pressure while maintaining the stability in the pressure range of 1–40 bar. In addition to the superior measurement stability, the FLIPS measurement provides a longer probe length (∼3 times compared to ns-LIBS) with a higher data acquisition speed (100 times compared to 10-Hz ns-LIBS).

Simultaneous high-speed quantitative imaging of mixture fraction and velocity is demonstrated using the fourth- and second-harmonic outputs, respectively, of a burst-mode Nd:YAG laser. A tenfold increase in the record length and 16-fold increase in per-pulse energy are achieved compared with previous measurements of mixture fraction using burst-mode and continuously pulsed diode-pumped solid-state lasers, respectively. The high output energy is used for quantitative, high-speed mixture-fraction imaging with acetone planar laser-induced fluorescence, which also enables simultaneous particle-based velocimetry without interference from particle scattering. A semiquantitative model is used to determine the limitations on fourth-harmonic output energy due to the effects of transient absorption and thermal phase mismatch over a range of repetition rates. Data are presented for mixing within a turbulent jet (Reynolds number of 15,000) and are validated by comparisons with known turbulent mixing laws and previously published data.

Active photothermal effects of aluminum nanoenergetics are demonstrated to control spatially and temporally the ignition of propane/air flames. High-speed chemiluminescence images of hydroxyl (OH * ) and methylidyne (CH * ) radicals and aluminum oxide (AlO) are employed to confirm the local flame ignition and propagation by activation of the nanoenergetics through the use of a single exposure from a xenon flash lamp. The photothermal effects of the nanoenergetics can efficiently activate the localized exothermal aluminum oxidation reactions, leading to sequential flame ignition. Most of the energy required for ignition is provided by the oxidation reaction of the aluminum nanoparticles such that the photothermal MIE (Minimum Ignition Energy) is at least 67 times less than that from regular spark ignition.